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  • 1.
    Bahaloo, Hassan
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Casselgren, Johan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Forsberg, Fredrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Capillary Bridge in Contact with Ice Particles Can Be Related to the Thin Liquid Film on Ice2024Ingår i: Journal of cold regions engineering, ISSN 0887-381X, E-ISSN 1943-5495, Vol. 38, nr 1, artikel-id 04023021Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We experimentally demonstrate the presence of a capillary bridge in the contact between an ice particle and a smooth aluminum surface at a relative humidity of approximately 50% and temperatures below the melting point. We conduct the experiments in a freezer with a controlled temperature and consider the mechanical instability of the bridge upon separation of the ice particle from the aluminum surface at a constant speed. We observe that a liquid bridge forms, and this formation becomes more pronounced as the temperature approaches the melting point. We also show that the separation distance is proportional to the cube root of the volume of the bridge. We hypothesize that the volume of the liquid bridge can be used to provide a rough estimate of the thickness of the liquid layer on the ice particle since in the absence of other driving mechanisms, some of the liquid on the surface must have been pulled to the bridge area. We show that the estimated value lies within the range previously reported in the literature. With these assumptions, the estimated thickness of the liquid layer decreases from nearly 56 nm at T = −1.7°C to 0.2 nm at T = −12.7°C. The dependence can be approximated with a power law, proportional to (TM − T)−β, where β < 2.6 and TM is the melting temperature. We further observe that for a rough surface, the capillary bridge formation in the considered experimental conditions vanishes.

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  • 2.
    Westin, E. M.
    et al.
    voestalpine Böhler Welding Austria GmbH, Böhler-Welding-Str. 1, 8605, Kapfenberg, Austria.
    Westerberg, Lars-Göran
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Evaluation of methods used for simulation of heat-affected zones in duplex stainless steels2024Ingår i: Welding in the World, ISSN 0043-2288, E-ISSN 1878-6669Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The weldability of duplex stainless steels partly depends on the ferritization of the high-temperature heat-affected zone (HT-HAZ). This area is rather narrow, and it can be challenging to visualize and determine its actual impact on the properties. To address this, various methods were applied to study the grain growth and austenite reformation in the HT-HAZ of the lean duplex grade UNS S32101. Thermo-mechanical Gleeble® simulations were conducted at 1360 °C with different holding times and cooling rates. Subsequently, the grain size and ferrite content were measured on polished and etched cross-sections. Bead-on-plate welds were performed on the same heat of 6-mm plate thickness using the gas tungsten arc welding (GTAW) process. The shielding gas was Ar + 0–8% N2 to illustrate the effect of nitrogen additions on the HT-HAZ morphology. The arc was either stationary, welding at one spot for 0.5–120 s, or travelling at different speeds to generate varying heat inputs and temperature gradients. The thermo-mechanical simulations approximated the results obtained by travelling arc welding and allowed for a more comprehensive investigation. Stationary arc welding was not suitable for HT-HAZ studies as it quickly caused nitrogen depletion and resulted in significantly higher ferrite contents compared to the travelling arc welds.

  • 3.
    Shiraghaee, Shahab
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sundström, Joel
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Raisee, Mehrdad
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran.
    Cervantes, Michel J.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Extending the operating range of axial turbines with the protrusion of radially adjustable flat plates: An experimental investigation2024Ingår i: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 225, artikel-id 120232Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The implementation of hydropower to stabilize electrical grids dictates more frequent off-design operations of these renewable energy resources. Flow instabilities under such conditions reduce the efficiency of hydro turbines. Part-load operation is particularly detrimental since the development of a rotating vortical structure termed rotating vortex rope (RVR) in the draft tube leads to periodic pressure pulsations that jeopardize turbine performance. This paper experimentally explores a novel solution involving the protrusion of flat plates into the turbine draft tube. Three flat plates equally separated by 120° were vertically installed on the draft tube wall. The plates were protruded up to 83% of the draft tube local radius under four different part-load conditions. Their impact was observed through time-resolved pressure measurements in the draft tube and vaneless space, as well as efficiency measurements. The results demonstrated successful RVR mitigation, achieving a maximum 85% reduction in pressure oscillation amplitudes. Protruding flat plates disrupted RVR periodicity and coherence, confining its orbit to the draft tube center. This approach proved particularly effective at lower part-load conditions, enhancing turbine hydraulic efficiency by increasing torque extraction. Reducing the adverse effects under part load, the proposed method appears promising in extending the operational range of hydraulic turbines.

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  • 4.
    Kalantar Neyestanaki, Mehrdad
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Dunca, Georgiana
    Department of Hydraulics, Hydraulic Machinery and Environmental Engineering, University Politehnica of Bucharest, Romania.
    Jonsson, Pontus
    Vattenfall AB, Sweden.
    Cervantes, Michel J.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Extending the pressure-time method to bend using 3D-CFD2024Ingår i: Flow Measurement and Instrumentation, ISSN 0955-5986, E-ISSN 1873-6998, Vol. 96, artikel-id 102535Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    According to the IEC 60041 standard, the pressure-time method (1D PTM) can be employed to determine the flow rate in hydraulic turbines. This method assumes a one-dimensional flow and applies to straight pipes with uniform cross-sections, with specific restrictions on the pipe length, fluid velocity, and distance between the measurement sections from any irregularities in the pipeline. However, challenges arise when applying this method in low-head hydropower plants due to the short lengths, irregularities like bends and developing flows in the intake. The present paper aims to improve the performance of the method in the presence of a bend. To this end, a test rig has been developed and measurements performed, including such geometry. The data are evaluated using the development of a newly proposed approach combining the 1D PTM based on an energy balance formulation and three-dimensional computational fluid dynamics (3D CFD) developed for axis-symmetrical accelerating flows. The updated methodology includes a correction of the experimental pressure measurements used in the 1D PTM to account for the effects of the Dean vortices present after the bend as well as the kinetic energy correction factors which deviate from known values in transient conditions. The results obtained under conditions involving the presence of bends either between or in close proximity to one show a significant improvement compared to the standard one-dimensional pressure-time method.

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  • 5.
    Kalantar Neyestanaki, Mehrdad
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Dunca, Georgiana
    Department of Hydraulicsm, Hydraulic Machinery and Environmental Engineering, University Politehnica of Bucharest , București 060042, Romania.
    Jonsson, Pontus
    Vattenfall AB, Sweden.
    Cervantes, Michel J.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Extending the Pressure-Time Method to Pipe With Variable Cross-Section With Three-Dimensional Numerical Simulations2024Ingår i: Journal of Fluids Engineering, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 146, nr 2, artikel-id 021305Artikel i tidskrift (Refereegranskat)
  • 6.
    Kalantar Neyestanaki, Mehrdad
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Extension of the Pressure Time Method to 3-Dimensional Flows2024Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Hydropower has stood as a clean and sustainable energy source since the late 19th century. Many turbines were built 50 to 70 years ago and require refurbishment. It is important to assess the efficiency of turbines before and after refurbishment to meet performance guarantees .However, the flow rate makes such estimation challenging. Moreover, determining the volumetric flow rate is crucial to specify the hydraulic performance characteristics of hydraulic turbines. The pressure-time method allows measuring the flow rate in hydraulic turbines, according to the IEC 60041 standard, based on transforming momentum into pressure during the deceleration of a liquid mass. The flow rate is obtained by integrating the differential pressure and the pressure loss history between two cross-sections.

    This method assumes a one-dimensional flow (1D) and is limited to straight pipes with a uniform cross-section and specific restrictions on length (L>10 m), velocity (U.L>50 m2s-1) and distance between the measurement sections from any irregularities in the pipeline. However, challenges arise when applying this method in low-head hydropower plants due to the short lengths, irregularities like bends, variation in cross section and developing flows in the intake. This thesis aims to improve the performance of the method out of IEC standards for conditions similar to low-head conditions.

    The thesis is divided into the numerical simulation of the fluid during the pressure-time method transient, experimental measurement, and a combination of both. The physics in the pressure-time method is studied to compare different assumptions to estimate the viscous losses for both developed and developing flow. Moreover, a test rig has been developed to extend the method’s applicability. The test rig is designed to study the pressure-time method for developing flow conditions, small measurement lengths, variable cross-section and the presence of bend close to measurement sections, which could be similar to low-head turbine conditions.

    Finally, the data are evaluated using the new approach combining the 1D pressure-time method and three-dimensional computational fluid dynamics (3D CFD).

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  • 7.
    Kumar, S.
    et al.
    Indian Inst Technol, Dept Mech & Ind Engn, Roorkee 247667, Uttarakhand, India.
    Cervantes, Michel J.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gandhi, B.K.
    Indian Inst Technol, Dept Mech & Ind Engn, Roorkee 247667, Uttarakhand, India.
    Flow Field Analysis of Francis Turbine Draft Tube using POD at Design and Part Load Operating Conditions2024Ingår i: Journal of Applied Fluid Mechanics, ISSN 1735-3572, E-ISSN 1735-3645, Vol. 17, nr 4, s. 770-784Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The hydraulic turbines, especially Francis turbines, frequently run at part load (PL) conditions to meet the dynamic energy needs. The flow field at the runner exit changes significantly with a change in the operating point. At PL, flow instabilities such as the Rotating Vortex Rope (RVR) form in the draft tube of the Francis turbine. The present paper compares the features of the velocity and vorticity field of the Francis turbine draft tube at the best efficiency point (BEP) and PL operations using the Proper Orthogonal Decomposition (POD) of the 2D-PIV data. The POD analysis decomposes the flow field into coherent and incoherent structures describing the spatiotemporal behavior of the flow field. A visual representation of the coherent structures and the turbulent length scales in the flow field is extracted and analyzed for BEP and PL, respectively. The study highlights the salient features of the draft tube flow field, which differentiate the BEP and PL operation. The fast Fourier transform of the temporal coefficients confirms the presence of RVR frequency (0.29 times the runner frequency) at PL. The phase portraits of different modes elucidate the relationship between different harmonics of the RVR frequency at PL.

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  • 8.
    Seifi, Zeinab
    et al.
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran.
    Raisee, Mehrdad
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran.
    Cervantes, Michel J.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Linear global stability analysis of a transient swirling flow leading to the formation of a helical precessing vortex breakdown in a straight diffuser2024Ingår i: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 103, s. 43-57Artikel i tidskrift (Refereegranskat)
  • 9.
    Bahaloo, Hassan
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Forsberg, Fredrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Casselgren, Johan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lycksam, Henrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Mapping of density-dependent material properties of dry manufactured snow using μCT2024Ingår i: Applied Physics A: Materials Science & Processing, ISSN 0947-8396, E-ISSN 1432-0630, Vol. 130, artikel-id 16Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Despite the significance of snow in various cryospheric, polar, and construction contexts, more comprehensive studies are required on its mechanical properties. In recent years, the utilization of μ CT has yielded valuable insights into snow analysis. Our objective is to establish a methodology for mapping density-dependent material properties for dry manufactured snow within the density range of 400–600 kg/m 3 utilizing μ CT imaging and step-wise, quasi-static, mechanical loading. We also aim to investigate the variations in the structural parameters of snow during loading. The three-dimensional (3D) structure of snow is captured using μ CT with 801 projections at the beginning of the experiments and at the end of each loading step. The sample is compressed at a temperature of − 18 o C using a constant rate of deformation (0.2 mm/min) in multiple steps. The relative density of the snow is determined at each load step using binary image segmentation. It varies from 0.44 in the beginning to nearly 0.65 at the end of the loading, which corresponds to a density range of 400–600 kg/m 3 . The estimated modulus and viscosity terms, obtained from the Burger’s model, show an increasing trend with density. The values of the Maxwell and Kelvin–Voigt moduli were found to range from 60 to 320 MPa and from 6 to 40 MPa, respectively. Meanwhile, the viscosity values for the Maxwell and Kelvin–Voigt models varied from 0.4 to 3.5 GPa-s, and 0.3–3.2 GPa-s, respectively, within the considered density range. In addition, Digital Volume Correlation (DVC) was used to calculate the full-field strain distribution in the specimen at each load step. The image analysis results show that, the particle size and specific surface area (SSA) do not change significantly within the studied range of loading and densities, while the sphericity of the particles is increased. The grain diameter ranges from approximately 100 μ m to nearly 400 μ m, with a mode of nearly 200 μ m. The methodology presented in this study opens up a path for an extensive statistical analysis of the material properties by experimenting more snow samples.

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  • 10.
    Bahaloohoreh, Hassan
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Forsberg, Fredrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lycksam, Henrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Casselgren, Johan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Material mapping strategy to identify the density-dependent properties of dry natural snow2024Ingår i: Applied Physics A: Materials Science & Processing, ISSN 0947-8396, E-ISSN 1432-0630, Vol. 130, nr 2, artikel-id 141Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The mechanical properties of natural snow play a crucial role in understanding glaciers, avalanches, polar regions, and snow-related constructions. Research has concentrated on how the mechanical properties of snow vary, primarily with its density; the integration of cutting-edge techniques like micro-tomography with traditional loading methods can enhance our comprehension of these properties in natural snow. This study employs CT imaging and uniaxial compression tests, along with the Digital Volume Correlation (DVC) to investigate the density-dependent material properties of natural snow. The data from two snow samples, one initially non-compressed (test 1) and the other initially compressed (test 2), were fed into Burger’s viscoelastic model to estimate the material properties. CT imaging with 801 projections captures the three-dimensional structure of the snow initially and after each loading step at -18C, using a constant deformation rate (0.2 mm/min). The relative density of the snow, ranging from 0.175 to 0.39 (equivalent to 160–360 kg/m), is determined at each load step through binary image segmentation. Modulus and viscosity terms, estimated from Burger’s model, exhibit a density-dependent increase. Maxwell and Kelvin–Voigt moduli range from 0.5 to 14 MPa and 0.1 to 0.8 MPa, respectively. Viscosity values for the Maxwell and Kelvin–Voigt models vary from 0.2 to 2.9 GPa-s and 0.2 to 2.3 GPa-s within the considered density range, showing an exponent between 3 and 4 when represented as power functions. Initial grain characteristics for tests 1 and 2, obtained through image segmentation, reveal an average Specific Surface Area (SSA) of around 55 1/mm and 40 1/mm, respectively. The full-field strain distribution in the specimen at each load step is calculated using the DVC, highlighting strong strain localization indicative of non-homogeneous behavior in natural snow. These findings not only contribute to our understanding of natural snow mechanics but also hold implications for applications in fields such as glacier dynamics and avalanche prediction.

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  • 11.
    Trieu, Hang
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Bergström, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Andreasson, Patrik
    Wildlife, fish and environmental studies, Swedish University of Agricultural Sciences.
    Lycksam, Henrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Natural surface floaters in image-based river surface velocimetry: Insights from a case study2024Ingår i: Flow Measurement and Instrumentation, ISSN 0955-5986, E-ISSN 1873-6998, Flow Measurement and Instrumentation, ISSN 0955-5986, Vol. 96, nr 102557Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This study focuses on utilizing image techniques for river velocity measurement, with a specific emphasis on natural surface floating patterns. Employing a multi-camera system, we conducted 3D measurements on river surfaces, including surface velocity and water surface reconstruction. A pattern-based tracking approach has been adopted to improve the performance of image measurements on different types of natural floating tracers. The study employs the following approaches: 3D Lagrangian Pattern Tracking Velocimetry (3D-LPTV), 2D Lagrangian Pattern Velocimetry (2D- LPTV), and Large-scale Particle Image Velocimetry (LSPIV), for surface velocity estimation. The outcomes revealed that all three approaches yielded consistent results in terms of averaged velocity. However, the LSPIV method produced about two times higher uncertainty in measured velocities compared to the other methods. A strategy to assess the quality of river surface patterns in velocity estimation is presented. Specifically, the sum of squared interrogation area intensity gradient (SSIAIG) was found to be strongly correlated with measurement uncertainty. Additionally, a term related to the peak sidelobe ratio (PSR) of the cross-correlation map was found as an effective constraint, ensuring the image-tracking process achieves high reliability. The precision of measurements increases corresponding to the increase of image intensity gradient and PSR.

  • 12.
    Khullar, Subodh
    et al.
    Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee, 247667, Roorkee, Uttarakhand, India.
    Kumar, Sandeep
    Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee, 247667, Roorkee, Uttarakhand, India.
    Singh, Krishna M
    Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee, 247667, Roorkee, Uttarakhand, India.
    Cervantes, Michel J
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gandhi, Bhupendra K
    Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee, 247667, Roorkee, Uttarakhand, India.
    Numerical study of peripheral air injection in draft tube of a Francis turbine at upper and normal part load operations2024Ingår i: Sadhana (Bangalore), ISSN 0256-2499, E-ISSN 0973-7677, Vol. 49, nr 1, artikel-id 81Artikel i tidskrift (Refereegranskat)
  • 13.
    Valizadeh, Ali
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap.
    Skoglund, Nils
    Department of Applied Physics and Electronics, Umeå University, SE-90187 Umeå, Sweden.
    Forsberg, Fredrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lycksam, Henrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Öhman, Marcus
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap.
    Role of surface morphology in bed particle layer formation on quartz bed particles in fluidized bed combustion of woody biomass2024Ingår i: Fuel, ISSN 0016-2361, E-ISSN 1873-7153, Vol. 357, nr part A, artikel-id 129702Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The influence of quartz bed particle surface morphology on the bed particle layer and crack layer formation process in fluidized bed combustion of woody biomass was investigated in this work. Bed material samples were collected at different sampling times from the startup with a fresh bed in industrial scale bubbling fluidized bed (BFB) and circulating fluidized bed (CFB) boilers, both utilizing woody biomass. X-ray microtomography (XMT) and scanning electron microscopy coupled with energy dispersive spectroscopy (SEM/EDS) were employed to characterize bed particle layers and crack layers in the samples. Results showed that there is a noticeable difference between the bed layer characteristics over the so-called “concave” and “convex”-shaped morphologies on the bed particle surface with respect to layer formation. The concave areas are mainly covered with a thin inner layer, whilst the convex display a comparably thick inner layer and an outer layer. In addition, 3D images of the particles revealed that the crack layers mainly originate from concave areas where the particle is less protected by an outer bed particle layer in conjunction with cracks in the inner layer.

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  • 14.
    Ghasemi Monfared, Zahra
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Umeki, Kentaro
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap.
    The Impact of Discrete Element Method Parameters on Realistic Representation of Spherical Particles in a Packed Bed2024Ingår i: Processes, E-ISSN 2227-9717, Vol. 12, nr 1, artikel-id 183Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Packed bed reactors play a crucial role in various industrial applications. This paper utilizes the Discrete Element Method (DEM), an efficient numerical technique for simulating the behavior of packed beds of particles as discrete phases. The focus is on generating densely packed particle beds. To ensure the model accuracy, specific DEM parameters were studied, including sub-step and rolling resistance. The analysis of the packed bed model extended to a detailed exploration of void fraction distribution along radial and vertical directions, considering the impact of wall interactions. Three different samples, spanning particle sizes from 0.3 mm to 6 mm, were used. Results indicated that the number of sub-steps significantly influences void fraction precision, a key criterion for comparing simulations with experimental results. Additionally, the study found that both loosely and densely packed beds of particles could be accurately represented by incorporating appropriate values for rolling friction. This value serves as an indicator of both inter-particle friction and friction between particles and the walls. An optimal rolling friction coefficient has been thereby suggested for the precise representation for the densely packed bed of spherical char particles.

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  • 15.
    Alnersson, Gustaf
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Gestamp Hardtech, Luleå, Sweden.
    Lejon, Erik
    Gestamp Hardtech, Luleå, Sweden.
    Zrida, Hana
    Gestamp Hardtech, Luleå, Sweden.
    Aitomäki, Yvonne
    RISE Sicomp, Öjebyn, Sweden.
    Ljung, Anna-Lena
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lundström, T. Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    3D flow and fibre orientation modelling of compression moulding of A-SMC: simulations and experimental validation in squeeze flow2023Ingår i: Functional Composite Materials, E-ISSN 2522-5774, Vol. 4, artikel-id 11Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Sheet Moulding Compound (SMC) based composites have a large potential in industrial contexts due to the possibility of achieving comparatively short manufacturing times. It is however necessary to be able to numerically predict both mechanical properties as well as manufacturability of parts.

    In this paper a fully 3D, semi-empirical model based on fluid mechanics for the compression moulding of SMC is described and discussed, in which the fibres and the resin are modelled as a single, inseparable fluid with a viscosity that depends on volume fraction of fibres, shear strain rate and temperature. This model is applied to an advanced carbon-fibre SMC with a high fibre volume fraction (35%). Simulations are run on a model of a squeeze test rig, allowing comparison to experimental results from such a rig. The flow data generated by this model is then used as input for an Advani-Tucker type of model for the evolution of the fibre orientation during the pressing process. Numerical results are also obtained from the software 3DTimon. The resulting fibre orientation distributions are then compared to experimental results that are obtained from microscopy. The experimental measurement of the orientation tensors is performed using the Method of Ellipses. A new, automated, accurate and fast method for the ellipse fitting is developed using machine learning. For the studied case, comparison between the experimental results and numerical methods indicate that 3D Timon better captures the random orientation at the outer edges of the circular disc, while 3D CFD show larger agreement in terms of the out-of-plane component. One of the advantages of the new image technique is that less work is required to obtain microscope images with a quality good enough for the analysis.

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  • 16.
    Valizadeh, Ali
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap.
    Skoglund, Nils
    Thermochemical Energy Conversion Laboratory, Department of Applied Physics and Electronics, Umeå University, SE-90187 Umeå, Sweden.
    Forsberg, Fredrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lycksam, Henrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Öhman, Marcus
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap.
    A comparative study in 3D of bed particle layer characteristics in quartz and K-feldspar from fluidized bed combustion of woody biomass using X-ray microtomography2023Ingår i: Fuel, ISSN 0016-2361, E-ISSN 1873-7153, Vol. 342, artikel-id 127707Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Bed particle layer and crack layer characteristics at different ages were studied for quartz and K-feldspar bed particles from a 30 MWth bubbling fluidized bed and a 90 MWth circulating fluidized bed, both using woody biomass as fuel. X-ray microtomography (XMT) was utilized to determine the bed particle layer distribution on the bed particles' surface. For each bed particle type, the average bed particle layer thickness as well as average volume fractions of the bed particle layer and crack layers to the entire bed particle volume were determined at three different bed particle ages by utilizing XMT analysis. Comparison of the two different bed particle types showed that K-feldspar retains a thinner bed particle layer in both conversion processes compared to quartz. Crack layers were observed extensively in quartz bed particles to the extent of 19.3 vol% and 32.1 vol% after 13 days in the BFB and the CFB, respectively, which could cause deposition of the bed particle fragments. On the contrary, K-feldspar has almost no tendency toward forming crack layers.

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  • 17.
    Moosavi, Amin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Ljung, Anna-Lena
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    A comparative study on thermo-fluid characteristics of free and wall-bounded cross-flow heat exchangers2023Ingår i: Thermal Science and Engineering Progress, ISSN 2451-9057, Vol. 40, artikel-id 101746Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In recent years, wall-bounded cross-flow heat exchangers have gained significant attention for battery cooling applications. Due to similarities in geometry, these systems are often evaluated based on the heat and flow knowledge of free cross-flow heat exchangers. To determine the reliability of this assumption, this study performs a numerical comparison of the thermo-fluid behavior of wall-bounded and free cross-flow heat exchangers. Both heat exchangers have similar dimensions, with transverse and longitudinal pitch ratios of 2.074 and 1.037, respectively, and are investigated at a Reynolds number of 40000 using the Unsteady Reynolds-Averaged Navier–Stokes (URANS) method. It is observed that the  transition model provides the most accurate predictions of the flow field when compared to available experimental data. The results suggest that for wall-bounded heat exchangers with an aspect ratio of 2 or larger, the flow behavior in the central flow region resembles that of a free heat exchanger, but with varying magnitudes due to the increase in velocity in the core region to counterbalance the reduction near the walls. The area-averaged mean Nusselt number from 2D and 3D models for free heat exchangers shows no significant difference compared to wall-bounded heat exchangers. However, there are considerable differences in the local Nusselt number distributions in the angular and spanwise directions. Overall, it is determined that certain conditions must be satisfied to ensure that applying the thermo-fluid characteristics of a free cross-flow heat exchanger to wall-bounded cross-flow heat exchangers in battery thermal management systems is accurate.

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  • 18.
    Kalantar Neyestanaki, Mehrdad
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Dunca, Georgiana
    Department of Hydraulics, Hydraulic Machinery and Environmental Engineering, University Politehnica of Bucharest, București, 060042, Romania.
    Jonsson, Pontus
    Vattenfall AB, 162 87 Stockholm, Sweden.
    Cervantes, Michel J.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    A Comparison of Different Methods for Modelling Water Hammer Valve Closure with CFD2023Ingår i: Water, E-ISSN 2073-4441, Vol. 15, nr 8, artikel-id 1510Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Water hammer is a transient phenomenon that occurs when a flowing fluid is rapidly decelerated, which can be harmful and damaging to a piping system. Three-dimensional computational fluid dynamics (CFD) with three-dimensional geometry is a common tool for studying water hammer, which is more accurate than numerical simulation with one-dimension approximation of the geometry. There are different methods with different accuracy and computational costs for valve closure modelling. This paper presents the result of water hammer 3D simulation with three main technics for modelling an axial valve closure: dynamic mesh, sliding mesh, and immersed solid methods. The variation of the differential pressure variation and the wall shear stress are compared with experimental results. Additionally, the 3D effects of the flow after the valve closure and the computational cost are addressed. The sliding mesh method presents the most physical results compared to the other two methods. The immersed solid method predicts a smaller pressure rise which may be the result of using a source term in the momentum equation instead of modelling the valve movement. The dynamic mesh method adds fluctuations to the primary phenomenon. Moreover, the sliding mesh is less expensive than the dynamic mesh method in terms of computational cost (approximately one-third), which was the primary method for axial valve closure modelling in the literature.

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  • 19.
    Goyal, Rahul
    et al.
    Department of Energy Science and Engineering, Indian Institute of Technology , Hauz Khas, New Delhi 110016, India.
    Cervantes, Michel J.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Masoodi, Faiz Azhar
    Department of Energy Science and Engineering, Indian Institute of Technology , Hauz Khas, New Delhi 110016, India.
    Sahu, Pallav
    Department of Energy Science and Engineering, Indian Institute of Technology , Hauz Khas, New Delhi 110016, India.
    A Study of the Velocity Field During Mitigation of Vortex Breakdown in Model Francis Turbine at High Load2023Ingår i: Journal of Fluids Engineering, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 145, nr 4, artikel-id 041203Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Off-design operation leads to the development of flow instabilities like vortex breakdown phenomenon which manifests as an enlarged vortex core in the draft tube at high load operating conditions. These flow instabilities are known to potentially have detrimental effects on turbine performance necessitating investigations on their formative and mitigation mechanisms. This work clarifies the evolving velocity maps characterizing vortex breakdown seen in a model Francis turbine draft tube during the transition from high load to best efficiency point. Velocity measurements have been performed inside a draft tube cone using a 2D particle image velocimetry system. Results show a wake-like velocity profile characterizing the vortex core in the draft tube cone at high load condition. The vortex core is a centrally located flow feature embodying a quasi-stagnant flow with recirculation regions. Surrounding the core, an axial outflow is seen with shear layers arising at the interface of core and outflow due to a substantial velocity gradient. Mitigation of this vortex core through a load rejection operation was further investigated. It is seen that as the flowrate approaches the best efficiency point, the shear layers between the outflow and central stagnation region break. The breakup leads to an axially dominated and streamlined flow. This is enabled by the reduction of the swirl until no central flow separation at the stagnation point occurs. The flow at the best efficiency point is thus devoid of the vortex core due to the absence of flow stagnation, the primary instability causing the core development.

  • 20.
    Moosavi, Amin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Ljung, Anna-Lena
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lundström, T. Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    A study on the effect of cell spacing in large-scale air-cooled battery thermal management systems using a novel modeling approach2023Ingår i: Journal of Energy Storage, ISSN 2352-152X, E-ISSN 2352-1538, Vol. 72, artikel-id 108418Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Recent studies have revealed that effective thermal management systems are necessary to maintain the performance, lifespan, and safety of lithium battery systems. A unique and novel modeling approach is presented in this work with the aim of estimating the thermal performance of air-based cooling systems for large-scale lithium battery packages. The overall model consists of sub-models, including an analytical model for battery cells and a numerical heat and flow model for the battery module, which are validated against experimental data and empirical correlations, respectively. The chosen approach implies that the sub-models can operate independently, allowing accurate transient simulations with reduced processing time. The model is employed to evaluate the effect of cell spacing on the thermal performance of an air-cooled battery system designed for a hybrid electric vehicle. The results demonstrate that the maximum temperature within the cells positively correlates with transverse and longitudinal pitch ratios; however, the maximum temperature difference in the module has a negative correlation with these pitch ratios. In contrast, temperature uniformity shows non-monotonic behavior, making it an applicable criterion to balance between temperature rise and thermal gradients. Moreover, considerable temperature non-uniformity is noted in the early rows, which becomes less significant as pitch ratios decrease.

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  • 21.
    Jouybari, Nima Fallah
    et al.
    Fibre Science and Communication Networks (FSCN), Mid Sweden University, 85170, Sundsvall, Sweden.
    Engberg, Birgitta
    Fibre Science and Communication Networks (FSCN), Mid Sweden University, 85170, Sundsvall, Sweden.
    Persson, Johan
    Fibre Science and Communication Networks (FSCN), Mid Sweden University, 85170, Sundsvall, Sweden.
    Berg, Jan-Erik
    Fibre Science and Communication Networks (FSCN), Mid Sweden University, 85170, Sundsvall, Sweden.
    Lundström, T. Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    An investigation of forces on a representative surface in a pulp flow through rotating and non-rotating grooves2023Ingår i: Journal of the Brazilian Society of Mechanical Sciences and Engineering, ISSN 1678-5878, E-ISSN 1806-3691, Vol. 45, nr 5, artikel-id 280Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Softwood pulp flow in rotating and non-rotating grooves is numerically simulated in the present study to investigate the fluid flow and the forces acting on a representative surface mounted in the groove. The viscosity of softwood pulp with various consistencies is available from the measurements reported in the literature providing the opportunity to examine the effects of fiber consistency on the velocity and pressure distribution within the groove. The simulations are carried out in OpenFOAM for different values of gap thickness, angular velocity and radial positions from which the pressure coefficient and shear forces values are obtained. It is found that the shear forces within the gap increase linearly with the angular velocity for all fiber consistencies investigated and in both grooves. Also, this behavior can be successfully predicted by modeling the gap flow as a Couette flow in a two-dimensional channel. Meanwhile, a more detailed analysis of the flow kinetic energy close to the stagnation point using Bernoulli’s principle is carried out to provide a better understanding of the pressure coefficient variation with angular velocity in the non-rotating groove. A comparison of pressure coefficients obtained numerically with those calculated by considering the compression effects revealed that the comparison effects are dominating in the pulp flow within the groove.

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  • 22.
    Bahaloohoreh, Hassan
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Casselgren, Johan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Forsberg, Fredrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Capillary bridge in contact of ice particles reveals the thin liquid film on ice2023Manuskript (preprint) (Övrigt vetenskapligt)
  • 23.
    Sjöstedt, Lovisa M.
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Andersson, Anders G.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    CFD Modelling as a Tool to Better Understand Fish Behavior2023Ingår i: Proceedings of the 40th IAHR World Congress - 2023: Rivers - Connecting Mountains and Coasts / [ed] Helmut Habersack; Michael Tritthart; Lisa Waldenberger, International Association for Hydro-Environment Engineering and Research (IAHR) , 2023, s. 2665-2672Konferensbidrag (Refereegranskat)
  • 24.
    Shiraghaee, Shahab
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sundström, Joel
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Mehrdad, Raisee
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran 11155-4563, Iran.
    Cervantes, Michel J.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Characterization of The Rotating Vortex Rope Pressure Oscillations in a Kaplan Model Turbine Draft Tube2023Ingår i: International Journal of Fluid Machinery and Systems, ISSN 1882-9554, Vol. 16, nr 2, s. 204-218Artikel i tidskrift (Refereegranskat)
  • 25.
    Sollén, Sofia
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Casselgren, Johan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Comparing floating car data regarding tire-to-road friction for different-sized operational areas during winter- and summertime in Sweden2023Ingår i: Pre-proceedings Prague 2023, 2023Konferensbidrag (Refereegranskat)
  • 26.
    Sollén, Sofia
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Casselgren, Johan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Comparison of methods for winter road friction estimation using systems implemented for floating car data2023Ingår i: International Journal of Vehicle Systems Modelling and Testing, ISSN 1745-6436, E-ISSN 1745-6444, Vol. 17, nr 2, s. 101-111Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Winter road maintenance is important for preventing accidents and enabling mobility. If the road friction gets low, there is a higher risk of road accidents. Therefore, it is vital to have information about road friction levels. Traditionally this is done by dedicated vehicles; however, using friction information from floating car data (FCD) would be more beneficial, as the coverage both in time and space increases. In this investigation, road friction data from three FCD suppliers, using only one test vehicle each, has been compared with a continuous method of road friction measurement. The test has been conducted on proving grounds covered with ice and snow, and on public roads covered with water, ice, snow, and slush; thereby both high friction and low friction surfaces have been evaluated. The investigation shows that the FCD provides a continuous method of friction measurement and is closer to the reality of road friction experienced by road users.

  • 27.
    Siddanathi, Likitha Sai
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Westerberg, Lars-Göran
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Åkerstedt, Hans O.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sepman, Alexey
    RISE Energy Technology Center, Piteå, Sweden.
    Wiinikka, Henrik
    RISE Energy Technology Center, Piteå, Sweden.
    Computational Analysis Of Different Non-Transferred Plasma Torch Geometries2023Konferensbidrag (Övrigt vetenskapligt)
    Abstract [en]

    A non-transferred plasma torch is a device used to generate a steady thermal plasma jet. These electric-driven plasma burners have the potential to be upscaled and used as heat sources in the process industry instead of the large-scale fossil fuel burners used today. A non-transferred plasma torch comprises the combination of a cathode, anode, and working gas flowing between the electrodes in order to generate a plasma jet. To enable the upscaling it is important to understand the effects of different electrode shapes and sizes on the plasma jet formation and its properties. There are two available cathode shapes: flat cathodes and conical cathodes, and the two shapes of anodes are cylindrical and stepped anodes. Therefore, to understand their effects computational analysis is performed on 2D axisymmetric geometries. Steady-state computational analysis is done using magnetohydrodynamic (MHD) modeling assuming the plasma to be fully ionized. The results obtained for the different cases are as follows: 

    1.    Cathode and anode shape study: The velocity and temperature of the plasma jet are analyzed, and the variation of arc formation is studied for all the cases.

    2.    Anode diameter: The anode diameter influences the arc formation and to understand its effects a case study for different anode diameters is conducted. The results obtained explained that the increase in anode diameter reduces the current in the arc.

    3.    Flow separation: One interesting observation made is flow separation at the tip of flat cathodes. The study explains the advantages or disadvantages of the observed flow separation. Further, if the flow separation is a disadvantage, methods to eliminate it are explained.

    4.    Conclusion: Based on the heat losses and the plasma torch efficiency, the work concludes by presenting the best cathode and anode geometries that can be used.

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  • 28.
    Siddanathi, Likitha Sai
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Westerberg, Lars-Göran
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Åkerstedt, Hans O.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Wiinikka, H.
    RISE AB, SE-941 38 Piteå, Sweden.
    Sepman, A.
    RISE AB, SE-941 38 Piteå, Sweden.
    Computational modeling and temperature measurements using emission spectroscopy on a non-transferred plasma torch2023Ingår i: AIP Advances, E-ISSN 2158-3226, Vol. 13, nr 2, artikel-id 025019Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A non-transferred plasma torch is a device used to generate a steady thermal plasma jet. Plasma torches have the potential to replace fossil fuel burners used as heat sources in the process industry. Today, however, the available plasma torches are of small scale compared to the power used in the burners in the process industry. In order to understand the effects of large scales on the plasma flow dynamics, it is essential to understand the operation of the plasma torch under different operating conditions and for different geometries. In this study, the analysis of a non-transferred plasma torch has been carried out using both computational and experimental methods. Computationally, the magnetohydrodynamic (MHD) equations are solved using a single-fluid model on a 2D axisymmetric torch geometry. The experiments are performed using emission spectroscopy to measure the plasma jet temperature at the outlet. This paper explains the changes in the arc formation, temperature, and velocity for different working gases and power inputs. Furthermore, the possibilities and disadvantages of the MHD approach, considering a local thermal equilibrium, are discussed. It was found that in general, the computational temperature obtained is supported by the experimental and equilibrium data. The computational temperatures agree by within 10% with the experimental ones at the center of the plasma torch. The paper concludes by explaining the significant impact of input properties like working gas and power input on the output properties like velocity and temperature of plasma jet. 

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  • 29.
    Siddanathi, Likitha Sai
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Westerberg, Lars-Göran
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Åkerstedt, Hans O.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Wiinikka, Henrik
    RISE AB, SE-941 38 Piteå, Sweden.
    Sepman, Alexey
    RISE AB, SE-941 38 Piteå, Sweden.
    Computational Modeling of a Plasma Torch Using Single-Fluid and Two-Fluid Modeling Approaches2023Ingår i: COMSOL Conference 2023, COMSOL , 2023Konferensbidrag (Refereegranskat)
    Abstract [en]

    Plasma, a complex fluid consisting of electrons, ions, neutrals, and excited species, exhibits both fluid-like behavior and electrical conductivity due to the presence of charge carriers. Consequently, computational modeling of plasma requires the integration of fluid and electrical models. This research paper presents a study on the steady-state computational modeling of a plasma torch with a 2D axisymmetric geometry using single-fluid and two-fluid modeling approaches in the COMSOL Multiphysics® software. The single-fluid modeling (SFM) approach combines the individual equations governing the behavior of different particles into a unified equation. Specifically, the SFM approach utilized in this study focuses on a fully ionized plasma and employs the Magnetohydrodynamic equations whose adaptation is equilibrium discharge interface (EDI) model available in COMSOL Multiphysics®. The EDI model solves the magnetohydrodynamic (MHD) equations, encompassing electric and magnetic fields, heat transfer in solids and fluids, and laminar models. By employing this approach, the researchers simulated and analyzed the behavior of the plasma torch. In contrast, the two-fluid modeling (TFM) approach separates the fluid equations for electrons and ions, considering a weakly ionized plasma. The TFM model is developed by deriving fluid equations based on kinetic theory for neutrals, ions, and electrons. These equations are then implemented in COMSOL Multiphysics®, utilizing models for the transport of diluted species, laminar flow, heat transfer in solids and fluids, and electric and magnetic fields. By adopting the TFM approach, the researchers aimed to gain insights into the behavior of the plasma torch. Throughout the study, various properties such as temperature, velocity, current density, and particle concentrations are analyzed within the plasma torch. Results obtained from both the single-fluid and two-fluid modeling approaches are compared and evaluated. This comparative analysis allows the researchers to highlight the advantages and challenges associated with each modeling approach. In conclusion, this study contributes to understanding plasma behavior by employing computational modeling techniques. The research presents and compares the outcomes of single-fluid and two-fluid modeling approaches applied to a plasma torch. By examining the advantages and challenges of each approach, the study offers valuable insights for future plasma modeling endeavors.

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  • 30.
    Sollén, Sofia
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Casselgren, Johan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Correlation between floating car data and road weather information implemented for winter road maintenance follow-up by monitoring theroad friction2023Konferensbidrag (Refereegranskat)
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  • 31.
    Raisee, Mehrdad
    et al.
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran, 11155-4563, Iran.
    Cervantes, Michel J.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Determination of all hydrodynamic added properties resulting from fluid structure interactions using singular value decomposition2023Ingår i: Journal of Fluids and Structures, ISSN 0889-9746, E-ISSN 1095-8622, Vol. 120, artikel-id 103898Artikel i tidskrift (Refereegranskat)
  • 32.
    Hedberg, P. A. Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Solheim, Nils
    Norwegian University of Science and Technology, 2, Trondheim, Norway.
    Experimental and computational evaluation of fish passageway with porous media boundary2023Ingår i: Proceedings of the 40th IAHR World Congress - 2023: Rivers - Connecting Mountains and Coasts / [ed] Helmut Habersack; Michael Tritthart; Lisa Waldenberger, International Association for Hydro-Environment Engineering and Research (IAHR) , 2023, s. 2422-2428Konferensbidrag (Refereegranskat)
  • 33.
    Hedberg, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Experimental and computational evaluations of parallel spillway outlets2023Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Experiments and computational simulations have been performed as part of a larger project to instil trust in computational methods for design of hydraulics flows in spillways. Presented in this licenciate is one manuscript and two conference papers. The first conference paper details experiments done at Älvkarleby of a multiple outlet spillway model with an inlet channel specifically designed to contain interesting hydraulic features. The results indicate that simulations agree well with experiments. In the second conference paper acoustic doppler velocimetry measurements (ADV) were done and compared to simulations of a racetrack flume with a fish passageway at Älvkarleby. The results showed agreement but due to inlet conditions of the experiment some discrepancies were noticed. The manuscript presents experiments of a wider range of flow in the experimental flume of the first conference paper, with additional ADV measurements. Preliminary conclusions are that discrepancies can be due to inlet conditions. A short summary of further work is included.

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  • 34.
    Joy, Jesline
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Raisee, Mehrdad
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, University of Tehran, Tehran, 1417935840, Iran.
    Cervantes, Michel J.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Experimental investigation of an adjustable guide vane system in a Francis turbine draft tube at part load operation2023Ingår i: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 210, s. 737-750Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Hydraulic turbines are increasingly used to regulate the electrical grid. They thus have to operate more often at off-design conditions where instabilities may develop leading to large pressure pulsations decreasing the life of the machines. In the present study, an adjustable guide vane system in the draft tube of a single regulated hydraulic turbine is developed. The aim is to mitigate the pressure pulsations when the turbine operates at part load operational regimes without significantly compromising the hydraulic efficiency of the turbine. The draft tube guide vane system, named DTGV, is composed of three guide vanes. Each guide vanes comprise two individual hydrofoils. The upper hydrofoil can rotate around an axis up to ±45°, and the lower is fixed. Therefore, the guide vanes are adjustable. The rotating vortex rope mitigation was investigated at two part-load operational regimes at a 12 m turbine head, a) Q/QBEP = 0.59, and b) Q/QBEP = 0.71. The results indicate that the DTGV system significantly mitigates the pressure pulsations, thereby improving the flexible operation of the turbine. The best mitigation outcome was for 0 GV configuration at both part-load operating conditions. The mitigation effects are better when the operating condition approaches BEP, and the rotating mode of the pressure pulsations tends to go towards the runner frequency (f/f0 = 1). The hydraulic efficiency is found to be marginally affected by the guide vane system at part load operations and is improved at the best efficiency point (Q/QBEP = 1) and high load Q/QBEP = 1.21.

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  • 35.
    Kalantar Neyestanaki, Mehrdad
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Dunca, Georgiana
    Department of Hydraulics, Hydraulic Machinery and Environmental Engineering, University Politehnica of Bucharest, Bucharest RO 060042, Romania.
    Jonsson, Pontus
    Vattenfall AB, Luleå SE 977 75, Sweden.
    Cervantes, Michel
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Experimental Study of The Pressure-Time Method With Potential Application for Low-Head Hydropower2023Ingår i: Journal of Fluids Engineering, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 145, nr 7, artikel-id 071205Artikel i tidskrift (Refereegranskat)
  • 36.
    Bahaloohoreh, Hassan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Experiments and simulations on the mechanics of ice and snow2023Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    In this study, experiments and simulations were conducted to investigate ice and snow. The ice sintering force as a function of temperature, pressing force (contact load), contact duration, and particle size during the primary stage of sintering was formulated using experimental methods along with an approximate, semi-analytic, close-form solution. It was shown that the ice sintering force increases nearly linear with increasing external pressing force but best approximated as a power law for dependency on both contact duration and particle size. Moreover, the exponent of the power law for size dependence is around the value predicted by general sintering theory. The temperature dependence of the sintering force is highly nonlinear and follows the Arrhenius equation. It was observed that at temperatures closer to the melting point, a liquid bridge is observed upon these paration of the contacted ice particles. The ratio of ultimate tensile strength of ice to the axial stress concentration factor in tension is found as an important factor in determining the sintering force, and a value of nearly 1.1 MPa was estimated to best catch the sintering force of ice in different conditions. From the temperature dependency, the activation energy is calculated to be around 41.4 kJ/mol, which is close to the previously reported value. Also, the results for the sintering force suggest that smaller particles are “stickier” than larger particles. Moreover, cavitation and surface cracking is observed during the formation of the ice particles and these can be one of the sources for the variations observed in the measured ice sintering force values.

    The presence of a capillary bridge in contact between an ice particle and a "smooth" (or rough) Aluminum surface at relative humidity around 50% and temperatures below the melting point was experimentally demonstrated. Experiments were conducted under controlled temperature conditions and the mechanical instability of the bridge upon separation of the ice particle from the Aluminum surface with a constant speed was considered. It was observed that a liquid bridge with a more pronounced volume at temperatures near the melting point is formed. It was showen that the separation distance is proportional to the cube root of the volume of the bridge. The volume of the liquidbridge is used to estimate the thickness of the liquid layer on the ice particle and the estimated value was shown to be within the range reported in the literature. The thickness of the liquid layer decreases from nearly 56 nm at -1.7◦C to 0.2 nm at -12.7◦C. The dependence can be approximated with a power law, proportional to (TM − T)−β, where β < 2.6. We further observe that for a rough surface, the capillary bridge formation in the considered experimental conditions vanishes.

    The Discrete Element Method (DEM) was employed to simulate the filling behavior of dry snow. Snow as a heterogeneous, hot material which is constituted from spherical ice particles which can form bonds. The bonding behavior of ice particles is important in determining the macroscopic behavior of snow. The bond diameter of ice-ice contacts as a function of time, compressive load, and strain rate is used and a DEM for dry snow was developed and programmed in MATLAB. A beam element with implemented damage model was used in the simulation. The simulated parameters were macroscopic angle of repose, packing density, and surface conditions as a function of temperature and fillingrate. The DEM results were able to verify the existing published experimental data. The simulation results showed that angle of repose of snow decreased with decreasing the temperature, the surface became irregular due to particles rotation and re-arrangement for lower falling speeds of particles, and density increased with depth of deposition.

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  • 37.
    Seifi, Zeinab
    et al.
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran.
    Raisee, Mehrdad
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran.
    Cervantes, Michel
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Global linear stability analysis of flow inside an axial swirl generator with a rotating vortex rope2023Ingår i: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 61, nr 1, s. 34-50Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In hydraulic turbines, either Francis or axial turbines, a high swirling flow is generated at PL conditions at the inlet of the draft tube resulting in the formation of a rotating vortex rope (RVR). This leads to pressure fluctuations which limit the operating range of single regulated turbines. In the present study, an axial hydraulic turbine has been numerically simulated by the k−ϵk−ϵ and SST-based Scale-Adaptive Simulation model (SST SAS) turbulent models to capture helical RVR. Considering the formation of the RVR as the result of a global instability, linear global stability analysis of the time-averaged turbulent flow field has been conducted. For the first time in axial hydraulic turbines, how boundary conditions affect the unstable mode and which frequency, plunging or rotating, is related to the vortex rope instability have been studied. It is found that the flow inside the draft tube is sensitive to the asymmetrical disturbances with a frequency close to the rotational component.

  • 38.
    Moosavi, Amin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Heat transfer in ordered porous media with application to batteries2023Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Environmental concerns, resource depletion, energy security, technological advancements, and global policies are just a few of the variables influencing the global energy perspective. In the case of technological advancement, lithium batteries play a key role in the development of a more sustainable energy infrastructure. The high energy density and long lifespan of lithium batteries make them ideal for usage in a broad range of applications, such as portable electronics, electric vehicles, and grid-scale energy storage for renewable energy sources. However, there are certain possible concerns regarding the safe operation and performance of lithium batteries, most of which are associated with the temperature sensitivity of lithium batteries. Hence, battery thermal management systems are an essential component of a battery package for regulating the temperature level in lithium batteries to avoid the aging process, poor performance, and safety issues.  

    Many studies have been conducted to develop battery thermal management systems with improved cooling performance. Within this framework, Paper A in this licentiate thesis considers how the design of a lithium battery cell may be improved to reduce the thermal load on the thermal management system. An analytical model based on the integral transform technique is developed to accurately and efficiently predict the thermal behavior of a cylindrical lithium battery cell. Following model validation, the thermal behavior of cylindrical lithium-ion battery cells with different jelly-roll layers and can sizes are compared. The results demonstrate that 21700 cylindrical battery cells outperform other types of cylindrical battery cells in terms of thermal performance. Furthermore, the thermally optimal thicknesses for positive active material, negative active material, positive current collector, and negative current collector are 180, 34, 21, and 20 um, respectively.

    After learning about design considerations to reduce thermal issues in lithium-ion battery cells and developing a proper tool for further studies, the focus was set on the flow behavior surrounding a cylindrical battery cell in an air-based cooling system. The cooling system under consideration is a wall-bounded cross-flow heat exchanger, the most common air-based cooling system for battery applications. Despite the importance of the cooling system in battery safety, few studies have been conducted to investigate the thermo-flow characteristics of wall-bounded cross-flow heat exchangers. Hence, in the battery research field, it is common to estimate the performance of wall-bounded cross-flow heat exchangers using the thermal characteristics of free cross-flow heat exchangers due to their geometrical similarities. In Paper B, this assumption is scrutinized by comparing the thermo-fluid characteristics of free and wall-bounded cross-flow heat exchangers. According to the results, flow through both heat exchangers shows almost similar thermo-fluid behavior in areas sufficiently far from the bounding walls. A turbulence model study suggests that the k-kl-omega transition model is a time-efficient and reliable turbulence model for capturing thermo-fluid characteristics in such heat exchangers. Moreover, it is observed that the two different heat exchangers have an almost identical area-averaged heat transfer rate despite the local changes in Nusselt number along the height of cells. This finding shows that it is possible to do two-dimensional simulations for applications that only require an area-averaged heat transfer rate on the battery cells.

    The findings in Paper A and Paper B may be used to investigate the cooling performance of a battery thermal management system with a practical design. Hence, in Paper C, a comprehensive yet simplified model is developed that can be used to study the thermal field of lithium battery cells in a large-scale air-based battery thermal management system. The model consists of the CFD model derived in Paper B, which predicts the flow behavior around cells in the inner region of the battery package, and the analytical model described in Paper A, which determines the thermal field within the battery cells. The area-averaged heat transfer coefficient interconnects the models, and a system of equations is employed to estimate the row-to-row variation of the thermal field. The model is employed to assess the effect of transverse and longitudinal pitch ratios on the thermal performance of an air-based battery thermal management system used in a hybrid electric vehicle.

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  • 39.
    Burman, Anton J.
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Andersson, Anders G.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hydraulic classification of hydropeaking stages in a river reach2023Ingår i: Rivers Research and Applications: an international journal devoted to river research and management, ISSN 1535-1459, E-ISSN 1535-1467, Vol. 39, nr 4, s. 692-702Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Hydropower is an important tool in the struggle for low-emission power production. In the Nordic countries, hydropower operating conditions are expected to change and work more in conjunction with intermittent power production. This in turn might increase the amount of hydropeaking events in the reaches downstream of hydropower plants. The current work investigates the influence of highly flexible, high-frequency hydropeaking on the hydrodynamics in the downstream reach. By quantifying four different dynamic stages in the study reach, the influence of the hydropeaking frequencies was investigated in the bypass reach of the Stornorrfors hydropower plant in the river Umeälven in northern Sweden. The hydrodynamics in the study reach were numerically modelled using the open source solver Delft3D. Eight different highly flexible future hydropeaking scenarios, varying from 12 to 60 flow changes per day, were considered. A method for identifying four hydropeaking stages—dewatering, dynamic, alternating and uniform —was introduced. The hydropeaking frequency directly decided the stage in most of the study reach. Furthermore, a Fourier analysis showed a significant difference between the stages and their corresponding power spectra. The classification of stages put forward in this work provides a novel, simple method to investigate the hydrodynamics due to hydropeaking in a river reach.

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  • 40.
    Burman, Anton
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hydraulic Modelling of Hydropeaking in Regulated Rivers2023Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    The role of hydropower is expected to change in the close future in the Nordic countries. In order to fulfill targets set by the Paris agreement, the reliance on low emission intermittent power production such as solar and wind power is likely to increase. This in turn means that the operating conditions of Nordic hydropower plants are expected to change to balance the volatility of intermittent power production. Hence, rapid flow fluctuations due to changes in power demand, so called hydropeaking, is likely to increase in the future. Hydropeaking is associated with many negative impacts on the reach downstream of the hydropower plant. It is with this background that the research consortium HydroFlex was created. One of the aims of HydroFlex is to investigate the effects of very high frequency hydropeaking on adjacent river reaches. In this work, several model scenarios involving high frequency hydropeaking in a bypass reach in the river Umeälven were investigated. The river dynamics was modeled using the solver Delft3D. By using LIDAR measurements in the winter, when the reach was mostly dry, it was possible to capture the bathymetry with high spatial resolution. A 2D model was then set up using the measured bathymetry and calibrated using eight pressure sensors placed along the reach. The transient response in the reach was then compared between the measured depth from the sensors and the model (Paper A). Once the calibration had been performed, the effects of an increase in hydropeaking frequency on the downstream reach, could be investigated. A hysteresis effect in the depth increase-decrease cycle was observed in both measurements and simulations. Additionally, high hydropeaking frequency led to a hydraulic stage in the river where steady state is never achieved (Paper B). This effect, and how it might affect downstream salmonid spawning areas, was then further investigated. It was observed that an increase in hydropeaking frequency could reduce the proportion of potential spawning areas in the downstream reach that would dry out. Furthermore, a general method of computing dewatering time for numerical models was provided. This method was then applied, in tandem with a stranding model, to investigate how different spill gate closing times affected salmonids propensity to stranding. The likelihood of stranding varied between species and life-stage. Moreover, the tendency of stranding decreased longitudinally downstream (Paper C). The findings of a hydraulic stage in (Paper B) merited further investigation. By investigating model scenarios, where the hydropeaking frequency varied between 10 and 80 flow changes per day, four different hydraulic stages that occur in the reach, were identified. A method of classifying the hydraulic stages was suggested. The identified hydraulic stages were: the dewatering stage, the alternating stage, the previously identified dynamic stage and lastly the uniform stage. The relation between the hydraulic stages and the hydropeaking frequency were then investigated using Fourier analysis. It was observed that the hydropeaking frequency is the dominant variable in the dynamic and uniform stages, while the alternating stage is more complex. Additionally, the dynamics of the stages could be predicted by performing a Fourier transform of one increase-decrease cycle. The Fourier spectrum could also be used as an alternative way of classifying the stage (Paper D). Numerical methods on their own is not a holistic tool to predict river flows, they require field measurements for calibration and validation. In (Paper E) an overview of the methods used in the field in this work is given. Moreover, a discussion on the limitations of 2D modeling for large scale river applications is presented.

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  • 41.
    Bahrami, Mohammad Amin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran.
    Cervantes, Michel J.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Raisee, Mehrdad
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran.
    Nourbakhsh, Ahmad
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran.
    Hydrodynamic damping in laminar, transient and turbulent regimes: Analytical and computational study2023Ingår i: Ocean Engineering, ISSN 0029-8018, E-ISSN 1873-5258, Vol. 289, nr part 1, artikel-id 116277Artikel i tidskrift (Refereegranskat)
  • 42.
    Höller, J. Bastian
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Andersson, Anders G.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Integrating Downstream Ecological, Social and Economic Effects of Hydropower to Hydraulic Modeling: A Review2023Ingår i: World Journal of Mechanics, ISSN 2160-049X, E-ISSN 2160-0503, Vol. 13, nr 8, s. 149-172Artikel, forskningsöversikt (Refereegranskat)
    Abstract [en]

    Hydropower gains increasing importance as a steerable and controllable power source in a renewable energy mix and deregulated markets. Although hydropower produces fossil-free energy, it has a significant impact on the local environment. This review investigates the effects of flow alterations by hydropower on the downstream river system and the possibilities to integrate these effects into hydraulic modeling. The results show that various effects of flow regulation on the ecosystem, but also social and economic effects on related communities were observed in the last decades. The application of hydraulic models for investigations of ecological effects is common. Especially hydraulic effects and effects on fish were extensively modeled with the help of hydraulic 1D- and 2D-simulations. Current applications to investigate social and economic effects integrated into hydraulic modeling are meanwhile limited. Approaches to realizing this integration are presented. Further research on the economic valuation of ecosystems and integration of social and economic effects to hydraulic models is necessary to develop holistic tools to support decision-making on sustainable hydropower.

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  • 43.
    Huber, Johannes Albert Josef
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Forsberg, Fredrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Local Stiffness of Softwood Based on Micro- and Macro-Scale Computed Tomography2023Ingår i: Proceedings ICM20: Experimental Mechanics in Engineering and Biomechanics: 20th International Conference on Experimental Mechanics / [ed] J.F. Silva Gomes, Institute of Science and Innovation in Mechanical and Industrial Engineering , 2023, s. 1001-1002, artikel-id 20161Konferensbidrag (Refereegranskat)
    Abstract [en]

    Wood is a discontinuous cellular structure on a microscopic scale, but its mechanical behaviour resembles a continuum on a macroscopic scale. The structure of both domains can be studied by X-ray computed tomography (CT). A challenge for accurate CT-based models of wood is to set the values of the orthotropic stiffness tensor locally based on density. Micro-CT scans under in-situ loading may be used to estimate local stiffness in wood, based on strain fields derived from digital volume correlation. The goal of the present paper is to study how micro-CT scans of clearwood under in-situ loading can be used to predict stiffness locally as a function of the apparent macroscopic density, to improve the fidelity of FE models based on macro-CT scans.

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  • 44.
    Joy, Jesline
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Mitigation of Rotating Vortex Rope in a Hydro Turbine using a Guide Vane System2023Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Using renewable energy, such as water or wind, to produce electricity has been proven highly effective in Sweden. The ability of these renewable resources to make clean output energy counters the adversities caused by non-renewable resources. Amongst these, hydro turbines are one of the most preferred methods for generating electricity from water. Today, the hydro-turbines are designed to run at their peak efficiency only at a single operating condition called the best efficiency point (BEP). However, the energy consumption increase necessitates the hydraulic turbine’s flexible operation. The issue of pressure pulsations in the draft tube of hydro-turbines, observed at part load operating conditions, has been unresolved for many years. These pressure pulsations are related to the rotating vortex rope (RVR) seen at part load operation and adversely affect the hydro turbine’s lifespan and performance. Several techniques have been investigated in the past to reduce the pressure pulsations in the draft tube of a hydro turbine at part load operation and enhance the operational flexibility of the turbine. However, there is no universal solution. The current study presents a proof-of-concept of mitigation techniques to suppress pressure pulsations in a mixed-flow reaction turbine. The technique involves implementing a variable guide vane system in the draft tube of the Francis-99 model turbine. Guide vanes are mechanical devices that can direct the flow in the desired direction. Therefore, they can suppress the pressure pulsations by re-directing the swirling flows in the draft tube at part load. The current research involved both numerical and experimental investigations. The guide vane system was initially designed using numerical methods. The geometry of the guide vanes, including the number of guide vanes, the chord length, the span, and the location of the guide vanes in the draft tube, is designed through numerical simulations. The results show that a set of three guide vane systems with a chord length of 86% runner radius and a leading-edge span of 30% runner radius is an ideal design that mitigates RVR when placed at some distance from the runner in the draft tube. For the experimental proof of concept, the guide vanes were tested on the Francis-99 model turbine at the Norwegian University of Science and Technology, Trondheim, Norway. During experiments, the guide vanes can rotate up to ±45°. There were six guide vane orientations selected between ±45°. The results indicate that the guide vanes can mitigate the pressure pulsations with a marginal efficiency loss in the turbine at part load operational regimes. However, there are slight efficiency improvements at BEP and high load.

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  • 45.
    Kranenbarg, Jelle
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Jonsson, Pontus P.
    Vattenfall AB, Luleå 97775, Sweden.
    Mulu, Berhanu G.
    Vattenfall AB, R&D, Älvkarleby 81426, Sweden.
    Cervantes, Michel Jose
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Mitigation of the Pressure Pulsations in an Axial Turbine at Speed-No-Load With Independent Guide Vanes Opening2023Ingår i: Journal of Fluids Engineering, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 45, nr 11, artikel-id 111204Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Hydraulic turbines are operated more frequently at no-load conditions, also known as speed-no-load (SNL), to provide a spinning reserve that can rapidly connect to the electrical grid. As intermittent energy sources gain popularity, turbines will be required to provide spinning reserves more frequently. Previous studies show vortical flow structures in the vaneless space and the draft tube and rotating stall between the runner blades of certain axial turbines operating at SNL conditions. These flow phenomena are associated with pressure pulsations and torque fluctuations which put high stress on the turbine. The origin of the instabilities is not fully understood and not extensively studied. Moreover, mitigation techniques for SNL must be designed and explored to ensure the safe operation of the turbines at off-design conditions. This study presents a mitigation technique with independent control of each guide vane. The idea is to open some of the guide vanes to the best efficiency point (BEP) angle while keeping the remaining ones closed, aiming to reduce the swirl and thus avoid the instability to develop. The restriction is to have zero net torque on the shaft. Results show that the flow structures in the vaneless space can be broken down, which decreases pressure and velocity fluctuations. Furthermore, the rotating stall between the runner blades is reduced. The time-averaged flow upstream of the runner is changed while the flow below the runner remains mainly unchanged.

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  • 46.
    Barestrand, Henrik A.
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Ljung, Anna-Lena
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Summers, Jon
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. RISE Research Institutes of Sweden, Luleå, Sweden.
    Lundström, T. Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Modeling Convective Heat Transfer of Air in a Data Center Using OpenFOAM: Evaluation of the Boussinesq Buoyancy Approximation2023Ingår i: OpenFOAM® Journal, E-ISSN 2753-8168, Vol. 3, s. 146-158Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Achieving energy and cooling efficiency in data center convective air flow and heat transfer can be a challenging task. Among different numerical methods to work with such issues is the Finite Volume Method in Computational Fluid Dynamics. This work evaluates the performance of two such solvers provided by OpenFOAM® in solving this type of convective heat-transfer problem, namely BuoyantBoussinesqPimpleFOAM and BuoyantPimpleFOAM. This is done for two different flow configurations of significantly different Richardson number. To sufficiently resolve the flow, grid sizing effects are elucidated by way of the kernel density estimate. It determines the volume distribution of the temperature in the data center configuration. For the k-epsilon turbulence model used here, it was found that the compressible solver performs faster and requires less grid resolution for both flow configurations. This is attributed to the nature of the boundary conditions which are set such that the mass flow conservation per server rack and cooling unit is achieved. Transient solutions are found to provide better iterative convergence for cases that involves buoyancy, compressibility and flow separation. This is, in comparison to steady-state solutions where artificial numerical pressure drop is found, to depend on the momentum relaxation factors for the convective case with a higher Richardson number.

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  • 47.
    Teng, Penghua
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Johansson, Fredrik
    Division of Soil and Rock Mechanics, KTH Royal Institute of Technology, Stockholm, SE-100 44, Sweden.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Modelling erosion of a single rock block using a coupled CFD-DEM approach2023Ingår i: Journal of Rock Mechanics and Geotechnical Engineering, ISSN 1674-7755, Vol. 15, nr 9, s. 2375-2387Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Rock block removal is the prevalent physical mechanism for rock erosion and could affect the stability of dam foundations and spillways. Despite this, understanding of block removal is still inadequate because of the complex interactions among block characteristics, hydraulic forces, and erosive processes acting on the block. Herein, based on a previously conducted physical experiment of erosion of a single rock block, the removal processes of two different protruding blocks are represented by a coupled computational fluid dynamics-discrete element model (CFD-DEM) approach under varied flow conditions. Additionally, the blocks could be rotated with respect to the flow direction to consider the effect of the discontinuity orientation on the block removal process. Simulation results visualize the entire block removal process. The simulations reproduce the effects of the discontinuity orientation on the critical flow velocity inducing block incipient motion and the trajectory of the block motion observed in the physical experiments. The numerical results present a similar tendency of the critical velocities at different discontinuity orientations but have slightly lower values. The trajectory of the block in the simulations fits well with the experimental measurements. The relationship between the dimensionless critical shear stress and discontinuity orientation observed from the simulations shows that the effect of block protrusion becomes more dominant on the block incipient motion with the increase of relative protrusion height. To our knowledge, this present study is the first attempt to use the coupled finite volume method (FVM)-DEM approach for modelling the interaction behavior between the block and the flowing water so that the block removal process can be reproduced and analyzed.

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  • 48.
    Teng, Penghua
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Johansson, Fredrik
    KTH Royal Institute of Technology, Stockholm, Sweden.
    Nilsson, Carl-Oscar
    Uniper – Sydkraft Hydropower AB, Östersund, Sweden.
    Modelling of erosion in rock spillway channels2023Konferensbidrag (Refereegranskat)
    Abstract [en]

    Erosion of rock channels downstream spillways may cause the significant deterioration of the spillways, raisingconcern for the safety of the dam. To evaluate the occurrence of rock erosion, this paper aims to employ a resolvedComputational Fluid Dynamics (CFD)-Discrete Element Method (DEM) approach to model the erosion process of asingle rock block and to investigate the effects of different parameters. The approach models the interaction behaviorsbetween flowing water and rock blocks. Simulation results visualize the erosion process of the rock block, whichprovides a reference to determine the threshold of the initiation of rock erosion. The results show that the incipientmotion of the rock block is significantly affected by the joint aperture. With the increase of the joint aperture size, thepressure distribution on the block surface is altered. Simulation results of the rough channel model are compared witha smooth model, which points out the influence of channel roughness on the rock erosion process.

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  • 49.
    Kranenbarg, Jelle
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Numerical investigation of the flow and instabilities at part-load and speed-no-load in an axial turbine2023Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Global renewable energy requirements rapidly increase with the transition to a fossil-free society. As a result, intermittent energy resources, such as wind- and solar power, have become increasingly popular. However, their energy production varies over time, both in the short- and long term. Hydropower plants are therefore utilized as a regulating resource more frequently to maintain a balance between production and consumption on the electrical grid. This means that they must be operated away from the design point, also known as the best-efficiency-point (BEP), and often are operated at part-load (PL) with a lower power output. Moreover, some plants are expected to provide a spinning reserve, also referred to as speed-no-load (SNL), to respond rapidly to power shortages. During this operating condition, the turbine rotates without producing any power.

    During the above mentioned off-design operating conditions, the flow rate is restricted by the closure of the guide vanes. This changes the absolute velocity of the flow and increases the swirl, which is unfavorable. The flow field can be described as chaotic, with separated regions and recirculating fluid. Shear layer formation between stagnant- and rotating flow regions can be an origin for rotating flow structures. Examples are the rotating-vortex-rope (RVR) found during PL operation and the vortical flow structures in the vaneless space during SNL operation, which can cause the flow between the runner blades to stall, also referred to as rotating stall. The flow structures are associated with pressure pulsations throughout the turbine, which puts high stress on the runner and other critical parts and shortens the turbine's lifetime.

    Numerical models of hydraulic turbines are highly coveted to investigate the detrimental flow inside the hydraulic turbines' different sections at off-design operating conditions. They enable the detailed study of the flow and the origin of the instabilities. This knowledge eases the design and assessment of mitigation techniques that expand the turbines' operating range, ultimately enabling a wider implementation of intermittent energy resources on the electrical grid and a smoother transition to a fossil-free society.

    This thesis presents the numerical study of the Porjus U9 model, a scaled-down version of the 10 MW prototype Kaplan turbine located along the Luleå river in northern Sweden. The distributor contains 20 guide vanes, 18 stay vanes and the runner is 6-bladed. The numerical model is a geometrical representation of the model turbine located at Vattenfall Research and Development in Älvkarleby, Sweden. The commercial software ANSYS CFX 2020 R2 is used to perform the numerical simulations.

    Firstly, the draft tube cone section of the U9 model is numerically studied to investigate the sensitivity of a swirling flow to the GEKO (generalized kω) turbulence model. The GEKO model aims to consolidate different eddy viscosity turbulence models. Six free coefficients are changeable to tune the model to flow conditions and obtain results closer to an experimental reference without affecting the calibration of the turbulence model to basic flow test cases. Especially, the coefficients affecting wall-bounded flows are of interest. This study aims to analyze if the GEKO model can be used to obtain results closer to experimental measurements and better predict the swirling flow at PL operation compared to other eddy viscosity turbulence models. Results show that the near-wall- and separation coefficients predict a higher swirl and give results closer to experimentally obtained ones.

    Secondly, a simplified version of the U9 model is investigated at BEP and PL operating conditions and includes one distributor passage with periodic boundary conditions, the runner and the draft tube. The flow is assumed axisymmetric upstream of the runner, hence the single distributor passage. Previous studies of hydraulic turbines operating at PL show difficulties predicting the flow's tangential velocity component as it is often under predicted. Therefore, a parametric analysis is performed to investigate which parameters affect the prediction of the tangential velocity in the runner domain. Results show that the model predicts the flow relatively well at BEP but has problems at PL; the axial velocity is overpredicted while the tangential is underpredicted. Moreover, the torque is overpredicted. The root cause for the deviation is an underestimation of the head losses. Another contributing reason is that the runner extracts too much swirl from the flow, hence the low tangential velocity and the high torque. Sensitive parameters are the blade clearance, blade angle and mass flow.

    Finally, the full version of the U9 model is analyzed at SNL operation, including the spiral casing, full distributor, runner and draft tube. During this operating condition, the flow is not axisymmetric; vortical flow structures extend from the vaneless space to the draft tube and the flow stalls between the runner blades. A mitigation technique with independent control of each guide vane is presented and implemented in the model. The idea is to open some of the guidevanes to BEP angle while keeping the remaining ones closed. The aim is to reduce the swirl and prevent the vortical flow structures from developing. Results show that the flow structures are broken down upstream the runner and the rotating stall between the runner blades is reduced, which decreases the pressure- and velocity fluctuations. The flow down stream the runner remains mainly unchanged.

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    Publikationen är tillgänglig i fulltext från 2024-03-31 10:00
  • 50.
    Kalantar Neyestanaki, Mehrdad
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Dunca, Georgiana
    Department Hydraulic Machinery and Environmental Engineering, University Politehnica of Bucharest 313 Splaiul Indepentei, S6, RO-060042, Bucharest, Romania.
    Jonsson, Pontus
    Vattenfall AB Aurorumvägen 12, 977 75 Luleå, Sweden.
    Cervantes, Michel
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Numerical Investigation of the Pressure-Time Method, Head loss in Developed and Developing Flows2023Ingår i: International Journal of Fluid Machinery and Systems, ISSN 1882-9554, Vol. 16, nr 4, s. 332-345Artikel i tidskrift (Refereegranskat)
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