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  • 1.
    A., Trubetskaya
    et al.
    National University of Ireland Galway.
    G. R., Surup
    University of Agder.
    Forsberg, Fredrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    T., Attard
    University of York.
    A., Hunt
    Khon Kaen University.
    V., Budarin
    University of York.
    V., Abdelsayed
    National Energy Technology Laboratory.
    D., Shekhawat
    National Energy Technology Laboratory.
    The Effect of Wood Composition and Supercritical CO2 Extraction on the Charcoal Production2019In: 2019 AIChE Annual Meeting proceedings, American Institute of Chemical Engineers, 2019, article id 552cConference paper (Other academic)
    Abstract [en]

    This work demonstrated that the coupling of supercritical carbon dioxide extraction with slow pyrolysis is effective to remove over half of extractives from low quality wood and to generate biochar from remaining solid wood fractions. The high yields of extractives from supercritical carbon dioxide extraction illustrates the potential utilizing of low quality wood as an alternative feedstock for the sustainable production of value-added chemicals. Results showed that supercritical carbon dioxide extraction has neither a strong impact on the physical properties of original wood nor on the yield of solid biochar. These results are promising as they show that the biochar obtained for this renewable feedstock could be used as an alternative to fossil-based coke in applications including ferroalloy industries. Moreover, the heat treatment temperature and supercritical carbon dioxide extraction had a significant impact on the tar yields, leading to the increase in naphthalene, polycyclic aromatic hydrocarbons, aromatic and phenolic fractions with the greater temperature. The differences in gasification reactivity and dielectric properties of solid biochars, composition and yields of liquid products of non-treated pinewood and extracted wood fraction emphasize the impact of supercritical carbon dioxide extraction on the pyrolysis process. 

  • 2.
    Aitomäki, Yvonne
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Moreno, Sergio
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Vacuum infusion of cellulose nanofibre network composites: Influence of porosity on permeability and impregnation2016In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 95, p. 204-211Article in journal (Refereed)
    Abstract [en]

    Addressing issues around the processing of cellulose nanofibres (CNF) composites is important in establishing their use as sustainable, renewable polymer reinforcements. Here, CNF networks of different porosity were made with the aim of increasing their permeability and suitability for processing by vacuum infusion (VI). The CNF networks were infused with epoxy using two different strategies. The permeability, morphology and mechanical properties of the dry networks and the resulting nanocomposites were investigated. Calculated fill-times for CNF networks with 50% porosity were the shortest, but are only less than the gel-time of the epoxy if capillary effects are included. In experiments the CNF networks were clearly wetted. However low transparency indicated that impregnation was incomplete. The modulus and strength of the dry CNF networks increased rapidly with decreasing porosity, but their nanocomposites did not follow this trend, showing instead similar mechanical properties to each other. The results demonstrated that increasing the porosity of the CNF networks to ≈ 50% gives better impregnation resulting in a lower ultimate strength, a higher yield strength and no loss in modulus. Better use of the flow channels in the inherently layered CNF networks could potentially reduce void content in these nanocomposites and thus increase their mechanical properties.

  • 3.
    Aitomäki, Yvonne
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Moreno, Sergio
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Vacuum Infusion of Nanocellulose Networks of Different Porosity2015In: 20th International Conference on Composite Materials: Copenhagen, 19-24th July 2015, ICCM , 2015, article id 4109-1Conference paper (Refereed)
    Abstract [en]

    Cellulose nanofibres (CNF) have shown good potential as sustainable, biobased reinforcing materials in polymer composites. Addressing issues around the processing of these composites is an important part of establishing their use in different applications. Here, CNF networks of different porosity are made from nanofibrillated hardwood kraft pulp with the aim of increasing the impregnation of the CNF networks and to allow vacuum infusion to be used. Two different vacuum infusion strategies: in-plane and out of plane were used to infuse the CNF networks with a low viscosity epoxy. The permeability, morphology and mechanical properties of the dry networks and the resulting nanocomposites were investigated and compared to a micro-fibre based network. Using the out-of-plane permeability measurements and Darcy’s law, the fill-time was calculated and showed that the CNF network with 40% porosity had the lowest fill-time when an out-of-plane impregnation strategy is used. However this exceeded the gel-time of the epoxy system. In experiments, the resin reached the other side of the network but low transparency indicated that wetting was poor. The dry CNF preforms showed a very strong dependence on the porosity with both modulus and strength increasing rapidly at low porosity. Interestingly, the composite based on the 60% porosity network showed good wetting particularly with the in-plane infusion strategy, exhibiting a much more brittle fracture and a high yield strength. This shows that in CNF composites produced by VI, lowering the fibre volume content of the CNF composites gives better impregnation resulting in a lower ultimate strength but higher yield strength and no loss in modulus.

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  • 4.
    Alam, Md Tabrez
    et al.
    Energy & Thermofluids Lab Department of Chemical and Biochemical Engineering Indian Institute of Technology Patna Amhara Road, Bihta Patna 801106, Bihar, India.
    Raj, Aashna
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Singh, Lalan K.
    Energy & Thermofluids Lab Department of Chemical and Biochemical Engineering Indian Institute of Technology Patna Amhara Road, Bihta Patna 801106, Bihar, India.
    Gupta, Anoop K.
    Energy & Thermofluids Lab Department of Chemical and Biochemical Engineering Indian Institute of Technology Patna Amhara Road, Bihta Patna 801106, Bihar, India.
    A comparative analysis on charging performance of triplex-tube heat exchanger under various configurations of composite phase change material2024In: Thermal Science and Engineering Progress, E-ISSN 2451-9049, Vol. 51, article id 102655Article in journal (Refereed)
    Abstract [en]

    In present work, the melting performance of triplex-tube latent heat thermal energy storage (LHTES) unit was numerically studied using equal volumes of PCM and metal foam composite PCM (CPCM) in various arrangements. For the n-eicosane (as PCM), the study was conducted at the fixed Rayleigh number (Ra) = 4.08x107, Prandtl number (Pr) = 62.9, and Stefan number (Ste) = 0.14. The results showed that positioning the metal foam on the bottom side and distributing segmented CPCM with alternating PCM zones effectively improved the system performance. Moreover, this also prevents the overheating of thermal layers in the LHTES unit. While the model labelled M2 exhibited the highest economic efficiency among all isotropic models, its low dimensionless thermal energy storage (TES) density (i.e., q’ ∼ 0.6) led this study to focus on models falling under the category having a TES density of ∼ 0.8. Compared to a pure PCM model, the configurations under equal volume ratio category demonstrated up to ∼ four times higher TES rate (p’) and the significant reduction of ∼ 75 % in melting time. The optimized isotropic model achieved the highest TES rate per unit cost with peak value of ∼ 3 at a price ratio (N) of 1. Lastly, the testing of metal foam anisotropy on the chosen design showed a substantial increase in melting/heat storage rates. The largest drop of ∼ 33 % in the total melting time was noticed for model M2 as compared to isotropic case.

  • 5.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Fabricius, John
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Flow in thin domains with a microstructure: Lubrication and thin porous media2017In: AIP Conference Proceedings, ISSN 0094-243X, E-ISSN 1551-7616, Vol. 1798, article id 020172Article in journal (Refereed)
    Abstract [en]

    This paper is devoted to homogenization of different models of flow in thin domains with a microstructure. The focus is on applications connected to the effect of surface roughness in full film lubrication, but a parallel to flow in thin porous media is also discussed. Mathematical models of such flows naturally include two small parameters. One is connected to the fluid film thickness and the other to the microstructure. The corresponding asymptotic analysis is a delicate problem, since the result depends on how fast the two small parameters tend to zero relative to each other. We give a review of the current status in this area and point out some future challenges.

  • 6.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Pellegrini, Barbara
    Department of Neurosciences, Biomedicine and Movement Sciences, University of Verona, Verona, Italy; CeRiSM, Sport Mountain and Health Research Centre, University of Verona, Verona, Italy.
    Lintzén, Nina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Emami, Nazanin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Holmberg, H-C
    Luleå University of Technology, Department of Health, Learning and Technology, Health, Medicine and Rehabilitation. School of Kinesiology, University of British Columbia, Vancouver, BC, Canada.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    A Scientific Perspective on Reducing Ski-Snow Friction to Improve Performance in Olympic Cross-Country Skiing, the Biathlon and Nordic Combined2022In: Frontiers in Sports and Active Living, E-ISSN 2624-9367, Vol. 4, article id 844883Article in journal (Refereed)
    Abstract [en]

    Of the medals awarded at the 2022 Winter Olympics in Beijing, 24% were for events involving cross-country skiing, the biathlon and Nordic combined. Although much research has focused on physiological and biomechanical characteristics that determine success in these sports, considerably less is yet known about the resistive forces. Here, we specifically describe what is presently known about ski-snow friction, one of the major resistive forces. Today, elite ski races take place on natural and/or machine-made snow. Prior to each race, several pairs of skis with different grinding and waxing of the base are tested against one another with respect to key parameters, such as how rapidly and for how long the ski glides, which is dependent on ski-snow friction. This friction arises from a combination of factors, including compaction, plowing, adhesion, viscous drag, and water bridging, as well as contaminants and dirt on the surface of and within the snow. In this context the stiffness of the ski, shape of its camber, and material composition and topography of the base exert a major influence. An understanding of the interactions between these factors, in combination with information concerning the temperature and humidity of both the air and snow, as well as the nature of the snow, provides a basis for designing specific strategies to minimize ski-snow friction. In conclusion, although performance on “narrow skis” has improved considerably in recent decades, future insights into how best to reduce ski-snow friction offer great promise for even further advances.

  • 7.
    Alnersson, Gustaf
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Gestamp Hardtech.
    Towards 3D modelling of Compression Moulding of SMC2021Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The automotive industry is facing ever increasing demands for reduced emissions, and lightweight solutions are thusly required. One field that has significant potential in this regard is composite materials, which can offer a good combination of weight reduction and mechanical properties. However, the rapid development cycles in the automotive industry mean that tools for numerical modeling are necessary, both regarding manufacturing processes and prediction of mechanical properties. 

    The material that has been of interest for this work is Sheet Moulding Compound (SMC). SMC consists of sheets of resin and chopped fibres. When used for manufacturing the sheets are cut into appropriate size and shape. The cut sheets are then placed in a pre-heated mould. When this mould is closed, the sheets melt and the fibre-filled resin flows out and fills the mould cavity; the resin then cures and solidifies. A significant advantage of SMC compared to other composite solutions is that the process has comparatively short cycle times, which is a necessity for automotive applications. However, it is a rather complicated process to model numerically for a number of reasons, including the complex rheological properties of the charge, the often rather significant temperature gradients throughout the thickness, often complicated three-dimensional effects in the flow, and the chopped fibres present in the charge. These fibres will move and change orientation as the charge is pressed, which is a significant challenge to model properly.

    The first part of this work is a review and discussion of the difficulties described above, and some solutions that have been suggested. The second part concerns a suggestion for a three-dimensional flow model for the compression moulding process, which takes into account factors that have been suggested to influence the flow behavior, such as temperature distribution and shear strain rate. Some simulation results are presented along with comparison to previous experimental results, and similar flow patterns are observed serving as a qualitative validation. The third part concerns the expansion of this model to include the effects of the flow on the fibre orientation.

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  • 8.
    Alnersson, Gustaf
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Gestamp Hardtech, Sweden.
    Aitomäki, Yvonne
    RISE AB, Sweden.
    Ljung, Anna-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, T. Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    3D Flow and Fibre Orientation Modelling of Compression Moulding of A-SMC: Simulation and Validation in Squeeze Flow2022In: ECCM 2022 - Proceedings of the 20th European Conference on Composite Materials: Composites Meet Sustainability / [ed] Vassilopoulos, Anastasios P.; Michaud, Véronique, EPFL Lausanne, Composite Construction Laboratory , 2022, Vol. 2, p. 250-255Conference paper (Refereed)
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  • 9.
    Alnersson, Gustaf
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. 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 University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, T. Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    3D flow and fibre orientation modelling of compression moulding of A-SMC: simulations and experimental validation in squeeze flow2023In: Functional Composite Materials, E-ISSN 2522-5774, Vol. 4, article id 11Article in journal (Refereed)
    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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  • 10.
    Alnersson, Gustaf
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Ljung, Anna-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    3D-flow and and fibre orientation modelling for compression moulding of SMCManuscript (preprint) (Other academic)
    Abstract [en]

    Two different numerical models for compression moulding of Sheet Moulding Compound are discussed and compared, with one being a more traditional flow model implemented in the 3DTimon commercial software, and the other being a more advanced flow model implemented in more general fluid dynamics software. The focus will be on comparing the resulting fibre orientations.

  • 11.
    Alnersson, Gustaf
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Gestamp Hardtech, Luleå, Sweden.
    Ljung, Anna-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, T. Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Fibre Orientation and Flow Modelling for Compression Moulding of SMC2022In: Svenska Mekanikdagar 2022 / [ed] Pär Jonsén; Lars-Göran Westerberg; Simon Larsson; Erik Olsson, Luleå tekniska universitet, 2022Conference paper (Refereed)
  • 12.
    Alnersson, Gustaf
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Gestamp Hardtech, Luleå, Sweden.
    Lundström, T. Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Ljung, Anna-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Numerical Study of the 3D-Flow Characteristics During Compression Moulding of SMC2019In: Proceedings of the 2019 International Conference on Composite Materials: Melbourne, Australia, August 11-16, 2019 / [ed] Adrian Mouritz; Chun Wang; Bronwyn Fox, RMIT University , 2019, p. 1571-1581, article id 305Conference paper (Other academic)
    Abstract [en]

    A numerical model for compression moulding of Sheet Moulding Compound is presented. The model is based on fluid mechanics and the SMC charge is modelled as a fluid with a viscosity that is dependent on the charge temperature, the fibre volume fraction of the material and the shear strain rate. Trends observed in the simulations regarding the shape of the flow front agree with experimental observations in previous studies. The simulations also yield that the type and magnitude of initial heating effects the initial flow to a large extent.

  • 13.
    Alnersson, Gustaf
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Gestamp Hardtech, Sweden.
    Tahir, M. Waseem
    Gestamp Hardtech, Sweden. Mechanical Engineering, University of Hull, UK.
    Ljung, Anna-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, T. Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Review of the Numerical Modeling of Compression Molding of Sheet Molding Compound2020In: Processes, ISSN 2227-9717, Vol. 8, no 2, article id 179Article, review/survey (Refereed)
    Abstract [en]

    A review of the numerical modeling of the compression molding of the sheet molding compound (SMC) is presented. The focus of this review is the practical difficulties of modeling cases with high fiber content, an area in which there is relatively little documented work. In these cases, the prediction of the flows become intricate due to several reasons, mainly the complex rheology of the compound and large temperature gradients, but also the orientation of fibers and the micromechanics of the interactions between the fluid and the fibers play major roles. The details of this during moldings are discussed. Special attention is given to the impact on viscosity from the high fiber volume fraction, and the various models for this. One additional area of interest is the modeling of the fiber orientation.

  • 14.
    Alrifaiy, Ahmed
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems. Institute of Neuroscience and Physiology, Section of physiology, Gothenburg University - Sahlgrenska Academy, Göteborg, 405 30, Sweden; CMTF, Centre for Biomedical Engineering and Physics, Luleå and Umeå, Sweden.
    Borg, Johan
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Embedded Internet Systems Lab.
    Lindahl, Olof A.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems. Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. CMTF, Centre for Biomedical Engineering and Physics, Luleå and Umeå, Sweden; Department of Radiation Sciences, Biomedical Engineering, Umeå, 901 87, Sweden.
    Ramser, Kerstin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems. CMTF, Centre for Biomedical Engineering and Physics, Luleå and Umeå, Sweden.
    A lab-on-a-chip for hypoxic patch clamp measurements combined with optical tweezers and spectroscopy: first investigations of single biological cells2015In: Biomedical engineering online, E-ISSN 1475-925X, Vol. 14, article id 36Article in journal (Refereed)
    Abstract [en]

    The response and the reaction of the brain system to hypoxia is a vital research subject that requires special instrumentation. With this research subject in focus, a new multifunctional lab-on-a-chip (LOC) system with control over the oxygen content for studies on biological cells was developed. The chip was designed to incorporate the patch clamp technique, optical tweezers and absorption spectroscopy. The performance of the LOC was tested by a series of experiments. The oxygen content within the channels of the LOC was monitored by an oxygen sensor and verified by simultaneously studying the oxygenation state of chicken red blood cells (RBCs) with absorption spectra. The chicken RBCs were manipulated optically and steered in three dimensions towards a patch-clamp micropipette in a closed microfluidic channel. The oxygen level within the channels could be changed from a normoxic value of 18% O 2 to an anoxic value of 0.0-0.5% O 2. A time series of 3 experiments were performed, showing that the spectral transfer from the oxygenated to the deoxygenated state occurred after about 227 ± 1 s and a fully developed deoxygenated spectrum was observed after 298 ± 1 s, a mean value of 3 experiments. The tightness of the chamber to oxygen diffusion was verified by stopping the flow into the channel system while continuously recording absorption spectra showing an unchanged deoxygenated state during 5400 ± 2 s. A transfer of the oxygenated absorption spectra was achieved after 426 ± 1 s when exposing the cell to normoxic buffer. This showed the long time viability of the investigated cells. Successful patching and sealing were established on a trapped RBC and the whole-cell access (Ra) and membrane (Rm) resistances were measured to be 5.033 ± 0.412 M Ω and 889.7 ± 1.74 M Ω respectively.

  • 15.
    Altorkmany, Lobna
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Architecture and Water.
    Kharseh, Mohamad
    Civil Environmental Engineering Department, Chalmers University of Technology, Sweden.
    Ljung, Anna-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Effect of Working Parameters of the Plate Heat Exchanger on the Thermal Performance of the Anti-Bact Heat Exchanger System to Disinfect Legionella in Hot Water Systems2018In: Applied Thermal Engineering, ISSN 1359-4311, E-ISSN 1873-5606, Vol. 141, p. 435-443Article in journal (Refereed)
    Abstract [en]

    The objective of the current study is to analyze the effect of different working parameters on the thermal performance of the Anti-Bact Heat Exchanger system (ABHE). The ABHE system is inspired by nature and implemented to achieve continuous disinfection of Legionella in different human-made water systems at any desired disinfection temperature. In the ABHE system, most of the energy is recovered using an efficient plate heat exchanger (PHE). A model by Engineering Equation Solver (EES) is set-up to figure out the effect of different working parameters on the thermal performance of the ABHE system. The study shows that higher supplied water temperature can enhance the regeneration ratio (RR), but it requires a large PHE area and pumping power (PP) which consequently increase the cost of the ABHE system. However, elevate temperature in use results in a reduced PHE area and PP, which accordingly reduce the cost of the ABHE system. On the other hand, the EES-based model is used to study the effect of the length and the width of the plates used in the PHE on the RR and the required area of the PHE. Finally, taking into account the geometrical parameters, flow arrangement and the initial operating conditions of the PHE, the EES-based model is used to optimize the PHE in which its area is minimized, and the RR of the ABHE system is maximized.

  • 16.
    Altorkmany, Lobna
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Architecture and Water.
    Kharseh, Mohamad
    Civil Environmental Engineering Department, Chalmers University of Technology.
    Ljung, Anna-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Experimental and Simulation Validation of ABHE for Disinfection of Legionella in Hot Water Systems2017In: Applied Thermal Engineering, ISSN 1359-4311, E-ISSN 1873-5606, Vol. 116, p. 253-265Article in journal (Refereed)
    Abstract [en]

    The work refers to an innovative system inspired by nature that mimics the thermoregulation system that exists in animals. This method, which is called Anti Bacteria Heat Exchanger (ABHE), is proposed to achieve continuous thermal disinfection of bacteria in hot water systems with high energy efficiency. In particular, this study aims to demonstrate the opportunity to gain energy by means of recovering heat over a plate heat exchanger. Firstly, the thermodynamics of the ABHE is clarified to define the ABHE specification. Secondly, a first prototype of an ABHE is built with a specific configuration based on simplicity regarding design and construction. Thirdly, an experimental test is carried out. Finally, a computer model is built to simulate the ABHE system and the experimental data is used to validate the model. The experimental results indicate that the performance of the ABHE system is strongly dependent on the flow rate, while the supplied temperature has less effect. Experimental and simulation data show a large potential for saving energy of this thermal disinfection method by recovering heat. To exemplify, when supplying water at a flow rate of 5 kg/min and at a temperature of 50 °C, the heat recovery is about 1.5 kW while the required pumping power is 1 W. This means that the pressure drop is very small compared to the energy recovered and consequently high saving in total cost is promising.

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  • 17.
    Amer, Eynas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Pulsed laser ablation studied using digital holographic interferometry2008Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Pulsed digital holographic interferometry has been used to investigate the plume and the shock wave generated in the ablation process of a Q-switched Nd-YAG (lambda= 1064 nm and pulse duration = 12 ns) laser pulse on a polycrystalline Boron Nitride (PCBN) target under atmospheric air pressure. A special set-up based on using two synchronised wavelengths from the same laser for simultaneous processing and measurement has been used. Digital holograms were recorded for different time delays using collimated laser light (lambda= 532 nm) passed through the volume along the target. Numerical data of the integrated refractive index field were calculated and presented as phase maps showing the propagation of the shock wave and the plume generated by the process. Radon inversion has been used to estimate the 3D refractive index fields measured from the projections assuming rotational symmetry. Verification of the point explosion model has been done. The amount of released energy i.e. the part of the incident energy of the laser pulse that is eventually converted to a shock wave has been estimated. Shock wave front densities have been calculated from the reconstructed refractive index fields using the Gladstone-Dale equation. A comparison of the shock front density calculated from the reconstructed data and that calculated using the point explosion model at different time delays has been done. The comparison shows quite good agreement between the model and the experimental data. Finally the reconstructed refractive index field has been used to estimate the electron number density distribution within the laser induced plasma. The results show that pulsed digital holographic interferometry is a promising technique to study laser ablation processes. Different materials and laser parameters like wavelength, focusing, number of pulses can be studied in combinations with other techniques.

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  • 18.
    Amer, Eynas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Pulsed laser ablation studied using digital holography2009Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Pulsed digital holographic interferometry has been used to study the plume and the shock wave generated in the laser ablation process on different targets under atmospheric air pressure. A pulsed Nd-YAG laser system (pulse duration 12 ns) has been used both for ablating the material (wavelength 1064 nm) and for measurement (wavelength 532 nm). Digital holograms were recorded for different time delays using collimated laser light passed through the volume along the target. Numerical data of the integrated refractive index field were calculated and presented as phase maps. The Radon inversion has been used to estimate the 3D refractive index fields measured from the projections assuming rotational symmetry. Intensity maps have been calculated from the recorded digital holograms and used to calculate the attenuation of the probing laser beam by the ablated plume. Qualitative and quantitative information have been extracted from both the phase map and the intensity map to help describing the laser ablation process. Also 3D information about the induced plume has been obtained by numerical reconstruction of the digital holograms at different planes along the plume. The amount of released energy due to laser impact on a PCBM target has been estimated using the point explosion model. The released energy is normalized by the incident laser pulse energy and the energy conversion efficiency between the laser pulse and the target has been calculated and it seems to be constant around 80 %. The 3D refractive index fields have been used to calculate the shock wave front density and the electron number density distribution within the induced plasma. The electron number densities are found to be in the order of 1018 cm-3 and decay at a rate of 3x1015 electrons/cm3ns. The effect of the laser spot diameter on the shock wave generated in the ablation process of a Zn target has been studied. The induced shock wave has an ellipsoidal shape that approaches a sphere for decreasing spot diameter. A model was developed that approaches the density distribution that facilitates the derivation of the particle velocity field. The method provides quantitative results that are discussed; in particular a comparison with the point explosion theory. The effect of the physical properties of the target on the laser ablation process has been studied. The comparison of the laser ablation of Zn and Ti shows that different laser ablation mechanisms are observed for the same laser settings and surrounding gas. At a laser fluence of 5 J/cm2, phase explosion appears to be the ablation mechanism in case of Zn, while for Ti normal vaporisation seems to be the dominant mechanism. The results show that pulsed digital holographic interferometry is a promising technique to give a physical picture and increase the understanding of the laser ablation process in a time resolved manner.

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  • 19.
    Amer, Eynas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Edenharder, Stefan
    Institut für Technische Optik, Universität Stuttgart.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Stimulated Raman scattering detection for chemically specific time-resolved imaging of gases2016In: Optics Express, E-ISSN 1094-4087, Vol. 24, no 9, p. 9984-9993Article in journal (Refereed)
    Abstract [en]

    stimulated Raman scattering (SRS) imaging technique based on spatial modulation of the pump beam has been used to study gases. The SRS gain signal was separated from the Stokes beam background in the spatial frequency domain. The SRS signal shows linear behaviour with the gas pressure at a range from 1.0 to 8.0 bars. The signal is linearly proportional to the pump beam intensity while it is enhanced with increasing the Stokes beam intensity to a certain limit than it saturates. Further, the chemical specificity of the technique has been investigated. Two sharp peaks with line width at half maximum of about 0.30 nm have been obtained at Stokes beam wavelengths of 629.93 nm and 634.05 nm corresponding to the methane and ethylene gases, respectively. The results show that SRS imaging is a promising technique to provide chemical specificity as well as spatial and temporal information of gaseous species

  • 20.
    Amer, Eynas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Edenharder, Stefan
    Institut für Technische Optik, Universität Stuttgart.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Stimulated Raman scattering holography for time-resolved imaging of methane gas2016In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 55, no 13, p. 3429-3434Article in journal (Refereed)
    Abstract [en]

    In this paper, pulsed digital holographic detection is coupled to the stimulated Raman scattering (SRS) process for imaging gases. A Q-switched Nd-YAG laser (532 nm) has been used to pump methane gas (CH4) at pressures up to 12 bars. The frequency-tripled (355 nm) beam from the same laser was used to pump an optical parametric oscillator (OPO). The Stokes beam (from the OPO) has been tuned to 629.93 nm so that the frequency difference between the pump (532 nm) and the Stokes beams fits a Raman active vibrational mode of the methane molecule (2922 cm(-1)). The pump beam has been spatially modulated with fringes produced in a Michelson interferometer. The pump and the Stokes beams were overlapped in time, space, and polarization on the gas molecules, resulting in a stimulated Raman gain of the Stokes beam and a corresponding loss of the pump beam through the SRS process. The resulting gain of the Stokes beam has been detected using pulsed digital holography by blending it with a reference beam on the detector. Two holograms of the Stokes beam, without and with the pump beam fringes present, were recorded. Intensity maps calculated from the recorded digital holograms showed amplification of the Stokes beam at the position of overlap with the pump beam fringes and the gas molecules. The gain of the Stokes beam has been separated from the background in the Fourier domain. A gain of about 4.5% at a pump beam average intensity of 4 MW/cm(2) and a Stokes beam intensity of 0.16 MW/cm(2) have been recorded at a gas pressure of 12 bars. The gain decreased linearly with decreasing gas pressure. The results show that SRS holography is a promising technique to pinpoint a specific species and record its spatial and temporal distribution

  • 21.
    Amer, Eynas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Sjödahl, Mikael
    Impact of an extended source in laser ablation using pulsed digital holographic interferometry and modelling2009In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 255, no 21, p. 8917-8925Article in journal (Refereed)
    Abstract [en]

    Pulsed digital holographic interferometry has been used to study the effect of the laser spot diameter on the shock wave generated in the ablation process of an Nd:YAG laser pulse on a Zn target under atmospheric pressure. For different laser spot diameters and time delays, the propagation of the expanding vapour and of the shock wave were recorded by intensity maps calculated using the recorded digital holograms. From the latter phase maps, the refractive index and the density field can be derived. A model was developed that approaches the density distribution, in particular the ellipsoidal expansion characteristics. The induced shock wave has an ellipsoid shape that approaches a sphere for decreasing spot diameter. The ellipsoidal shock waves have almost the same centre offset towards the laser beam and the same aspect ratio for different time steps. The model facilitates the derivation of the particle velocity field. The method provides valuable quantitative results that are discussed, in particular in comparison with the simpler point source explosion theory.

  • 22.
    Amer, Eynas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Shaer, M. El
    Department of Engineering Physics and Mathematics, Faculty of Engineering, Zagazig University.
    Comparison of the laser ablation process on Zn and Ti using pulsed digital holographic interferometry2010In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 256, no 14, p. 4633-4641Article in journal (Refereed)
    Abstract [en]

    Pulsed digital holographic interferometry has been used to compare the laser ablation process of a Q-switched Nd-YAG laser pulse (wavelength 1064 nm, pulse duration 12 ns) on two different metals (Zn and Ti) under atmospheric air pressure. Digital holograms were recorded for different time delays using collimated laser light (532 nm) passed through the volume along the target. Numerical data of the integrated refractive index field were calculated and presented as phase maps. Intensity maps were calculated from the recorded digital holograms and are used to calculate the attenuation of the probing laser beam by the ablated plume. The different structures of the plume, namely streaks normal to the surface for Zn in contrast to absorbing regions for Ti, indicates that different mechanisms of laser ablation could happen for different metals for the same laser settings and surrounding gas. At a laser fluence of 5 J/cm2, phase explosion appears to be the ablation mechanism in case of Zn, while for Ti normal vaporisation seems to be the dominant mechanism.

  • 23.
    Amer, Eynas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Ramser, Kerstin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Measurement of selective species concentration using spectroscopic holography2018In: Speckle 2018: VII International Conference on Speckle Metrology / [ed] M. Kujawińska ; L. R. Jaroszewicz, SPIE - International Society for Optical Engineering, 2018, Vol. 10834, article id 108340PConference paper (Refereed)
    Abstract [en]

    Spectroscopic holography refers to techniques in which the detected hologram contains information about specific species in the medium under study. In general, at least two lasers are required with wavelengths chosen carefully to fit the interaction process utilized. In this process, energy from the shorter wavelength laser beam is transferred to the longer wavelength coherently through the process of stimulated emission. Two interaction mechanisms are considered; Stimulated Laser Induced Fluorescence (LIF) and Stimulated Raman Scattering (SRS), which both are species specific with the ability of coherent interaction. In this paper, the fundamental properties of spectroscopic holography is presented and demonstrated with a few idealized experiments. These validation experiments are performed in a gas chamber in which different gases may be blended and the gas pressure changed between 1-12 bars. In addition, two examples of applications are presented. In the first set of experiments, LIF holography is used to image light absorption and laser heating in a dye simultaneously. The second set of experiments is performed in a ow of methane gas. It is demonstrated that the combination of holographic phase measurements and SRS gain images may be used for calibration. This calibration may further be used to measure absolute concentration in a burning flame.

  • 24.
    Amer, Eynas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjödahl, Mikael
    Laser-ablation-induced refractive index fields studied using pulsed digital holographic interferometry2009In: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 47, no 7-8, p. 793-799Article in journal (Refereed)
    Abstract [en]

    Pulsed digital holographic interferometry has been used to investigate the plume and the shock wave generated in the ablation process of a Q-switched Nd-YAG (λ=1064 nm and pulse duration=12 ns) laser pulse on a polycrystalline boron nitride (PCBN) target under atmospheric air pressure. A special setup based on two synchronised wavelengths from the same laser for simultaneous processing and measurement has been used. Digital holograms were recorded for different time delays using collimated laser light (λ=532 nm) passed through the volume along the target. Numerical data of the integrated refractive index field were calculated and presented as phase maps showing the propagation of the shock wave and the plume generated by the process. Radon inversion has been used to estimate the 3D refractive index fields measured from the projections assuming rotational symmetry. The shock wave density has been calculated using the point explosion model and the shock wave condition equation and its behaviour with time at different power densities ranging from 1.4 to 9.1 GW/cm2 is presented. Shock front densities have been calculated from the reconstructed refractive index fields using the Gladstone-Dale equation. A comparison of the shock front density calculated from the reconstructed data and that calculated using the point explosion model at different time delays has been done. The comparison shows quite good agreement between the model and the experimental data. Finally the reconstructed refractive index field has been used to estimate the electron number density distribution within the laser-induced plasma. The electron number density behaviour with distance from the target at different power densities and its behaviour with time are shown. The electron number densities are found to be in the order of 1018 cm-3 and decay at a rate of 3×1015 electrons/cm3 ns.

  • 25.
    Amer, Eynas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjödahl, Mikael
    Pulsed laser ablation studied using digital holographic interferometry2009In: Svenska mekanikdagarna: Södertälje 2009, Stockholm: Svenska nationalkommittén för mekanik , 2009, p. 87-Conference paper (Other academic)
  • 26.
    Amer, Eynas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjödahl, Mikael
    Shock wave generation in laser ablation studied using pulsed digital holographic interferometry2008In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 41, no 21Article in journal (Refereed)
    Abstract [en]

    Pulsed digital holographic interferometry has been used to study the shock wave induced by a Q-switched Nd-YAG laser (λ = 1064 nm and pulse duration 12 ns) on a polycrystalline boron nitride (PCBN) ceramic target under atmospheric air pressure. A special setup based on using two synchronized wavelengths from the same laser for processing and measurement simultaneously has been introduced. Collimated laser light (λ = 532 nm) passed through the volume along the target and digital holograms were recorded for different time delays after processing starts. Numerical data of the integrated refractive index field were calculated and presented as phase maps showing the propagation of the shock wave generated by the process. The location of the induced shock wave front was observed for different focusing and time delays. The amount of released energy, i.e. the part of the incident energy of the laser pulse that is eventually converted to a shock wave has been estimated using the point explosion model. The released energy is normalized by the incident laser pulse energy and the energy conversion efficiency between the laser pulse and PCBN target has been calculated at different power densities. The results show that the energy conversion efficiency seems to be constant around 80% at high power densities.

  • 27.
    Amer, Eynas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Single-shot polarization digital holography for recording stimulated Raman scattering signal for time-resolved measurement of gaseous species2017In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 56, no 36, p. 10016-10023Article in journal (Refereed)
    Abstract [en]

    The stimulated Raman scattering (SRS) process is sensitive to the relation between the polarization direction of the two laser beams (the pump and the Stokes) that generate it. In this paper, we made use of the polarization sensitivity of the SRS process and used polarization-resolved pulsed digital holography to record the signal from one single-shot hologram. The pump beam polarization was kept vertical, while the Stokes beam polarization was 45 deg. The two polarization components of the Stokes beam were recorded in a single hologram by blending the Stokes beam with two reference beams with orthogonal polarization on the detector. The two components of the Stokes beam were separated in the Fourier domain, and the corresponding intensity maps were calculated. The vertically polarized component of the Stokes beam was amplified due to the SRS process, while the horizontal component experienced no gain. The difference between the vertically and horizontally polarized intensity maps, respectively, was calculated and Fourier transformed to separate the gain signal in the spatial frequency domain. The method was demonstrated on methane (CH4) gas at a pressure of 12 bars. Results show that SRS polarization holography is a promising technique for recording the SRS signal from one single-shot hologram for time-resolved monitoring of specific species. 

  • 28.
    Amer, Eynas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Stimulated laser induced fluorescence holography for imaging fluorescent species2013In: Optics Communications, ISSN 0030-4018, E-ISSN 1873-0310, Vol. 311, p. 124-128Article in journal (Refereed)
    Abstract [en]

    In this paper pulsed digital holographic detection is coupled to the stimulated laser induced fluorescence (LIF) effect for imaging fluorescent species. A frequency tripled Q-switched Nd-YAG laser (wavelength 355 nm, pulse duration 12 ns) has been used to pump Coumarin 153 dye solved in ethanol. Simultaneously a frequency doubled pulse (532 nm) from the same laser is used to probe the solvent resulting in a gain through stimulated emission. The resulting gain of the probe beam is recorded using digital holography by blending it with a reference beam on the detector. Intensity maps were calculated from the recorded digital holograms and used to calculate the gain of the probe beam due to stimulated fluorescence emission which is coupled to the concentration of the dye. The results show that the amplification of the probe beam (532 nm) due to stimulated LIF emission is seen in the intensity maps. The gain is about 40% at a dye concentration of 0.32 g/L and decreases to be about 20% at a dye concentration of 0.04 g/L for a probe beam energy density of 0.1 mJ/cm2. Spectroscopic measurements have been carried out to confirm the holographic results. The results show that stimulated LIF holography is a promising technique for quantitative imaging of fluorescent species.

  • 29.
    Amer, Eynas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Stimulated Raman scattering imaging for studying specific species2015In: SPECKLE 2015: VI International Conference on Speckle Metrology : 24-26 August 2015, Guanajuato, México / [ed] Fernando Mendoza Santoyo, Washington: SPIE - International Society for Optical Engineering, 2015, article id 96600VConference paper (Refereed)
    Abstract [en]

    The stimulated Raman scattering (SRS) signal in diffuse light has been recorded using an optical imaging technique based on spatial modulation. A frequency doubled Q-switched Nd-YAG laser (wavelength 532 nm) has been used to pump a polymethyl methacrylate (PMMA) cylinder. The frequency tripled (355 nm) beam from the same laser is used to pump an optical parametric oscillator (OPO). The Stokes beam (from the OPO) has been tuned to 631.27 nm so that the frequency difference between the pump and the Stokes beams fits a Raman active vibrational mode of the PMMA molecule (2956 cm-1). The two laser beams were overlapped in time and space on a PMMA cylinder resulting in a gain of the Stokes beam through the SRS process of about 4.0 %. For separating the SRS signal, the pump beam was spatially modulated with fringes produced in a Michelson interferometer. The gain of the Stokes beam due to SRS was separated from the Stokes beam background in the Fourier domain. The intensity image has been calculated from an inverse Fourier transform of the separated gain signal. The intensity image shows a gain of the Stokes beam at the area of overlap between the pump beam fringes and the Stokes beam compared to the undisturbed surrounding. The results show that spatial modulation of the pump beam is a promising method to separate the weak SRS signal from the Stokes beam background. This technique can be applied to pin-point specific species and record its spatial and temporal distribution

  • 30.
    Amer, Eynas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Stimulated Raman scattering signals recorded by the use of an optical imaging technique2015In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 54, no 20, p. 6377-6385Article in journal (Refereed)
    Abstract [en]

    In this paper, stimulated Raman scattering (SRS) signals have been recorded by an optical imaging technique that is based on spatial modulation. A frequency doubled Q-switched Nd:YAG laser (532 nm) was used to pump a polymethyl methacrylate (PMMA) target. The frequency tripled (355 nm) beam from the same laser was used to pump an optical parametric oscillator (OPO). The Stokes beam (from the OPO) was tuned to 631.27 nm so that the frequency difference between the pump and the Stokes beams fit the Raman active vibrational mode of the PMMA molecule (2956 cm(-1)). The pump beam has been spatially modulated with fringes produced in a Michelson interferometer. The pump and the Stokes beams were overlapped on the target resulting in a gain of the Stokes beam of roughly 2.5% and a corresponding loss of the pump beam through the SRS process. To demodulate the SRS signal, two images of the Stokes beam without and with the pump beam fringes present were recorded. The difference between these two images was calculated and Fourier transformed. Then, the gain of the Stokes beam was separated from the background in the Fourier domain. The results show that spatial modulation of the pump beam is a promising method to separate the weak SRS signal from the background.

  • 31.
    Amer, Eynas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    UV laser interaction with a fluorescent dye solution studied using pulsed digital holography2013In: Optics Express, E-ISSN 1094-4087, Vol. 21, no 21, p. 25316-25323Article in journal (Refereed)
    Abstract [en]

    A frequency tripled Q-switched Nd-YAG laser (wavelength 355 nm, pulse duration 12 ns) has been used to pump Coumarin 153 dye solved in ethanol. Simultaneously, a frequency doubled pulse (532 nm) from the same laser is used to probe the solvent perpendicularly resulting in a gain through stimulated laser induced fluorescence (LIF) emission. The resulting gain of the probe beam is recorded using digital holography by blending it with a reference beam on the detector. Two digital holograms without and with the pump beam were recorded. Intensity maps were calculated from the recorded digital holograms and used to calculate the gain of the probe beam due to the stimulated LIF. In addition numerical data of the local temperature rise was calculated from the corresponding phase maps using Radon inversion. It was concluded that about 15% of the pump beam energy is transferred to the dye solution as heat while the rest is consumed in the radiative process. The results show that pulsed digital holography is a promising technique for quantitative study of fluorescent species

  • 32.
    Amer, Eynas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Stenvall, Jonas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Stimulated LIF studied using pulsed digital holography and modelling2013In: Optical Measurement Systems for Industrial Inspection VIII / [ed] Peter H. Lehmann; Wolfgang Osten; Armando Albertazzi, SPIE - International Society for Optical Engineering, 2013Conference paper (Refereed)
    Abstract [en]

    A frequency tripled Q-switched Nd-YAG laser (wavelength 355 nm, pulse duration 12 ns) has been used to pump Coumarin 153 dye solved in ethanol. The laser induced fluorescence (LIF) spectrum has been recorded using a spectrometer at different dye concentrations. The frequency doubled 532 nm beam from the same laser is used as a probe beam to pass through the excited volume of the dye. Because of stimulated emission an increase of the probe (532 nm) beam energy is recorded and a reduction of the spontaneous fluorescence spectrum intensity is observed. A model was developed that approaches the trend of the gain as a function of the probe beam energy at low dye concentrations (less than 0.08 g/L). The stimulated LIF is further recorded using digital holography. Digital holograms were recorded for different dye concentrations using collimated laser light (532 nm) passed through the dye volume. Two holograms without and with the UV laser beam were recorded. Intensity maps were calculated from the recorded digital holograms and are used to calculate the gain of the green laser beam due to the stimulated fluorescence emission which is coupled to the dye concentration. The gain of the coherent 532 nm beam is seen in the intensity maps and its value is about 40% for a dye concentration of 0.32 g/L and decreases with the decrease of the dye concentration. The results show that pulsed digital holography can be coupled to the stimulated LIF effect for imaging fluorescent species

  • 33.
    Amiri, Kaveh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    An experimental investigation of flow in a Kaplan runner: steady-state and transient2014Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Water turbines are since some years widely used for grid stabilization purposes according to their exceptional load variation capability which gives them the ability to compensate grid fluctuations initiated by the customer’s consumption or intermittent electricity production systems such as wind and solar power. Different renewable power generation technologies can be combined in mini-grids to electrify isolated villages and extend existing grid networks. In these occasions, small hydro units are also a good option to reduce the overall variability of supply to low levels and provide low‑cost, local electrification solutions. Hence, initially designed hydropower turbines for steady operation at on-design operating condition experience many off-design, start/stop and load variations during their life time according to the nowadays on-demand energy market and introduction of intermittent power generation systems to the electricity market.Start/stop and load variations can be harsh for the turbines due to the time dependent forces exerted on different parts of the turbines, especially rotating parts. Off-design performance of hydropower systems may also result in unfavorable and harmful periodic forces on the rotating parts. Therefore, investigations are required to study these working conditions and consider them in design of new hydropower plants and refurbished turbines. This was the motivation for the experimental investigation of a Kaplan turbine during on-design, off-design and transient operations with focus on the turbine’s rotor. The test case was a 1:3.1 scaled model of Porjus U9; a Kaplan turbine. The first paper deals with pressure measurements on the runner blades of the model under steady state operating conditions to find and quantify the sources of pressure fluctuations exerted on the runner at different operating points. The goal was to investigate the turbine’s performance at the best efficiency point with concentration on the performance of the water supply system and compare it to operations at high load and part load for a constant blades angle. The model results are compared with prototype measurements to corroborate the findings. The second paper presents the model investigations during load acceptance and load rejection. The model was investigated with pressure measurement on the stationary and rotating parts of the turbine under different load variations between part load, high load and best efficiency point. The third paper focuses on velocity measurements in the runner blade channels and at the runner outlet under on-design and off-design operating conditions. The velocity measurements are performed with a laser Doppler anemometry (LDA) system. The results of the model investigations along two propeller curves are presented to investigate the runner blade angle effects on the turbine’s performance.

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    FULLTEXT01
  • 34.
    Amiri, Kaveh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Experimental investigation of a Kaplan runner under steady-state and transient operations2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Hydropower is a key part in electricity production nowadays. Hydropower electricity production rose to 3579.5 TWh in 2013, ranked as the second source of electricity production in the world after fossil fuels. It is the principle source of renewable electricity production, producing 16.2% of the electricity in 2013, accounting for 78% of the renewable electricity production in the world. Specifically in Sweden, hydropower is the main source of electricity production producing 47.5% of the required electricity. Nuclear, biomass, and wind placed in the following positions in the ranking in 2013 with 38.4%, 6.5%, and 4.3%, respectively.Besides meeting electricity demand with an environmental-friendly method, hydropower has a unique and important role which is grid regulation: balancing electricity production and consumption. Gas turbines and hydraulic turbines called “Primary reserves” are the only electricity production systems that can be used for fast regulations due to their short start-up time from 1 to 60 s. The obvious environmental problems, air pollution, and costs associated with gas turbines make hydropower a prime alternative whenever applicable. In Sweden, the share of fossil fuels in electricity production is small; 2.8% in 2012 with an average annual growth of -0.7% in the period 2002-2012. Hence, hydropower is practically the only available source used to regulate the grid fluctuations resulting from deregulated market and fast growth of intermittent power generation systems, i.e., solar and wind energy. Hydraulic turbines are subject to frequent off-design and transient operations because of their grid regulation responsibility. Such operating conditions decrease turbine’s efficiency and affect its lifetime significantly. Off-design and transient operation of hydraulic turbines may induce unpredicted pressure fluctuations on the stationary and rotating parts of the turbine. Special attention should be dedicated to the effects of such phenomena on the runner blades because of their importance on the efficiency of the turbine, and their vulnerability to the pressure fluctuations.This thesis presents an experimental investigation on the effects of off-design and transient operation of an axial hydraulic turbine on velocity fields and pressure fluctuations exerted on the runner and the draft tube of a turbine. The investigation was performed on a 1:3.1 scaled model of a Kaplan turbine known as Porjus U9. The main objective was to investigate the effect of operating point on pressure and velocity fluctuations in the runner and the draft tube. Another objective was to study the effect of transient operation on pressure fluctuations exerted on the runner and the draft tube, to investigate the formation and mitigation process of a rotating vortex rope (RVR) within the draft tube. Finally, the effect of the swirl leaving the runner and the draft tube bend on the performance of the turbine was investigated. The study involves pressure measurements on the runner blades and draft tube walls of the turbine, laser Doppler anemometry (LDA) measurements within and after the runner, and particle image velocimetry (PIV) measurements within the draft tube.The pressure and LDA results acquired during steady state operation of the turbine showed different sources of fluctuations on the runner at different operating points resulting in symmetric and asymmetric fluctuating forces on the runner. The pressure measurements during transient operating conditions exhibited pressure fluctuations exerted on the runner during load variations and elucidated some aspects of formation and mitigation process of RVR within the draft tube. The PIV measurements performed after the draft tube bend of the turbine focuses on the physical phenomena resulting in flow asymmetry after the draft tube bend of hydraulic turbines affecting their efficiency.

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  • 35.
    Amiri, Kaveh
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Department of Energy and Process Engineering, Water Power Laboratory, Norwegian University of Science and Technology, Trondheim, Norway.
    Mulu, Berhanu
    Vattenfall Research and Development, Älvkarleby, Sweden.
    Experimental investigation of the hydraulic loads on the runner of a Kaplan turbine model and the corresponding prototype2015In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 53, no 4, p. 452-465Article in journal (Refereed)
    Abstract [en]

    This paper reports on unsteady pressure measurements on the runner blades of a Kaplan turbine model as well as torque and radial load bearing measurements on the corresponding prototype at several operating points to investigate the sources of periodic loads exerted on the runner when operating at the best efficiency point and off design. Pressure measurements on the model runner blades indicated that the spiral casing delivers a poorly conditioned flow to the guide vanes close to the lip-entrance junction, resulting in flow separation on the guide vanes. The asymmetric flow delivered to the runner induces large oscillations with respect to the guide vane passing frequency, runner frequency and its harmonics to the runner blades. The torque measurements on the prototype also revealed an asymmetric flow at the distributor outlet. The bearing radial load measurements performed on the prototype support the torque measurement results. The asymmetric hydraulic loads on the runner result in shaft wobbling, and the oscillatory forces exerted on the blades are transferred to the main shaft and bearings. Another source of oscillating forces exerted on the runner blades is the rotating vortex rope (RVR) formation that occurs at part-load operation of the turbine and induces pressure fluctuations at two sub-synchronous frequencies to the runner.

  • 36.
    Amiri, Kaveh
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Mulu, Berhanu
    Vattenfall Research & Development.
    Raisee, Mehrdad
    Mechanical Engineering Department, University of Tehran.
    Experimental study of fluid structure interaction in a Kaplan turbine runner blade2013Conference paper (Refereed)
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  • 37.
    Amiri, Kaveh
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Raisee, M.
    School of Mechanical Engineering, University of Tehran.
    Effects of flow unsteadiness on the wall shear stress2012In: 26th IAHR Symposium on Hydraulic Machinery and Systems: 19–23 August 2012, Beijing, China, IOP Publishing Ltd , 2012, p. 62033-Conference paper (Refereed)
    Abstract [en]

    Measurements were performed on pulsating fully turbulent flows in a pipe test rig with a diameter of 100 mm. Sinusoidal oscillatory flow at different frequencies was superimposed on a mean flow of averaged Reynolds number Re=20000 based on the pipe diameter. The measurements have been performed at different forcing frequencies (0.001 < ω+ < 0.08) covering all the oscillatory regimes; quasi-steady, relaxation, quasi laminar and high frequency. The amplitude of the flow oscillation was small enough to allow a linear response in the measurements, i.e., all flow parameters showed an oscillatory behavior at the frequency of the flow. The amplitude of the oscillatory flow was about 10% of the mean velocity in all cases. The results include mean and phase averaged values of different parameters. The centerline velocity was measured by a 2D LDA system. Hot film and constant temperature anemometry system was used to determine the wall shear stress. Bulk velocity and pressure gradient along the pipe were also acquired. The results showed a good agreement with the previous analytical, experimental and numerical results available in the literature.

  • 38.
    Amiri, Kaveh
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Mulu, Berhanu
    Vattenfall Research and Development, Älvkarleby.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Experimental Investigation of the Interblade Flow in a Kaplan Runner at Several Operating Points Using Laser Doppler Anemometry2016In: Journal of Fluids Engineering, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 138, no 2, article id 21106Article in journal (Refereed)
    Abstract [en]

    This paper presents laser Doppler anemometry (LDA) measurements within the runner blade channels and at the runner outlet of a Kaplan turbine model. The model was investigated at six operating points located on two propeller curves of the turbine to study the flow condition during on-cam and off-cam operations. Main and secondary flows within and after the runner were analyzed, and the effects of the hub and tip clearances on the velocity fields within and after the runner were evaluated. Operation of the turbine at flow rates that are lower than the designed rate for the corresponding propeller curve resulted in vortex breakdown and the formation of a rotating vortex rope (RVR). The RVR formation produced an asymmetrical velocity distribution within and after the runner. The results demonstrated the occurrence of an oscillating flow with the same frequency as the vortex rope within the blade channels located upstream of the RVR. This results in an asymmetric flow through the runner and oscillating forces on the runner blades. The measured velocities indicated that the geometrical asymmetries in the runner manufacturing process resulted in various flow asymmetries at the measured sections. The asymmetries were up to 3% within the runner and 7% at the runner outlet

  • 39.
    Amiri, Kaveh
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Mulu, Berhanu
    Vattenfall Research and Development, Älvkarleby, 814 70, Sweden.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Department of Energy and Process Engineering, Water Power Laboratory, Norwegian University of Science and Technology, Trondheim, 7491, Norway.
    Raisee, Mehrdad
    Mechanical Engineering Department, University of Tehran, 14155-6448, Tehran, Iran.
    Effects of load variation on a Kaplan turbine runner2016In: The International Journal of Fluid Machinery and Systems, E-ISSN 1882-9554, Vol. 9, no 2, p. 182-193Article in journal (Refereed)
    Abstract [en]

    Introduction of intermittent electricity production systems like wind and solar power to electricity market together with the deregulation of electricity markets resulted in numerous start/stops, load variations and off-design operation of water turbines. Hydraulic turbines suffer from the varying loads exerted on their stationary and rotating parts during load variations they are not designed for such operating conditions. Investigations on part load operation of single regulated turbines, i.e., Francis and propeller, proved the formation of a rotating vortex rope (RVR) in the draft tube. The RVR induces pressure pulsations in the axial and rotating direction called plunging and rotating modes, respectively. This results in oscillating forces with two different frequencies on the runner blades, bearings and other rotating parts of the turbine. This study investigates the effect of transient operations on the pressure fluctuations exerted on the runner and mechanism of the RVR formation/mitigation. Draft tube and runner blades of the Porjus U9 model, a Kaplan turbine, were equipped with pressure sensors for this purpose. The model was run in off-cam mode during different load variations. The results showed that the transients between the best efficiency point and the high load occurs in a smooth way. However, during transitions to the part load a RVR forms in the draft tube which induces high level of fluctuations with two frequencies on the runner; plunging and rotating mode. Formation of the RVR during the load rejections coincides with sudden pressure change on the runner while its mitigation occurs in a smooth way.

  • 40.
    Amiri, Kaveh
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Mulu, Berhanu G.
    Vattenfall Research and Development, Älvkarleby.
    Raisee, Mehrdad
    Mechanical Engineering Department, University of Tehran.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Effects of upstream flow conditions on runner pressure fluctuations2017In: Journal of Applied Fluid Mechanics, ISSN 1735-3572, E-ISSN 1735-3645, Vol. 10, no 4, p. 1045-1059Article in journal (Refereed)
    Abstract [en]

    The rotor-stator interaction and the corresponding pressure fluctuations represent one of the sources of pressure and load fluctuations on the rotating parts of rotating machineries. The high-Reynolds flow is subject to rotation in the comparably large vaneless space of axial turbines, causing wake interaction and wake dissipation in this region. This increases the level of flow complexity in this region. This study examined the effect of the flow condition entering the spiral casing on the flow condition within the distributor and the runner and the physical source of pressure fluctuations exerted on the runner of a Kaplan turbine model. Simulations were performed within the water supply system, including the upstream tank, penstock, and the Francis turbines, the level of entering the spiral casing; the results were compared with laser Doppler anemometry (LDA) results. The results were considered as the inlet boundary condition for simulation of the turbine model from the spiral inlet to the draft tube outlet to investigate the flow condition within the distributor and the runner. The CFD simulations showed that the water supply system induces inhomogeneity to the velocity distribution at the spiral inlet. However, the flow condition does not affect the pressure fluctuations exerted on the runner blades due to the rotor-stator interactions. Moreover, the dominant frequencies exerted on the runner blades were accurately approximated although the amplitudes of the fluctuations were underestimated.

  • 41.
    Amiri, Kaveh
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Mulu, Berhanu
    Vattenfall AB, Älvkarleby, Sweden.
    Raisee, Mehrdad
    School of Mechanical Engineering, College of Engineering, University of Tehran, P.O. Box: 11155-4563, Tehran, Iran.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Department of Energy and Process Engineering, Water Power Laboratory, Norwegian University of Science and Technology, Trondheim, Norway.
    Experimental study on flow asymmetry after the draft tube bend of a Kaplan turbine2016In: Advances and Applications in Fluid Mechanics, ISSN 0973-4686, Vol. 19, no 2, p. 441-472Article in journal (Refereed)
    Abstract [en]

    Flow condition in a Kaplan turbine draft tube is investigated using laser Doppler anemometry (LDA) and particle image velocimetry(PIV). The investigated draft tube is composed of a cone followed by an elbow and a straight diffuser. The three velocity components were measured after the elbow at two different locations across the straight diffuser to quantify the flow asymmetry as well as the secondary flows formed in this region. The velocity profiles at the draft tube inlet are measured using a 2D LDA system allowing estimation of the draft tube inlet swirl. The results are presented at three operating points of the turbine. The flow condition after the draft tube bend was shown to be highly dependent on the vortex structures within the straight draft tube; namely Dean vortices and the swirl leaving the runner. At operating points with high flow rates and low swirl, Dean vortices dominate the upstream swirl; a symmetric but inhomogeneous flow resembling flow after a pipe bend forms within the straight diffuser. At part load operating points with high swirl and low flow rate, the flow after the bend is dominated by the upstream swirl resulting in asymmetric flow after the draft tube bend. The flow asymmetry is shown to be a 2nd order function of the swirl-to-Dean ratio.

  • 42.
    Amiri, Kaveh
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Mulu, Berhanu
    Vattenfall Research & Development.
    Raisee, Mehrdad
    School of Mechanical Engineering, University of Tehran, Mechanical Engineering Department, University of Tehran.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Load variation effects on the pressure fluctuations exerted on a Kaplan turbine runner2014In: 27th IAHR Symposium on Hydraulic Machinery and Systems, IAHR 2014: Montreal, Canada, 22- 26 September 2014 / [ed] A-M Giroux, IOP Publishing Ltd , 2014, article id 32005Conference paper (Refereed)
    Abstract [en]

    Introduction of intermittent electricity production systems like wind power and solar systems to electricity market together with the consumption-based electricity production resulted in numerous start/stops, load variations and off-design operation of water turbines. The hydropower systems suffer from the varying loads exerted on the stationary and rotating parts of the turbines during load variations which they are not designed for. On the other hand, investigations on part load operation of single regulated turbines, i.e., Francis and propeller, proved the formation of rotating vortex rope (RVR) in the draft tube. The RVR induces oscillating flow both in plunging and rotating modes which results in oscillating force with two different frequencies on the runner blades, bearings and other rotating parts of the turbine. The purpose of this study is to investigate the effect of transient operations on the pressure fluctuations on the runner and mechanism of the RVR formation/mitigation. Draft tube and runner blades of the Porjus U9 model, a Kaplan turbine, were equipped with pressure sensors. The model was run in off-cam mode during different load variation conditions to check the runner performance under unsteady condition. The results showed that the transients between the best efficiency point and the high load happens in a smooth way while transitions to/from the part load, where rotating vortex rope (RVR) forms in the draft tube induces high level of fluctuations with two frequencies on the runner; plunging and rotating mode of the RVR.

  • 43.
    Amiri, Kaveh
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Mulu, Berhanu
    Vattenfall Research and Development, Älvkarleby.
    Raisee, Mehrdad
    School of Mechanical Engineering, University of Tehran, Mechanical Engineering Department, University of Tehran.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Unsteady pressure measurements on the runner of a Kaplan turbine during load acceptance and load rejection2016In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 54, no 1, p. 56-73Article in journal (Refereed)
    Abstract [en]

    The paper addresses unsteady pressure measurements on the blades and stationary parts of a Kaplan turbine model (Porjus U9) during load variation. The turbine was studied in various load acceptance and load rejection scenarios in off-cam mode to investigate the effect of the transients on the turbine performance. The formation and mitigation processes for the rotating vortex ropes and their effects on the forces exerted on the runner were also investigated. The results show a smooth transition during load variations between high load and the best efficiency point, at which no rotating vortex ropes form in the draft tube. However, load variation to part load resulted in a draft tube surge and the formation of a rotating vortex rope with two fluctuating components: rotating and plunging. The rotating vortex ropes began to form at the end of the draft tube cone during the closure of the guide vanes and travelled upstream with further guide vane closure. The plunging mode induced flow oscillation throughout the entire turbine conduit, whereas the rotating mode resulted in local pressure fluctuations. The rotating vortex ropes induced wide-band pressure fluctuations on the suction side of the runner close to the hub section. The formation of the rotating vortex ropes near the runner resulted in a sudden change in the pressure exerted on the suction side of the blades, whereas the rotating vortex rope mitigation process proceeded in a smooth manner.

  • 44. Andersson, A.
    et al.
    Fernberg, P.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Optical methods to study fracture of notched glass mat composites2000In: Proceedings / International Conference on Trends in Optical Nondestructive Testing: May 3 - 5, 2000, Hotel de la Paix, Lugano, Switzerland / [ed] Pramod K. Rostagi, Lugano: Ecole Polytechnique Fédérale de Lausanne , 2000Conference paper (Refereed)
  • 45.
    Andersson, Anders G.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Modelling flow with free and rough surfaces in the vicinity of hydropower plants2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Flow with free and rough surfaces near hydropower stations is of interest for both engineering and environmental applications. Here, Computational Fluid Dynamics simulations of free surface flow and flow over rough surfaces in regulated rivers were performed in applications such fish migration, spillway design and flow over rough surfaces as in hydropower tunnels or natural channels. For all the investigated applications it is typical with very large geometrical scales, high flow rates and highly turbulent flow. Modelling boundaries such as free water surfaces and rough walls presents a challenge and was given special attention as well as the treatment of turbulence. Validation of the numerical simulations was performed in all cases with methods such as acoustic measurements with an Acoustic Doppler Current Profiler (ADCP), Acoustic Doppler Velocimeter (ADV) and optical measurements with Particle Image Velocimetry (PIV).Numerical simulations have been used to evaluate the flow downstream the Stornorrfors hydropower plant in Umeälven with regards to upstream migrating fish. Field measurements with an ADCP were performed and the measurements were used to validate the simulations. By adding a fish ladder in the simulations it was possible to investigate the attraction water created from the fishway at different positions and angles. An additional possibility to create better attraction water and improve the conditions for upstream migrating fish was simulated by guiding the spill water from the hydropower dam through a smaller passage from the old river bed.Fish population data from the same location was compared with flow fields from numerical simulations. The population data was compared with variables such as velocity, vorticity and turbulence intensity. A correlation between fish detections and turbulence intensity was shown.Simulations on the spilling from a dam were performed and compared to experimental results from a physical scale model. ADV was used to measure the velocity and validate the simulations. Two different spillway configurations were considered and simulations with both the Rigid Lid model and the Volume of Fluids method were carried out. Water levels, velocities and the shape of the water surface were compared between simulations and experiments. The simulations capture both qualitative features such as a vortex near the outlet and show good quantitative agreement with the experiments.A wall with large surface roughness was created by laser scanning a tunnel. One of the side walls was down-scaled and used to create a rough wall in a channel with rectangular cross-section for both a numerical model and an experimental model. Numerical simulations were performed and validated by PIV-measurements in the experimental model.The resolution of the geometry for the rough surface was lowered in two steps and numerical simulations were performed for flow over all three surfaces. The difference in flow fields in the bulk and near wall region was investigated as well as the difference in turbulent quantities which can provide good input for a new model for surface roughness in applications with very large surface roughness and high velocities such as flow in hydropower tunnels or natural channels and rivers.

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  • 46.
    Andersson, Anders G.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Simulations and measurements of free surface flow in regulated rivers2010Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Open channel flow near hydropower stations is of interest for both engineering and environmental applications. In this research project Computational Fluid Dynamics simulations of free surface flow in regulated rivers were applied with both fish migration and validation of numerical simulations in focus. In the first paper, numerical simulations has been used to evaluate the flow downstream a hydropower plant with regards to upstream migrating fish. Field measurements with an Acoustic Doppler Current Profiler were performed and the measurements were used to validate the simulations. In the second paper ,simulations on the spilling from a dam were performed and compared to experimental results from a physical scale model. The third paper deals with more in depth analysis of field measurements where the variations in the flow downstream a hydropower plant was examined.

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  • 47.
    Andersson, Anders G.
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Andreasson, Patrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Hellström, J. Gunnar I.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, T. Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Modelling and validation of flow over a wall with large surface roughness2012Conference paper (Refereed)
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  • 48.
    Andersson, Anders G.
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Andreasson, Patrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, T. Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    CFD-modelling and validation of free surface flow during spilling of reservoir in down-scale model2013In: Engineering Applications of Computational Fluid Mechanics, ISSN 1994-2060, E-ISSN 1997-003X, Vol. 7, no 1, p. 159-167Article in journal (Refereed)
    Abstract [en]

    Fully three dimensional modelling of the spilling from a reservoir with relatively complex geometry were performed and compared to experimental results from a physical scale model with the aim to advance the science of numerical modelling of free surface flow of real reservoirs. In the set-up in focus the water was spilled from the reservoir through three gates that could be manoeuvred separately. In the first case two of the gates were closed and the third gate was partly opened. In this experimental set-up the water surface in the reservoir was close to horizontal. Therefore it was here meaningful to compare a rigid lid modelling approximation to the more computational heavy method of Volume of Fluids. In the second case, all three gates were open, resulting in a nonhorizontal varied flow surface profile in the reservoir upstream critical sections at the spillway crests. This case was simulated with Volume of Fluids and the position of the air-water interface was derived for two turbulence models, the standard k-ε and SSG. Water levels, velocities and the shape of the water surface were compared to experiments. The simulation results capture qualitative features such as a vortex near the outlet and show good quantitative agreement with the experiments regardless of method used to simulate the free surface. In general, simulations with the standard k-ε and the more advanced SSG turbulence models give the same results with respect to the averaged quantities measured.

  • 49.
    Andersson, Anders G.
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Andreasson, Patrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, T. Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Modellering av avbördning med fri vattenyta och validering i en skalmodell2011Conference paper (Refereed)
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  • 50.
    Andersson, Anders G.
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Hellström, J. Gunnar I.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Andreasson, Patrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, T. Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Effect of spatial resolution of rough surfaces on numerically computed flow fields with application to hydraulic engineering2014In: Engineering Applications of Computational Fluid Mechanics, ISSN 1994-2060, E-ISSN 1997-003X, Vol. 8, no 3, p. 373-381Article in journal (Refereed)
    Abstract [en]

    In numerical simulations of flow over rough surfaces, the roughness is often not resolved but represented by a numerical model. The validity of such an assumption is investigated in this paper by Reynolds-Averaged Navier-Stokes simulations of flow over a surface with a large roughness. The surface was created from a high-resolution laser scanning of a real rock blasted tunnel. By reducing the geometrical resolution of the roughness in two steps, the importance of an appropriate surface description could be examined. The flow fields obtained were compared to a set-up with a geometrical flat surface where the roughness was represented by a modified form of the Launder and Spalding wall-function. The flow field over the surface with the lowest resolution was substantially different from those of the two finer resolutions and rather close to the results from the set-up with the wall-function. The results also yield that the finer the resolution is the more vorticity is formed close to the rough surface and more turbulence is generated.

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