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  • 1.
    Benti, Gudeta Berhanu
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Rondon, David
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Gustavsson, Rolf
    Vattenfall AB Research and Development, Älvkarleby, SE 814 26, Sweden.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Numerical and experimental study on the dynamic bearing properties of a four-pad and eight-pad tilting pad journal bearings in a vertical rotor 2022In: Journal of energy resources technology, ISSN 0195-0738, E-ISSN 1528-8994, Vol. 144, no 1Article in journal (Refereed)
    Abstract [en]

    In this paper, the dynamics of tilting pad journal bearings with four and eight pads are studied and compared experimentally and numerically. The experiments are performed on a rigid vertical rotor supported by two identical bearings. Two sets of experiments are carried out under similar test setup. One set is performed on a rigid rotor with two four-pad bearings, while the other is on a rigid rotor with two eight-pad bearings. The dynamic properties of the two bearing types are compared with each other by studying the unbalance response of the system at different rotor speeds. Numerically, the test rig is modeled as a rigid rotor and the bearing coefficients are calculated based on Navier-Stokes equation. A nonlinear bearing model is developed and used in the steady state response simulation. The measured and simulated displacement and force orbits show similar patterns for both bearing types. Compared to the measurement, the simulated mean value and range (peak-to-peak amplitude) of the bearing force deviate with a maximum of 16 % and 38 %, respectively. It is concluded that, unlike the eight-pad TPJB, the four-pad TPJB excite the system at the third and fifth-order frequencies, which are due to the number of pads, and the amplitudes of these frequencies increase with the rotor speed. 

  • 2. Domaschuk, L.
    et al.
    Knutsson, Sven
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Mining and Geotechnical Engineering.
    Shields, D. H.
    Rahman, M. G.
    Creep of frozen sand under isotropic and deviatoric components of stress1985In: Journal of energy resources technology, ISSN 0195-0738, E-ISSN 1528-8994, Vol. 107, no 2, p. 199-203Article in journal (Refereed)
    Abstract [en]

    The creep behavior of frozen soil is being studied by examining its response to changes in the mean normal stress and changes in the deviatoric components of stress, using separate tests.^The two response components will then be coupled to provide a model for a general stress state.^The paper contains the results of three triaxial compression creep tests on frozen sand (T=-3 C) in which the samples were subjected to changes in the deviatoric components of stress only. Stepwise stress increases were applied for periods up to 1700 hr during which volumetric and axial strains were measured. The samples underwent attenuating creep when subjected to low stress levels and accelerating creep when subjected to higher stress levels, with no indication of a significantly long period of steady-state creep.The samples underwent volume reduction when the axial strains were attenuating and dilated when the axial strain rates began to accelerate.

  • 3.
    Lazzaretto, Andrea
    et al.
    Department of Mechanical Engineering, University of Padova.
    Toffolo, Andrea
    A critical review of the thermoeconomic diagnosis methodologies for the location of causes of malfunctions in energy Systems2006In: Journal of energy resources technology, ISSN 0195-0738, E-ISSN 1528-8994, Vol. 128, no 4, p. 335-341Article in journal (Refereed)
    Abstract [en]

    Thermoeconomic diagnosis procedures in the literature rely on the assumption that specific consumptions of resources in the components are the key to interpret the effects of malfunctions and then to trace a path towards the sources of anomalies. The main obstacle to a successful application of these approaches is represented by the actual interactions existing among components which cause a propagation of the alteration of component specific consumptions and therefore mask those effects that would allow a direct identification of the origin of malfunction. This paper presents an extensive discussion of potentialities and limits of diagnosis procedures proposed in the literature in distinguishing the effects induced by component interactions from those that are intrinsically generated by the anomaly, which is considered here as the main task to locate effectively causes of malfunctions in energy systems

  • 4.
    Lazzaretto, Andrea
    et al.
    Department of Mechanical Engineering, University of Padova.
    Toffolo, Andrea
    Zanon, Frederico
    Department of Mechanical Engineering, University of Padova.
    Parameter sttetting for a tubular SOFC simulation model2004In: Journal of energy resources technology, ISSN 0195-0738, E-ISSN 1528-8994, Vol. 126, no 1, p. 40-46Article in journal (Refereed)
    Abstract [en]

    Several empirical assumptions deriving from observations and measurements of the physical processes are involved in the modeling of Solid Oxide Fuel Cells (SOFCs). An insight of the main models proposed in the literature is given to present the characteristics and limits of these assumptions for the various existing configurations. The basic structure and equations of the models are discussed in details, focusing particularly on the parameters that are to be set to achieve reliability and accuracy. According to this discussion, a zero-dimensional model for a tubular Solid Oxide Fuel Cell (SOFC) is then presented. The model demonstrates good capability in predicting SOFC characteristic curves as they appear in the literature.

  • 5.
    Mesfun, Sennai
    et al.
    International Institute for Applied Systems Analysis (IIASA), Laxenburg.
    Lundgren, Joakim
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Toffolo, Andrea
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Lindbergh, Göran
    School of Chemical Science and Engineering, KTH Royal Institute of Technology.
    Lagergren, Carina
    School of Chemical Science and Engineering, KTH, Stockholm.
    Engvall, Klas
    School of Chemical Science and Engineering, KTH, Stockholm.
    Integration of an electrolysis unit for producer gas conditioning in a bio-synthetic natural gas plant2019In: Journal of energy resources technology, ISSN 0195-0738, E-ISSN 1528-8994, Vol. 141, no 1, article id 012002Article in journal (Refereed)
    Abstract [en]

    Producer gas from biomass gasification contains impurities like tars, particles, alkali salts, and sulfur/nitrogen compounds. As a result, a number of process steps are required to condition the producer gas before utilization as a syngas and further upgrading to final chemicals and fuels. Here, we study the concept of using molten carbonate electrolysis cells (MCEC) both to clean and to condition the composition of a raw syngas stream, from biomass gasification, for further upgrading into synthetic natural gas (SNG). A mathematical MCEC model is used to analyze the impact of operational parameters, such as current density, pressure and temperature, on the quality and amount of syngas produced. Internal rate of return (IRR) is evaluated as an economic indicator of the processes considered. Results indicate that, depending on process configuration, the production of SNG can be boosted by approximately 50-60% without the need of an additional carbon source, i.e., for the same biomass input as in standalone operation of the GoBi-Gas plant. Copyright

  • 6.
    Morandin, Matteo
    et al.
    Chalmers University of Technology, Department of Energy and Environment, Division of Heat and Power Technology.
    Toffolo, Andrea
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Lazzaretto, Andrea
    Department of Industrial Engineering, University of Padova.
    Superimposition of elementary thermodynamic cycles and separation of the heat transfer section in energy systems analysis2013In: Journal of energy resources technology, ISSN 0195-0738, E-ISSN 1528-8994, Vol. 135, no 2, article id 21602Article in journal (Refereed)
    Abstract [en]

    In a wide variety of thermal energy systems, the high integration among components derives from the need to correctly exploit all the internal heat sources by a proper matching with the internal heat sinks. According to what has been suggested in previous works to address this problem in a general way, a "basic configuration" can be extracted from the system flowsheet including all components but the heat exchangers, in order to exploit the internal heat integration between hot and cold thermal streams through process integration techniques. It was also shown how the comprehension of the advanced thermodynamic cycles can be strongly facilitated by decomposing the system into elementary thermodynamic cycles which can be analyzed separately. The advantages of the combination of these approaches are summarized in this paper using the steam injected gas turbine (STIG) cycle and its evolution towards more complex system configurations as an example of application. The new concept of "baseline thermal efficiency" is introduced to combine the efficiencies of the

  • 7.
    Rondon, David
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Benti, Gudeta Berhanu
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Gustavsson, Rolf
    Vattenfall AB Research and Development, SE 814 26 Älvkarleby, Sweden.
    Rotordynamic Characterization of Tilting-Pad Bearings With Eight Pads in Vertical Rotors 2022In: Journal of energy resources technology, ISSN 0195-0738, E-ISSN 1528-8994, Vol. 144, no 1, article id 010902Article in journal (Refereed)
    Abstract [en]

    It has been documented that stiffness and damping for a four-pad bearing are dependent not only on the magnitude of the load but also on the position of the rotor in the bearing. However, eight-pad bearings are not commonly employed on horizontal turbines, and the presence of several pads in the bearing will decisively affect the dynamics of the system. This paper evaluates the stiffness and damping coefficients of tilting-pad bearings with eight pads and explores the main frequencies acting on the forced response of a vertical rotor. The bearing properties were modeled as a function of eccentricity and position in the stationary coordinate system by Navier–Stokes equations whose results are taken from commercial software. The simulated unbalanced response is compared to experimental results; the changing position of the shaft produces a periodic stiffness and damping, which is dependent on the number of pads. Cross-coupled coefficients influence is discussed, showing that their absence makes an accurate model for the mean values. The results indicate that simulation of vertical rotors with eight-pad bearings can be simplified which allow more effective simulations and dynamic analysis. 

  • 8.
    Tabatabaei, Narges
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gantasala, Sudhakar
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Wind Turbine Aerodynamic Modeling in Icing Condition: Three-Dimensional RANS-CFD Versus Blade Element Momentum Method2019In: Journal of energy resources technology, ISSN 0195-0738, E-ISSN 1528-8994, Vol. 141, no 7, article id 071201Article in journal (Refereed)
    Abstract [en]

    Icing limits the performance of wind turbines in cold climates. The prediction of the aerodynamic performance losses and their distribution due to ice accretion is essential. Blade element momentum (BEM) is the basis of blade structural studies. The accuracy and limitations of this method in icing condition are assessed in the present study. To this purpose, a computational study on the aerodynamic performance of the full-scale NREL 5 MW rotor is performed. Three-dimensional (3D) steady Reynolds-averaged Navier–Stokes (RANS) simulations are performed for both clean and iced blade, as well as BEM calculations using two-dimensional (2D) computational fluid dynamics (CFD) sectional airfoil data. The total power calculated by the BEM method is in close agreement with the 3D CFD results for the clean blade. There is a 4% deviation, while it is underestimated by 28% for the iced one. The load distribution along the clean blade span differs between both methods. Load loss due to the ice, predicted by 3D CFD, is 32% in extracted power and the main loss occurs at the regions where the ice horn height exceeds 8% of the chord length.

  • 9.
    Tabatabaei, Narges
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Raisee, Mehrdad
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran, Iran.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Uncertainty Quantification of Aerodynamic Icing Losses in Wind Turbine With Polynomial Chaos Expansion2019In: Journal of energy resources technology, ISSN 0195-0738, E-ISSN 1528-8994, Vol. 141, no 5, article id 051210Article in journal (Refereed)
    Abstract [en]

    Icing of wind turbine blades poses a challenge for the wind power industry in cold cli-mate wind farms. It can lead to production losses of more than 10%of the annual energyproduction. Knowledge of how the production is affected by icing is of importance. Com-plicating this reality is the fact that even a small amount of uncertainty in the shape ofthe accreted ice may result in a large amount of uncertainty in the aerodynamic perform-ance metrics. This paper presents a numerical approach using the technique of polyno-mial chaos expansion (PCE) to quantify icing uncertainty faster than traditionalmethods. Time-dependent bi-dimensional Reynolds-averaged Navier–Stokes computa-tional fluid dynamics (RANS-CFD) simulations are considered to evaluate the aerody-namic characteristics at the chosen sample points. The boundary conditions are based onthree-dimensional simulations of the rotor. This approach is applied to the NREL 5 MWreference wind turbine allowing to estimate the power loss range due to the leading-edgeglaze ice, considering a radial section near the tip. The probability distribution functionof the power loss is also assessed. The results of the section are nondimensionalized andassumed valid for the other radial sections. A correlation is found allowing to model theload loss with respect to the glaze ice horn height, as well as the corresponding probabil-ity distribution. Considering an equal chance for any of the ice profiles, load loss is esti-mated to be lower than 6.5%for the entire blade in half of the icing cases, while it couldbe roughly 4–6 times in the most severe icings.

  • 10. Toffolo, Andrea
    Fuzzy expert systems for the diagnosis of component and sensor faults in complex energy systems2009In: Journal of energy resources technology, ISSN 0195-0738, E-ISSN 1528-8994, Vol. 131, no 4, p. 0420021-04200210Article in journal (Refereed)
    Abstract [en]

    Locating the causes of malfunctions in complex energy systems is an extremely difficult task, since more than one fault mode may produce similar and possibly undistinguishable patterns of effects. This paper shows how fuzzy expert systems can exploit the available measurements from the data acquisition system to identify different component and sensor fault modes. Real sensor data (mass flow rates, pressures, temperatures, and key operating parameters) are compared with the expected values of the same quantities that are calculated using numerical models of local subsystems. This comparison simply determines if the differences between measured and expected values are "negative," "zero," or "positive" in fuzzy logic terms. The final objective is to verify the existence of some patterns of these attributes that univocally identify the considered fault modes. These patterns are then implemented as the set of rules forming the knowledge base of a fuzzy expert system. The proposed diagnostic methodology is tested on the gas section of a real combined-cycle cogeneration plant, and the effect of measurement noise is also discussed

  • 11. Toffolo, Andrea
    et al.
    Lazzaretto, Andrea
    Department of Mechanical Engineering, University of Padova.
    A global and a local approach with evolutionary algorithms to locate malfunction causes in energy systems2009In: Journal of energy resources technology, ISSN 0195-0738, E-ISSN 1528-8994, Vol. 131, no 4, p. 0420011-0420017Article in journal (Refereed)
    Abstract [en]

    Energy system performance may differ from the expected one during actual operation because of the effects of faults, anomalies, and wear and tear due to normal use. One of the main issues of diagnosis, i.e., the procedure to discover the causes of malfunctions, is to find the way back from measured altered performance to the original cause. Several procedures were proposed in the literature to solve the diagnostic problem, usually based on the comparison between a reference nonmalfunctioning condition and an actual, possibly malfunctioning, condition. A different strategy is suggested in the paper. A direct search of the possible causes of malfunctions is performed by means of an evolutionary algorithm: a component fault is arbitrarily introduced in a model of the healthy system by substituting the reference characteristic curve with an altered one, and the algorithm is used to search for a combination of different kinds of performance modifiers that generates the same measured effects of the actual anomaly. A global and a local approach are proposed and applied to a real test case plant, also in presence of measurement noise. The local approach demonstrates to be more effective in terms of accuracy and computational effort.

  • 12. Toffolo, Andrea
    et al.
    Lazzaretto, Andrea
    Department of Mechanical Engineering, University of Padova.
    A new thermoeconomic method for the location of causes of malfunctions in energy systems2007In: Journal of energy resources technology, ISSN 0195-0738, E-ISSN 1528-8994, Vol. 129, no 1, p. 1-9Article in journal (Refereed)
    Abstract [en]

    Diagnosis procedures primarily aim at locating the control volumes where anomalies occurred. This is not a simple task since the effects of anomalies generally propagate through the whole system and affect the behavior of several components. Some components may therefore present a reduced efficiency, although they are not sources of operation anomalies, due to nonflat efficiency curves. These induced effects are a big obstacle in the use of thermoeconomic techniques for the search of the origin of the anomalies. On the other hand, the real cause of the alteration of component behavior is the modification of its characteristic curve, due to degradation or failures. According to this concept, a new approach, based on an indicator measuring the alteration of the characteristic curve of the component affected by the operation anomaly, is proposed and applied to a test case power plant

  • 13.
    Toffolo, Andrea
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Rech, S.
    Department of Industrial Engineering, University of Padova, Italy.
    Lazzaretto, A.
    Department of Industrial Engineering, University of Padova, Italy.
    Generation of Complex Energy Systems by Combination of Elementary Processes2018In: Journal of energy resources technology, ISSN 0195-0738, E-ISSN 1528-8994, Vol. 140, no 11, article id 112005Article in journal (Refereed)
    Abstract [en]

    The fundamental challenge in the synthesis/design optimization of energy systems is the definition of system configuration and design parameters. The traditional way to operate is to follow the previous experience, starting from the existing design solutions. A more advanced strategy consists in the preliminary identification of a superstructure that should include all the possible solutions to the synthesis/design optimization problem and in the selection of the system configuration starting from this superstructure through a design parameter optimization. This top–down approach cannot guarantee that all possible configurations could be predicted in advance and that all the configurations derived from the superstructure are feasible. To solve the general problem of the synthesis/design of complex energy systems, a new bottom–up methodology has been recently proposed by the authors, based on the original idea that the fundamental nucleus in the construction of any energy system configuration is the elementary thermodynamic cycle, composed only by the compression, heat transfer with hot and cold sources and expansion processes. So, any configuration can be built by generating, according to a rigorous set of rules, all the combinations of the elementary thermodynamic cycles operated by different working fluids that can be identified within the system, and selecting the best resulting configuration through an optimization procedure. In this paper, the main concepts and features of the methodology are deeply investigated to show, through different applications, how an artificial intelligence can generate system configurations of various complexity using preset logical rules without any “ad hoc” expertise.

  • 14.
    Ytrehus, Jan David
    et al.
    Sintef Petroleum AS.
    Taghipour, Ali
    Sintef Petroleum AS.
    Golchin, Arash
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Saasen, Arild
    Department of Petroleum Engineering, University of Stavanger.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    The Effect of Different Drilling Fluids on Mechanical Friction2017In: Journal of energy resources technology, ISSN 0195-0738, E-ISSN 1528-8994, Vol. 139, no 3, article id 034502Article in journal (Refereed)
    Abstract [en]

    A very important aspect in highly inclined wellbores is the mechanical friction. For extended reach drilling (ERD) and through tubing extended reach drilling (TTERD) this can be a limiting factor. Friction caused by the contact between the drill string and the well casing or borehole is dependent on the drilling weight and fluid properties. Drilling fluids play an important role in determining mechanical friction. The use of oil-based drilling fluids with higher lubricity can reduce torque and drag behavior and minimize stick and slip. Reducing mechanical friction will improve drilling efficiency in general, and will in particular enable longer reach for ERD wells. This paper presents results from experimental laboratory tests where the mechanical friction has been investigated. Friction behavior was investigated for different drilling fluids; water-based and oil-based drilling fluids both with and without solid particles. A pin on disk setup was used for these experiments where a spherical ended steel pin was slid against a rotating disk made of granite. The test results show that the mechanical friction in general is smaller with oil-based than water-based drilling fluids in the presence of solid particles.

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