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Artykuły w czasopismach na temat "Transport/Diffusion Equivalence"
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Liu, Zhi Bin, Jin Ma, Bing Shu Wang i Xin Hui Duan. "The Study on Homogeneous Parameters of Light Water Reactor by the Nodal Diffusion Method". Applied Mechanics and Materials 666 (październik 2014): 144–48. http://dx.doi.org/10.4028/www.scientific.net/amm.666.144.
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Recent progress in the development of coarse-mesh nodal methods for the numerical solution of the neutron diffusion and transport equations is reviewed. Compared with earlier nodal simulators, more recent nodal diffusion methods are characterized by the systematic derivation of spatial coupling relationships which are entirely consistent with the multi-group diffusion equation. These relationships most often are derived by developing approximations to the one-dimensional equations obtained by integrating the multidimensional diffusion equation over directions transverse to each coordinate axis. The objective of this research is to develop accurate and efficient spatial homogenization method for coarse mesh analysis of light water reactors. More sophisticated methods for computing approximate equivalence parameters are also introduced and make use of nonlinear iterations between homogenized reactor calculations and local fixed-source calculations to compute equivalence parameters. This special feature induces the need for the study on homogeneous parameters of light water reactor which takes phenomena of different scale and their interaction into account by means of the nodal diffusion method.
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Rapaport, Alain, Alejandro Rojas-Palma, Jean-Raynald de Dreuzy i Hector C. Ramirez. "Equivalence of Finite Dimensional Input–Output Models of Solute Transport and Diffusion in Geosciences". IEEE Transactions on Automatic Control 62, nr 10 (październik 2017): 5470–77. http://dx.doi.org/10.1109/tac.2017.2701150.
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Buonocore, Salvatore, Mihir Sen i Fabio Semperlotti. "Stochastic scattering model of anomalous diffusion in arrays of steady vortices". Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 476, nr 2238 (czerwiec 2020): 20200183. http://dx.doi.org/10.1098/rspa.2020.0183.
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We investigate the occurrence of anomalous transport phenomena associated with tracer particles propagating through arrays of steady vortices. The mechanism responsible for the occurrence of anomalous transport is identified in the particle dynamic, which is characterized by long collision-less trajectories (Lévy flights) interrupted by chaotic interactions with vortices. The process is studied via stochastic molecular models that are able to capture the underlying non-local nature of the transport mechanism. These models, however, are not well suited for problems where computational efficiency is an enabling factor. We show that fractional-order continuum models provide an excellent alternative that is able to capture the non-local nature of anomalous transport processes in turbulent environments. The equivalence between stochastic molecular and fractional continuum models is demonstrated both theoretically and numerically. In particular, the onset and the temporal evolution of heavy-tailed diffused fields are shown to be accurately captured, from a macroscopic perspective, by a fractional diffusion equation. The resulting anomalous transport mechanism, for the selected ranges of density of the vortices, shows a superdiffusive nature.
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Wassmer, Dominik, Bruno Schuermans, Christian Oliver Paschereit i Jonas P. Moeck. "Measurement and modeling of the generation and the transport of entropy waves in a model gas turbine combustor". International Journal of Spray and Combustion Dynamics 9, nr 4 (24.04.2017): 299–309. http://dx.doi.org/10.1177/1756827717696326.
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Indirect combustion noise is caused by entropy spots that are accelerated at the first turbine stage. These so-called entropy waves originate from the equivalence ratio fluctuations in the air–fuel mixture upstream of the flame. As entropy waves propagate convectively through the combustion chamber, they are subject to diffusion and dispersion. Because of the inherent difficulty of accurately measuring the burned gas temperature with sufficient temporal resolution, experimental data of entropy waves are scarce. In this work, the transfer function between equivalence ratio fluctuations and entropy fluctuations is modeled by a linearized reactor model, and the transport of entropy waves is investigated based on a convection-diffusion model. Temperature fluctuations are measured by means of a novel measurement technique at different axial positions downstream of the premixed flame, which is forced by periodic fuel injection. Experiments with various flow velocities and excitation frequencies enable model validation over a wide range of parameters.
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Kather, Vincent, Finn Lückoff, Christian O. Paschereit i Kilian Oberleithner. "Interaction of equivalence ratio fluctuations and flow fluctuations in acoustically forced swirl flames". International Journal of Spray and Combustion Dynamics 13, nr 1-2 (czerwiec 2021): 72–95. http://dx.doi.org/10.1177/17568277211015544.
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The generation and turbulent transport of temporal equivalence ratio fluctuations in a swirl combustor are experimentally investigated and compared to a one-dimensional transport model. These fluctuations are generated by acoustic perturbations at the fuel injector and play a crucial role in the feedback loop leading to thermoacoustic instabilities. The focus of this investigation lies on the interplay between fuel fluctuations and coherent vortical structures that are both affected by the acoustic forcing. To this end, optical diagnostics are applied inside the mixing duct and in the combustion chamber, housing a turbulent swirl flame. The flame was acoustically perturbed to obtain phase-averaged spatially resolved flow and equivalence ratio fluctuations, which allow the determination of flux-based local and global mixing transfer functions. Measurements show that the mode-conversion model that predicts the generation of equivalence ratio fluctuations at the injector holds for linear acoustic forcing amplitudes, but it fails for non-linear amplitudes. The global (radially integrated) transport of fuel fluctuations from the injector to the flame is reasonably well approximated by a one-dimensional transport model with an effective diffusivity that accounts for turbulent diffusion and dispersion. This approach however, fails to recover critical details of the mixing transfer function, which is caused by non-local interaction of flow and fuel fluctuations. This effect becomes even more pronounced for non-linear forcing amplitudes where strong coherent fluctuations induce a non-trivial frequency dependence of the mixing process. The mechanisms resolved in this study suggest that non-local interference of fuel fluctuations and coherent flow fluctuations is significant for the transport of global equivalence ratio fluctuations at linear acoustic amplitudes and crucial for non-linear amplitudes. To improve future predictions and facilitate a satisfactory modelling, a non-local, two-dimensional approach is necessary.
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Takasugi, Cole, Nicolas Martin, Vincent Labouré, Javier Ortensi, Kostadin Ivanov i Maria Avramova. "Preservation of kinetics parameters generated by Monte Carlo calculations in two-step deterministic calculations". EPJ Nuclear Sciences & Technologies 9 (2023): 15. http://dx.doi.org/10.1051/epjn/2022056.
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The generation of accurate kinetic parameters such as mean generation time Λ and effective delayed neutron fraction βeff via Monte Carlo codes is established. Employing these in downstream deterministic codes warrants another step to ensure no additional error is introduced by the low-order transport operator when computing forward and adjoint fluxes for bilinear weighting of these parameters. Another complexity stems from applying superhomogenization (SPH) equivalence in non-fundamental mode approximations, where reference and low-order calculations rely on a 3D full core model. In these cases, SPH factors can optionally be computed for only part of the geometry while preserving reaction rates and K-effective, but the impact of such approximations on kinetics parameters has not been thoroughly studied. This paper aims at studying the preservation of bilinearly-weighted quantities in the Serpent–Griffin calculation procedure. Diffusion and transport evaluations of IPEN/MB-01, Godiva, and Flattop were carried out with the Griffin reactor physics code, testing available modeling options using Serpent-generated multigroup cross sections and equivalence data. Verifying Griffin against Serpent indicates sensitivities to multigroup energy grid selection and regional application of SPH equivalence, introducing significant errors; these were demonstrated to be reduced through the use of a transport method together with a finer energy grid.
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Heinz, Stefan, Jakob Heinz i Jonathan A. Brant. "Mass Transport in Membrane Systems: Flow Regime Identification by Fourier Analysis". Fluids 7, nr 12 (30.11.2022): 369. http://dx.doi.org/10.3390/fluids7120369.
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The numerical calculation of local mass distributions in membrane systems by computational fluid dynamics (CFD) offers indispensable benefits. However, the concept to calculate such distributions in response to separate variations of operation conditions (OCs) makes it difficult to address overall, flow-physics-related questions, which require the consideration of the collective interaction of OCs. It is shown that such understanding-related relationships can be obtained by the analytical solution of the advection–diffusion equation considered. A Fourier series model (FSM) is presented, which provides exact solutions of an advection–diffusion equation for a wide range of OCs. On this basis, a new zeroth-order model is developed, which is very simple and as accurate as the complete FSM for all conditions of practical relevance. Advection-dominated blocked and diffusion-dominated unblocked flow regimes are identified (depending on a Péclet number which compares the flow geometry with a length scale imposed by the flow), which implies relevant requirements for the use of lab results for pilot- and full-scale applications. Analyses reveal the equivalence of variations of OCs, which offers a variety of options to accomplish desired flow regime changes.
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Rahnema, Farzad, i Piero Ravetto. "On the Equivalence of Boundary and Boundary Condition Perturbations in Transport Theory and Its Diffusion Approximation". Nuclear Science and Engineering 128, nr 2 (luty 1998): 209–23. http://dx.doi.org/10.13182/nse98-a1952.
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Gazizov, R. K., A. A. Kasatkin i S. Yu Lukashchuk. "Symmetry properties of fractional order transport equations". Proceedings of the Mavlyutov Institute of Mechanics 9, nr 1 (2012): 59–64. http://dx.doi.org/10.21662/uim2012.1.010.
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In the paper some features of applying Lie group analysis methods to fractional differential equations are considered. The problem related to point change of variables in the fractional differentiation operator is discussed and some general form of transformation that conserves the form of Riemann-Liouville fractional operator is obtained. The prolongation formula for extending an infinitesimal operator of a group to fractional derivative with respect to arbitrary function is presented. Provided simple example illustrates the necessity of considering both local and non-local symmetries for fractional differential equations in particular cases including the initial conditions. The equivalence transformation forms for some fractional differential equations are discussed and results of group classification of the wave-diffusion equation are presented. Some examples of constructing particular exact solutions of fractional transport equation are given, based on the Lie group methods and the method of invariant subspaces.
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Yan, Sheng, Zhili Zou i Zaijin You. "Eulerian Description of Wave-Induced Stokes Drift Effect on Tracer Transport". Journal of Marine Science and Engineering 10, nr 2 (12.02.2022): 253. http://dx.doi.org/10.3390/jmse10020253.
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The wave-induced Stokes drift plays a significant role on mass/tracer transport in the ocean and the evolution of coastal morphology. The tracer advection diffusion equation needs to be modified for Eulerian ocean models to properly account for the surface wave effects. The Eulerian description of Stokes drift effect on the tracer transport is derived in this study to show that this effect can be accounted for automatically in the wave-averaged advection-diffusion equation. The advection term in this equation is the wave-averaged concentration flux produced by the interaction between fluctuations of linear wave orbital velocity and tracer concentration, and the advection velocity is the same as the Stokes drift velocity. Thus, the effective dispersion of tracers by surface gravity waves is calculated due to the Stokes drift effect and the corresponding dispersion coefficient in the depth-integrated equation is then derived. The Eulerian description of Stokes drift effect of tracer concentration is illustrated by the direct numerical simulation of the advection–diffusion equation under simple linear waves. The equivalence between both the Eulerian and Lagrangian descriptions is also verified by particle tracking method. The theoretical analysis is found to agree well with the wave-induced dye drift velocity observed outside the surf zone in a longshore current experiment.
Rozprawy doktorskie na temat "Transport/Diffusion Equivalence"
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Rispo, Adrien. "Modélisation neutronique avancée des interfaces avec la chaîne SCIENCE V2 pour la remontée axiale du flux dans les réacteurs REP". Electronic Thesis or Diss., université Paris-Saclay, 2024. http://www.theses.fr/2024UPASP034.
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L'objectif de la thèse est de comprendre puis réduire les biais sur la distribution axiale de puissance de la chaîne de calcul SCIENCE de FRAMATOME en comparaison aux simulations Monte-Carlo. L'enjeu industriel est d'estimer précisément la remontée de puissance en extrémité de colonne fissile, à l'interface avec le réflecteur. Cette surpuissance locale est nommée PIC1, pour pic de puissance dans le premier centimètre de combustible. Ce phénomène local étant très dépendant du type d'assemblage, les comparaisons sont réalisées pour six types d'assemblages différents représentatifs du parc français, afin de quantifier les écarts sur le PIC1. Une analyse des différentes sources de biais permet de déterminer une nouvelle structure du schéma actuel de SCIENCE, notamment sur l'équivalence réalisée dans le calcul des données du réflecteur axial, afin de réduire ces biais.Un nouveau schéma d'équivalence réflecteur est développé durant la thèse, permettant un gain en précision sur le PIC1 sur les configurations utilisées précédemment, ainsi que sur des configurations perturbées et avec un environnement. Il permet également un couplage avec le Monte-Carlo, en utilisant les sections efficaces homogénéisées comme données d'entrée pour l'échelle cœur. L'utilisation de plusieurs codes de transport déterministes montre l'importance du traitement 3D dans la résolution de l'équation de Boltzmann.Le nouveau schéma apporte également un gain de précision en évolution en comparant l'apport du nouveau schéma par rapport aux calculs équivalents avec SCIENCE de production et le Monte-Carlo. Une discussion sur la méthode d'implémentation de ce nouveau schéma dans SCIENCE de production est discutée, notamment sur la méthode de génération des sections efficaces à l'échelle réseauThe aim of this thesis is to understand and then reduce the biases in the axial power distribution between FRAMATOME's SCIENCE calculation chain and Monte-Carlo simulations. The industrial challenge is to accurately estimate the power increase at the bottom of the fissile column, at the interface with the reflector. This local overpower is called PIC1, for peak power in the first centimeter of fuel. As this local phenomenon highly depends on the assembly type, PIC1 comparisons are made for six different assembly types, all representative of the French fleet. An analysis of the various sources of bias is performed to determine a new calculation scheme for SCIENCE.A new reflector scheme based on an improved equivalence was developed during the thesis, enabling a gain in PIC1 accuracy for all configurations, but also on perturbed and with an environment configurations. It also enables a coupling with Monte Carlo, using homogenized cross sections as input data for the core level. The use of several deterministic transport codes demonstrates the importance of the 3D contribution to solve the Boltzmann equation.The new scheme also increases accuracy for depletion calculation. The way to implement the new scheme in SCIENCE for industrial studies is discussed, with an emphasis on the method to generate macroscopic cross-sections at the lattice level
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Penny, Melissa. "Mathematical modelling of dye-sensitised solar cells". Queensland University of Technology, 2006. http://eprints.qut.edu.au/16270/.
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This thesis presents a mathematical model of the nanoporous anode within a dyesensitised solar cell (DSC). The main purpose of this work is to investigate interfacial charge transfer and charge transport within the porous anode of the DSC under both illuminated and non-illuminated conditions. Within the porous anode we consider many of the charge transfer reactions associated with the electrolyte species, adsorbed dye molecules and semiconductor electrons at the semiconductor-dye- electrolyte interface. Each reaction at this interface is modelled explicitly via an electrochemical equation, resulting in an interfacial model that consists of a coupled system of non-linear algebraic equations. We develop a general model framework for charge transfer at the semiconductor-dye-electrolyte interface and simplify this framework to produce a model based on the available interfacial kinetic data. We account for the charge transport mechanisms within the porous semiconductor and the electrolyte filled pores that constitute the anode of the DSC, through a one- dimensional model developed under steady-state conditions. The governing transport equations account for the diffusion and migration of charge species within the porous anode. The transport model consists of a coupled system of non-linear differential equations, and is coupled to the interfacial model via reaction terms within the mass-flux balance equations. An equivalent circuit model is developed to account for those components of the DSC not explicitly included in the mathematical model of the anode. To obtain solutions for our DSC mathematical model we develop code in FORTRAN for the numerical simulation of the governing equations. We additionally employ regular perturbation analysis to obtain analytic approximations to the solutions of the interfacial charge transfer model. These approximations facilitate a reduction in computation time for the coupled mathematical model with no significant loss of accuracy. To obtain predictions of the current generated by the cell we source kinetic and transport parameter values from the literature and from experimental measurements associated with the DSC commissioned for this study. The model solutions we obtain with these values correspond very favourably with experimental data measured from standard DSC configurations consisting of titanium dioxide porous films with iodide/triiodide redox couples within the electrolyte. The mathematical model within this thesis enables thorough investigation of the interfacial reactions and charge transport within the DSC.We investigate the effects of modified cell configurations on the efficiency of the cell by varying associated parameter values in our model. We find, given our model and the DSC configuration investigated, that the efficiency of the DSC is improved with increasing electron diffusion, decreasing internal resistances and with decreasing dark current. We conclude that transport within the electrolyte, as described by the model, appears to have no limiting effect on the current predicted by the model until large positive voltages. Additionally, we observe that the ultrafast injection from the excited dye molecules limits the interfacial reactions that affect the DSC current.
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Penny, Melissa. "Mathematical modelling of dye-sensitised solar cells". Thesis, Queensland University of Technology, 2006. https://eprints.qut.edu.au/16270/1/Melissa_Penny_Thesis.pdf.
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This thesis presents a mathematical model of the nanoporous anode within a dyesensitised solar cell (DSC). The main purpose of this work is to investigate interfacial charge transfer and charge transport within the porous anode of the DSC under both illuminated and non-illuminated conditions. Within the porous anode we consider many of the charge transfer reactions associated with the electrolyte species, adsorbed dye molecules and semiconductor electrons at the semiconductor-dye- electrolyte interface. Each reaction at this interface is modelled explicitly via an electrochemical equation, resulting in an interfacial model that consists of a coupled system of non-linear algebraic equations. We develop a general model framework for charge transfer at the semiconductor-dye-electrolyte interface and simplify this framework to produce a model based on the available interfacial kinetic data. We account for the charge transport mechanisms within the porous semiconductor and the electrolyte filled pores that constitute the anode of the DSC, through a one- dimensional model developed under steady-state conditions. The governing transport equations account for the diffusion and migration of charge species within the porous anode. The transport model consists of a coupled system of non-linear differential equations, and is coupled to the interfacial model via reaction terms within the mass-flux balance equations. An equivalent circuit model is developed to account for those components of the DSC not explicitly included in the mathematical
model of the anode. To obtain solutions for our DSC mathematical model we develop code in FORTRAN for the numerical simulation of the governing equations. We additionally employ regular perturbation analysis to obtain analytic approximations to the solutions of the interfacial charge transfer model. These approximations facilitate a reduction in computation time for the coupled mathematical model with no significant loss of accuracy. To obtain predictions of the current generated by the cell we source kinetic and transport parameter values from the literature and from experimental measurements associated with the DSC commissioned for this study. The model solutions we obtain with these values correspond very favourably with experimental data measured from standard DSC configurations consisting of titanium dioxide porous films with iodide/triiodide redox couples within the electrolyte. The mathematical model within this thesis enables thorough investigation of the interfacial reactions and charge transport within the DSC.We investigate the effects of modified cell configurations on the efficiency of the cell by varying associated parameter values in our model. We find, given our model and the DSC configuration investigated, that the efficiency of the DSC is improved with increasing electron diffusion, decreasing internal resistances and with decreasing dark current. We conclude that transport within the electrolyte, as described by the model, appears to have no limiting effect on the current predicted by the model until large positive voltages. Additionally, we observe that the ultrafast injection from the excited dye molecules limits the interfacial reactions that affect the DSC current.
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Patel, Amin. "Transport-theory-equivalent diffusion coefficients for node-homogenized neutron diffusion problems in CANDU lattices". Thesis, 2010. http://hdl.handle.net/10155/87.
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Calculation of the neutron flux in a nuclear reactor core is ideally performed by solving the neutron transport equation for a detailed-geometry model using several tens of energy groups. However, performing such detailed calculations for an entire core is prohibitively expensive from a computational perspective. Full-core neutronic calculations for CANDU reactors are therefore performed customarily using two-energy-group diffusion theory (no angular dependence) for a node-homogenized reactor model. The work presented here is concerned with reducing the loss in accuracy entailed when going from Transport to Diffusion. To this end a new method of calculating the diffusion coefficient was developed, based on equating the neutron balance equation expressed by the transport equation with the neutron balance equation expressed by the diffusion equation. The technique is tested on a simple twelve-node model and is shown to produce transport-like accuracy without the associated computational effort.UOIT
Streszczenia konferencji na temat "Transport/Diffusion Equivalence"
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Kumar, Ankan, i Sandip Mazumder. "Investigation of Approximate Diffusion Models for the Prediction of Heterogeneous Combustion in Monolith Tubes". W ASME/JSME 2007 Thermal Engineering Heat Transfer Summer Conference collocated with the ASME 2007 InterPACK Conference. ASMEDC, 2007. http://dx.doi.org/10.1115/ht2007-32073.
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In the case of heterogeneous reactions, diffusion is the only mechanism, locally, of transport of species to and from a surface. Thus, accurate prediction of diffusive transport is a prerequisite for accurate prediction of the operation of devices in which heterogeneous reactions occur. Three different diffusion models are examined from the standpoint of both accuracy and efficiency. Two of these models, namely the Dilute Approximation (DA) model and the Schmidt Number (SN) model, are approximate models, and are compared against a rigorous Multi-Component Diffusion (MCD) model derived from the Stefan-Maxwell equation. Both hydrogen-air and methane-air combustion in a monolith channel are studied. Inlet equivalence ratio, Reynolds number (flow rate), and wall temperature are considered as parameters. The results show that both the DA model and the SN model are accurate within 2% irrespective of the equivalence ratio or fuel—the worst accuracy being for hydrogen combustion. The DA model and the SN model produce almost identical results. In comparison to the MCD model, the DA model is approximately twice as computationally efficient, while the SN model is 3–4 times more efficient. The accuracy and efficiency of the SN model, in conjunction with its simplicity, makes it an attractive choice for the treatment of diffusion in catalytic combustion calculations.
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Zhao, Jing, Zhihong Liu, Chunlin Wei i Yongming Hu. "Analysis of Heterogeneous Cores Using SN Code With Discontinuity Factor". W 2014 22nd International Conference on Nuclear Engineering. American Society of Mechanical Engineers, 2014. http://dx.doi.org/10.1115/icone22-30165.
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Solving diffusion equations with the equivalence homogenization theory is the common method in reactor neutronics. But for some case, as for stronger absorbers, the diffusion equations will bring great errors and the transport method will be more suitable. The discontinuity factor theory has been successfully used in core diffusion computation programs and effectively reduced the homogenization error. The method of using the discontinuity factor in the transport method were studied. The result shows that higher accuracy was obtained from the discrete ordinates core transport computation program with discontinued factor.
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Kaiser, Thomas Ludwig, i Kilian Oberleithner. "Modeling the Transport of Fuel Mixture Perturbations and Entropy Waves in the Linearized Framework". W ASME Turbo Expo 2020: Turbomachinery Technical Conference and Exposition. American Society of Mechanical Engineers, 2020. http://dx.doi.org/10.1115/gt2020-14715.
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Abstract In this paper a new method is introduced to model the transport of entropy waves and equivalence ratio fluctuations in turbulent flows. The model is based on the Navier-Stokes equations and includes a transport equation for a passive scalar, which may stand for entropy or equivalence ratio fluctuations. The equations are linearized around the mean turbulent fields, which serve as the input to the model in addition to a turbulent eddy viscosity, which accounts for turbulent diffusion of the perturbations. Based on these inputs, the framework is able to predict the linear response of the flow velocity and passive scalar to harmonic perturbations that are imposed at the boundaries of the computational domain. These in this study are fluctuations in the passive scalar and/or velocities at the inlet of a channel flow. The code is first validated against analytic results, showing very good agreement. Then the method is applied to predict the convection, mean flow dispersion and turbulent mixing of passive scalar fluctuations in a turbulent channel flow, which has been studied in previous work with Direct Numerical Simulations (DNS). Results show that our code reproduces the dynamics of coherent passive scalar transport in the DNS with very high accuracy and low numerical costs, when the DNS mean flow and Reynolds stresses are provided. Furthermore, we demonstrate that turbulent mixing has a significant effect on the transport of the passive scalar fluctuations. Finally, we apply the method to explain experimental observations of transport of equivalence ratio fluctuations in the mixing duct of a model burner.
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Saxena, Priyank, i Forman A. Williams. "Experimental and Numerical Studies of Ethanol Flames". W ASME Turbo Expo 2006: Power for Land, Sea, and Air. ASMEDC, 2006. http://dx.doi.org/10.1115/gt2006-90326.
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This paper reports results of experimental and numerical investigations of ethanol-air diffusion flames and partially premixed flames at an air-side strain rate of 100 s−1, in a counterflow geometry. The diffusion flame consists of prevaporized fuel, with mole fraction of 0.3, diluted with nitrogen in the fuel stream, and plant air as the oxidizer stream. The partially premixed flame includes prevaporized fuel in air partially premixed to an equivalence ratio of 2.3 in the fuel stream, and plant air as the oxidizer stream. Temperature profiles were measured by thermocouple, and concentration profiles of the stable species C2H5OH, CO, CO2, H2, H2O, O2, N2, CH4, C2H6, and C2H2+C2H4 were measured by gas chromatography of samples withdrawn by a fine probe. Computational studies involved numerical integration of the conservation equations, with detailed chemistry, transport and radiation effects included, to calculate the structures of the counterflow flames. A chemical-kinetic mechanism consisting of 235 elementary steps and 46 species with recently published reaction-rate parameters was developed and tested for these flames. The proposed mechanism, which produces reasonable agreement with previous measurements of ignition, freely propagating premixed flames and diffusion-flame extinction, also yields good agreement with much of the present data, although there are quite noticeable differences between predicted and measured peak C2H6 concentrations. These differences and the desirability of additional tests of other predictions and of tests under other conditions motivate further research.
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Rohde, U., S. Mittag, U. Grundmann, P. Petkov i J. Ha´dek. "Application of a Step-Wise Verification and Validation Procedure to the 3D Neutron Kinetics Code DYN3D Within the European NURESIM Project". W 17th International Conference on Nuclear Engineering. ASMEDC, 2009. http://dx.doi.org/10.1115/icone17-75446.
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A generic strategy of core physics codes benchmarking was elaborated within the European NURESIM code platform development. In this paper, the application of this step-wise procedure to benchmarking the 3D neutron kinetics code DYN3D for applications to VVER-type reactors is described. Numerical and experimental benchmark problems were considered for code verification and validation. Examples of these benchmarks including benchmark set-up and results obtained by use of DYN3D in comparison with other codes are given. First, mathematical problems with given cross sections are used for the verification of the mathematical methods applied e.g. in nodal codes against finite difference solutions. Discretization errors were quantified. After minimisation of numerical errors, modelling errors have to be considered. Diffusion approximation and homogenisation errors are due to simplified physical approaches and can be estimated by comparing diffusion solutions with more accurate Monte Carlo or deterministic transport solutions. Methods to reduce these errors are outlined. A series of 2D whole core benchmarks for different core loadings and operational conditions for VVER-1000 reactors was defined for this purpose. Reference transport solutions were calculated by the MARIKO and APOLLO codes based on Method of Characteristics. Homogenised two-group and few-group diffusion parameters were derived from the reference solutions and used as cross section data for the nodal diffusion code DYN3D. The DYN3D solutions were compared to the reference solution. It was shown that the homogenisation error can be significantly reduced by using Assembly Discontinuity Factors (ADF) and Reflector Discontinuity Factors (RDF) which are obtained from the transport solution by applying equivalence theory. A study using the multi-group version of DYN3D has shown that increasing the number of groups in the considered cases has only a small effect in comparison with homogenisation error. After reducing modelling errors by choosing appropriate physical approximations, the code have to be validated against reality. Experimental problems are used for code validation. Experimental data for VVER reactors, which were used for the benchmarking of the DYN3D code within NURESIM, are power distribution measurements at the full-size (VVER-1000) experimental facility V-1000, which have been well documented within the EC project VALCO, and kinetic experiments at the LR-0 zero power reactor in NRI Rˇezˇ. The code DYN3D, being one of the NURESIM platform codes, has proved to be an effective tool for steady-state and kinetics core calculations. The high accuracy of the code is based on the advanced nodal method “HEXNEM2”, multi-group approach, applying discontinuity factors, and intra-nodal flux reconstruction.
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Yücel, Fatma Cansu, Fabian Habicht, Alexander Jaeschke, Finn Lückoff, Kilian Oberleithner i Christian Oliver Paschereit. "Investigation of the Fuel Distribution in a Shockless Explosion Combustor". W ASME Turbo Expo 2020: Turbomachinery Technical Conference and Exposition. American Society of Mechanical Engineers, 2020. http://dx.doi.org/10.1115/gt2020-16210.
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Abstract Shockless explosion combustion is a promising concept for implementing pressure gain combustion into a conventional gas turbine cycle. This concept aims for a quasi-homogeneous autoignition that induces a moderate rise in pressure. By this, considerable losses due to entropy generation by inherent shock waves of detonation-based concepts can be avoided. Since the ignition is not triggered by an external source but driven by chemical kinetics only, the homogeneity of the autoignition is very sensitive to local perturbations in equivalence ratio, temperature, and pressure that produce undesired local premature ignition. Therefore, the precise injection of a well-defined fuel profile into an convecting air flow is crucial to ensure a quasi-homogeneous ignition of the entire flammable mixture. The objective of this work is to demonstrate that the injected fuel profile is preserved throughout the entire measurement section. For this, two different control trajectories are investigated. Optical measurement techniques are used to illustrate the effect of turbulent transport and dispersion caused by boundary layer effects on the fuel concentration profile inside the combustor. Results from line-of-sight measurements by tunable diode laser absorption spectroscopy indicate that the transport of the fuel-air mixture is dominated by turbulent diffusion. However, comparisons to numerical calculations reveal the effect of dispersion towards the bounds of the fuel concentration profile. The spatially resolved distributions of the fuel concentration inside the combustor gained from acetone planar laser induced fluorescence replicates a typical velocity distribution of turbulent pipe flow in radial direction visualizing boundary layer effects. Comparing both methods provide deep insights into the transport processes that have an impact on the operation of the shockless explosion combustor.
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Amin, E. M., G. E. Andrews, M. Pourkashnian, A. Williams i R. A. Yetter. "A Computational Study of Pressure Effects on Pollutants Generation in Gas Turbine Combustors". W ASME 1995 International Gas Turbine and Aeroengine Congress and Exposition. American Society of Mechanical Engineers, 1995. http://dx.doi.org/10.1115/95-gt-304.
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A numerical study of the effect of pressure on the formation of NOx and soot in an axisymmetric 30° counter rotating axial swirler lean low NOx gas turbine combustor has been conducted. This has previously been studied experimentally and this CFD investigation was undertaken to explain the higher than expected NOx emissions. The combustion conditions selected for the present study were 300 deg K inlet air, 0.4 overall equivalence ratio, and pressures of 1 and 10 bar. The numerical model used here involved the solution of time-averaged governing equations using an elliptic flow-field solver. The turbulence was modelled using algebraic stress modelling (ASM), The Thermo-chemical model was based on the laminar flamelet formulation. The conserved scalar/assumed pdf approach was used to model the turbulence chemistry interaction. The study was for two pressure cases at 1 and 10 bar. The turbulence-chemistry interaction is closed by assumption of a Clipped Gaussian function form for the fluctuations in the mixture fraction. The kinetic calculations were done separately from the flowfield solver using an opposed laminar diffusion flame code of SANDIA. The temperature and species profiles were made available to the computations through look-up tables. The pollutants studied in this work were soot and NO for which three more additional transport equations are required namely; averaged soot mass fraction, averaged soot particle number density, and finally averaged NO mass fraction. Soot oxidation was modelled using molecular oxygen only and a strong influence of pressure was predicted. Pressure was shown to have a major effect on soot formation.
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Go¨ll, Stephan, i Manfred Piesche. "Characterization of Micro-Fluidic Exhaust Sensors Through Fully Coupled Modeling of Multi-Component Gas Transport and Reaction". W ASME 2008 6th International Conference on Nanochannels, Microchannels, and Minichannels. ASMEDC, 2008. http://dx.doi.org/10.1115/icnmm2008-62141.
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Transport and reaction of gas mixtures in porous media are common phenomena in many chemical engineering applications. One favored method of modeling the transport processes is to notionally substitute a uniform bundle of tortuous channels for the irregular porous structure. Then, accurate equations of motion for the gas flow and diffusion inside these small-sized channels can be used. This advantage comes at the cost of two additional parameters that enter into the equations, the tortuosity factor and the equivalent capillary diameter. In this work, an existing model for transient transport of multi-component gas mixtures has been expanded to comprise heterogeneous fluid domains and chemical reaction. It can be applied to fluid domains that partially or completely enclose porous regions. The potential of the present model is demonstrated by simulating the electro-chemically induced and transport-limited signal formation inside an exhaust gas sensor.
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Vallet, Christophe, Didier Lasseux, Philippe Sainsot i Hassan Zahouani. "Numerical Determination of Sealing Performance of a Rough Contact: Real Versus Synthetic Fractal Surfaces". W ASME 2008 Pressure Vessels and Piping Conference. ASMEDC, 2008. http://dx.doi.org/10.1115/pvp2008-61213.
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In this work, we adress the issue of sealing performance of metal gaskets using a deterministic approach that allows the analysis of creeping viscous flow and diffusion through a tight contact between rough surfaces taking into account surface deformation. Our analysis is focused on rough surfaces exhibiting fractal properties, and our purpose is to study the validity of the use of synthetic fractal surfaces as a representation of real ones. Two kinds of real surfaces, obtained from two machining processes — lapping and sand-blasting — are considered. After checking the fractal nature of these surfaces, equivalent fractal ones are synthesized. Distributions of contact areas on the one hand, and transport properties K (for viscous flow) and D (for diffusion) on the other, obtained from real and synthetic surfaces are compared for a wide range of tightening. This comparison leads to the conclusion that the fractal representation is adequate to predict mechanical and transport properties of a contact between lapped or sand-blasted surfaces. Finally, using synthetic surfaces, it is shown that sealing performance of a rough contact decreases when the arithmetic roughness Ra and the fractal dimension Df increase.
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Roy, Arnab, Ugur Pasaogullari, Michael W. Renfro i Baki M. Cetegen. "Validation and Calibration of a Proton Exchange Membrane Fuel Cell Model Against Dynamic Partial Pressure Data". W ASME 2010 8th International Conference on Fuel Cell Science, Engineering and Technology. ASMEDC, 2010. http://dx.doi.org/10.1115/fuelcell2010-33336.
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Transient experimental validation and investigation of the effect of diffusivity of porous layers on the dynamic water vapor partial pressure profiles of a proton exchange membrane fuel cell (PEMFC) during load change is presented. A three dimensional, isothermal, transient, single-phase computational fluid dynamics based model is developed to validate with the water partial pressure profiles experimentally measured during start-up conditions earlier in a 50 cm2 PEMFC having a single serpentine flow path in counter-flow configuration. The fluid flow within the serpentine channel geometry is simulated using a straight channel fuel cell model with total channel length equivalent to the stretched length of the entire serpentine path incorporating the same amount of pressure drop from inlet to outlet. The model equations are solved using a multi-domain approach incorporating water transport through membrane and multi-component species transport through porous diffusion layer. The transient model predictions of water partial pressure profiles of anode and cathode channels are found to be in good agreement within the error bounds of the experimental results. This validation is also indicative of the two different time scales i.e. initial anode dip due to electro-osmotic drag and recovery due to back diffusion from cathode to anode. Steady state model predictions are compared to check for accuracy simultaneously. The model also delineates the significance of effective diffusivity of porous Gas Diffusion Layers (GDL) and Catalyst Layers (CL) on transient characteristics. In order to come up with best parameters to validate with experimental data, a sensitivity analysis with parametric variations of effective porosity of GDL and CL is performed with a single experimental data set and then applied to the remaining sets. Results show that the CL diffusivity has a more pronounced effect on water accumulation as well as on temporal water transport than GDL diffusivity. The numerical simulation thus provides a validated set of quantitative model parameters along with an insight to the underlying physics of water transport phenomena in a PEMFC.
Raporty organizacyjne na temat "Transport/Diffusion Equivalence"
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Kirchhoff, Helmut, i Ziv Reich. Protection of the photosynthetic apparatus during desiccation in resurrection plants. United States Department of Agriculture, luty 2014. http://dx.doi.org/10.32747/2014.7699861.bard.
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In this project, we studied the photosynthetic apparatus during dehydration and rehydration of the homoiochlorophyllous resurrection plant Craterostigmapumilum (retains most of the photosynthetic components during desiccation). Resurrection plants have the remarkable capability to withstand desiccation, being able to revive after prolonged severe water deficit in a few days upon rehydration. Homoiochlorophyllous resurrection plants are very efficient in protecting the photosynthetic machinery against damage by reactive oxygen production under drought. The main purpose of this BARD project was to unravel these largely unknown protection strategies for C. pumilum. In detail, the specific objectives were: (1) To determine the distribution and local organization of photosynthetic protein complexes and formation of inverted hexagonal phases within the thylakoid membranes at different dehydration/rehydration states. (2) To determine the 3D structure and characterize the geometry, topology, and mechanics of the thylakoid network at the different states. (3) Generation of molecular models for thylakoids at the different states and study the implications for diffusion within the thylakoid lumen. (4) Characterization of inter-system electron transport, quantum efficiencies, photosystem antenna sizes and distribution, NPQ, and photoinhibition at different hydration states. (5) Measuring the partition of photosynthetic reducing equivalents between the Calvin cycle, photorespiration, and the water-water cycle. At the beginning of the project, we decided to use C. pumilum instead of C. wilmsii because the former species was available from our collaborator Dr. Farrant. In addition to the original two dehydration states (40 relative water content=RWC and 5% RWC), we characterized a third state (15-20%) because some interesting changes occurs at this RWC. Furthermore, it was not possible to detect D1 protein levels by Western blot analysis because antibodies against other higher plants failed to detect D1 in C. pumilum. We developed growth conditions that allow reproducible generation of different dehydration and rehydration states for C. pumilum. Furthermore, advanced spectroscopy and microscopy for C. pumilum were established to obtain a detailed picture of structural and functional changes of the photosynthetic apparatus in different hydrated states. Main findings of our study are: 1. Anthocyan accumulation during desiccation alleviates the light pressure within the leaves (Fig. 1). 2. During desiccation, stomatal closure leads to drastic reductions in CO2 fixation and photorespiration. We could not identify alternative electron sinks as a solution to reduce ROS production. 3. On the supramolecular level, semicrystalline protein arrays were identified in thylakoid membranes in the desiccated state (see Fig. 3). On the electron transport level, a specific series of shut downs occur (summarized in Fig. 2). The main events include: Early shutdown of the ATPase activity, cessation of electron transport between cyt. bf complex and PSI (can reduce ROS formation at PSI); at higher dehydration levels uncoupling of LHCII from PSII and cessation of electron flow from PSII accompanied by crystal formation. The later could severe as a swift PSII reservoir during rehydration. The specific order of events in the course of dehydration and rehydration discovered in this project is indicative for regulated structural transitions specifically realized in resurrection plants. This detailed knowledge can serve as an interesting starting point for rationale genetic engineering of drought-tolerant crops.