CTTC - Centre Tecnològic de la Transferència de Calor
http://hdl.handle.net/2117/3190
2019-10-18T22:10:54ZA dynamic load balancing method for the evaluation of chemical reaction rates in parallel combustion simulations
http://hdl.handle.net/2117/168954
A dynamic load balancing method for the evaluation of chemical reaction rates in parallel combustion simulations
Muela Castro, Jordi; Borrell Pol, Ricard; Ventosa Molina, Jordi; Jofre Roca, Lluís; Lehmkuhl Barba, Oriol; Pérez Segarra, Carlos David
The development and assessment of an efficient parallelization method for the evaluation of reaction rates in combustion simulations is presented. Combustion simulations where the finite-rate chemistry model is employed are computationally expensive. In such simulations, a transport equation for each species in the chemical reaction mechanism has to be solved, and the resulting system of equations is typically stiff. As a result, advanced implicit methods must be applied to obtain accurate solutions using reasonable time-steps at expenses of higher computational resources than explicit or classical implicit methods. In the present work, a new algorithm aimed to enhance the numerical performance of the time integration of stiffsystems of equations in parallel combustion simulations is presented. The algorithm is based on a runtime load balancing mechanism, increasing noteworthy the computational performance of the simulations, and consequently, reducing significantly the computer time required to perform the numerical combustion studies.
2019-09-30T17:37:54ZMuela Castro, JordiBorrell Pol, RicardVentosa Molina, JordiJofre Roca, LluísLehmkuhl Barba, OriolPérez Segarra, Carlos DavidThe development and assessment of an efficient parallelization method for the evaluation of reaction rates in combustion simulations is presented. Combustion simulations where the finite-rate chemistry model is employed are computationally expensive. In such simulations, a transport equation for each species in the chemical reaction mechanism has to be solved, and the resulting system of equations is typically stiff. As a result, advanced implicit methods must be applied to obtain accurate solutions using reasonable time-steps at expenses of higher computational resources than explicit or classical implicit methods. In the present work, a new algorithm aimed to enhance the numerical performance of the time integration of stiffsystems of equations in parallel combustion simulations is presented. The algorithm is based on a runtime load balancing mechanism, increasing noteworthy the computational performance of the simulations, and consequently, reducing significantly the computer time required to perform the numerical combustion studies.Thermal and fluid dynamic analysis of Direct Steam Generation Parabolic Trough Collectors
http://hdl.handle.net/2117/167910
Thermal and fluid dynamic analysis of Direct Steam Generation Parabolic Trough Collectors
Ferchichi, Souha; Kessentini, Hamdi; Morales Ruíz, Sergio; Rigola Serrano, Joaquim; Bouden, Chiheb; Oliva Llena, Asensio
Direct Steam Generation (DSG) has attracted considerable attention in comparison with conventional technologies based on Heat Transfer Fluids (HTF) for electricity generation due to its higher temperatures. In this context, a detailed numerical model has been developed, not only for comparison and assessment purposes but also for optimization analysis, in order to obtain an accurate comprehensive analysis of this type of systems. This paper explains in detail a numerical model developed to accurately predict the phase change and the heat transfer in the DSG Parabolic Trough Collector (PTC) system. The whole numerical model has been validated using experimental data of Direct Solar Steam (DISS) test facility from scientific literature. The model results have been compared also to the measurements of an experimental DSG PTC setup installed at Ecole Nationale d’Ingénieurs de Tunis (ENIT) running under different working and climate conditions. In all cases, both numerical results and experimental data present a reasonable agreement. The numerical code is used to perform a parametric study pointing out the influence of inlet conditions on the performance of DSG PTCs. Emphasis is placed on the determination of the optimal mass flow rate range leading to a maximum thermal efficiency of the solar collectors. Annual thermal performance of the solar collectors is featured including useful energy and thermal efficiency evaluated under Tunisian climatic conditions.
2019-09-04T09:26:07ZFerchichi, SouhaKessentini, HamdiMorales Ruíz, SergioRigola Serrano, JoaquimBouden, ChihebOliva Llena, AsensioDirect Steam Generation (DSG) has attracted considerable attention in comparison with conventional technologies based on Heat Transfer Fluids (HTF) for electricity generation due to its higher temperatures. In this context, a detailed numerical model has been developed, not only for comparison and assessment purposes but also for optimization analysis, in order to obtain an accurate comprehensive analysis of this type of systems. This paper explains in detail a numerical model developed to accurately predict the phase change and the heat transfer in the DSG Parabolic Trough Collector (PTC) system. The whole numerical model has been validated using experimental data of Direct Solar Steam (DISS) test facility from scientific literature. The model results have been compared also to the measurements of an experimental DSG PTC setup installed at Ecole Nationale d’Ingénieurs de Tunis (ENIT) running under different working and climate conditions. In all cases, both numerical results and experimental data present a reasonable agreement. The numerical code is used to perform a parametric study pointing out the influence of inlet conditions on the performance of DSG PTCs. Emphasis is placed on the determination of the optimal mass flow rate range leading to a maximum thermal efficiency of the solar collectors. Annual thermal performance of the solar collectors is featured including useful energy and thermal efficiency evaluated under Tunisian climatic conditions.A level-set model for mass transfer in bubbly flows
http://hdl.handle.net/2117/167909
A level-set model for mass transfer in bubbly flows
Balcázar Arciniega, Néstor; Antepara Zambrano, Óscar; Rigola Serrano, Joaquim; Oliva Llena, Asensio
A level-set model is presented for simulating mass transfer or heat transfer in two-phase flows. The Navier-Stokes equations and mass transfer (or heat transfer) equation are discretized using a finite volume method on a collocated unstructured mesh, whereas a multiple marker level-set approach is used for interface capturing in bubble swarms. This method avoids the numerical coalescence of the fluid particles, whereas the mass conservation issue inherent to standard level-set methods is circumvented. Furthermore, unstructured flux-limiter schemes are used to discretize the convective term of momentum transport equation, level-set equations, and chemical species concentration equation, to avoid numerical oscillations around discontinuities, and to minimize the numerical diffusion. A convection-diffusion-reaction equation is used as a mathematical model for the chemical species mass transfer at the continuous phase. Because the mathematical analogy between dilute mass transfer and heat transfer, the same numerical model is applicable to solve both phenomena. The capabilities of this model are proved for the diffusion of chemical species from a sphere, external mass transfer in the buoyancy-driven motion of single bubbles and bubble swarms. Results are extensively validated by comparison with analytical solutions and empirical correlations from the literature.
2019-09-04T07:55:31ZBalcázar Arciniega, NéstorAntepara Zambrano, ÓscarRigola Serrano, JoaquimOliva Llena, AsensioA level-set model is presented for simulating mass transfer or heat transfer in two-phase flows. The Navier-Stokes equations and mass transfer (or heat transfer) equation are discretized using a finite volume method on a collocated unstructured mesh, whereas a multiple marker level-set approach is used for interface capturing in bubble swarms. This method avoids the numerical coalescence of the fluid particles, whereas the mass conservation issue inherent to standard level-set methods is circumvented. Furthermore, unstructured flux-limiter schemes are used to discretize the convective term of momentum transport equation, level-set equations, and chemical species concentration equation, to avoid numerical oscillations around discontinuities, and to minimize the numerical diffusion. A convection-diffusion-reaction equation is used as a mathematical model for the chemical species mass transfer at the continuous phase. Because the mathematical analogy between dilute mass transfer and heat transfer, the same numerical model is applicable to solve both phenomena. The capabilities of this model are proved for the diffusion of chemical species from a sphere, external mass transfer in the buoyancy-driven motion of single bubbles and bubble swarms. Results are extensively validated by comparison with analytical solutions and empirical correlations from the literature.Simulation of Household Refrigerators with a Flexible Numerical Tool
http://hdl.handle.net/2117/166575
Simulation of Household Refrigerators with a Flexible Numerical Tool
Ablanque Mejía, Nicolás; Oliet Casasayas, Carles; Rigola Serrano, Joaquim; Lehmkuhl Barba, Oriol; Pérez Segarra, Carlos David
This work presents a flexible numerical platform to simulate a whole household refrigeration unit taking into account both the refrigeration cycle itself and the refrigerated compartments network. The methodology implemented to achieve the transient simulation of the whole system combines a transient-state approach for the refrigerated chambers network and a steady-state approach for the refrigerant loop. The latter includes the simulation of a capillary-tube/suction-line heat exchanger (to prevent liquid refrigerant from entering into the compressor), and the simulation of a receiver (to store excess refrigerant in its liquid state). In addition, the global system resolution includes two significant features, namely, a specific numerical method to predict the system dynamics when the compressor is switched off, and a control system to regulate the compartments inner temperatures by modifying a damper position (open/closed) and/or the compressor state (on/off).
In this work, the major numerical aspects of the platform are briefly described. Furthermore, an illustrative numerical simulation of a household refrigerator including most of the model features is shown in order to see the model potential.
2019-07-23T08:01:21ZAblanque Mejía, NicolásOliet Casasayas, CarlesRigola Serrano, JoaquimLehmkuhl Barba, OriolPérez Segarra, Carlos DavidThis work presents a flexible numerical platform to simulate a whole household refrigeration unit taking into account both the refrigeration cycle itself and the refrigerated compartments network. The methodology implemented to achieve the transient simulation of the whole system combines a transient-state approach for the refrigerated chambers network and a steady-state approach for the refrigerant loop. The latter includes the simulation of a capillary-tube/suction-line heat exchanger (to prevent liquid refrigerant from entering into the compressor), and the simulation of a receiver (to store excess refrigerant in its liquid state). In addition, the global system resolution includes two significant features, namely, a specific numerical method to predict the system dynamics when the compressor is switched off, and a control system to regulate the compartments inner temperatures by modifying a damper position (open/closed) and/or the compressor state (on/off).
In this work, the major numerical aspects of the platform are briefly described. Furthermore, an illustrative numerical simulation of a household refrigerator including most of the model features is shown in order to see the model potential.Numerical Model of Capillary Tubes: Enhanced Performance and Study of Non-Adiabatic Effects
http://hdl.handle.net/2117/166043
Numerical Model of Capillary Tubes: Enhanced Performance and Study of Non-Adiabatic Effects
Ablanque Mejía, Nicolás; Oliet Casasayas, Carles; Rigola Serrano, Joaquim; Pérez Segarra, Carlos David
In the present work a numerical model to simulate the thermal and fluid-dynamic phenomena inside non-adiabatic capillary tubes is presented. It consists of an improved version of the distributed model detailed in Ablanque et al. (2010). The model is based on a pseudo-homogeneous two-phase flow model where the governing equations (continuity, momentum, energy and entropy) are integrated over the discretized fluid domain and solved by means of a step-by-step scheme. The main novelty of the improved algorithm is its enhanced capability to address common convergence issues typically found in distributed models for non-adiabatic capillary tubes (i.e. flow discontinuities caused by the refrigerant re-condensation). In addition, the newest version of the model allows the simulation of both concentric and lateral configurations.
This work presents the detailed numerical aspects of the new model, the implementation and validation of the lateral configuration, and a detailed study of its performance when addressing the aforementioned numerical difficulties.
2019-07-11T12:01:17ZAblanque Mejía, NicolásOliet Casasayas, CarlesRigola Serrano, JoaquimPérez Segarra, Carlos DavidIn the present work a numerical model to simulate the thermal and fluid-dynamic phenomena inside non-adiabatic capillary tubes is presented. It consists of an improved version of the distributed model detailed in Ablanque et al. (2010). The model is based on a pseudo-homogeneous two-phase flow model where the governing equations (continuity, momentum, energy and entropy) are integrated over the discretized fluid domain and solved by means of a step-by-step scheme. The main novelty of the improved algorithm is its enhanced capability to address common convergence issues typically found in distributed models for non-adiabatic capillary tubes (i.e. flow discontinuities caused by the refrigerant re-condensation). In addition, the newest version of the model allows the simulation of both concentric and lateral configurations.
This work presents the detailed numerical aspects of the new model, the implementation and validation of the lateral configuration, and a detailed study of its performance when addressing the aforementioned numerical difficulties.Experimental investigation of inlet distortion effect on performance of a micro gas turbine
http://hdl.handle.net/2117/165326
Experimental investigation of inlet distortion effect on performance of a micro gas turbine
Naseri, Alireza; Sammak, Shervin; Boroomand, Masoud; Alihosseini, Alireza; Tousi, Abolghasem M.
An experimental study has been carried out to determine how inlet total-pressure distortion affects the performance of a micro gas turbine. An inlet simulator is designed and developed to produce and measure distortion patterns at the inlet to the gas turbine. An air jet distortion generator (AJDG) is used to produce nonuniform flow patterns and total pressure probes are installed to measure steady-state total-pressure distribution at the inlet. A set of wind tunnel tests have been performed to confirm the fidelity of distortion generator and measuring devices. Tests are carried out with the gas turbine exposed to inlet flow with 60¿deg, 120¿deg, and 180¿deg circumferential distortion patterns with different distortion intensities. The performance of the gas turbine has been measured and compared with that of clean inlet flow case. Results indicate that the gas turbine performance can be affected significantly facing with intense inlet distortions.
2019-07-01T09:14:17ZNaseri, AlirezaSammak, ShervinBoroomand, MasoudAlihosseini, AlirezaTousi, Abolghasem M.An experimental study has been carried out to determine how inlet total-pressure distortion affects the performance of a micro gas turbine. An inlet simulator is designed and developed to produce and measure distortion patterns at the inlet to the gas turbine. An air jet distortion generator (AJDG) is used to produce nonuniform flow patterns and total pressure probes are installed to measure steady-state total-pressure distribution at the inlet. A set of wind tunnel tests have been performed to confirm the fidelity of distortion generator and measuring devices. Tests are carried out with the gas turbine exposed to inlet flow with 60¿deg, 120¿deg, and 180¿deg circumferential distortion patterns with different distortion intensities. The performance of the gas turbine has been measured and compared with that of clean inlet flow case. Results indicate that the gas turbine performance can be affected significantly facing with intense inlet distortions.A time-average filtering technique to improve the efficiency of two-layer wall models for large eddy simulation in complex geometries
http://hdl.handle.net/2117/133647
A time-average filtering technique to improve the efficiency of two-layer wall models for large eddy simulation in complex geometries
Calafell Sandiumenge, Joan; Trias Miquel, Francesc Xavier; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio
Two-Layer wall models have been recurrently studied since they represent a good physical model for Large Eddy Simulations with underresolved wall regions. Specifically, those based on the Reynolds Averaged Navier-Stokes equations are of special interest, since they can be applied to a wide range of conditions including non-equilibrium flows. Nonetheless, these models are affected by two recurrent problems, the “log-layer mismatch” and the resolved Reynolds stresses inflow, which until now, have been dealt with separated techniques. In this work, a time-filtering methodology is applied to tackle both issues at once with a single and low-computational-cost step, easily applicable to complex three-dimensional geometries. The time-filtering technique has already been applied to other types of wall models to mitigate the “log-layer mismatch.” Now, it is applied for the first time in the Two-Layer wall model context, showing its ability not only in avoiding the mismatch issue but also in blocking the resolved Reynolds stress inflow, dramatically improving the wall model performance and generality compared to other existing implementations. A methodology to determine the necessary temporal filter length is proposed and validated in equilibrium and non-equilibrium conditions. Additionally, the filter size influence on large-scale unsteady flow motions is assessed. Good results are obtained in steady and unsteady flow regimes by suppressing the LES highest frequencies while taking into account large-scale temporal effects.
2019-05-29T13:13:54ZCalafell Sandiumenge, JoanTrias Miquel, Francesc XavierLehmkuhl Barba, OriolOliva Llena, AsensioTwo-Layer wall models have been recurrently studied since they represent a good physical model for Large Eddy Simulations with underresolved wall regions. Specifically, those based on the Reynolds Averaged Navier-Stokes equations are of special interest, since they can be applied to a wide range of conditions including non-equilibrium flows. Nonetheless, these models are affected by two recurrent problems, the “log-layer mismatch” and the resolved Reynolds stresses inflow, which until now, have been dealt with separated techniques. In this work, a time-filtering methodology is applied to tackle both issues at once with a single and low-computational-cost step, easily applicable to complex three-dimensional geometries. The time-filtering technique has already been applied to other types of wall models to mitigate the “log-layer mismatch.” Now, it is applied for the first time in the Two-Layer wall model context, showing its ability not only in avoiding the mismatch issue but also in blocking the resolved Reynolds stress inflow, dramatically improving the wall model performance and generality compared to other existing implementations. A methodology to determine the necessary temporal filter length is proposed and validated in equilibrium and non-equilibrium conditions. Additionally, the filter size influence on large-scale unsteady flow motions is assessed. Good results are obtained in steady and unsteady flow regimes by suppressing the LES highest frequencies while taking into account large-scale temporal effects.A second-order time accurate semi-implicit method for fluid–structure interaction problems
http://hdl.handle.net/2117/133394
A second-order time accurate semi-implicit method for fluid–structure interaction problems
Naseri, Alireza; González Acedo, Ignacio; Amani, Ahmad; Pérez Segarra, Carlos David; Oliva Llena, Asensio
This paper is concerned with numerical solution of fluid-structure inter-action (FSI) problems involving an incompressible viscous flow and an elasticstructure. A semi-implicit partitioned method with second-order temporalaccuracy is proposed. The method separates the pressure term of the fluidequations and strongly couples it to the structure, while the remaining fluidterms and the geometrical nonlinearities are treated explicitly. A second-order projection method is used to solve the fluid equations and also as aframework for the FSI coupling. Particular attention is paid to the boundaryconditions for fluid equations and the accuracy of the fluid pressure on thecommon interface. The proposed coupling method retains the second-orderaccuracy for fully-coupled nonlinear FSI problems. Extensive numerical testsare carried out on a number of benchmark FSI problems and the second-ordertemporal accuracy for all the variables of interest (fluid velocity and pressure,and structural displacement) is demonstrated.
2019-05-23T15:35:02ZNaseri, AlirezaGonzález Acedo, IgnacioAmani, AhmadPérez Segarra, Carlos DavidOliva Llena, AsensioThis paper is concerned with numerical solution of fluid-structure inter-action (FSI) problems involving an incompressible viscous flow and an elasticstructure. A semi-implicit partitioned method with second-order temporalaccuracy is proposed. The method separates the pressure term of the fluidequations and strongly couples it to the structure, while the remaining fluidterms and the geometrical nonlinearities are treated explicitly. A second-order projection method is used to solve the fluid equations and also as aframework for the FSI coupling. Particular attention is paid to the boundaryconditions for fluid equations and the accuracy of the fluid pressure on thecommon interface. The proposed coupling method retains the second-orderaccuracy for fully-coupled nonlinear FSI problems. Extensive numerical testsare carried out on a number of benchmark FSI problems and the second-ordertemporal accuracy for all the variables of interest (fluid velocity and pressure,and structural displacement) is demonstrated.Numerical study of binary droplets collision in the main collision regimes
http://hdl.handle.net/2117/133392
Numerical study of binary droplets collision in the main collision regimes
Amani, Ahmad; Balcázar Arciniega, Néstor; Gutiérrez Álvarez, Enrique; Oliva Llena, Asensio
Direct numerical simulation of binary droplets collision is done using a conservative level-set method. The Navier-Stokes and level-set equations are solved using a finite-volume method on collocated grids. A novel lamella stabilization approach is introduced to numerically resolve the thin lamella film appeared during a broad range of collision regimes. This direction-independent method proves to be numerically efficient and accurate compared with experimental data. When the droplets collide, the fluid between them is pushed outward, leaving a thin gas layer bounded by the surface of two droplets. This layer progressively gets thinner and depending on the collision regime, may rupture resulting in coalescence of the droplets or may linger resulting in bouncing-off the droplets. Embedded ghost-nodes layer makes it possible to mimic both bouncing and coalescence phenomena of the droplets collision. The numerical tools introduced are validated and verified against different experimental results for a wide range of collision regimes. A very good agreement is observed between the results of this paper and experimental data available in the literature. A detailed study of the energy budget for different shares of kinetic and dissipation energies inside of the droplet and matrix, in addition to the surface tension energy for studied cases, is provided. Supplementary quantitative values of viscous dissipation rate inside of the matrix and droplet, and also the radial expansion of the droplet are presented as well.
2019-05-23T14:39:58ZAmani, AhmadBalcázar Arciniega, NéstorGutiérrez Álvarez, EnriqueOliva Llena, AsensioDirect numerical simulation of binary droplets collision is done using a conservative level-set method. The Navier-Stokes and level-set equations are solved using a finite-volume method on collocated grids. A novel lamella stabilization approach is introduced to numerically resolve the thin lamella film appeared during a broad range of collision regimes. This direction-independent method proves to be numerically efficient and accurate compared with experimental data. When the droplets collide, the fluid between them is pushed outward, leaving a thin gas layer bounded by the surface of two droplets. This layer progressively gets thinner and depending on the collision regime, may rupture resulting in coalescence of the droplets or may linger resulting in bouncing-off the droplets. Embedded ghost-nodes layer makes it possible to mimic both bouncing and coalescence phenomena of the droplets collision. The numerical tools introduced are validated and verified against different experimental results for a wide range of collision regimes. A very good agreement is observed between the results of this paper and experimental data available in the literature. A detailed study of the energy budget for different shares of kinetic and dissipation energies inside of the droplet and matrix, in addition to the surface tension energy for studied cases, is provided. Supplementary quantitative values of viscous dissipation rate inside of the matrix and droplet, and also the radial expansion of the droplet are presented as well.Numerical study of rising bubbles with path instability using conservative level-set and adaptive mesh refinement
http://hdl.handle.net/2117/133387
Numerical study of rising bubbles with path instability using conservative level-set and adaptive mesh refinement
Antepara Zambrano, Óscar; Balcázar Arciniega, Néstor; Rigola Serrano, Joaquim; Oliva Llena, Asensio
This paper focuses on three-dimensional direct numerical simulations of rising bubbles in the wobbling regime, and the study of its dynamical behavior for Eötvös number 1 ≤ Eo ≤ 10 and Morton number 1e−11 ≤ M ≤ 1e−9. The computational methodology is based on a mass Conservative Level-Set method, whereas the spatial discretization of the computational domain employs an Adaptive Mesh Refinement strategy for the reduction of computational resources. The Navier–Stokes equations are discretized using the finite-volume approach on a collocated unstructured mesh; the pressure-velocity coupling is solved using a classical fractional-step projection method. This methodology is applied to a series of verification and validation tests, which are compared with experiments and numerical results from the literature. Finally, buoyancy bubbles rising in the wobbling regime are researched at moderate to high Reynolds numbers (100 < Re < 3000). Terminal Reynolds number, drag coefficient and frequency of path oscillations are compared with empirical correlations and numerical studies from the literature. Results show the discharge of alternate oppositely-oriented hairpin vortex structures. Moreover, depending on the characteristics numbers of the system, different path features, bubble shape, and vortical structures in the wake are reported.
2019-05-23T14:06:41ZAntepara Zambrano, ÓscarBalcázar Arciniega, NéstorRigola Serrano, JoaquimOliva Llena, AsensioThis paper focuses on three-dimensional direct numerical simulations of rising bubbles in the wobbling regime, and the study of its dynamical behavior for Eötvös number 1 ≤ Eo ≤ 10 and Morton number 1e−11 ≤ M ≤ 1e−9. The computational methodology is based on a mass Conservative Level-Set method, whereas the spatial discretization of the computational domain employs an Adaptive Mesh Refinement strategy for the reduction of computational resources. The Navier–Stokes equations are discretized using the finite-volume approach on a collocated unstructured mesh; the pressure-velocity coupling is solved using a classical fractional-step projection method. This methodology is applied to a series of verification and validation tests, which are compared with experiments and numerical results from the literature. Finally, buoyancy bubbles rising in the wobbling regime are researched at moderate to high Reynolds numbers (100 < Re < 3000). Terminal Reynolds number, drag coefficient and frequency of path oscillations are compared with empirical correlations and numerical studies from the literature. Results show the discharge of alternate oppositely-oriented hairpin vortex structures. Moreover, depending on the characteristics numbers of the system, different path features, bubble shape, and vortical structures in the wake are reported.