http://hdl.handle.net/2117/3191 2013-05-24T13:08:01Z http://hdl.handle.net/2117/18780 Title: Numerical simulation of wind flow around a parabolic trough solar collector Authors: Amine Hachicha, Ahmed; Rodríguez Pérez, Ivette María; Castro González, Jesús; Oliva Llena, Asensio Abstract: The use of parabolic trough solar technology in solar power plants has been increased in recent years. Such devices are located in open terrain and can be the subject of strong winds. As a result, the stability of these devices to track accurately the sun and the convection heat transfer from the receiver tube could be affected. In this paper, a detailed numerical aerodynamic and heat transfer model based on Large Eddy Simulations (LES) modelling for these equipments is presented. First, the model is verified on a circular cylinder in a cross-flow. The drag forces and the heat transfer coefficients are then validated with available experimental measurements. After that, simulations are performed on an Eurotrough solar collector to study the fluid flow and heat transfer around the solar collector and its receiver. Computations are carried out for a Reynolds number of Re W = 3.6 x 10(5) (based on the aperture) and for various pitch angles (h=0,45,90, 135, 80, 270). The aerodynamic coefficients are calculated around the solar collector and validated with measurements performed in wind tunnel tests. Instantaneous velocity field is also studied and compared to aerodynamic coefficients for different pitch angles. The time-averaged flow is characterised by the formation of several recirculation regions around the solar collector and the receiver tube depending on the pitch angle. The study also presents a comparative study of the heat transfer coefficients around the heat collector element with the circular cylinder in a cross-flow and the effect of the pitch angle on the Nusselt number. 2013-04-12T13:17:16Z http://hdl.handle.net/2117/17446 Title: Parallel algorithms for Sn transport sweeps on unstructured meshes Authors: Colomer, Guillem; Borrell Pol, Ricard; Trias Miquel, Francesc Xavier; Rodríguez Pérez, Ivette María Abstract: The Boltzmann Transport Equation is solved on unstructured meshes using the Discrete Ordinates Method. The flux for each ordinate is swept across the computational grid, within a source iteration loop that accounts for the coupling between the different ordinates. In this paper, a spatial domain decomposition strategy is used to divide the work among the available CPUs. The sequential nature of the sweep process makes the parallelization of the overall algorithm the most challenging aspect. Several parallel sweep algorithms, which represent different options of interleaving communications and calculations in the solution process, are analysed. The option of grouping messages by means of buffering is also considered. One of the heuristics proposed consistently stands out as the best option in all the situations analyzed, which include different geometries and different sizes of the ordinate set. With this algorithm, good scalability results have been achieved regarding both weak and strong speedup tests with up to 2560 CPUs. 2013-01-21T16:36:47Z http://hdl.handle.net/2117/15762 Title: Assessment of the symmetry-preserving regularization model on complex flows using unstructured grids Authors: Lehmkuhl Barba, Oriol; Borrell Pol, Ricard; Rodríguez Pérez, Ivette María; Pérez Segarra, Carlos David; Oliva Llena, Asensio Abstract: The main objective of the present paper is the assessment of symmetry-preserving regularization models on unstructured meshes. Three different test cases have been studied: the impinging jet flow, the flow past a circular cylinder and a simplified Ahmed car. The properties of the filters and their performance on general unstructured meshes have also been considered. A detailed analysis considering the Gaussian and the Helmholtz differential filters is presented. 2012-04-25T13:27:10Z http://hdl.handle.net/2117/15021 Title: Object-oriented simulation of reciprocating compressors: Numerical verification and experimental comparison Authors: Mohan Damle, Rashmin; Rigola Serrano, Joaquim; Pérez Segarra, Carlos David; Castro González, Jesús; Oliva Llena, Asensio Abstract: Numerical simulation of reciprocating compressors is important for the design, development, improvement and optimization of the elements constituting the compressor circuit. In this work, an object-oriented unstructured modular numerical simulation of reciprocating compressors is presented. Pressure correction approach is applied for the resolution of tubes, chambers and compression chambers, while valve dynamics are modelled assuming a spring-mass system having single degree of freedom. The modular approach offers advantages of handling complex circuitry (e.g. parallel paths, multiple compressor chambers, etc.), coupling different simulation models for each element and adaptability to different configurations without changing the program. The code has been verified with some basic tests for assuring asymptotic behaviour to guarantee error free code and physically realistic results. Cases with different compressor configurations and working fluids (R134a, R600a and R744) have also been worked out. Numerical results are compared with experimental data and illustrative cases of multi-stage compression are also presented. 2012-02-08T13:39:07Z http://hdl.handle.net/2117/14990 Title: Numerical investigation of the location of maximum erosive wear damage in elbow: Effect of slurry velocity, bend orientation and angle of elbow Authors: Zhang, Hao; Tan, Yuanqiang; Yang, Dongmin; Trias Miquel, Francesc Xavier; Jiang, Shengqiang; Sheng, Yong; Oliva Llena, Asensio 2012-02-07T11:07:28Z http://hdl.handle.net/2117/13326 Title: Hybrid MPI+OpenMP parallelization of an FFT-based 3D Poisson solver with one periodic direction Authors: Gorobets, Andrei; Trias Miquel, Francesc Xavier; Borrell Pol, Ricard; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio Abstract: This work is devoted to the development of efficient parallel algorithms for the direct numerical simulation (DNS) of incompressible flows on modern supercomputers. In doing so, a Poisson equation needs to be solved at each time-step to project the velocity field onto a divergence-free space. Due to the non-local nature of its solution, this elliptic system is the part of the algorithm that is most difficult to parallelize. The Poisson solver presented here is restricted to problems with one uniform periodic direction. It is a combination of a block preconditioned Conjugate Gradient (PCG) and an FFT diagonalization. The latter decomposes the original system into a set of mutually independent 2D systems that are solved by means of the PCG algorithm. For the most ill-conditioned systems, that correspond to the lowest Fourier frequencies, the PCG is replaced by a direct Schur-complement based solver. The previous version of the Poisson solver was conceived for single-core (also dual-core) processors and therefore, the distributed memory model with message-passing interface (MPI) was used. The irruption of multi-core architectures motivated the use of a two-level hybrid MPI + OpenMP parallelization with the shared memory model on the second level. Advantages and implementation details for the additional OpenMP parallelization are presented and discussed in this paper. Numerical experiments show that, within its range of efficient scalability, the previous MPI-only parallelization is slightly outperformed by the MPI + OpenMP approach. But more importantly, the hybrid parallelization has allowed to significantly extend the range of efficient scalability. Here, the solver has been successfully tested up to 12800 CPU cores for meshes with up to 109 grid points. However, estimations based on the presented results show that this range can be potentially stretched up until 200,000 cores approximately. Finally, several examples of DNS simulations are briefly presented to illustrate some potential applications of the solver. 2011-09-26T10:19:29Z http://hdl.handle.net/2117/13309 Title: A self-adaptive strategy for the time integration of navier-stokes equations Authors: Trias Miquel, Francesc Xavier; Lehmkuhl Barba, Oriol Abstract: An efficient self-adaptive strategy for the explicit time integration of Navier-Stokes equations is presented. Unlike the conventional explicit integration schemes, it is not based on a standard CFL condition. Instead, the eigenvalues of the dynamical system are analytically bounded and the linear stability domain of the time-integration scheme is adapted in order to maximize the time step. The method works independently of the underlying spatial mesh; therefore, it can be easily integrated into structured or unstructured codes. The additional computational cost is minimal, and a significant increase of the time step is achieved without losing accuracy. The effectiveness and robustness of the method are demonstrated on both a Cartesian staggered and an unstructured collocated formulation. In practice, CPU cost reductions up to more than 4 with respect to the conventional approach have been measured. 2011-09-23T07:56:17Z http://hdl.handle.net/2117/11984 Title: Parameter-free symmetry-preserving regularization modeling of a turbulent differentially heated cavity Authors: Trias Miquel, Francesc Xavier; Verstappen, R.W.C.P.; Gorobets, Andrei; Soria Guerrero, Manel; Oliva Llena, Asensio Abstract: Since direct numerical simulations of buoyancy driven flows cannot be computed at high Rayleigh numbers, a dynamically less complex mathematical formulation is sought. In the quest for such a formulation, we consider regularizations (smooth approximations) of the non-linearity: the convective term is altered to reduce the production of small scales of motion by means of vortex stretching. In doing so, we propose to preserve the symmetry and conservation properties of the convective terms exactly. This requirement yielded a novel class of regularizations [Comput Fluids 2008;37:887] that restrain the convective production of smaller and smaller scales of motion in an unconditionally stable manner, meaning that the velocity cannot blow up in the energy-norm (in 2D also: enstrophy-norm). The numerical algorithm used to solve the governing equations preserves the symmetry and conservation properties too. In the present work, a criterion to determine dynamically the regularization parameter (local filter length) is proposed: it is based on the requirement that the vortex stretching must stop at the scale set by the grid. Therefore, the proposed method constitutes a parameter-free turbulence model. The resulting regularization method is tested for a 3D natural convection flow in an air-filled (Pr = 0.71) differentially heated cavity of height aspect ratio 4. Direct comparison with DNS results at Rayleigh number 6.4 X 10 8 ≤ Ra ≤ 10 11 shows fairly good agreement even for very coarse grids. Finally, the robustness of the method is tested by performing simulations with Ra up to 10 17. A 2/7 scaling law of Nusselt number has been obtained for the investigated range of Ra. 2011-03-21T11:48:09Z http://hdl.handle.net/2117/10256 Title: Flamelet mathematical models for non-premixed laminar combustion Authors: Carbonell Sánchez, Daniel; Pérez Segarra, Carlos David; Coelho, P.J.; Oliva Llena, Asensio Abstract: Detailed numerical calculations based on the solution of the full transport equations have been compared with flamelet calculations in order to analyse the flamelet concept for laminar diffusion flames. The goal of this work is to study the interactive (Lagrangian Flamelet Model and Interactive Steady Flamelet Model), and non-interactive (Steady Flamelet Model and Enthalpy Defect Flamelet Model) flamelet models considering both differential diffusion and non-differential diffusion situations, and adiabatic and nonadiabatic conditions. Moreover, a new procedure has been employed to obtain enthalpy defects in the flamelet library, the application of which has been found to be encouraging. The effect of using insitu, local or stoichiometric scalar dissipation rate conditions, and also the effect of using local or stoichiometric conditions to evaluate the flamelet-like time has been analysed. To improve slow species predictions using the non-interactive models, their transport equations are solved with the reaction terms calculated from the flamelet library, also considering local or stoichiometric conditions in the so-called Extended Flamelet Models. 2010-11-11T18:12:47Z http://hdl.handle.net/2117/9980 Title: Implementation of two-equation soot flamelet models for laminar diffusion flames Authors: Carbonell Sánchez, Daniel; Oliva Llena, Asensio; Pérez Segarra, Carlos David Abstract: The two-equation soot model proposed by Leung et al. [K.M. Leung, R.P. Lindstedt, W.P. Jones, Combust. Flame 87 (1991) 289–305] has been derived in the mixture fraction space. The model has been implemented using both Interactive and Non-Interactive flamelet strategies. An Extended Enthalpy Defect Flamelet Model (E-EDFM) which uses a flamelet library obtained neglecting the soot formation is proposed as a Non-Interactive method. The Lagrangian Flamelet Model (LFM) is used to represent the Interactive models. This model uses direct values of soot mass fraction from flamelet calculations. An Extended version (E-LFM) of this model is also suggested in which soot mass fraction reaction rates are used from flamelet calculations. Results presented in this work show that the E-EDFM predict acceptable results. However, it overpredicts the soot volume fraction due to the inability of this model to couple the soot and gas-phase mechanisms. It has been demonstrated that the LFM is not able to predict accurately the soot volume fraction. On the other hand, the extended version proposed here has been shown to be very accurate. The different flamelet mathematical formulations have been tested and compared using well verified reference calculations obtained solving the set of the Full Transport Equations (FTE) in the physical space. 2010-10-25T16:33:05Z http://hdl.handle.net/2117/7863 Title: Direct numerical simulation of a differentially heated cavity of aspect ratio 4 with Rayleigh numbers up to 10(11) - Part II: Heat transfer and flow dynamics Authors: Trias Miquel, Francesc Xavier; Gorobets, Andrei; Soria Guerrero, Manel; Oliva Llena, Asensio Abstract: This is the second of a two-part paper on five direct numerical simulations of a differentially heated cavity of aspect ratio 4 with adiabatic horizontal walls (Rayleigh numbers based on the cavity height. The numerical methods and the time-averaged flow results were presented in the Part I. The heat transfer and the flow dynamics, including the turbulent statistics, the global kinetic energy balances and the internal waves motion phenomenon, are herewith described and discussed. The power-law scalings of the total kinetic dissipation rate and the Nusselt number suggest that a state of transition to a new scaling regime has been reached for the highest Ra. 2010-06-28T10:34:58Z http://hdl.handle.net/2117/7861 Title: Direct numerical simulation of a differentially heated cavity of aspect ratio 4 with Rayleigh numbers up to 10(11) - Part I: Numerical methods and time-averaged flow Authors: Trias Miquel, Francesc Xavier; Gorobets, Andrei; Soria Guerrero, Manel; Oliva Llena, Asensio Abstract: A set of direct numerical simulations of a differentially heated cavity of aspect ratio 4 with adiabatic horizontal walls is presented. The five configurations selected here (Rayleigh numbers based on the cavity height View the MathML source and View the MathML source) cover a relatively wide range of Ra from weak to fully developed turbulence. A short overview of the numerical methods and the methodology used to verify the code and the simulations is presented. The time-averaged flow results are presented and discussed in this first part. Significant changes are observed for the two highest Ra for which the transition point at the boundary layers clearly moves upstream. Such displacement increases the top and bottom regions of disorganisation shrinking the area in the cavity core where the flow is stratified. Consequently, thermal stratification values are significantly greater than unity (1.25 and 1.41, respectively). 2010-06-28T10:23:08Z http://hdl.handle.net/2117/7183 Title: Comparison of the performance of falling film and bubble absorbers for air-cooled absorption systems Authors: Castro González, Jesús; Oliet Casasayas, Carles; Rodríguez Pérez, Ivette María; Oliva Llena, Asensio Abstract: Small capacity, air-cooled NH3–H2O absorption systems are becoming more attractive in applications where the input energy can be obtained for free (e.g., solar energy, exhaust gases of engines, etc.), due to the increasing price of the primary energy. One of the main difficulties for a wider use of absorption machines is the necessary high initial investment. For this reason, the development of air-cooled systems could be an important achievement for low capacity applications. In this work, two types of air-cooled absorber have been modelled: (i) falling film flow; (ii) bubble flow. The two models have been validated with experimental data obtained from a developed testing device and published numerical results of other authors from another model. The agreement is acceptable for both cases. Finally, a parametric study has been done for air-conditioning and refrigeration in a mobile application taking advantage of the exhaust gases of the engine. In both cases, the performance of the bubble absorber has been higher. Description: The original publication is available at: doi:10.1016/j.ijthermalsci.2009.01.003 2010-05-12T13:22:32Z http://hdl.handle.net/2117/6894 Title: Numerical analysis of two-phase flow in condensers and evaporators with special emphasis on single-phase/two-phase transition zones Authors: Morales Ruíz, Sergio; Rigola Serrano, Joaquim; Pérez Segarra, Carlos David; García-Valladares, O Abstract: A numerical study of the thermal and fluid-dynamic behaviour of the two-phase flow in ducts under condensation or evaporation phenomena is presented. The numerical simulation has been developed by means of the finite volume technique based on a one-dimensional and transient integration of the conservative equations (continuity, momentum and energy). The discretized governing equations are solved using the Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) which allows back flow phenomena. Special emphasis is performed on the treatment of the transition zones between the single-phase and two-phase flow. The empirical inputs of single-phase and two-phase flow, including sub-cooled boiling and dry-out, have been adapted by means of adequate splines in the transition zones where the heat transfer correlations available in the literature are not suitable. Different numerical aspects have been evaluated with the aim of verifying the quality of the numerical solution. The mathematical model has been validated by comparison with experimental data obtained from literature considering condensation and evaporation processes. This comparison shown the improvements in the numerical solution not only in the transition zone but also in all condenser and evaporator ducts, when the special treatment for transitions is used. Illustrative results on double-pipe heat exchanger are also presented. 2010-04-09T11:32:07Z http://hdl.handle.net/2117/6892 Title: Analysis of the dynamic behavior of refrigerated spaces using air curtains Authors: Jaramillo Ibarra, Julian Ernesto; Pérez Segarra, Carlos David; Oliva Llena, Asensio; Oliet Casasayas, Carles Abstract: This article is devoted to the study of air curtains applied to reduce the refrigerated chambers heat gains. The proposed strategy is based on the numerical simulation of air curtains by means of computational fluid dynamics (CFD) using RANS modeling and their corresponding experimental validation. Further work on the reduction of the detailed numerical results into overall energetic parameters is also presented. Unsteady three-dimensional numerical parametric studies are carried out, simulating the process of refrigerated chamber sudden door opening. The numerical solutions are verified and the influence of the turbulence model used is also investigated. The studies are centered on the influence of air curtain location, the air suction combination, and both the air discharge velocity and the discharge angle. 2010-04-09T11:04:19Z