http://hdl.handle.net/2117/3190 2013-05-20T17:58:47Z http://hdl.handle.net/2117/19357 Title: Use of a Low-Mach model on a CFD&HT solver for the elements of an object oriented program to numerically simulate hermetic refrigeration compressors Authors: López Mas, Joan; Lehmkuhl Barba, Oriol; Rigola Serrano, Joaquim; Pérez Segarra, Carlos David Abstract: A powerful object oriented approach for the simulation of generic thermal systems (Damle et al., 2008) is used as a framework to numerically simulate the thermal and fluid behavior of hermetic reciprocating compressors. A physical abstraction of the compressor system provides a vertex-edge graph, defining the elements and the neighborhood relations of the system to be solved. Each one of these resulting elements is modeled in order to be solved by itself by giving their respective boundary conditions. Since each element provides its own solver tool, the coupled system can be solved in an integrated form. Into previous works, an unstructured and parallel object oriented Computational Fluid Dynamics and Heat Transfer code (from now on CFD&HT) for accurate and reliable solving of turbulent industrial flow, called TermoFluids (Lehmkuhl et al., 2007), was used to provide with CFD&HT capability the system elements (López et al., 2010). In this work, a Low-Mach based CFD&HT module (Chiva et al., 2011) implemented within the TermoFluids software has been used solve the fluid domain existing inside the shell of a reciprocating compressor, which is identified as one of the compressor elements in the abstraction stage. This improvement allows us to numerically simulate the recirculation flow inside the shell of a reciprocating compressor, providing detailed information about suction area of the compressor and allowing study of new geometric configurations of such part. Furthermore, in comparison with previously tested CFD&HT modules, the Low-Mach model allows better treatment of the compressibility effects generated at the inner elements of the compressor such as chambers, tubes and undoubtedly the compression chamber. 2013-05-20T15:15:42Z http://hdl.handle.net/2117/18870 Title: Numerical simulation of wrap scroll temperature for refrigeration and air conditioning compressors Authors: Rovira Casals, Jordi; Rigola Serrano, Joaquim; Pérez Segarra, Carlos David; Oliva Llena, Asensio Abstract: Being part of a model which simulates the whole consecutive overall compression process in a scroll compressor by solving equations of mass, momentum and energy balance for fluid refrigerant (Rovira et al. ,2006), an updated version is presented. In this new model, an energy balance over the scroll wraps is implemented; where temperatures and heat fluxes are obtain ed dividing the wall into 36 parts (slices) each turn. The scroll wrap is divided into different solid slices; energy balance is carried out, taking into account: i) conduction along the scroll wrap; ii) convection heat transfer between each slice and each fluid chamber, with special attention on solid slice - fluid chamber contact at each time step. The numerical model shows the one dimensional and transient temperature, pressure and mass flow rate, at each fluid chamber along the scroll compressor, among detailed solid wrap temperature distribution. The whole numerical model has been experimentally validated against experimental data from technical literature (Halm,1997)(Chen et al.,2004a)(Chen et al.,2004b), comparing mass flow rate, discharge temperature, compression work and power consumption. Finally, the influence of wall temperatures and wall heat fluxes on the compressor performance and other output variables is analyzed. 2013-04-18T13:51:51Z http://hdl.handle.net/2117/18868 Title: Numerical simulation of the turbulent fluid flow through valves based on low mach models Authors: Rigola Serrano, Joaquim; Lehmkuhl Barba, Oriol; Ventosa, Jordi; Pérez Segarra, Carlos David; Oliva Llena, Asensio Abstract: The aim of the present paper is to carry out a group of numerical experiments over the fluid flow through the valve reed, using the CFD&HT code TermoFluids, an unstructure d and parallel object-oriented CFD code for accurate and reliable solving of industrial flows (Lehmkuhl, O. et al. 2007 ) with special attention on incompressible hypothesis against low Mach compressible flow modeling, as a critic al numerical aspect depending on Reynolds number and gap thickness conditions. In all studied cases a multi-dimensional explicit finite volume fractional-step based algorithm extended to simulate low Mach fluxes using a Runge-Kutta/Crank-Nicholson time integration scheme, with a symmetry preserving discretization has been used. When turbulence modeling is needed, an extension of the WALE (Wall Adapting Local Eddy-viscosity) (Nicoud, F. and Ducros, F., 1999) model to non-structured meshes is applied. The pressure equation is solved by means of parallel Fourier Schur decomposition solver which is an efficient direct solver for loosely coupled PC clusters (Borrell, R. et al. 2011). In a two dimensional periodic way the fluid flow is approach ed by two parallel phenomena (an entrance flow through a channel and a free jet through a surface). In that sense, the present paper is focused on the numeri cal simulation model of the fluid flow through the valve reeds, considering a simplified geometry of an axial hole plus a radial diffuser. The numerical results presented are based on a specific geome try – valve diameter D is 3 times orifice diameter d, while s/d ratio is 0.6 – considering high Reynolds number at the entrance as boundary condition. The studied cases show the influence from laminar to turbulent flow from incompressible assumption to lower subsonic conditions and/or chocked flow. 2013-04-18T13:32:40Z http://hdl.handle.net/2117/18866 Title: Modular simulation of vapour compression systems with an object oriented tool Authors: Ablanque Mejía, Nicolás; Oliet Casasayas, Carles; Rigola Serrano, Joaquim; Lehmkuhl Barba, Oriol; Pérez Segarra, Carlos David Abstract: The objective of this work is to simulate vapour compression refrigeration systems through a modular approach by means of an object - oriented numerical tool called NEST. For this purpose, the global system is modeled as a collection of different elements which are linked between them. Each element represents a specific part of the system (e.g. heat exchanger, compressor, expansion device, tube, cavity, wall, etc.) and can be independently solved for given boundary conditions. The global resolution procedure is carried out by solving all the elements iteratively, transferring information between them, until a converged solution is reached. The system is easily modified by adding, subtracting or substituting any of its elements. This feature gives great flexibility to the model, not only because the configuration of the system can be clearly altered, but also because the numerical model of any element can be easily replaced allowing different levels of simulation. In this work the object - oriented methodology together with the elements description and their resolution procedures are presented. The model is validated against experimental data obtained from a refrigeration cycle working with isobutane. In addition to this, an illustrative case is presented in order to show the system capabilities. 2013-04-18T13:12:59Z 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/2117/18233 Title: Parallelization of the coupling between CFD models for airflow and building energy simulation with an object-oriented infrastructure Authors: Damle, Rashmin; Lehmkuhl Barba, Oriol; López Mas, Joan; Rigola Serrano, Joaquim; Oliva Llena, Asensio Abstract: Integrating CFD & HT models with the general building program raises the computational time of building simulations as these simulations are usually performed over a period of one year. Within this context, our aim is to couple a object-oriented modular building program with CFD & HT for airflow and parallelize the simulation with numerous processors for reducing computational time. Also the modular nature of the code will allow to resolve selective critical zones with CFD & HT models while employing simple models for airflow in less critical zones. Thus, there are different levels of modelling different rooms/elements of the building system depending on the requirements of a specific case. 2013-03-12T16:34:32Z http://hdl.handle.net/2117/18220 Title: Combined heat and moisture transfer in buildings systems Authors: Damle, Rashmin; Lehmkuhl Barba, Oriol; Rigola Serrano, Joaquim; Oliva Llena, Asensio Abstract: Temperature and humidity are the two main parameters indicating the comfort level of the building occupants. Although the effect of temperature is taken into account in thermal simulation of buildings, the moisture transfer through the rooms and porous building walls is sometimes neglected. The level of humidity can give different sensations of thermal comfort. It is necessary to take into account both heat and moisture transport in and around buildings to predict the hygrothermal behavior of rooms and building walls so as to calculate the energy demands correctly. In this work some benchmark exercises are worked out to see the performance of the heat and moisture transfer model implemented for rooms and porous walls. Finally, numerical results are compared with the measured data for a room exposed to varying outdoor conditions. 2013-03-12T15:10:08Z http://hdl.handle.net/2117/18010 Title: Modelling of absorption of H2O vapor in falling film of lbr aqueous solution in vertical tubes with presence of non-condensables Authors: García-Rivera, Eduardo; Castro González, Jesús; Farnós Baulenas, Joan; Oliva Llena, Asensio Abstract: One of the main reasons of the discrepancies between theoretical predictions made by models of absorbers of H2O-LiBr absorption chillers when they are compared with experimental results under real conditions is the presence of non-condensables gases. These non-condensables gases are inside the shell of the absorption chiller mainly for two reasons: i) air leakages (Oxygen-Nitrogen); ii) gases produced by corrosion (Hydrogen). A mathematical model of falling film absorption of H2O by LiBr aqueous solutions which considers the influence of non-condensable gases has been implemented. The model is semi-empirical, based on Navier Stokes equations together with energy and mass species simplified under the boundary layer hypotheses. Under such conditions, the differential system of equations in partial derivatives, becomes parabolic and could be solved by means of finite difference method in a step by step procedure. Detailed heat and mass transfer balances are applied at the interface to specify the boundary conditions between liquid and gas phases. In order to calculate gradient of air at the interface, the penetration theory is applied in order to avoid a detailed calculation of the gas phase. Numerically the presence of air in the interface results in a pressure drop and consequently in a reduction in heat and mass transfer rates. 2013-02-27T16:26:57Z http://hdl.handle.net/2117/17948 Title: Spectrally-consistent regularization modeling of wind farm boundary layers Authors: Trias Miquel, Francesc Xavier; Folch, David; Gorobets, Andrei; Oliva Llena, Asensio Abstract: The incompressible Navier-Stokes equations constitute an excellent mathematical modelization of turbulence. Unfortunately, attempts at performing direct simulations are limited to relatively low-Reynolds numbers because of the almost numberless small scales produced by the non-linear convective term. Alternatively, a dynamically less complex formulation is proposed here. Namely, regularizations of the Navier-Stokes equations that preserve the symmetry and conservation properties exactly. To do so, both convective and diffusive term are altered in the same vein. In this way, the convective production of small scales is effectively restrained whereas the modified diffusive term introduces a hyperviscosity effect and consequently enhances the destruction of small scales. In practise, the only additional ingredient is a self-adjoint linear filter whose local filter length is determined from the requirement that vortex-stretching must stop at the smallest grid scale. In the present work, the performance of the above-mentioned recent improvements is assessed through application to homogeneous isotropic turbulence, a turbulent channel flow and a turbulent boundary layer. As a final application, regularization modelling will be applied for large-scale numerical simulation of the atmospheric boundary layer through wind farms. 2013-02-22T15:08:48Z http://hdl.handle.net/2117/17885 Title: Numerical simulation of heat transfer and fluid flow in a flat plate solar collector with TIM and ventilation channel Authors: Kessentini, Hamdi; Capdevila Paramio, Roser; Lehmkuhl Barba, Oriol; Castro González, Jesús; Oliva Llena, Asensio Abstract: Flat plate solar collector with plastic transparent insulation materials and ventilation channel as overheating protection system inserted between the absorber and the back insulation has been studied numerically. First, a general object-oriented unsteady model of this solar collector is developed and presented. It allows solving, in parallel way, every component separately and interacting with its neighbors to set the boundary conditions in every time step of the simulation. Every component can be simulated using its own mesh and the number of CPUs necessary (depending on the simulation level needed). Second, the numerical simulations of the fluid flow and heat transfer by natural convection in the bottom part (ventilation channel) and the upper part (air gap + TIM) of the collector are done separately. The simulation has taken into account the different operation modes of the channel (opened at high operation temperatures and closed in normal operations). A three dimensional parallel turbulent CFD model based on Large Eddy Simulation is used in the simulations. The obtained numerical results are validated with experimental and benchmark results found in the literature. 2013-02-19T15:02:35Z http://hdl.handle.net/2117/17884 Title: Numerical study of the incompressible Richtmyer-Meshkov instability. Interface tracking methods on general meshes Authors: Jofre Cruanyes, Lluís; Balcázar Arciniega, Néstor; Lehmkuhl Barba, Oriol; Castro González, Jesús; Oliva Llena, Asensio Abstract: The numerical simulation of interfacial and free surface flows is a vast and interesting topic in the areas of engineering and fundamental physics, such as the study of liquid-gas interfaces, formation of droplets, bubbles and sprays, combustion problems with liquid and gas reagents, study of wave motion and others. Many different methods for interface tracking exist, but Volume-of-Fluid and Level-Set methods are two of the most important. The Volume-of-Fluid preserves mass in a natural way but requires large computational resources. On the other hand, the Level-Set is not as accurate and mass conservative as the Volume-of-Fluid but is a faster way to track interfaces, representing them by the middle contour of a signed distance function. The objective of this work is to analyze the advantatges and drawbacks of the Volume-of-Fluid and Level-Set methods by solving the incompressible two-liquid Richtmyer-Meshkov instability and to compare the results to experimental data. 2013-02-19T14:33:54Z http://hdl.handle.net/2117/17883 Title: A filtered kinetic energy preserving finite volumes scheme for compressible flows Authors: Baez Vidal, Aleix; Lehmkuhl Barba, Oriol; Pérez Segarra, Carlos David; Oliva Llena, Asensio Abstract: The Kinetic Energy Preserving Scheme (KEP) for compressible flows has been shown to solve 1-D shock waves without smearing them. The method does not add numerical diffusion to solutions but, as a counterpart, it needs very dense meshes in order to be Local Variations Diminishing (LVD) and stable. For 2-D and 3-D geometries, the method needs unaffordable meshes. A Filtered Kinetic Energy Preserving Method (FKEP) that partially solves this issue is presented in this document. The method filters the solution obtained by a KEP at each time step. It is shown that the use of filters does not significantly change the low frequencies of the motion while it enables the use of much coarser meshes.FKEP is tested on the 1-D shock tube. 2013-02-19T14:19:03Z http://hdl.handle.net/2117/17861 Title: Solid-liquid phase change with turbulent flow Authors: Galione, Pedro; Rigola Serrano, Joaquim; Castro González, Jesús; Rodríguez Pérez, Ivette María Abstract: The present paper describes a numerical simulation of the solid-liquid phase change Computational Fluid Dynamics and Heat Transfer (CFD&HT) model developed in order to account for natural convection inside Phase Change Materials (PCMs) taking into account turbulent effects. 2013-02-18T16:24:36Z 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/2117/17441 Title: Detailed numerical model for the resolution of molten salt storage tanks for CSP plants Authors: Pérez Segarra, Carlos David; Rodríguez Pérez, Ivette María; Oliva Llena, Asensio; Torras Ortíz, Santiago; Lehmkuhl Barba, Oriol Abstract: Considering the state-of-the-art in TES technologies, two-tank indirect system using molten salt is the most widespread within CSP plants. However, current techniques for design and optimization, as well as, for assessing the behaviour of these systems are mainly based on not-to-scale costly experimental set-ups. In this paper, a detailed numerical methodology modelling molten salt thermal storage tanks is presented. This methodology considers the transient behaviour of the molten-salt fluid, the gas ullage, the molten-salt free surface, the tank walls and insulation, different material in the foundation, radiation exchange between the salt and the tank walls in the ullage, the passive cooling in the foundation is proposed. Results for different configuration which allows an optimal design of the tank walls, insulation materials and tank foundations are presented. 2013-01-21T14:51:05Z