C.D. Pérez-Segarra

TermoFluids: A new Parallel unstructured CFD code for the simulation of turbulent industrial problems on low cost PC Cluster

The main features of TermoFluids are presented. It is a new unstructured and parallel objectoriented CFD code for accurate and reliable solving of industrial flows. The more relevant aspects from a parallel computing point of view, such as communication between CPUs and parallel direct and iterative algebraic solvers that allow TermoFluids to run efficiently on looselycoupled parallel computers are presented. Also, the different approaches for turbulence modelling implemented in TermoFluids (RANS, LES and hybrid LES/RANS models) are pointed out. Illustrative results of numerical simulation of industrial problems, as the thermal optimisation of the nacelle of a wind turbine, are also presented.

Numerical simulation of capillary-tube expansion devices

Abstract A numerical method has been developed to simulate the thermal and fluid-dynamic behaviour of capillary-tube expansion devices. The governing equations of the flow (continuity, momentum and energy), written in one-dimensional and transient form over finite control volumes, have been solved using an implicit step-by-step numerical technique. This formulation requires the use of additional equations for the evaluation of convective heat transfer, shear stress and void fraction. A special treatment of the point of transition between single-phase and two-phase fluid flow has been implemented together with a Newton-Raphson algorithm for the evaluation of the inlet mass flow rate corresponding to the given boundary conditions. The numerical model allows analysis of aspects such as geometry, type of fluid, critical and non-critical flow conditions, boundary conditions (heat transfer, inlet and discharge pressures etc.) and transient aspects. Good agreement between numerical and experimental data from three different authors (Bolstand and Jordan, Whitesel, Mikol) has been obtained. Some illustrative results are presented to indicate the main characteristics of the numerical model.

Numerical Study of Plane and Round Impinging Jets using RANS Models

In this article, different impinging jet configurations are studied by means of time-averaged Navier-Stokes simulations. In this technique, explicit algebraic Reynolds stress models and both nonlinear and linear eddy–viscosity models are explored jointly with k − ε and k − ω platforms. The main object of this work is to study numerical performance and accuracy of models when they are used in the simulation of both plane and round impinging jets. With this purpose, results from numerical simulations, using different models, are compared among them and with experimental data available in the literature. Comparisons are performed in terms of mean and fluctuating velocities and global parameters, i.e., the local Nusselt number. A verification procedure is applied in order to ensure the credibility of the numerical solutions.

Numerical simulation of capillary tube expansion devices behaviour with pure and mixed refrigerants considering metastable region. Part I: mathematical formulation and numerical model

A detailed one-dimensional steady and transient numerical simulation of the thermal and fluid-dynamic behaviour of capillary tube expansion devices working with pure and mixed refrigerants has been developed. The discretised governing equations are coupled using an implicit step by step method. A special treatment has been implemented in order to consider transitions (subcooled liquid region, metastable liquid region, metastable two-phase region and equilibrium two-phase region). All the flow variables (enthalpy, temperatures, pressures, vapour quality, velocities, heat fluxes, etc.) together with the thermophysical properties are evaluated at each point of the grid in which the domain is discretised. The numerical model allows analysis of aspects such as geometry, type of fluid (pure substances and mixtures), critical or non-critical flow conditions, metastable regions, adiabatic or non-adiabatic capillary tubes and transient aspects. Comparison of the numerical simulation with experimental data presented in the technical literature will be shown in part II of the present article.

DIRECT NUMERICAL SIMULATION OF A THREE-DIMENSIONAL NATURAL-CONVECTION FLOW IN A DIFFERENTIALLY HEATED CAVITY OF ASPECT RATIO 4

The majority of the direct numerical simulations of turbulent and transition natural- convection flows in cavities assume two-dimensional behavior. To investigate the effect of the three-dimensional fluctuations, a complete direct numerical simulation has been carried out, in a cavity with aspect ratio 4, Raz = 6.4 × 108 , and Pr = 0.71, using a low-cost PC cluster. A description of the parallel algorithm and the methodology used to verify the code and the accuracy of the statistics obtained is presented. The main features of the two- and three-dimensional flows are described and compared. Several first- and second-order statistic distributions have been evaluated, including the Reynolds stress tensor. Significant differences are observed between the second-order statistics of the two- and three- dimensional simulations.

Symmetry-preserving discretization of Navier-Stokes equations on collocated unstructured grids

A fully-conservative discretization is presented in this paper. The same principles followed by Verstappen and Veldman (2003) 3] are generalized for unstructured meshes. Here, a collocated-mesh scheme is preferred over a staggered one due to its simpler form for such meshes. The basic idea behind this approach remains the same: mimicking the crucial symmetry properties of the underlying differential operators, i.e., the convective operator is approximated by a skew-symmetric matrix and the diffusive operator by a symmetric, positive-definite matrix. A novel approach to eliminate the checkerboard spurious modes without introducing any non-physical dissipation is proposed. To do so, a fully-conservative regularization of the convective term is used. The supraconvergence of the method is numerically showed and the treatment of boundary conditions is discussed. Finally, the new discretization method is successfully tested for a buoyancy-driven turbulent flow in a differentially heated cavity.

Numerical simulation of capillary-tube expansion devices behaviour with pure and mixed refrigerants considering metastable region. Part II: experimental validation and parametric studies

Abstract A detailed one-dimensional steady and transient numerical simulation of the thermal and fluid-dynamic behaviour of capillary-tube expansion devices working with pure and mixed refrigerants has been developed and presented in part I of this article. The accuracy of the detailed simulation model is demonstrated in this paper by comparison with experimental data from the technical literature. Results presented include both metastable flow modelling and non-metastable flow modelling, homogeneous and separated flow model for metastable flow and the used of different empirical correlation needed in the numerical model. Comparisons of model prediction between various approaches are discussed. Numerical results and parametric studies for concentric capillary tube-suction line heat exchangers have also been presented.

A DIRECT PARALLEL ALGORITHM FOR THE EFFICIENT SOLUTION OF THE PRESSURE-CORRECTION EQUATION OF INCOMPRESSIBLE FLOW PROBLEMS USING LOOSELY COUPLED COMPUTERS

The numerical simulation of time-accurated complex flows needs large computational resources. For the case of incompressible flows, the solution of the pressure-correction equation is typically the main bottleneck, especially on loosely coupled parallel computers such as PC clusters. An algorithm intended to solve this problem is presented. It is a variant of the Schur complement method that uses direct solvers for each subdomain and for the interface equation. The inverse of the interface matrix is evaluated and stored in parallel. Simulation of turbulent natural convection is used as a benchmark to show its potential and limitations.

THREE-DIMENSIONAL NUMERICAL STUDY OF MELTING INSIDE AN ISOTHERMAL HORIZONTAL CYLINDER

The problem of melting inside an isothermal horizontal cylinder has been numerically studied, considering its three-dimensional behavior. The presented numerical results correspond to two different cases with Rayleigh numbers of 3.68 × 105 and 1.22 × 105, Stefan numbers of 0.04 and 0.133, and a common Prandtl number of 56.9. The flow pattern developed has been numerically visualized, and the melted mass fraction and the Nusselt number on the surface cylinder have been computed. The numerical results presented have been compared with a two-dimensional numerical prediction and with some experimental data.

Thermal and Fluid Dynamic Simulation of Automotive Fin-and-Tube Heat Exchangers, Part 1: Mathematical Model

The aim of this paper is to present a developed detailed model for the simulation of automotive fin-and-tube heat exchangers (Compact Heat Exchanger Simulation Software, CHESS). The simulation strategy and the mathematical methodology are described in detail. The model is based on a 3D discretization around the tubes as small heat exchangers, where the appropriate governing equations (mass, momentum and energy) are applied for each control volume on the air-side, the solid elements, and the coolant side. Some verification and illustrative results are also provided to show the features of the model. A comparison between numerical simulation results and experimental data is presented in a companion paper (Part 2).