CFD Techniques and Thermo-Mechanics Applications

The present paper discusses an experimentally validated three-dimensional CFD analysis of the flow and thermal processes in a laboratory drying oven with a forced air circulation. The thermal field within an oven has significant impact on the quality of cooked food and reliable predictions are important for a robust design and performance evaluation of an oven. A numerical simulation was carried out to predict the three-dimensional isothermal airflow in an industrial electrical forced convection oven using a computational fluid dynamics code. The CFD model is based on the fundamental equations for the conservation of mass, momentum, and the k-ε turbulence model. The performance of the CFD model was assessed by means of point measurements of the velocity with a directionally hot-film velocity sensor. The simulated results were consistent with the actual velocity measurements from the industrial oven. The calculation error was on average 18.14% of the actual velocity, caused by the limitations in turbulence modeling and numerical grid density.

Concentrating solar thermal (CST) technologies are focused on the production of both electricity and heat by the concentration of a direct beam part of the sunlight. Thus, solar thermal electricity (STE) plants collect and concentrate the solar energy which is converted into heat using a heat transfer fluid (HTF) in the solar receiver, and, in a second step, the heat is transformed into electricity by a power block. The selection of an appropriate HTF is important for increasing both the solar receiver efficiency and that of the overall STE plant. In addition, the solar receiver relative cost can be minimised by the selection of an HTF capable of achieving higher temperatures or, as a result, higher receiver efficiencies. A higher working fluid temperature is associated with a greater thermal efficiency in both receiver and power cycle if a suitable HTF and receiver design are defined. Commercial HTFs present some limitations that do not allow the improvement of thermal efficiencies in STE plants. Therefore, innovative HTFs should be studied to enhance the facility performance. This chapter is focused on the evaluation of a new HTF (supercritical CO₂) used in a solar central receiver compared to a commercial one (molten salt) using a CFD model. In this context, the results related to the operating conditions for the innovative HTF and to the adaptation of the solar receiver design have been discussed and analysed.

A two-dimensional model based on computational fluid dynamics was developed to analyze the thermal performance of an Earth-to-Air Heat Exchanger (EAHE) in three cities of Mexico. The climatic data correspond to a temperate climate (México City), a humid-hot climate (Mérida, Yucatán), and an extreme weather (Juárez City, Chihuahua). The EAHE optimal burial depth was reached for the three cities. Temperature, velocity, and cooling and heating potential were presented for each study case. The results show that the cooling and heating potential change with the tube burial depth. In a warm season, a temperature difference between the smallest depth and the biggest reached 13.3, 10.6, and 9.1 °C for Mérida, Mexico City, and Juárez City, respectively. In a cold season, the temperature increased in greater proportion in Juárez City, an average of 2.4 °C per each 0.5 m of depth increment. In the case of Mexico City and Mérida, the temperature increased to 1.8 °C. Thus, there was a temperature difference of 19.2, 14.3, and 14.5 °C for Juárez City, Mexico City, and Mérida, respectively. When analyzing an entire day using the optimum depth, it was found that the maximum heating and cooling capacity of the EAHE were 17.4 and 6.3 °C for Reynolds 100 and 1500, respectively in Juárez City.

In this paper, the parabolic trough collector (PTC) is numerically examined with the aim to choose the most appropriate receiver tube configuration. In order to carry out this study, the adopted method comprises two major steps. In the first step, the concentrated solar heat flux densities in a focal zone are calculated by SOLTRACE software. In the second step, computational fluid dynamics (CFD) simulations are performed to analyze and optimize the thermal performance of the tube receiver. The calculated heat flux densities by the SOLTRACE software are used as heat flux wall boundary conditions for the receiver tube. The effect of the absorber tube cross section shape on the performance of the PTC system is analyzed. Triangular-, rectangular-, and circular-shaped tubes are tested and results are compared.

Nowadays open-source CFD codes provide suitable environments for the implementation and testing low-dissipative algorithms typically used for turbulence simulation. Therefore in this research work, we have developed a CFD solver for incompressible fluid flow and forced convection heat transfer based on high-order diagonally implicit Runge–Kutta (RK) schemes for time integration. In particular, an iterated PISO-like procedure based on Rhie–Chow correction was used for handling pressure–velocity coupling within each RK stage. It is worth emphasizing that for space discretization, the numerical technology available within the well-known OpenFOAM library was used. The first aim of this work was to explore the reliability and effectiveness of OpenFOAM library for convective heat transfer problems using high-fidelity numerics. This is a point of interest since we cannot find similar papers in the available literature. The accuracy of the considered algorithm was evaluated studying several flow benchmarks. Hence, we also provide a further contribution to the literature involving forced convection heat transfer around bluff bodies at low Reynolds numbers. Lastly, this paper is only a first step toward turbulent heat transfer simulation in complex configurations by means of DNS/LES techniques.

The development of computational gas dynamics (CFD) and computer technologies makes it possible to design and implement methods for computing unsteady three-dimensional viscous compressible flows in regions of complex geometry. Multigrid and preconditioning techniques allowing to speed up CFD calculations on unstructured meshes were discussed in this chapter. Flow solution was provided using cell-centered finite volume formulation of unsteady three-dimensional compressible Navier–Stokes equations on unstructured meshes. The CFD code uses an edge-based data structure to give the flexibility to run on meshes composed of a variety of cell types. The fluxes were calculated on the basis of flow variables at nodes at either end of an edge or an area associated with that edge (edge weight). The edge weights were precomputed and took into account the geometry of the cell. The nonlinear CFD solver works in an explicit time-marching fashion, based on a multistep Runge–Kutta stepping procedure and piecewise parabolic method (PPM). The governing equations were solved with MUSCL-type scheme for inviscid fluxes, and the central difference scheme of the second order for viscous fluxes. Convergence to a steady state was accelerated by the use of multigrid techniques, and by the application of block Jacobi preconditioning for high-speed flows, with a separate low Mach number preconditioning method for use with low-speed flows. The capabilities of the developed approaches were demonstrated through solving some benchmark problems on structured and unstructured meshes.

Numerical and experimental studies were devoted to the thin delta wings aerodynamics with “Privileged Angles”. These studies focused only on observations and visualizations in the wind tunnel. They suggested that the delta wings with “privileged” apex can influence the delta wing aerodynamic characteristics and consequently could have repercussions on the aircraft performances. In addition, these studies revealed that the apex vortex which develops on the suction face of this type of wings occupies positions corresponding to values of quantified angles, called “Privileged Angles”. The delta wing vortex lift is mainly due to the depression generated on its extrados part (suction face) by a three-dimensional (3D) flow resulting from the complex swirling structure, which occurs at the leading edge of the wing. Relatively, the topology of this type of flow is well known, but the character of the mechanism remains to specify. The present study aimed to validate these phenomenological aspects delivered by visualizations, through the measurement of the pressure coefficient Cp, determined for the considered configurations (delta wings with the apex angles β = 75°, 80°, 85° and their combinations with fuselage of diameter d). The same study was reproduced with the Computational Fluid Dynamics software Fluent 6.1.22. The experimental and numerical results obtained are encouraging, but the interaction mechanisms between the values of the apex angle and the flow are still too badly known to consider the optimization of aerodynamic form for a real application. The principal objective of our present work was, then, to reproduce numerically the role of the delta wings with privileged angles, observed in the experimental visualizations and measurements.

This study aimed to investigate the effect of the overlap on the aerodynamic characteristics of the flow around a Savonius wind rotor. A numerical simulation was developed using a commercial CFD code. The considered numerical model is based on the resolution of the Navier–Stokes equations in conjunction with the k-ε turbulence model. These equations were solved by a finite volume discretization method. We were particularly interested in visualizing the velocity field, the mean velocity, the static pressure, the dynamic pressure, the turbulent kinetic energy, the dissipation rate of the turbulent kinetic energy, and the turbulent viscosity. The comparison of the numerical results with previous results shows a good agreement.

This chapter aimed to optimize the geometry of the collector in a solar chimney power plant (SCPP). Particularly, we were interested in the study of the collector diameter effect on the SCPP output. A two-dimensional steady model with the standard k-ε turbulence model has been developed using the commercial computational fluid dynamics (CFD) code “ANSYS Fluent 17.0.” A numerical simulation was achieved to study the local characteristics of the air flow inside the SCPP. Four collector diameters, equal to D = 100 cm, D = 200 cm, D = 300 cm, and D = 400 cm, were considered in this study. The local flow characteristics were presented and discussed for each case. The comparison shows that the collector diameter is an important parameter for the optimization of the solar setup.