Numerical simulation of radiative heat transfer in turbulent flows
- Autores: Roser Capdevila Paramio
- Directores de la Tesis: Assensi Oliva Llena (dir. tes.), Carlos David Pérez Segarra (codir. tes.)
- Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2012
- Idioma: inglés
- Tribunal Calificador de la Tesis: José Fernández Seara (presid.), Joaquim Rigola Serrano (secret.), María Manuela Prieto González (voc.)
- Materias:
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- Resumen
Radiative heat transfer becomes more important with rising temperature levels, playing a key role in many relevant high-temperature applications. Besides, there are many natural and engineering problems, such as combustion, glass manufacture, atmosphere, photovoltaic cells, etc., where radiative exchange takes place not only between surfaces but also the medium, either gas, liquid or solid, is involved in this exchange (i.e. the medium is participating). Although radiation may be dominant at very high temperatures, in most of the applications takes place together with conduction and convection. Because of these aspects, the work presented in this Thesis is focussed on the numerical simulation of radiative heat transfer in participating media, either alone or coupled with turbulent fluid flows. The first part of the Thesis is devoted to the resolution of radiative heat transfer in participating media without the presence of fluid flow. The basics of thermal radiation and a detailed mathematical derivation of the equation that describes the radiative heat transfer in participating media, i.e. the Radiative Transfer Equation (RTE), is presented. Two of the most popular numerical methodologies for the resolution of the RTE, the Finite Volume Method (FVM) and the Discrete Ordinates Method (DOM), are presented using unstructured grids. The numerical solutions are verified with benchmark results of the literature or available analytical solutions . Afterwards, a set of numerical schemes are developed in order to reduce one of the main drawbacks of FVM and DOM, the false scattering. These schemes are designed specifically for 3D unstructured meshes by means of the extrapolation of nodal values of intensity on the studied radiative direction. One of them takes also into account the formal solution of the RTE. They are tested in different 3D benchmark cases including transparent and non-transparent grey medium situations. The proposed numerical schemes reduce the false scattering. Those schemes that do not follow the parabolic nature of the RTE, present convergence problems. Interaction between ray-effect and false-scattering is detailed. In the second part of the Thesis radiative heat transfer is coupled with turbulent natural convection using two different turbulent approaches, the Reynolds-Averaged Navier-Stokes (RANS) and the Large Eddy Simulation} (LES). For the first approach, RANS, turbulent stresses and heat fluxes are predicted with two low-Reynolds number two-equation eddy-viscosity turbulent models, k-E and k-w. Two benchmark cases with experimental data available in the literature are analysed considering the influence of radiation on the fluid flow and the effect of different boundary conditions. The first and second case correspond to differentially heated cavities of 28.6 and 5 with 1.92e10 and 4.48e10 Rayleigh numbers, respectively. The obtained results show that radiation does not affect the fluid flow of the first case due to its large aspect ratio. On the contrary, in the second case the flow is greatly affected: the flow becomes more turbulent and decreases thermal stratification. Although radiation approaches the numerical results to the experimental data, differences still exist. For the second approach, LES, symmetry-preserving discretizations and three different subgrid-scale (SGS) models are used in the simulation of the previous benchmark case of aspect ratio 5. Firstly the ability of LES to simulate turbulent natural convection without radiation has been analysed and secondly the effect of the surface and gas radiation on the main variables of the fluid flow is studied. For the case without radiation, LES results accurately reproduce the first statistics variables predicted by DNS results, that have been recently published in the literature, and the fluid flow is different from that obtained with RANS.
