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RADIATIVE HEAT TRANSFER IN TECHNICAL APPLICATIONS

Stefan Braun
Fluent Deutschland GmbH, Birkenweg 14a, D-64295 Darmstadt, Germany

Ingo Cremer
Fluent Deutschland GmbH, Birkenweg 14a, D-64295 Darmstadt, Germany

Abstract

Fluid flow with heat transfer is common in most Computational Fluid Dynamics (CFD) applications. In addition to the transport mechanisms convection and conduction, radiation plays an important role in many of these cases. This paper explains, how CFD can be applied effectively to predict radiative heat transfer in technical applications accurately.
First, the physical effects of radiation are discussed, namely emission, absorption, scattering, reflection and semi transparence.
Second, five suitable radiation models taking one or more effects of radiation into account are highlighted. The most simple one, called "Rosseland approach", leads to an additional diffusive source in the energy equation. The so called "Pn approximations" use spherical harmonics truncated after n terms. In both methods radiation is treated to be continuous throughout the domain. A visibility matrix for all surfaces is the base of view factor models that are often referred to as "Surface to Surface models". A Lagrangian approach can be followed to calculate discrete rays departing from surfaces. These "ray tracing models" are common in light design. Adding a statistical approach to determine the outgoing rays is leading to Monte-Carlo methods. The "Discrete Ordinate Model (DOM)" is a model that combines continuous and Lagrangian elements. Space is divided into discrete ordinates each ruled by its own transport equation for radiation energy. By increasing the number of spatial divisions, accuracy can be enhanced quite naturally.
Finally all models are demonstrated for a "raclette party" example using the commercial CFD software FLUENT. It will be shown that each model has its special area of usage.

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