Multiscale Modeling of Turbulence, Radiation, and Combustion Interactions in Turbulent Flames

Abstract

Traditional modeling of radiative transfer in reacting flows has ignored turbulence–radiation interactions (TRI), due to difficulties caused by their inherent nonlinearities and their vast range of length scales and time scales. The state-of-the-art of modeling TRI is reviewed, and some results are presented, in which TRI are calculated from basic principles from the composition PDF method and from DNS calculations. The results show that, in turbulent jet flames, TRI are always of great importance, and that they are dominated by the correlation between the absorption coefficient and the radiative Planck function. Introduction Most fires as well as commercial combustion processes, such as boilers, internal combustion engines, gas turbines, etc., involve high temperatures and, therefore, thermal radiation usually contributes a significant fraction to the overall heat transfer rates. At the same time, the vast majority of these combustion applications occur under turbulent conditions, which is known to enhance heat transfer. Radiation, chemical kinetics and turbulence individually are among the most challenging fundamental and practical problems of computational science and engineering, due to their inherent nonlinearities and their vast range of length scales and time scales. In turbulent combustion, these phenomena are coupled in interesting and highly nonlinear ways, leading to entirely new classes of interactions. In much the same way as convection is aided by turbulence, so is radiation, which in the presence of chemical reactions may increase several fold due to turbulence interactions. While coupling between turbulence and chemistry has received a great deal of attention over the years [1, 2], turbulence-radiation interaction (hereafter TRI) has until recently been ignored by virtually all investigations due to its extreme complexity, even though its importance has been widely recognized [3–7]. Preliminary and state-of-the-art calculations have shown that TRI always increases the heat loss from a flame, and that this additional heat loss can reach 60% of the total and more, leading to a reduction in the local gas temperature of 200 C or more. Therefore,