Interaction of turbulence and radiation in confined diffusion flames

A new approach for modeling turbulence-radiation interaction in confined diffusion flames is proposed. In addition to the balance equations of velocity and mixture fraction f, equations of mean, valiance, and covariance of the heat-release rate g and mixture fraction are solved. Coupling with the chemistry model is achieved by means of a two-dimensional PDF of mixture fraction and heat-release rate. The proposed approach is open for improvement by more sophisticated submodels. Validity of this approach is illustrated by simulation of a 20-kW axisymmetric confined swirling combustion system. The results are compared to experimental data from LDV and Raman/Rayleigh measurements obtained from the same combustion system. Particularly, investigations of the influence of different coupling models on the local source terms of the radiation and of the nitrogen oxide production are discussed. The main results of this study are (1) the thin eddy assumption is valid for this system; (2) there are considerable fluctuations of the heat-release rate g with constant mixture fraction f at all locations because of the individual history of the burned gas eddies: (3) the coupling model has only negligible influence on the spatial temperature-, velocity, and mixture-fraction fields and on the overall integrated radiation-transfer power; (4) the coupling model has strong influence on the local nitrogen oxide production and on the total nitrogen oxide emission. The present study demonstrates thai for middle-sized enclosed diffusion flames, the complex phenomenon of turbulence-radiation interaction can be simulated efficiently without use of tedious stochastic approaches.