Flamelet modeling of NO formation in laminar and turbulent diffusion flames

The applicability of the laminar flamelet concept for the formation and des truction of nitric oxides in laminar and turbulent diffusion flames has bee n studied. In a first step, temperatures and species concentrations in an a xisymmetric laminar diffusion flame have been calculated (i) by solving the detailed conservation equations and (ii) by applying the laminar flamelet concept. The main purpose of this step was the identification of difference s between results from both approaches. It turned out that for highly tempe rature sensitive or relatively slow chemical processes, the inclusion of th e full range of the prevailing scalar dissipation rates plays a major role for the calculated species concentrations. This behavior is obvious from th e concept of the laminar flamelet model, where the scalar dissipation rate can be discussed in terms of the reciprocal of a residence time for attaini ng chemical equilibrium. In a second step, flamelet modeling of NO, formati on was extended to a turbulent hydrogen diffusion flame. In both the steps, the flow fields of the flames were calculated by solving the Navier-Stokes equations in axisymmetric formulation using the SIMPLER algorithm. For the turbulent flow, Favre-averaged equations have been used and turbulence was modeled with the standard k-epsilon model including a correction term for axisymmetric systems. The averaging of the species concentrations was accom plished with presumed shape probability density functions (pdfs). The pdf u f the mixture fraction was described with a beta -function whereas that of the scalar dissipation rate was assumed to be log-normal. Buoyancy effects have been taken into account. The calculated temperatures and concentration s were compared with data from different experiments. (C) 2001 Elsevier Sci ence Ltd. All rights reserved.