A Flamelet Model of Turbulent Non-Premixed Combustion

Abstract

A flamelet model for thermodynamic state in turbulent non-premixed flames is described in which the microscopic element in the turbulent ensemble has the structure of an undisturbed laminar diffusion flame. The profiles of scalar properties through this flame uniquely define instantaneous mixture state and hence provide an alternative thermochemical relationship to that of chemical equilibrium, commonly employed in flowfield calculations. The need for a flowfield model is avoided by using published measurements of mean temperature and RMS temperature fluctuation in a turbulent flame to determine the necessary mixture fraction statistics, so that the instantaneous laminar flamelet composition profiles can be appropriately averaged.Comparison is made between species prediction using the alternative thermochemical assumptions, and experiments as reported in the literature. Substantial improvements in prediction are achieved in the fuel-rich regions of an open turbulent flame, notably in the sensitive prediction of CO concentration levels. The paper illustrates how thermodynamic sub-models may be validated by comparison with measurement without the implementation of a complete flowfield calculation.