Transient response of a radiating flamelet to changes in global stoichiometric conditions

The effects of changes in global stoichiometric conditions on radiating fla melets are numerically investigated by varying the boundary values of react ant (fuel/oxidizer) concentrations. The flame response to both step and sin usoidal time variations about a mean value of reactant concentration for va rious strain rates is examined. Flames having nonunity Lewis number are als o studied. The objective is to help understand the effect of turbulent fluc tuations on a flamelet embedded in the flow held and the interaction of the flame with the radiative heat loss from the combustion products (CO2 and H 2O). The peak flame temperature, heat release rate, and the radiative heat losses are used to describe the flame response. The results show that the f lame responds to fluctuations with a time delay. The effect of the frequenc y of fluctuation is found to be more important than its amplitude. At low f luctuation frequencies, the flame responds quasi-steadily and it becomes gr adually insensitive at high frequencies. This insensitivity is due to effec tive neutralization of high-frequency fluctuations by diffusion processes. In addition to the frequency, the flame response also depends on the strain rate. Due to the enhanced role of convection, flame response increases wit h increasing strain rate. The present results are used to identify a criter ion, based on a modified Strouhal number, to predict the flame response to imposed fluctuations a;nd to determine when transient effects should be inc orporated into flamelet models. Inclusion of transient effects will improve the accuracy of turbulent flame calculations, particularly the prediction of NOx and other trace species. (C) 2000 by The Combustion Institute.