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

Abstract

The effects of changes in global stoichiometric conditions on radiating flamelets are numerically investigated by varying the boundary values of reactant (fuel/oxidizer) concentrations. The flame response to both step and sinusoidal time variations about a mean value of reactant concentration for various strain rates is examined. Flames having nonunity Lewis number are also studied. The objective is to help understand the effect of turbulent fluctuations on a flamelet embedded in the flow field and the interaction of the flame with the radiative heat loss from the combustion products (CO 2 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 flame responds to fluctuations with a time delay. The effect of the frequency of fluctuation is found to be more important than its amplitude. At low fluctuation frequencies, the flame responds quasi-steadily and it becomes gradually insensitive at high frequencies. This insensitivity is due to effective 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 with increasing strain rate. The present results are used to identify a criterion, based on a modified Strouhal number, to predict the flame response to imposed fluctuations and to determine when transient effects should be incorporated into flamelet models. Inclusion of transient effects will improve the accuracy of turbulent flame calculations, particularly the prediction of NO x and other trace species.