Response of counterflow premixed and diffusion flames to strain rate variations at reduced and elevated pressures

The thermal structure of counterflow premixed and diffusion flames was experimentally and computationally studied to examine the response of flame structure to strain rate and pressure variations. The temperature profiles were experimentally measured as a function of strain rate at reduced and elevated pressures by using spontaneous Raman scattering, and were found to agree well with the independently computed profiles using detailed reaction mechanisms and transport properties. For both near-equidiffusive and non-equidiffusive premixed flames, results show that variation of the thermal structures is much smaller than that of the strain rate, and the structural sensitivity decreases with increasing pressure due to the reduced flame thickness. For diffusion flames, results show that the flame structure at different pressures largely scales with the density-weighted strain rate instead of the strain rate alone. Further analysis was conducted to extract the global flame parameters from the thicknesses of stretched non-equidiffusive flames in response to strain rate and pressure variations. It is demonstrated that the effects of stretch vary linearly with the imposed strain rate for the flames studied herein, and that the stretch effects can be predicted with good accuracy by knowing the laminar flame speeds of the one-dimensional planar flame as a function of the system pressure and equivalence ratio and by knowing the mixture Lewis number as a function of the equivalence ratio.