Coherent anti-Stokes Raman spectroscopy of a premixed ethylene-air flame in a dual-mode scramjet

As part of a broader effort to provide detailed measurements of turbulent flames in dual-mode scramjets, promote a deeper understanding of the relevant combustion physics, and aid appropriate computational model development, a high-subsonic cavity-stabilized premixed ethylene-air flame (typical of ramjet operation) is studied using coherent anti-Stokes Raman spectroscopy. This technique provides simultaneous measurements of temperature, separate O2 temperature, and mole fraction of N2, O2, CO, CO2, and C2H4. The experiments reveal a highly unsteady turbulent flame, approximately two-dimensional in the mean, which propagates downstream from the cavity and towards the observation wall. Measurements in the flame region reveal a flow that is largely divided into reactants (freestream fluid) and (nearly) equilibrium products, separated by flames that cannot be resolved by the CARS measurement volume, which is about 1 mm long. Mean and standard deviation of the resolved fluctuations of the temperature and mole fractions of species are quantified. The peak standard deviation in each profile across the flame occurs where the mean gradient is steepest, and is about 37% the difference between reactants (freestream) and products conditions. Several cases were investigated including limiting combustion cases near the lean fuel and low air temperature blowouts; in all cases the flame propagation angles are the same and distributions of suitably normalized mean and fluctuation parameters are similar at all locations and for all cases.