Concentration Distribution in Supersonic Flow Recirculation Region

Flameholding in supersonic flow depends on the local conditions in the recirculation region and the mass transfer into and out of this region. Large gradients in local gas composition and temperature exist in the recirculation region; hence, stability parameter correlations developed for premixed flames cannot be used to determine the blowout stability limits for nonpremixed flames encountered in practical devices. In the present investigation, mixture samples were extracted at different locations in the recirculation region and shear layer formed behind a rearward-facing step and were analyzed by a mass spectrometer to determine the distribution of species concentration in the region. Both nonreacting flow tests and combustion experiments were performed for a range of fuel-related parameters such as injection location, injection pressure, and fuel type. The difference between the local fuel mole fraction within the recirculation region determined from mass spectrometry and the global fuel mole fraction based on the total moles of air and fuel injected was identified. Planar-laser-induced fluorescence was used in nonreacting cases to provide a two-dimensional image of fuel distribution and complement the mass spectrometry measurements. Differences between local concentrations and estimates based on overall global injected mass flows were large, amounting to an order of magnitude. This implies significant differences in flame stability limits of a nonpremixed flame in supersonic flow compared to premixed flame.