MEASURED LENGTHS OF SUPERSONIC HYDROGEN-AIR JET FLAMES - COMPARED TO SUBSONIC FLAME LENGTHS - AND ANALYSIS

Measurements of flame length are reported that help to quantify the ov erall fuel-air mixing process that occurs within a jet-like flame in t he supersonic regime. A nonpremixed, turbulent, hydrogen-air jet flame is stabilized along the axis of a Mach 2.2 coflowing air stream. Supe rsonic flame lengths are compared to measured lengths of subsonic hydr ogen-air flames with coflow that were stabilized on the axis of a subs onic wind tunnel for a range of fuel/air velocity and density ratios. The supersonic dames are found to be significantly shorter than (i.e., typically half as long as) corresponding subsonic flames, providing t hat both the velocity and density ratios are matched for the two cases . This difference implies that for axisymmetric jet geometries with co mbustion, mixing rates are larger in the supersonic case. Compressibil ity effects art not believed to be significant since typical convectiv e Mach numbers are less than 0.45. One possible reason why the superso nic flames are relatively short is that the radial velocity, which con trols entrainment, can be altered by compression/expansion waves that are inherent to the jet geometry. Both subsonic and supersonic flame l engths increase as the normalized fuel mass flux increases, which is p redicted to occur by a scaling analysis. The density of the supersonic airstream was decreased in order to simulate an increase in altitude, and the supersonic flames became longer, as predicted by the analysis . Increasing the air stagnation temperature to 600K shortens the super sonic flames, which is not explained by the mixing-limited scaling ana lysis and may be due to finite-rate chemistry or partially premixed co mbustion, both of which are temperature dependent processes.