Experimental Scramjet Combustion Modes of Hydrocarbon Mixtures at Mach 8 Flight Conditions

Abstract

Shock tunnel experiments are presented that examine combustion modes of ethylene and a 64/36% ethylene/methane mixture under Mach 8 flight-representative conditions within an axisymmetric scramjet cavity combustor. Combustion modes (scram-mode and dual-mode) are presented via streamwise pressure measurements and visualized via planar laser-induced fluorescence (PLIF) of the hydroxyl (OH) combustion radical in the combustor exhaust plume. Ethylene-fuelled experiments developed scram-mode combustion under lean fuelling, whereas increased fuelling anchored combustion to the fuel injector wake. Further fuelling increases to stoichiometric levels developed dual-mode combustion. Exhaust OH PLIF indicated ethylene scram-mode combustion developed combustion products primarily in the wall-adjacent regions, whereas asymmetric flowfields were noted for the dual-mode. The addition of methane inhibited reactions, with no scram-mode combustion observed at even fuel-rich conditions. Higher fuelling rates developed dual-mode combustion with comparable pressure rises to the ethylene-fuelled dual-mode cases. OH PLIF indicated reduced concentration of OH for the ethylene/methane mixture. This work confirms the retarding influence of methane upon the reaction pathways of ethylene and reaffirms that fuels do not react independently in gaseous mixtures.