Mixing characteristics of cracked gaseous hydrocarbon fuels in a scramjet combustor

High-performance hydrocarbon-fuelled scramjet engines require efficient fuel-air mixing due to the relatively short flow residence time through the combustor. At high temperatures, hydrocarbon fuels react endothermically and absorb thermal energy from the surroundings. The process known as cracking becomes essential at high Mach numbers to increase the total heat-sink capacity of the fuel. This study presents the results of chemically frozen numerical simulations that investigate the mixing characteristics of cracked gaseous heavy hydrocarbon fuels injected through a circular, flush-wall porthole injector. The mixing characteristics of fuel compositions representing cracking efficiencies ranging from 0 to 100% are investigated. The mixing rates and flow structures are found to change with fuel compositions. As the cracking increases, the mixing and streamwise circulation increase for an injectant. However, the jet penetration and stagnation pressure losses decrease. The streamwise circulation is found to have a strong influence on the mixing, the injection pressure on the jet penetration and the strength of the bow shock on stagnation pressure losses. Overall, it is shown that there are mixing benefits to be gained by injecting cracked hydrocarbon fuels compared to heavy uncracked fuels in scramjets.