Effects of Oxygen Enrichment on Scramjet Performance

This investigation quantifies the beneficial effects of oxygen enrichment on the combustion efficiency and specific thrust of a simplified planar scramjet engine fueled with gaseous hydrogen. Numerical modeling was used to examine the influence of fuel equivalence ratio and enrichment percentage. Broad agreement with experimental data is reported, but the present simulations provide considerably more detail, revealing aspects of the flow physics that could not be discerned from experimental data. Multiple Reynolds-averaged Navier-Stokes simulations were performed at Mach 12 equivalent freestream conditions with fuel equivalence ratios of 0.8-1.4 and enrichment percentages up to 20%. Trends in combustion efficiency with enrichment percentage are formulated for use in trajectory studies. The combustion efficiency and specific thrust are found to be significantly increased by enriching fuel with oxygen. The increases are greater than those expected from the additional propellant and complete consumption of premixed oxygen. We found that this is due to oxygen enrichment modifying the mixing layer properties within the scramjet such that the production of turbulence and therefore fuel/air mixing is increased. It is also shown that an optimal combustor length exists beyond which internal losses outweigh contributions from combustion.