Mode Change Characteristics of a Three-Dimensional Scramjet at Mach 8

Experiments were undertaken to investigate the variation in thrust and combustion efficiency of a three-dimensional scramjet operating in different modes. The scramjet flowpath included a rectangular-to-elliptical shape transition inlet and a divergent elliptical combustion chamber, and the experiments replicated conditions for flight at Mach 8 and 32 km altitude. Two combustion chamber configurations were tested with the same overall length and area ratio, but with different lengths of constant area section before divergence. Injection of the hydrogen fuel was through portholes at the combustion chamber entrance. Both engine configurations underwent a significant change in operating character with increasing equivalence ratio that was consistent with a change from "supersonic" to "separated" combustion in a dual-mode scramjet. The configuration with longer constant area section changed mode at an equivalence ratio just less than one, with an abrupt increase in thrust. This was accompanied by a significant increase in the calculated fuel-based combustion efficiency from 25 to 72%. Mode change occurred at a significantly higher equivalence ratio for the combustor with shorter constant area section, indicating that small changes in combustor area distribution can have important effects on scramjet operability and performance.