Effects of Changing Cavity Length-to-Depth Ratio on a Scramjet Operating With a Pre-Combustion Shock Train

The design of a cavity-based flameholder in a scramjet engine is critical to the operation of the combustor over a wide range of Mach numbers and altitudes. The characteristic dimensions are generally depth, length-to-depth ratio (L/d), and closeout angle, with L/d controlling mass exchange rate and, ultimately, heat release in the combustor. An experimental study of the effect that varying the L/d of a cavity-based flameholder has on an axisymmetric scramjet combustor operating with a pre-combustion shock train is presented. The combustor was fueled with gaseous ethylene through fuel injectors upstream of the cavity flameholder. Supersonic air flow was provided through a Mach-2 facility nozzle with a fixed stagnation temperature and pressure of 600 K and 240 kPa, respectively. With a cavity step height of 19 mm, L/d values of 4, 6, and 8 were tested in a new modular scramjet flowpath that conserved the isolator and combustor geometries. An increase in L/d from 4 to 6 resulted in decreased heat release in the combustor such that more fuel was required to achieve the same shock train leading edge position with the L/d = 6 configuration as the L/d = 4 configuration. The L/d = 8 configuration resulted in similar operating conditions as the L/d = 4 configuration, revealing a non-monotonic trend with increasing L/d. Local cavity equivalence ratio (Φ) measurements in all three configurations under non-burning conditions are presented and revealed lower cavity Φ in the L/d = 6 cavity than the L/d = 4 or 8 cavities, signifying a connection to the reduced performance of the L/d = 6 configuration.