Hypervelocity Fuel/Air Mixing in a Shcramjet Inlet

Abstract

The mixing of fuel with air in the inlet of a shock-induced combustion ramjet (shcramjet) is presented. The study is limited to nonreacting hydrogen/air mixing in an external-compression inlet at a flight Mach number of 11 and at a dynamic pressure of 1400 psf (67,032 Pa), with use of an array of cantilevered ramp injectors. Results are obtained using the WARP code solving the Favre-averaged Navier‐Stokes equations closed by the Wilcox kω turbulence model and the Wilcox dilatational dissipation correction, discretized by the Yee‐Roe flux-limited scheme. Because of the fuel being injected at a very high speed, fuel injection in the inlet is found to increase the thrust potential considerably, with a gain exceeding the losses by 40‐120%. Losses due to skin friction are seen to play a significant role in the inlet, because they are estimated to make up as much as 50‐70% of the thrust potential losses. The use of a turbulence model that can predict the wall shear stress accurately is, hence, crucial in assessing the losses accurately in a shcramjet inlet. Substituting the second inlet shock by a Prandtl‐Meyer compression fan is encouraged because it decreases the thrust potential losses and reduces the risk of premature ignition by reducing the static temperature, while decreasing the mixing efficiency by a mere 6%. One approach that is observed to be successful at increasing the mixing efficiency in the inlet is alternating the injection angle along the injector array. The use of two injection angles of 9 and 16 deg is seen to result in a 32% increase in the mixing efficiency at the expense of a 14% increase in the losses when compared to a single injection angle of 10 deg. When alternating injection angles are used, the mixing efficiency reaches as much as 0.47 at the inlet exit.