Effect of fuel-jet injection angle variation on the overall performance of a SCRAMJET engine

Air-fuel mixing in a SCRAMJET engine is augmented by the interaction of the transverse fuel jet with the incoming supersonic air. The strong bow shock created by this interaction aids in mixing and increases the fuel residence time, but it also leads to loss in performance of the SCRAMJET engine through the loss of stagnation pressure, and rise in entropy in the combustor. One of the ways to address this issue is to weaken the bow shock by changing the angle of injection of fuel into the combustor. In the present study, the effect of variation of the injection angle, measured in the direction of the cross-flow from a line perpendicular to it (and the wall), has been numerically studied and analyzed on a 3-D SCRAMJET combustor of generic design with dual injectors, using Menter's SST model for turbulence on an in-house 3-D unstructured grid RANS solver. The angle for each injector is independently varied between 0° and 45° with an increment of 15°, while the jet positions are kept fixed at locations previously found to be optimum for the chosen flow conditions and zero angle (i.e. transverse) injection. It is observed that in every case that positive non-zero angles of injection, in the direction of the crossflow, increase thermodynamic efficiency, while the negative non-zero angles, opposing the crossflow, augment mixing. As mixing is of paramount importance in the SCRAMJET engine, due to high speeds and low residence times, we conclude that the best option is to have the angle of fuel jet injection in the direction opposing the incoming flow – a recommendation that has not been seen yet in the research literature. The degree to which the injection is slanted towards the incoming flow can be decided on the basis of the desired rate of the simultaneous penetration of the fuel into the recirculating flame-holder, which increases with increasing angle