The cost of sending a payload into low earth orbit remains high, yet there is a growing global reliance on space based technology. Hypersonic air breathing propulsion systems such as the Scramjet engine are a viable means of significantly reducing this cost, however their performance can be adversely influenced by many factors. One such factor is the high viscous drag encountered during the atmospheric part of the flight, which leads to reduced performance. Although numerous methods of drag reduction presently exist, mass injection into the boundary layer with the aim of either film cooling or combustion induced drag reduction is one of the more viable approaches, and is the subject of this study.
A numerical study was performed to investigate whether a new approach to injecting fuel into a high speed boundary layer would reduce viscous drag. Hydrogen fuel was injected on a flat plate through an array consisting of four streamwise aligned flush circular portholes into a Mach 4.5 crossflow. The effect of injectant mass flow rate and streamwise jet-to-jet spacing were investigated using the Reynolds-Averaged Navier-Stokes equations with Menter's Shear Stress Transport turbulence model and a 13 species 33 reaction hydrogen-air model. The study involved four areas of investigation including jet in crossflow simulation sensitivities, fundamental flow behaviour of multiple jets in crossflow, the drag reduction possible through film cooling and finally the drag reduction in the presence of boundary layer combustion.
The results of the sensitivity study found the numerical solution to be very sensitive to choice of turbulence model and value of the turbulent Schmidt number. Sensitivities to the boundary conditions imposed on the injector pipe walls and injector configuration were noted where injection conditions were held constant. Finally, variation of the freestream turbulence intensity was found to produce only small effects. In terms of the main study, the multi-port injector array was found to produce a very complex jet interaction flow field. Variation of streamwise injector port spacing, along with jet-to-freestream dynamic pressure ratio, induced a wide variety of flow structures in the cases investigated. At low injection mass flow rates, coupling of adjacent injectors was small, whereas high mass flow rates increased the effect of jet-to-jet coupling. Variations in the streamwise jet-to-jet spacing was also found to play a critical role in the flow behaviour. At very close spacings, intense interactions coupled the behaviour of the individual jets, however at increased spacings, the larger spatial freedom allowed individual jets to develop more naturally, leading to less jet-to-jet interactions. At the maximum spacing investigated, the jet interactions behaved more like discrete jets in crossflow.
The various flow features were found to have subtle effects on the overall system performance with the injection system acting as a non-reacting film cooling device. Total viscous drag reductions of up to 60% over a plate length of 0.5m were achieved, with local drag reductions of over 90% in the near-field. Significant wall heat transfer reductions were also found in all cases. Drag reduction and wall heat transfer rates were strongly influenced by injectant mass flow rate, and only moderately effected by streamwise spacing. The maximum drag reduction performance was found for the highest injectant mass flow rate and closest streamwise jet-to-jet spacing. Mixing performance generally improved with reduced mass flow rate and increased streamwise spacing. In the presence of boundary layer combustion, total viscous drag reductions of up to 80% were achieved over the 0.5m plate length. Even with the presence of combustion in the boundary layer, significant wall heat transfer reductions were also found in all cases compared to no injection.