A significant challenge of hypersonic vehicle design is the efficiency of the scramjet propulsion system. The high flow speeds and high density, due to inlet compression, encountered within scramjet inlets and combustors result in significant drag and high temperatures. Reductions in length and area contraction offer improved performance through viscous drag reduction, improved thermal management and higher available thrust.
One approach to combustor drag reduction proposed by Gardner, Paull and McIntyre (2002), in a two-dimensional planar scramjet configuration, was through inlet fuel injection resulting in fuel-air mixing beginning upstream of the combustion chamber. Experimental results indicated the combustion chamber could be shortened, lowering the flow-path viscous drag, as the mixing length within the combustor was reduced. To ensure fuel-air ignition did not occur in the inlet, the contraction ratio was low. This resulted in a relatively cold mainstream flow through the engine precluding fuel auto ignition. To overcome the challenge of auto ignition, the inlet was used to create a non-homogeneous combustor flow field characterised by a series of high pressure and high temperature regions. These regions allowed for fuel-air ignition, and sustained combustion, in a flow field that was typically too mild. The concept was termed radical farming. A reduction in inlet contraction reduces the viscous and inviscid drag of the inlet, and lowers the compression ratio. This in turn reduces viscous drag within the combustor and provides higher temperature rise availability for thrust generation.
This research project investigated the application of the concept of radical farming and the use of inlet fuel injection to an axisymmetric Busemann-like scramjet configuration. The investigation focused on: the experimental establishment of stable supersonic combustion; the experimental limitations of the scramjet configuration to the key parameters of dynamic pressure, angle-of-attack and fuel-air equivalence ratio; and the computational investigation of the dominant flow structures and their coupling to the ignition streamline combustion kinetics.
Two scramjet flow-paths were designed, as part of this study, with inlet geometric contraction ratios of Ac=6.7 and Ac=4.58 for Mach 8 flight at a nominal altitude of 28 kilometres. An experimental investigation was then undertaken in The University of Queensland’s T4 free-piston reflected shock tunnel. Variations in free-stream dynamic pressure, angle-of-attack and fuel-air equivalence ratio were investigated across test runs. Computational reconstructions of three experimental conditions were shown to be in good agreement with experimental results and, on that basis, were used to probe the flow structures. The coupling of these structures to the ignition combustion kinetics was investigated through a streamline finite-rate chemical analysis.
Significant and stable supersonic combustion pressure rise was demonstrated for dynamic pressures ranging from qf=74 kPa to qf=147 kPa, as well as for angles-of-attack from 0° to 3.15°, and fuel-air equivalence ratios up to φ=1.2. Based on these results, an envelope of operation is presented. Variation in angle-of-attack was found to produce inlet and combustor boundary layer separations and retard combustion. This was primarily due to the non-uniform oblique conical shock generated from the inlet leading edge. Variation in free-stream dynamic pressure was found to affect ignition and combustion length as the conditions within the radical farm structure varied. At the lowest free-stream dynamic pressure of qf=47 kPa, ignition was not achieved.
Finite-rate combusting computational fluid dynamic (CFD) and analytical calculations compared to the experimental results indicated full combustion heat release was not achieved in the low inlet contraction configuration. Through the CFD reconstruction and finite-rate chemical analysis, fuel-air ignition was found to be coupled physically and chemically to inlet fuel injection, where the as-designed radical farm structures were disrupted and new regions of elevated temperature and pressure formed. Chemical activity in the ignition streamline, within the inlet, resulted in radical production upstream of the ignition point.
The experimental results accomplished in this investigation have formed the fundamental basis of several additional areas of research and the designed flow-path was implemented as the primary experiment on two hypersonic flight vehicles.