One of the major challenges to scramjet propulsion is the ability to operate over a wide range of Mach numbers. The dual-mode scramjet has the advantage that it can operate much like a ramjet at low Mach numbers, with subsonic flow entering the combustion chamber, and as a conventional scramjet at higher Mach numbers where the flow remains supersonic throughout the engine. A dual-mode scramjet is able to operate as a ramjet by allowing a shock train to form in the section between the inlet and combustor known as the isolator. This pre-combustion shock train, which is a series of intersecting shock and expansion waves, provides additional compression of the flow allowing subsonic flow to be supplied to the combustion chamber.Free-piston shock tunnels, such as Stalker Tubes, are typically used to test scramjet engines due to the high enthalpy flows which these facilities can produce. However these impulse type wind tunnels have short test times of the order of milliseconds. As the pre-combustion shock train involves large regions of separated flow, which take longer than attached flow to establish, there is some question as to whether dual mode can be modelled in Stalker Tubes.This work investigates whether pre-combustion shock trains can be studied in shock tunnels at the upper end of the dual-mode regime and if so, how this phenomenon affects scramjet performance. Experiments were conducted in the T4 Stalker Tube at The University of Queensland at a condition which represents flight at Mach 8 at an altitude of 26 km. This corresponds to a dynamic pressure of 105 kPa and a total enthalpy of 3.1 MJ kg −1 .The experiments were conducted using a simple axi-symmetric scramjet, which comprised a short inlet, an isolator, fuel injectors and three interchangeable combustion chambers. The set of combustion chambers consisted of a constant area and two divergent combustors with half-cone angles of 1 • and 2 • . The different combustion chambers were tested to assess the effect which area expansion has on the shock train and on the overall combustion efficiency of the engine. The model was arranged in a semi-direct connect configuration to produce the desired conditions at the isolator entrance. Gaseous hydrogen was used as the fuel and was injected via six port-hole injectors equally spaced around the circumference. Wall static pressures were measured along the isolator and combustor walls. A quasi-one-dimensional cycle analysis code, ii which is capable of modelling separated flow, is used to model the flow through the engine and provides estimates of the combustion efficiency and distribution of heat release.Robust combustion was observed in the experiments over a range of fuel equivalence ratios between 0.5 and 1.35, for all three combustor configurations. Results for the constant area combustor show that at an equivalence ratio approximately equal to 0.7 the boundary layer separates due to the pressure rise generated from combustion, forming a shock train upstream of fuel injection. With increasing equivalence...

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This work is a fundamental investigation of the behavior of pre-combustion shock trains in the T4 Stalker Tube at the upper end of the dual-mode regime. Experiments were conducted at a condition which represents flight at Mach 8. The test model was a simple axi-symmetric duct, which comprised a short diffuser, an isolator (L/D = 4.1), fuel injectors and a constant area combustor. Gaseous hydrogen was used as the fuel and was via port-hole injection. From wall pressure measurements, it has been demonstrated that a stable pre-combustion shock train can form over a range of equivalence ratios within the short test time of the T4 Stalker Tube. From the transient data, the isolator establishment time (test) and pre-combustion shock train length (S0) were determined. Spatial distributions of the time-averaged measurements show the pressure profile throughout the duct and clearly show the position of the shock train within the isolator.

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