Investigation of pre-combustion shock trains in a sramjet using a shock tunnel at Mach 8 flight conditions

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 ratio the shock train grows in scale. The transient data reveal that there is sufficient time for the shock train to establish at equivalence ratios at or below 1.35. This is indicated by the steady pressure at the rear of isolator. At equivalence ratios above 1.35, the pressure in the rear of the isolator never reaches a steady level and for these cases the test time is not sufficiently long for the shock train to establish. The transient characteristics of the shock train are similar for all three combustors with the initial separation occurring at similar fuelling levels. The pressure distributions also show that the shock train length and position are similar for all three combustors. This indicates that the area divergence of the two divergent combustors does not have an effect on pre-combustion shock train for this engine. Cycle analysis of the experimental data reveals a number of aspects. First, for lean mixtures up to an equivalence ratio of 0.9, all three combustors have combustion efficiencies around 100%. Above this there is a steady decrease in combustion efficiency. For the non-separated cases, the combustion length is similar to the combustion chamber length. However, for the separated cases, there is a dramatic shortening of the combustion length. This indicates that the shock train improves combustion through higher compression of the combustor inflow but also through enhanced mixing. The results from the cycle analysis reveal that for the separated cases both the reattachment point and point of complete combustion are well upstream of the divergence point of both divergent combustors. This is consistent with the observation from the experiments that the area divergence had no effect on the shock train for either of the divergent combustors.