The development of scramjet propulsion systems for sustained hypersonic air-breathing flight remains an area of active research. Inlet unstart is one of the main transient phenomena affecting scramjet operation, whereby the inlet shock system rapidly diverges from its design condition. One cause of unstart is heat release above the Rayleigh limit, which causes the formation of an unsteady, upstream-propagating shock system.

This thesis aims to investigate the detailed behavior of unsteady shock-system propagation in a constant-area duct with heat release, representative of a simple scramjet combustor, from both numerical and experimental perspectives. A one-dimensional, unsteady method of characteristics scheme is developed to study the shock-system propagation in an idealized manner. Connected-pipe experiments of the flow configuration are developed and conducted in a small-scale shock tunnel with injection of gaseous Hydrogen into a 40% Oxygen freestream using both porthole and slot injectors. Schlieren imaging and pressure sensor measurements are used to provide shock-speed measurements and reveal the development and propgation of the unsteady shock system.

Numerical studies predict the slowing of the shock as it propagates upstream, in close agreement with calculations made using a previously proposed quasi-steady model, except in cases of larger heat-release ratios or rapid changes in heat-release distributions which are unlikely to occur in practical combustors. The extension of the model to include a finite-rate chemical model is presented and discussed.

Experimental results show that the flow phenomena associated with combustion, even for simple geometries, are highly complex and dynamic. For equivalence ratios of approximately 0.5, a close-coupled flow regime between the normal shock and the heat release in the post-shock flow is observed, which transitions to a loosely coupled regime further upstream; this latter regime was seen throughout the shock-train development at lower equivalence ratios. Although it is generally observed that the shock speed decreases with upstream propagation in accordance with a simplified analysis, in some cases the shock is observed to re-accelerate mid-combustor at a location corresponding to mean-flow structures. Schlieren velocimetry enabled measurement of the mean velocity distributions at different injection conditions, showing that slot injection results in lower velocities in the lower-wall region at low equivalence ratios compared to porthole injection.