Transient Fluid-Combustion Phenomena in a Model Scramjet

Abstract

An experimental and numerical investigation of the unsteady phenomena induced in a hydrogen-fuelled scramjet combustor under high equivalence-ratio conditions is carried out, focusing on the processes leading up to unstart. The configuration for the study is the fuelled flowpath of the HyShot II flight experiment. Experiments are performed in the HEG reflected-shock wind tunnel, and results are compared with those obtained from unsteady numerical simulations. High-speed Schlieren and OH* chemiluminescence visualization, together with time-resolved surface pressure measurements, allow links to be drawn between the experimentally observed flow and combustion features. The transient flow structures signaling the onset of unstart are seen to take the form of an upstreampropagating shock train. Both the speed of propagation and the downstream location at which the shock train originates depend strongly on the equivalence ratio; however, the physical nature of the incipient shock system appears to be similar for different equivalence ratios. Both experiments and computations indicate that the primary mechanism responsible for the transient behaviour is thermal choking, though localised boundary-layer separation is observed to accompany the shock system as it moves upstream. In the numerical simulations, the global choking behavior is dictated by the limited region of maximum heat release near the shear layer between the injected hydrogen and the main flow: this leads to the idea of “local” thermal choking and results in a lower choking limit than suggested by a simple integral analysis. Such localised choking makes it possible for new quasi-steady flow topologies to arise, and these are observed in both experiments and simulation. Finally, a quasi-unsteady one-dimensional model is proposed to explain elements of the observed choking behaviour.