Performance Variation of Scramjet Nozzle at Various Nozzle Pressure Ratios

An experimental study of scramjet nozzle was conducted to investigate how the nozzle flow is affected by ambient pressure. In order to elucidate the aerodynamic properties of nozzle flow, detailed measurements of thrust and wall pressure were carried out using cold nitrogen. Nozzle flow was also visualized using a shear sensitive liquid crystal. Wall pressure and shear stress distributions in an overexpanded nozzle showed that nozzle flow includes a crossing shock wave made at the side-fences. This flowfield can be approximated as a supersonic inlet flow compressed by side walls. The high-pressure region on the nozzle ramp generated by the shock waves results in a higher performance in scramjet nozzle than that estimated for a two-dimensiona l separation from the ramp. bient or "back" pressure in flight. The change in back pressure can significantly alter the thrust and pitching moment gen- erated in the nozzle, especially when the nozzle's internal flow is affected by external conditions. Thus, investigation of noz- zle performance in off-design conditions is important for es- timation of the feasibility of SERN for hypersonic vehicles. Both experimental and numerical studies on SERNs have been carried out to investigate the interaction between the nozzle internal flow and the external conditions. Simulated flight conditions in these studies have recently been extended to the hypersonic regime. Ruff in et al.4 calculated the flow- field in SERN in an underexpanded condition with an implicit Navier-Stokes solver and compared the results with experi- mental data. Watanabe5 investigated the flowfield in SERN in both underexpanded and overexpanded conditions exper- imentally. He also examined the effect of side-fence shapes on the flowfield. Zeutzius and Beylich6 conducted an exper- imental investigation on the effects of parameters such as aspect ratio and scarf angle on the flowfield in a scarfed SERN. These studies, however, concentrated on the investigation of the flowfield in SERNs, while the effect of the interaction, namely the separation, on SERN performance remains un- investigated. The objective of the present study is to investigate the variation of nozzle performance with the change in external conditions. A scarfed nozzle6 was selected to minimize the three dimensionality of the flow on the nozzle ramp. The effect of ambient pressure was measured in terms of nozzle pressure ratio (NPR), defined as the ratio of the nozzle total pressure to the ambient pressure. Total pressure of the nozzle flow was varied to change the NPR. Thrust delivered by a scramjet nozzle was measured in both underexpanded and overexpanded conditions. Obtained thrust efficiency was compared with predicted values. A simple prediction method based on the assumption that flowfield is two dimensional was adopted to predict the variation of nozzle performance. In the overexpanded condition, the treatment of interference of the internal flow with the external flow, viz., the separation of flow, becomes very important in the prediction of the noz- zle performance. In the present study, two-dimensional sep- aration was assumed. The flowfield in the nozzle in the over- expanded condition was observed to investigate the nature of the interference between the nozzle internal flow and the external conditions.