An Investigation On Starting Characteristics Of Hypersonic Inlets

The scramjet engine powered air breathing hypersonic vehicle enable a low-cost way for both high-speed air travel and space access. The status of related researches is now approaching to a phase of flight tests. The scramjet technologies may firstly achieve a success in hypersonic missile application, and make changes in defense sys-tem in the future. The hypersonic inlet is an important and first-step component of the scramjet engine. For efficient operation, the scramjet engine requires the inlet to operate in a starting mode. However, the capability to accurately predict the inlet start-ing characteristic is still lacking, due to the complexity of inlet flows. In some hy-personic air breathing propulsion flight tasks, the test vehicles encountered inlet un-starting and the flight ended prematurely. The prediction and test method of hyper-sonic inlet starting characteristic, such as starting, self-starting, hysteresis in starting, and shock wave oscillations, are still hot and difficult issues. The understanding in hypersonic inlet starting/un-starting and shock wave oscillations are important for the prediction/detection and suppression of inlet un-starting and shock wave oscillations, the improvement of inlet design and performance, and extension of inlet stable oper-ation range. Therefore, additional thorough and detailed wind tunnel experiments are necessary. In the present study, a simple way to test various behaviors of hypersonic inlets was developed in a shock tunnel (pulse facility). Then, the flow patterns of inlet starting/un-starting and shock wave oscillations were observed and discussed with the combination of CFD.The flow establishment in a shock tunnel was examined at first. To shorten the starting process of the tunnel nozzle, a very low initial pressure is usually used in the dump tank, which results in an unsteady flow with a high Mach number dominating the nozzle starting process. The coupling of a shock tunnel starting process and an inlet starting process were studied, to cover the shortage of some earlier investigations in literature. The effects of initial pressure in the dump tank are shown. It appears that the high Mach number unsteady flow has a capability of aiding the inlet to start. It is also found that the lower the initial pressure is, the faster the nozzle starting process is resulted, and the stronger the aid-starting capability is achieved.Based on the previous coupling study, a new test method was developed to suppress the aid-starting capability of the shock tunnel without affecting much of the test time. It adopts a light obstacle as a flow plug, that can be rapidly blown out of the inlet, to choke the inlet flow for un-starting at the initial stage of the shock tunnel operation. Simultaneous high speed Schlieren imaging and surface pressure measurements were applied to determine whether the inlet could be restarted after the obstacle action. The initiation of un-start and restart process were observed. A generic hypersonic inlet that was able to self-start was archived in the shock tunnel. With the help of the newly developed test method, the flow characteristics in starting/un-starting, known as dual-solution area, were also observed in an inlet with a large internal concentration ratio. The internal contraction ratio limits for inlets self-starting and un-starting were obtained and compared with the literature. The effects of leading edge bluntness on the inlet starting characteristic were also examined. It appears that the leading edge bluntness play a quite important role in the inlet starting process.The usable test time of the shock tunnel was extended, and a wedge shaped flow plug was pre-mounted near the isolator exit. A set of two-dimensional hypersonic inlets with different internal contraction ratios was tested with various exit throttling ratios. The results indicate that the backpressure generated by the throttling device can be toler-ated and the inlet can maintain the starting mode at low throttling ratios, whereas un-start flows are initiated from the near-choke throttling ratios and a shock wave oscillation ap-pears. The Schlieren movie demonstrates that the upstream-propagating shocks in the duct play important roles during the oscillation cycles and that the formations of the upstream-propagating shocks are related to the downstream-propagating compression waves/shock waves that encounter the throttling section. The frequency of shock wave oscillation increases with increasing exit throttling ratios, primarily because of the ac-celeration of the upstream propagation. The critical throttling ratio at which the shock wave oscillations occur varies with the inlet internal concentration ratios, and the shock wave oscillation flow patterns and frequencies also relate to the inlet configurations.Based on these experiments, shock tunnels with sufficiently long running times can be a useful tool to investigate various flow characteristics in hypersonic inlets.