Experimental study on the operational characteristics of a scramjet engine under pressure-varying inflow conditions

The scramjet engine is a promising air-breathing propulsion system, suitable for future hypersonic aircraft and space-earth round-trip vehicles. Facing the demand for wide-speed-range and large-airspace flight, the scramjet engine encounters rapidly changing incoming flow conditions, including significant drops or rises in pressure. Therefore, this paper examines the impact of a single factor, namely the incoming flow pressure, on the operational characteristics of the scramjet engine. This is achieved through the establishment of incoming flow simulation system for a variable total pressure and ground direct-connect test. Ground direct-connect tests are conducted under the conditions of a simulated flight Mach number of 6, a total temperature of 1650 K, and a fuel injection equivalence ratio of 1.0 (based on initial inflow conditions). The experimental results reveal that variations in incoming flow pressure significantly influence the overall pressure level within the engine's flow passage. However, the pressure changes at different locations within the flow passage are not synchronized with the changes in incoming flow pressure. Depending on the degree of pressure that change at various locations with varying incoming flow pressure, the engine flow passage can be categorized into five regions: following zone, isolation zone, strong disturbance zone, core combustion zone, and weak disturbance zone. The mismatch between the pressure changes in the core heat release area and the total pressure change of the inflow can result in rapid shock train movement across a wide range of the flow passage in a dynamic equilibrium state, leading to single-peak and double-peak pressure distribution patterns. These changes in pressure distribution patterns are accompanied by significant variations in both the intensity and frequency of pressure oscillations. Specifically, under single-peak and double-peak pressure distribution patterns, the oscillation frequency in the core heat release area exhibits a single dominant frequency and a double dominant frequency, respectively.