Autoignited combustion testing in a water-cooled scramjet combustor

A water-cooled scramjet combustor was tested to study the effects of pressure, combustor length, fuel-injection style, and wall temperature on autoignited combustion performance. The tests were conducted with an inflow Mach number of 2.5; total pressure of air of 1, 1.5, and 2 MPa; and total temperature of air from 1200 to 2600 K. High-enthalpy air was produced using a vitiation heater. The combustion condition was detected by an increase of temperature in the combustor, which was related to a local combustion condition. When hydrogen fuel was supplied transversely to the combustor wall downstream of the backward-facing step, the autoignited combustion performance degraded with an increase of the airflow pressure under a condition of low total temperature of the vitiated air. The ignition region was around the second explosion limit. The presence of H2O in the air further retarded ignition under the high-pressure condition. In high-temperature conditions, the combustion performance was improved by the increase of pressure. With the long combustor with a downstream extension, the autoignited combustion limit on the air temperature became lower. A long separation region was presumed to exist downstream of the transverse fuel jet. Under such conditions, the residence time of the mixture increased, and the ability of autoignited combustion could be improved. This effect of the extension of the combustor length decreased with the increase of pressure. Parallel fuel injection from the step base showed low autoignited combustion ability. In the parallel injection, autoignition would initiate at the base of the step, where the pressure was low and the size of the base was small. In the water-cooled combustor, the autoignited combustion limit on the air temperature was higher than that in the uncooled combustor. This fact suggested that the ignition source existed near the wall, for example, in the separation region.