Experimental Study of Vitiation Effects on Scramjet Mode Transition

Abstract

M ANY ground test facilities for studying dual-mode scramjet (DMSJ) propulsion rely on stagnation enthalpy simulationvia combustion of hydrogen or hydrocarbon fuel. This heating process results in levels of vitiates in the test flow, such as water and carbon dioxide, that are not present in atmospheric air. The effects of these vitiates on DMSJ performance and operability must therefore be understood if accurate extrapolations to flight can be made. The Supersonic Combustion Facility [1] provides a unique opportunity to study such effects since the facility is electrically heated and supplies a test gas that is free of combustion generated vitiates. However, water vapor, in the form of steam, and carbon dioxide can be added to the flow to examine the effects these vitiates have on a DMSJ. Previous work to study vitiation effects has been performed using up to 7% H2O and 2.5% CO2, both by mole [1]. However, these results were only obtained over a limited range of fuel equivalence ratios and the study did not fully investigate mode-transition with a precombustion shock train due to the lack of a flowpath isolator. The present study was conducted with vitiate levels closer to those encountered in combustion heated facilities and with an isolator installed. Reference [2] provides details of the new configuration. Basically it consists of the DMSJ of [1], a rectangular, direct connect combustor with a single ramp fuel injector, and a new constant area, rectangular isolator that is ten duct heights long. This Note reports on experimental pressure measurements in the scramjet combustor at twovitiated conditions and compares the resultswith that of clean air. Water and carbon dioxide levels are examined such that the effects of water vapor vitiation can be isolated for conditions that are representative of a methane combustion heated facility operating at Mach 5 simulation. Trends in the data are examined, particularly with respect to the effect vitiates have on the mode-transition characteristics of the DMSJ.