Experimental investigation of vitiation effects on supersonic combustor performance

Abstract

A significant problem for use of combustion heated facilities in ground laboratory studies of scramjet propulsion is that the resulting high enthalpy test air are seriously vitiated by several species, for example, H2O, CO2, CO, H, OH, O, and NO, which are not of representative or very few in actual atmosphere, so-called vitiation air relative to pure air. Combustion in such vitiation air stream can be influenced by chemical and physical effects due to the different species from actual atmosphere. Therefore, the ground-test results from such vitiated facilities should be properly analyzed and corrected before extrapolated to atmospheric flight condition. The primary goal of the present efforts is to assess the net effects of vitiation air on combustion process in a supersonic combustor. Based on the direct-connected test facility of Northwestern Polytechnical University, an experimental system is developed for comparative investigation of supersonic combustion in vitiation airstream and clean airstream, respectively. Specific species at well-controlled concentration are added to the clean airstream generated from resistance heater to synthesize the vitiation airstream, duplicating the test media in a combustion heated facility. The air total temperature at combustor entrance is about 850K, typically simulating the Mach 4 flight condition. Details of the experiment system are present in this paper. With the newly constructing system, hydrogen, ethylene and kerosene fueled supersonic combustions with clean air and vitiation air stream are investigated. Individual and combined influences of H2O and CO2 at various concentrations are considered over a range of experiment condition. The combustion characteristics with clean and vitiation air stream are compared, and the influences of H2O and H2O/CO2 on supersonic combustion processes are discussed. Results show that, the combustion induced pressure rise can be significantly inhibited by H2O and/or CO2 vitiation . The direct extrapolation of vitiated test results to flight condition may possibly result in over-fueling combustion, even inlet unstart. The H2O and/or CO2 vitiation also influence the shock waves in the isolator, resulting in the tendency of supersonic combustion mode.