Experimental investigation of test medium vitiation effects on supersonic combustion

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 etc., 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 it is extrapolated to actual 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 airflow and clean airflow respectively. Specific species at well-controlled concentration are added to the clean airflow generated from resistance heater to synthesize the vitiation airflow, duplicating the test media in a combustion heated facility. The air total temperature at combustor entrance is about 850K, typically simulating about 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 inlet clean air and vitiation airflow are investigated. Individual and combined influences of H₂O and CO₂ at various concentrations are considered over a range of experiment condition. The total temperature, total pressure, Mach number, fuel/air equivalence ratio, O₂ concentration of supersonic combustor inlet airflow are matched, the combustion characteristics with clean and vitiation inlet flow are compa- ed, and the influences of H₂O and H₂O/CO₂ on supersonic combustion processes are discussed. The top and bottom wall pressures of supersonic combustor are measured at difference inlet airflow vitiation case. Test results show that, the combustion induced pressure rise can be significantly inhibited by H₂O and H₂O/CO₂ vitiation. The wall pressure rise in clean airflow is higher than in vitiation airflow. The direct extrapolation of vitiated test results to flight condition may possibly result in over-fueling combustion, even inlet unstart. The H₂O and H₂O/CO₂ vitiation also influence the shock train in the isolator and supersonic combustion mode.