Influencing factors on the mode transition in a dual-mode scramjet

An experimental investigation was performed to characterize the effects of fuel type, injector configuration, inflow total temperature and fuel injection distribution on the mode transition in a dual-mode scramjet combustor. High enthalpy vitiated air was heated to three different total temperatures by hydrogen-oxygen combustion, entering the isolator entrance at a Mach number of 2.0. The fuel was injected through a four-hole aero-ramp or transverse injector, and ignited by a gas-pilot flame. At an inflow stagnation condition of P ₀=0-85MPa and T_o=1200K, three combustion modes, namely Pure Scram Mode, Dual-Mode Scram Mode and Dual-Mode Ram Mode, were classified in detail by wall static pressure distributions, optical visualization and one-dimensional performance analysis code. The transition point is defined as the critical fuel equivalence ratio at which pre-combustion shock train occurred in the isolator. The experimental results demonstrated that the transition point in the hydrogen case was higher than that in the ethylene case. The transition point in the aero-ramp case was slightly higher than that in the transverse injector case. The transition point increased with inflow total temperature increasing. However, the inflow total temperature had no influence on the required compression in the isolator that would lead to the transition from Dual-Mode Scram Mode to Dual-Mode Ram Mode. Hysteresis was observed in the mode transition region, illustrating the unstable characteristics of Dual-Mode Scram Mode. The mode transition was also accomplished by adjusting the fuel distributions along the combustor under the same overall fuel equivalence ratio. An analytical equation was introduced to analyze the influencing factors on the transition point. The transition point calculated by the analytical equation agreed well with the experimental results. Key words: combustion mode; transition point; hysteresis.