Scaling laws for scramjets : an experimental and theoretical investigation to determine the scaling laws for scramjet engine performance at hypersonic flight conditions

Abstract

A scaling study was performed to determine the scaling laws for the performance of scramjets engines.Experiments were performed on two centrally injected scramjets in the reflected shock tunnel, T4. The scramjets had a 5:1 scale to all their dimensions. The binary scaling (pressure scaling) law was tested on the two scramjets meaning the product of the static pressure in the duct and characteristic length was held constant. A number of test conditions were used to test the scaling law. Five test conditions were at approximately the same nozzle stagnation pressure (36.9 MPa) however the stagnation enthalpy was varied from 3.59 to 10.7 MJ/kg). Additionally five conditions were held at approximately the same stagnation enthalpy (5.61 MJ/kg) while the nozzle stagnation pressure was varied from 5.9 to 37.5 MPa. For each of these conditions great care was taken to ensure that the Mach number, temperature and species concentrations were the same as nearly as possible in the two scramjets.Pressure and heat transfer measurements were taken for the tare, mixing and combustion runs performed at each condition in the two scramjets. From the heat transfer results it was clear that the boundary layers was laminar for most of the conditions tested. The pressure results showed that a number of conditions were in the mixing-limited regime of combustion while a number were in the reaction-limited regime. As the stagnation enthalpy was increased the ignition point in the duct moved upstream. As the nozzle stagnation pressure was decreased the ignition point moved downstream until at the lowest nozzle stagnation pressure condition ignition did not occur in the available scramjet duct length.The scaling of the effects occurring in a scramjet duct were theoretically determined and compared to the experimental results. The scaling of the heat transfer for the tare and combustion runs, presented in terms of Stanton number, was shown to obey the pressure length scaling reasonably well as long as the boundary layer in one of the scramjets did not become transitional. The scaling of the viscous effects (tare runs) shows good agreement with the pressure-length scaling law as was theoretically predicted. The scaling of the mixing effects (mixing runs) showed good agreement with the pressure-length scaling law as well consistent with the theoretical predictions. The pressure rise due to combustion, however, was not predicted as well by the pressure-length scaling law. In contrast the scaled pressure rise was larger in the large scramjet than in the small scramjet. A possible explanation for this effect is put forward and empirical scaling laws for this effect are presented.The results therefore indicate that the pressure-length scaling law is effective as a first approximation. However, there are second order effects which do not scale according to this law.