A study of scramjet scaling

Abstract

A scaling study was performed to determine the scaling laws for the performance of scramjet 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 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. The pressure distributions obtained show combustion in the mixing- limited regime and the reaction-limited regime. The ignition time was found to scale well by the pressure- length scaling law. The rate of pressure rise due to combustion was found to be more sudden in the large scramjet for the reaction-limitedcases than it was in the small scramjet. The scaled pressure rise due to combustion was slightly larger in the large scramjet than in the small scramjet for all but the highest enthalpy condition. Possible explanations for the above two effects are put forward. The results indicate that the pressure-length scaling law is effective as a first approximation in predicting scramjet performance.