REDUCTION OF SKIN FRICTION DRAG IN HYPERSONIC FLOW BY BOUNDARY LAYER COMBUSTION

The high levels of viscous drag that are produced at hypersonic velocities pose a considerable barrier to the successful development and operation of scramjet flight vehicles. The higher density flow that passes through a scramjet combustor produces a large component of the overall viscous drag of the vehicle. The present study investigates a skin friction reduction method, in which hydrogen is injected and combusted within a turbulent boundary layer, over a range of conditions in the hypersonic regime. Results from an experimental and numerical study of skin friction levels obtained when hydrogen is injected into turbulent boundary layers are presented. Measurements are reported from experiments in the T4 free-piston reflected shock tunnel. Hydrogen was injected from a 3 mm high slot into the boundary layer on the flat surface of one of the walls of a duct 100 mm wide, 60 mm high, and 1745 mm long. The experiments were conducted at Mach numbers ranging from 4.2 to 5.9, flow nozzle-supply enthalpies of 4.8 MJ/kg to 9.5 MJ/kg, and Reynolds numbers of 3.7 × 106 m-1 to 17.2 × 106 m-1. Fuel was injected at mass flow rates ranging from 0 kg/s/m to 0.62 kg/s/m, and nozzle area ratios of 1.0 to 4.2 for test flows of air. Combustion occurred at most flow conditions with results indicating a maximum reduction in skin friction coefficient of approximately 80% of the level measured with no injection. Skin friction reductions of approximately 60% were obtained at two other test flows. Generally, the experimental results showed that when the pressure rise due to combustion was larger, the reduction in local skin friction coefficient was larger. At high nozzle-supply enthalpies of test flows, reductions in skin friction due to combustion reduced as a result of decreasing heat release from combustion. Hydrogen injection into a test flow of nitrogen was also trialled at all flow conditions to compare with the results obtained when fuel was injected into an air flow in order to identify the effects of combustion. In general, the results showed that reductions in local skin friction coefficient were greater when combustion occured than when fuel was injected and did not burn. In addition, measured heat transfer levels were found to be comparable with levels obtained without injection for most of the experimental conditions. Reynolds analogy was also explored at high stagnation enthalpy and high Mach number over a broad range of skin friction coefficients. Results for Reynolds analogy factors at higher skin friction coefficients than previous studies are presented. Results with nitrogen as a test gas to suppress real-gas effects are also included. Shock tunnel measurements of skin friction and heat transfer rates show a trend of decreasing Reynolds analogy factor with increasing skin friction coefficient. This trend is apparently independent of stagnation enthalpy, unit Reynolds number and Mach number for the range of conditions that are examined in the present study