Skin Friction Reduction in Hypersonic Turbulent Flow by Boundary Layer Combustion

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 4.0 to 4.5, flow stagnation enthalpies of 4.8 MJ/kg to 9.5 MJ/kg, static pressures of 75 kPa to 110 kPa, and fuel equivalence ratios of 0.3 and 0 for test flows of air. Combustion occurred at all flow conditions with results indicating a maximum reduction in skin friction coefficient, of approximately 77% of the level measured with no injection. Skin friction reductions of approximately 60% were obtained at two other test flows. Measured heat transfer levels were found to be comparable with levels obtained without injection, for most of the experimental conditions. 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.