Experimental Investigation of Film Cooling Effectiveness at Different Enthalpies

Abstract Experiments were carried out to assess the effectiveness of opposing jet film cooling at hypersonic flow conditions by utilising a simple flat-faced cylindrical injector. Two different enthalpy conditions, 2.8 MJ/kg and 4.9 MJ/kg, were generated using the shock tunnel and expansion tunnel facilities, with air as the test gas. The flat-faced cylinder had a 2 mm orifice at its centre, through which the nitrogen gas coolant was injected. Coaxial surface junction thermocouples were used to obtain the convective heat flux values. The analysis of the flowfield and shock structures was done using high-speed schlieren imaging. Coolant gas was injected at pressure ratios (PR) of 3.3 and 5 into the freestream. It was observed that the injection of coolant successfully caused reductions in the heat flux for all the cases, and an improvement in cooling effectiveness was observed as the pressure ratio was increased. While comparing low and high enthalpy conditions at the same pressure ratio (PR= 5), it came to light that cooling effectiveness was higher for the low enthalpy case, which is attributed to the higher coolant mass flow rate and higher diffusivity. Additionally, while comparing two different enthalpy cases having similar coolant mass flow rates, namely the PR=3.3 low-enthalpy case and the PR=5 high-enthalpy case, it was observed that the cooling effectiveness was contingent upon how well the coolant gas diffused across the flowfield.