The current work is an investigation of supersonic film cooling effectiveness including interactions with a two-dimensional shock wave. Air and helium, which are either heated or cooled, are injected at Mach numbers between 1.2 and 2.2 into a Mach 2.4 air freestream. The adiabatic wall temperature is measured directly. The injection velocity and mass flux are varied by changing the total temperature and Mach number while maintaining matched pressure conditions.
Heated injection, with the injectant to freestream velocity ratios greater than 1, exhibit a rise in wall temperature downstream of the slot yielding effectiveness values greater than one. The temperature rise, which also occurs for cooled injection, is attributed to the merging of the injectant boundary layer and the lip-wake. As a result comparisons between heated and cooled injection may not be valid. With the exception of heated helium runs, larger injection Mach numbers slightly increase the effective cooling length per mass injection rate. The results for helium injection indicate an increase in effectiveness as compared to that for air injection. The experimental results are compared with studies in the literature.
Flow profiles at several axial locations, up to 90 slot heights, indicate that for the same Mach number the helium injections induce a larger wake and a thicker boundary layer than air injection.
The influence of the shock impingement on the recovery temperature is not large if the flow remains attached. Once separation occurs the temperature changes drastically with downstream distance. The shock strength for incipient separation is smaller when helium is injected than when no film coolant is present. However, the converse is true with air injection even though, for the same Mach number, the momentum flux for the air injection is less than that for the helium injection. The induced separation in the case of helium is attributed to the reduced fullness of its momentum flux profile prior to interaction. This research demonstrates how the performance of supersonic film cooling for thermal control is undermined by the susceptibility to shock induced separation, and raises concerns about hydrogen film cooling for N.A.S.P.
