Experimental Investigation of Supersonic Film Cooling Flow

In this study, the interaction of shock waves with supersonic film cooling flows is investigated experimentally. A laminar jet is tangentially injected into a turbulent boundary layer at a freestream Mach number Ma1 = 2.45. Two injection species, i.e., air and helium, are considered. The total temperature of the cooling film either matches the freestream condition or is reduced to obtain total temperature ratios in the range 0.75 < T0,i/T0,1 < 0.8. For air injection, the injection Mach numbers are Mai = 1.2 and Mai = 1.8, and for helium injection, the cooling film is injected at Mai = 1.3. A deflection of = 5° and 8° generates a shock that impinges upon the film cooling flow. Three shock impingement positions are considered for air injection, whereas for helium injection, a single shock impingement position is investigated. The flow is analyzed by high-speed 2C-2D particle-image velocimetry and temperature measurements. For the helium cooling film, the helium mass fraction fluctuations, the turbulent mass flux, and the turbulent Schmidt number are determined qualitatively and the results are compared with a large-eddy simulation (LES) with comparable flow configuration. The flow structure of the shock/cooling-film interaction is highly sensitive to the near-wall mass flux. An increased near-wall mass flux, i.e., increased injection Mach number or decreased injection temperature, stabilizes the flow. This drastically reduces the separation bubble size or even prevents mean separation and leads to reduced turbulent mixing downstream of the shock interaction region. In consequence, the turbulent heat transport towards the wall decreases which implies a higher cooling effectiveness. The findings also show that flow cases with different injection temperatures but similar near-wall mass flux yield separation bubbles with similar streamwise extent. For cases with strong flow separation, the additional free shear layer and the near-wall jet of the film cooling flow reduce the large-scale, low-frequency motion of the separation shock by one order of magnitude compared to the oscillation in standard shock/boundary-layer interaction. The streamwise and wall-normal turbulent mass fluxes for the cases with helium injection are in qualitative agreement with the LES data. The turbulent Schmidt number differs significantly from unity. Without shock interaction, the turbulent Schmidt number is in the range 0.5 < Sct < 1.5 which is in agreement with the literature. With shock interaction, the turbulent Schmidt number varies drastically in the vicinity of the shock interaction. Thus, the experimental results confirm the numerical data showing a massively varying turbulent Schmidt number in supersonic film cooling flows, i.e., the standard assumption of a constant turbulent Schmidt number is valid neither without nor with shock interaction.