Research on supersonic film cooling of hypersonic optical window under different nozzle pressure ratios

When optical imaging-guided aircraft flies at hypersonic speeds in the atmosphere, the optical window withstands severe aerodynamic heating. Conducting the thin film resistance thermometer measurements in a hypersonic gun wind tunnel with a Mach number of 7.1 and total temperature of 670 K, the study investigates the effect of nozzle pressure ratio (NPR = film exit static pressure/nearby mainstream static pressure) on supersonic (Mach 2.43) film cooling for the hypersonic optical window. By combining the flow information near the window obtained using the three-dimensional compressible Reynolds-averaged Navier–Stokes method, the study reveals the mechanism of the effect of NPR on film cooling. The results indicate that increasing NPR can enhance the momentum of the unit volume film and improve the film's ability to resist mainstream mixing. Moreover, the film with a large NPR can better maintain its own momentum, leading to an increase in the film effective cooling length and film cooling effectiveness. The film effective cooling length corresponding to the unit mass flow rate of the cooling gas increases with the increase in NPR. It verifies the nonlinear relationship between the film cooling performance and the coolant mass flow rate, indicating the additional benefits of increasing NPR on the film cooling performance. Through research, it is found that increasing NPR can increase the film thickness, thereby enhancing its ability to isolate the mainstream. Moreover, as NPR increases, the cooling film expands, objectively leading to the widening of the film flow channel, allowing the Mach number of the supersonic film to increase continually. This further reduces the static temperature of the film in the flow field, thereby enhancing its cooling capability for the mainstream.