Comparison of Transient Thermal Measurement Techniques for Film Cooling on a Flat Plate in Transonic Flow

Abstract Adiabatic effectiveness, as a primary indicator of film cooling performance, still suffers from the disputable measurement principle in transonic flow. The two transient thermal measurement techniques in the state-of-the-art linear regression method and dual linear regression technique (DLRT), produce different results, due to their distinct treatment of local recovery temperature. As the first-of-its-kind work to resolve this controversy, this paper established a criterion for judging the two methods based on the similarity principle, then evaluated the correctness of the two methods through transient thermal measurements for transonic flow over a flat plate. During heat transfer experiments, relevant dimensionless numbers (Mach number, Reynolds number, Prandtl number of the mainstream, blowing ratio, momentum ratio, and temperature ratio between coolant and mainstream) were all fixed. Only the values of mainstream and coolant temperature varied by 8 K and 14 K, respectively, so according to the judging criterion, adiabatic effectiveness should be invariant. It is found that adiabatic effectiveness obtained by linear regression method is indeed roughly unaltered at different mainstream and coolant temperatures, when the coolant-to-mainstream temperature ratio is fixed. However, results measured by the dual linear regression technique display noticeable variation (by a maximum of 0.3) under the same circumstances. This is because the assumption of this technique conflicts with the similarity principle. Therefore, the linear regression method aligns much better with the judging criterion based on the similarity principle than the dual linear regression technique and is hence recommended to measure adiabatic effectiveness in transonic flow.