Experimental investigation of film cooling effectiveness deviation on a transonic blade: Oxygen mixture correction for pressure sensitive paint measurement

Traditional pressure sensitive paint (PSP) based measurements for the film cooling effectiveness assume identical pressure fields between the air and foreign gas (FG) coolant jets, which introduces fundamental deviation when the FG and air have different densities. This study compares the film cooling effectiveness results on a transonic turbine blade using a novel oxygen mixture (OM) correction method and the traditional method, to obtain the film cooling effectiveness deviation distribution. Furthermore, the OM method has been employed to measure the surface pressure distribution around jets with a density higher than air on the blade surface. Experiments were conducted on both suction and pressure surfaces of a blade with single/double-row holes, varying blowing ratios (BR) and density ratios (DR = 1.5, 2.0). Key findings include: (1) Suction-side film cooling exhibits substantial negative deviations near downstream hole regions due to localized low-pressure zones induced by coolant jets, exacerbated by higher BR and DR. (2) In multi-row configurations, the superposition of film coverage and deviation distribution leads to cumulative errors, resulting in misinterpretation of coolant “lift-off” phenomena. (3) Pressure-side deviations remain negligible due to continuous pressure gradients, contrasting sharply with suction-side results. The OM method effectively resolves pressure-field discrepancies, enabling accurate PSP-based cooling effectiveness measurements in transonic flows. This work proves the necessity of OM method in suction side, especially in multi-row film.