This paper examines the effect of aerofoil surface film cooling on the aerodynamic efficiency of an annular cascade of transonic nozzle guide vanes. A dense foreign gas (SF6/Ar mixture) is used to simulate engine representative coolant-to-mainstream density ratios under ambient conditions. The flowfield measurements have been obtained using a four-hole pyramid probe in a short duration blowdown facility which correctly models engine Reynolds and Mach numbers, as well as the inlet turbulence intensity.
The use of foreign gas coolant poses specific challenges not present in an air-cooled cascade, and this paper addresses two. Firstly, a novel method for the determination of mass flow from pneumatic probe data in a heterogeneous gas environment is presented which eliminates the need to measure concentration in order to determine loss. Secondly, the authors argue on the grounds of dimensionless similarity that momentum flux ratio is to be preferred to blowing rate for the correct parametrisation of film cooling studies with varying coolant densities.
Experimental results are presented as area traverse maps, from which values for loss have been calculated. It is shown that air and foreign gas at the same momentum flux ratio give very similar results, and that the main difference between cooled and uncooled configurations is an increase in wake width. Interestingly, it is shown that an increase in the momentum flux ratio above the design value with foreign gas coolant reduces the overall loss compared with the design value. The data has been obtained both for purposes of design and for CFD code validation.