Heat Transfer Coefficient and Adiabatic Effectiveness on a Film-Cooled Pressure Side: Results and Assessment of the IR-Based Measurement Technique Reliability

Abstract

The precise knowledge of the external heat transfer coefficient and of the film cooling coverage characteristics is crucial to an efficient design of a turbine nozzle guide vane. From an experimental point of view, thermal methods allow to retrieve both the heat transfer coefficient and the adiabatic wall temperature, but their reliability is generally questionable, as they suffer from problems as thermal conduction. In the present work, a well-known literature nozzle guide vane (VKI LS89 profile) was provided with a simplified cooling system made by a single row of cylindrical holes on the pressure side. A transient thermal technique was employed to characterize both the adiabatic effectiveness and the heat transfer coefficient. The uncertainty and the reliability of thermal technique, which are generally a weak point, were evaluated. The former aspect was addressed by means of virtual experiments, considering both the effect of the measured temperature noise and of the inaccuracies introduced by the physical model adopted for the post-processing. For the latter aspect, adiabatic effectiveness findings were compared to the ones achieved on the same test article from Pressure Sensitive Paint measurements, which have a limited and known degree of uncertainty. The overall results showed that, once a proper approach is taken, the thermal technique is able to accurately retrieve reliable results with an acceptable resolution, even on a test case with dimensions close to real engines. Different fluid-dynamic conditions were investigated to assess the effect of blowing ratio and free-stream turbulence on film cooling performances. The results indicate that the introduction of film cooling on the vane surface significantly enhances the heat transfer coefficient with respect to the uncooled case. Net Heat Flux Reduction was also computed, showing that film effectiveness can not always compensate for the HTC augmentation, highlighting the necessity of properly considering the heat transfer coefficient enhancement.