Film cooling of gas turbine blades

An experimental apparatus was designed and built to study the film cooling effectiveness from a single row of holes at various angles and hole spacings, using a foreign gas technique. Mixtures of Freon 12 and air were injected into an air mainstream to give a range of density ratios encompassing the values found in a gas turbine. The density ratio was found to be of importance and none of the commonly used parameters - e.g. blowing parameters - can be used to scale results, unless the density ratio is correctly modelled. The boundary layer thickness was varied independently of other parameters, and an increase in thickness was found to decrease the effectiveness, for normal and angled injection geometries for 20 hole diameters downstream. Favourable and adverse pressure gradients over the injection holes were tested and found to have little effect on the film cooling effectiveness. Changing the hole spacing produced considerable variations, with the smallest hole spacing giving the best performance in all respects. A hole spacing of greater than 3.75 diameters was found to be the maximum to give overall coverage above 0.10 effectiveness. The injection angle was also investigated and for low blowing rates the shallow angles gave the best results; but at high blowing rates, i.e. greater than 1.4, normal injection gave the best performance as the shallow angles rapidly became detached from the surface with increasing velocity ratio. The normal injection was also superior in terms of lateral distribution of coolant at all values of blowing rate. A correlation was proposed that included the density and velocity ratios and hole spacing for normal injection and, in a modified form, for angled injection at 3 diameter spacings. This was found to work well for the experimental results obtained here and by other researchers