A comperative experimental study between the film effectiveness of trench and diffusion film holes

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Gas turbine airfoils have been consistently exposed to very high temperature gases to extract maximum power. Therefore, there is a growing need to protect the airfoil surfaces from the potentially surface damaging high temperature gases. Film cooling holes and trenches are one possible solution to the aforementioned problem. This thermal protection scheme which has been in practice for some considerable time, is an active area for optimization and will be our major theme of this research paper. Trenches as the name suggests are troughs on the surfaces to be cooled that are constantly filled with the coolant. The cooling air is carried over the hot surfaces by the passing hot gases to create a thin blanket over the surface. In order to achieve the best possible coolant spread over the hot surfaces, a new trench geometry that combines the positives of a slanted circular feed hole and a multi-sided trench has been adopted in this study. That geometry with its axis parallel to the flow direction served as our baseline geometry. The second geometry was identical to the baseline in all aspects except that the trenches and feed holes were rotated about the normal to the test plate by 30° to create a compound-angle-trench. The third geometry was a 7o-7o-7o diffusion hole. Pressure Sensitive Paint (PSP) technique was utilized to test these geometries for their film cooling effectiveness. Major conclusions of this study are: a) The streamwise as well as spanwise coverage of trench geometries are superior to those of 7-7-7 diffusion holes, b) The compound-angle-trench geometry produced the highest film effectiveness in both streamwise and spanwise directions and c) the blowing ratio of 1 was found to be the optimum value for the best streamwise and spanwise film effectiveness distribution.