Effect of Partial Coating Blockage on Film Cooling Effectiveness

Abstract

A simple experimental study has been undertaken to determine the effects of typical turbine airfoil protective coatings on film cooling effectiveness due to the partial blockage of film hole exits by the coatings. Two types of film holes have been tested with and without coatings, a single row of axial round holes and a single row of axial shaped diffuser holes, each at an angle of 30-degrees to the surface. Both film rows have been manufactured with electro-discharge machining in flat plates, and have nominal throat diameters of 1.778 mm and a pitch-to-diameter spacing of 3.5. A series of tests was first performed without the use of protective surface coatings to provide the baseline film effectiveness performance. Each plate was then coated first with a layer of metallic bondcoat and then a layer of air-plasma sprayed thermal barrier coating (TBC), and the film tests repeated for the same coolant flow rates. Downstream thermocouple measurements are used to show the adiabatic film effectiveness along the midspan centerline location. Micrographs of the coated hole cross-sections depict the extent and form of coating blockage. Results show that significant degradation to film performance can result from coatings which are deposited in the hole exit regions, or inside the holes themselves, during the spray application process. Results also show that shaped film holes are generally very tolerant of coatings. The results of this study demonstrate the need for a film cooling and heat transfer database which accounts for the effects of film hole blockage as experienced in actual turbine operation.