The Determination of In-Situ Film Hole Flow Rates Using a Transient Thermal Inertia Method

Abstract

Film hole flow rates are conventionally characterized by a discharge coefficient relating the actual mass flow rate to the theoretical ideal flow rate based upon some measured effective hole diameter or flow area. These discharge coefficients are typically measured on controlled test plates that contain the particular size, shape, and fabrication method for an individual film hole type. Such discharge coefficients are then assumed to apply to all of the in-situ film holes of that type which are machined or formed in the as-fabricated cooled turbine component. The thermal-mechanical analysis of the component is then performed using these assumed values to calculate film hole flow rates. In practice however, every film hole in a cooled airfoil is different due to machine tool wear, surface curvatures, laser drift, coating variations, and local flow supply behavior. A new method has been developed and demonstrated which allows determination of the individual film hole flow rates in-situ for an as-fabricated component, thus avoiding the need for assumed discharge coefficients or highly detailed flow checks. This method uses the thermal transient characteristics of external surface points near an active film hole to determine the flow rate through the hole. An imaging Infrared system is used to record the component response to an induced thermal cooling transient in which the film hole internal heat transfer dominates the local thermal transient behavior. The characteristic of the non-dimensionalized thermal decay is related to the flow rate within each individual film hole using a limited calibration function. This method allows the rapid inspection and quantification of detailed film hole flows for actual parts, which data may then be used in the analysis and health monitoring of parts in operation.