Thermal imaging as flow visualization for gas-turbine film cooling

Abstract

Flow-visualization experiments related to turbine film cooling have been conducted. These have investigated the fluid mechanics of coolant ejection from the leading edge of a turbine blade using a large-scale, cylindrical model at engine-representative Reynolds numbers in a low-speed tunnel with ambient-temperature mainstream flow. The coolant trajectories were captured using a fine nylon mesh covered with thermochromic liquid crystal (TLC), allowing the measurement of gas temperature (hence non-dimensional effectiveness) contours in planes perpendicular to the flow. A study was undertaken to determine an appropriate mesh to support the TLC experiments: the pressure loss coefficient and TLC colour-intensity response was measured for five different meshes, and from this two were chosen for further investigation; one with the smallest loss coefficient and the other with the best intensity response. Key features of the coolant jet (i.e. the shape of the contours, the approximate size, and the jet's inclination) were identified using this flow-visualization method. The coolant films featured a core of high gas effectiveness with a strong gradient in temperature at the edges of the jet. The coolant footprints were observed to lift off the surface at high momentum-flux ratios. Cross-stream injection featured jets with an asymmetric structure due to the lateral component of momentum of the coolant; these jets exhibited a kidney-shaped cross-section created by a vortex structure in the flow. The flow visualization experiments also assessed the interaction of coolant plumes from different rows of holes, clearly illustrating that upstream holes affected the size, shape, and position of downstream films. The shape and trajectory of the coolant films were also shown to depend upon the local acceleration of the mainstream and corresponding boundary-layer thickness.