This thesis presents an investigation into the base pressure of a turbine blade when coolant is ejected from the trailing edge with a significant span-wise component. An understanding of the base pressure of turbines is extremely important when improving the efficiency of cooled turbine blades. Previous investigations into the loss of a turbine blade when coolant is ejected with a span-wise component have shown that the loss decreases as the span-wise ejection angle is increased. The current study aims to measure the base pressure for similar conditions to determine if the variation of base pressure with span-wise ejection angle contributes to this decrease in loss.
The base pressure experiments were performed in the low speed wind tunnel in the Department of Mechanical Engineering at the University of Queensland. Before the experiments could be made the central blade in the cascade was modified to include internal tapings that could measure the base pressure at three span-wise positions along the trailing edge. The central blade is equipped with an internal cooling system that enables coolant to be ejected through interchangeable trailing-edge slots at span-wise angles of 0°, 15°, 30° and 45°. A final design for the internal tapings was chosen so that the base pressure could be measured faithfully, not interfere with the internal coolant flow, or the interchangeable trailing-edge pieces.
Base pressure experiments were performed for coolant mass flow rates between 0 and 5% of the mainstream flow at span-wise ejection angles of 0°, 15°, 30° and 45°. The base pressure coefficient was observed to increase approximately linearly with increasing coolant flow rates for all span-wise ejection angles. However the rate of increase of base pressure coefficient decreases with increasing span-wise ejection angle and coolant flow rate. Linear approximations of the results were fitted to a previous analytical model to determine loss. With the current experimental results the theory followed previous experimental loss data for all span-wise ejection angles up to a coolant flow rate ratio of 1.7%. At higher coolant flow rates significant differences between theoretical and experimental loss were still observed.
This investigation suggests that the base pressure does vary with span-wise coolant ejection but not to a degree that explains the differences in loss coefficient shown by previous investigations when coolant is ejected with a significant span-wise component.