The measurement of axial turbine tip clearance flow phenomena in a moving wall annular cascade and in a linear cascade.

Abstract

On unshrouded axial flow turbine rotors, the tip clearance, required for thermal expansion and manufacturing limitations, allows fluid to leak from the pressure side to the suction side of the blade. This flow across the blade tip causes a large proportion of the overall rotor loss. In this work, the flow was visualized, microscopic static pressures taken and flow field measurements were done in the blade tip region to investigate the complex nature of tip clearance flows. An annular turbine cascade with a rotating outer casing was used to simulate the relative motion at the tip of an axial rotor. It was found that relative motion did not have a significant effect on the basic structure of the micro-flow, even though it reduced the leakage mass flow rate which is important as far as mixing loss formation is concerned. The existence of a narrow, very low pressure depression, caused by the flow remaining attached around the sharp pressure corner edge, was confirmed. The width and pressure of the separation bubble were found to be strongly dependent on gap size but the relationship was not linear. The point at which the separation bubble reattaches was seen to coincide with a slight rise in static pressure. The separation bubble which caused the majority of the internal gap loss, and which was thought to contribute to the mixing loss, was shown to disappear when the pressure corner was given a radius of 2,5 gap widths.A linear cascade was used to evaluate the performance of two blade tip shapes that substantially reduced internal gap loss and to compare them to a standard sharp or flat tip blade. A method whereby linear cascade data was analyzed as if it were a rotor with work transfer, was used to evaluate the performance of the various blade tip geometries. It was found that both modified tips increased the mixing loss due to the extra leakage mass flow rate. The first tip with the radiused pressure corner was seen to have a lower efficiency than the flat tip blade. A second tip that was contoured to shed flow in a radial direction and thus decrease the leakage mass flow rate through the gap was seen to significantly increase the overall efficiency.