The Microflows Within the Tip Clearance Gap of an Unshrouded Axial Turbine and Their Effects on Loss Development

The flows occurring in the tip clearance region of an axial turbine is studied in a linear cascade with flow visualisation, surface pressures and with flow probes. The internal gap flow is dominated by a "separation" bubble within which intense static pressure driven shear flows takes place giving rise to 38% of the overall loss. This microflow pattern was shown to be uninfluenced by blade relative motion but could be eliminated by contouring and radiusing the blade tip to also improve rotor performance determined by a method of converting stationary cascade data into a simulated rotor. The development of loss on the endwall is almost unaffected by clearance up to mid chord. Thereafter loss growth is very rapid which could be due to the influence of suction corner diffusion on the high loss tip clearance vortex.