An Investigation into Turbine Blade Tip Leakage Flows at High Speeds

Authors: Z. Saleh, E. J. Avital, T. Korakianitis

Abstract:

The effect of the blade tip geometry of a high pressure gas turbine is studied experimentally and computationally for high speed leakage flows. For this purpose two simplified models are constructed, one models a flat tip of the blade and the second models a cavity tip of the blade. Experimental results are obtained from a transonic wind tunnel to show the static pressure distribution along the tip wall and provide flow visualization. RANS computations were carried to provide further insight into the mean flow behavior and to calculate the discharge coefficient which is a measure of the flow leaking over the tip. It is shown that in both geometries of tip the flow separates over the tip to form a separation bubble. The bubble is higher for the cavity tip while a complete shock wave system of oblique waves ending with a normal wave can be seen for the flat tip. The discharge coefficient for the flat tip shows less dependence on the pressure ratio over the blade tip than the cavity tip. However, the discharge coefficient for the cavity tip is lower than that of the flat tip, showing a better ability to reduce the leakage flow and thus increase the turbine efficiency.

Keywords: Gas turbine, blade tip leakage flow, transonic flow.

Digital Object Identifier (DOI): doi.org/10.5281/zenodo.1071962

References:

[1] Bunker RS, (2004), "Blade tip heat transfer and cooling techniques", VKI Lecture Series 2004-02.
[2] Bryant RAA (1960) "The Hydraulic Analogy for External Flow", Journal of the Aerospace Sciences, Vol. 27, No. 2 , pp. 148-149.
[3] Moore J, Elward KM, (1993) "Shock formation in over expanded tip leakage flow", ASME Journal of turbomachinary, vol.115, pp 392-399
[4] Chen G, Dawes WN, Hodson HP, (1993) "A numerical and experimental investigation of turbine tip gap flow" 29th Joint Propulsion Conference and Exhibit, AIAA 93-2253.
[5] Wheeler, A.P.S., Atkins N., and He, L. 2009 "Turbine blade tip heat transfer in low speed and high speed flows", GT 2009-59404, presented at the ASME Turbo Expo 2009, Orlando, Florida.
[6] Korakianitis T, Hamakhan I, Rezaienia MA, Wheeler APS, Avital EJ and Williams JJR (2012), Design of high-efficiency turbomachinery blades for energy conversion devices with the 3D prescribed surface curvature distribution blade design (CIRCLE) method. Applied Energy, Vol. 89 No. 1, pp. 215-227.
[7] Doulgeris G, Korakianitis T, Avital EJ, Pilidis P. and Laskaridis P., Effect of jet noise reduction on gas turbine engine efficiency Proc IMechE Part G: J Aerospace Engineering, 0954410012456925, first published on September 3, 2012.