Experimental Study of the Heat Transfer on a Squealer Tip Transonic Turbine Blade with Purge Flow

The objective of this work is to examine the flow structure and heat transfer distribution of a squealer tip rotor blade with purge flow cooling and provide a comparison with a basic flat tip rotor blade without purge flow cooling, under transonic conditions and high inlet free stream turbulence intensity. The blade design was provided by Solar Turbines Inc., and consists of a double squealer around the pressure and suction sides, two purge flow blowing holes located downstream of the leading edge and mid-chord, four ribs in the mid-chord region and a trailing edge bleeder exiting on the pressure side. Blade cavity depth is 2.29 mm (0.09 in.) and the total blade turning angle is 107.5°. Tests were performed in a blow-down facility at a turbulence intensity of 12%, in a seven bladed 2-D linear cascade at transonic conditions. Experiments were conducted at isentropic exit Mach numbers of 0.85 and 1.05, corresponding to Reynolds numbers based on axial chord of 9.75x10^5 and 1.15x10^6, respectively, and tip clearance gaps of 1% and 2% of the scaled engine blade span. A blowing ratio of 1.0 was used in the squealer tip experiments. Detailed heat transfer coefficient and film cooling effectiveness distributions were obtained using an infrared thermography technique, while oil flow visualization was used to investigate the flow patterns in the blade tip region.

With the addition of a squealer tip, leakage flow was found to decrease, as compared to a flat tip blade. With increasing tip clearance gap, the heat transfer coefficients within the cavity and along the squealer rim were found to decrease and increase, respectively. Film cooling effectiveness decreased with increasing tip clearance gap and was mainly observed within the squealer cavity. The maximum heat transfer coefficient was observed on the leading edge, however, comparatively large values were observed on the mid-chord ribs. The presence of the ribs, greatly affected the flow structure and heat transfer distributions within the cavity and downstream towards the trailing edge.