Turbine Blade Tip Heat Transfer in Low Speed and High Speed Flows

Abstract

In this paper, high and low speed tip flows are investigated for a high-pressure turbine blade. Previous experimental data are used to validate a computational fluid dynamics (CFD) code, which is then used to study the tip heat transfer in high and low speed cascades. The results show that at engine representative Mach numbers, the tip flow is predominantly transonic. Thus, compared with the low speed tip flow, the heat transfer is affected by reductions in both the heat-transfer coefficient and the recovery temperature. The high Mach numbers in the tip region (M>1.5) lead to large local variations in recovery temperature. Significant changes in the heat-transfer coefficient are also observed. These are due to changes in the structure of the tip flow at high speed. At high speeds, the pressure side corner separation bubble reattachment occurs through supersonic acceleration, which halves the length of the bubble when the tip-gap exit Mach number is increased from 0.1 to 1.0. In addition, shock/boundary-layer interactions within the tip gap lead to large changes in the tip boundary-layer thickness. These effects give rise to significant differences in the heat-transfer coefficient within the tip region compared with the low speed tip flow. Compared with the low speed tip flow, the high speed tip flow is much less dominated by turbulent dissipation and is thus less sensitive to the choice of turbulence model. These results clearly demonstrate that blade tip heat transfer is a strong function of Mach number, an important implication when considering the use of low speed experimental testing and associated CFD validation in engine blade tip design.

Thermal Performance of Transonic Cooled Tips in a Turbine Cascade

“…This value is higher than those obtained in the low-speed case, which were normally less than 0.7 (e.g., Zhou and Hodson [4]). As observed by Wheeler et al [17], the size of the separation zone near the pressureside edge of an uncooled flat tip reduces as the Mach number increases. The flow chokes near the pressure-side inlet over the separation zone.…”

“…Over the flat tip, relatively high turbulence viscosity appears near the leading edge, and this is related to the region of the high heat transfer coefficient near the leading edge of the blade tip. From midchord to the trailing edge, the turbulence viscosity within the tip gap is low due to flow acceleration, as explained by Wheeler et al [17] and Zhang et al [18]. The coolant injection significantly increases the turbulence viscosity over the flat tip.…”

“…18a, the heat transfer coefficient on the uncooled flat tip from blade midchord to the trailing edge is low. This is because the turbulent viscosity reduces as the flow accelerates over the tip from subsonic to transonic (Wheeler et al [17] and Zhang et al [18]). The reduction of the turbulent viscosity reduces the heat transfer coefficient on the blade tip.…”

“…The distribution of heat transfer coefficient on the uncooled flat tip was found to be different at subsonic and transonic conditions. On the flat tip, Wheeler et al [17] found that, when the Mach number over the blade tip increases, the size of the separation zone near the pressure-side edge of the tip reduces. At a high Mach number, the flow accelerates over the separation zone and becomes supersonic.…”

Squealer Geometry Effects on Aerothermal Performance of Tip-Leakage Flow of Cavity Tips

“…The shock structures create large local variations in the heat transfer of a flat tip. Wheeler et al [23] compared the heat transfer of a flat tip at high speed and low speed. Increasing the Mach number reduced the size of the separation zone near the pressure-side inlet of the tip gap.…”