Overtip Shock Wave Structure and Its Impact on Turbine Blade Tip Heat Transfer

Abstract

In this paper, the transonic flow pattern and its influence on heat transfer on a high-pressure turbine blade tip are investigated using experimental and computational methods. Spatially resolved heat transfer data are obtained at conditions representative of a single-stage high-pressure turbine blade (Mexit=1.0, Reexit=1.27×106, gap=1.5% chord) using the transient infrared thermography technique within the Oxford high speed linear cascade research facility. Computational fluid dynamics (CFD) predictions are conducted using the Rolls-Royce HYDRA/PADRAM suite. The CFD solver is able to capture most of the spatial heat flux variations and gives prediction results, which compare well with the experimental data. The results show that the majority of the blade tip experiences a supersonic flow with peak Mach number reaching 1.8. Unlike other low-speed data in the open literature, the turbine blade tip heat transfer is greatly influenced by the shock wave structure inside the tip gap. Oblique shock waves are initiated near the pressure-side edge of the tip, prior to being reflected multiple times between the casing and the tip. Supersonic flow within the tip gap is generally terminated by a normal shock near the exit of the gap. Both measured and calculated heat transfer spatial distributions illustrate very clear stripes as the signature of the multiple shock structure. Overall, the supersonic part of tip experiences noticeably lower heat transfer than that near the leading-edge where the flow inside the tip gap remains subsonic.

Overtip Choking and Its Implications on Turbine Blade-Tip Aerodynamic Performance

“…Most recently, the over tip choking and shock structure were reported for a shroudless flat tip (Zhang et al [26]), and even a modern winglet tip design (O'Dowd et al [27]). Similar results have also been reported by Shyam et al [28,29] in their numerical study for a highly The present computational analysis using an extensively developed CFD code which has been recently validated for transonic blade tip configurations [25], [26], [31] should shed some light on this primary issue. Furthermore, one may ask, are there any advantageous implications if a leakage flow is made choked?…”

“…Zhang et al [25] reported the first-of-the-kind detailed experimental evidences of the heat transfer stripe distributions caused by shock wave structures over the tip. Backed up by the experimental results, their virtual Schlieren visualizations in a CFD analysis clearly demonstrated that the stripe variations of heat flux correspond to those flow features associated with the over-tip shock system.…”

Aerothermal Performance of Shroudless Turbine Blade Tips with Relative Casing Movement Effects

“…Wheeler et al [25] showed tip flow to be largely transonic and significantly different to low-speed cascade tests; showing almost the opposite in spatial trends. Experimentally, Zhang et al [26] illustrated that the local heat transfer was directly affected by shock waves in a striped pattern within the tip gap. Zhang and He [27] also explored the differences of low and high-speed flows noting the difference in flow structures and tip leakage loss.…”

Overtip Choking and Its Implications on Turbine Blade-Tip Aerodynamic Performance

“…Again with a cautionary note on the absolute loss values predicted, the attention should be paid to the qualitatively consistent At this point it should be commented that there might be some other beneficial aspects of a high-loading blading design. On the heat transfer side, the heat transfer coefficient was much lower within the transonic tip region than the subsonic region due to the low turbulence diffusion, as reported by Zhang et al [25,26]. A fully choked rotor tip would then have less cooling requirement due to the heat load reduction.…”