Numerical simulations, verified against experimental results, were used to study the stalling process of a transonic compressor. The method of topological analysis was used to study the distribution of the flow field on the blade surface in detail. In this manner, the steady three-dimensional vortex structure of the transonic compressor stator under transition conditions was established, and the mechanism of the stalling process was revealed from the perspective of the vortex structure evolution. The results show that in the process of stalling, as the back pressure increases, the separation areas on the suction side of the stator and the scale and strength of the separation vortex also increase, thereby weakening other vortices and causing a large amount of low-energy fluid to accumulate in the passage. Thus, the passage is blocked, and the through-flow capability of the compressor is reduced. In addition, because of the complex movement of the vortices and the transport of low-energy fluid clusters, the boosting capacity of the compressor is reduced, thus causing the compressor to stall.
This paper numerically investigated the impact of the holes and their location on the flow and tip internal heat transfer in a U-bend channel (aspect ratio = 1:2), which is applicable to the cooling passage with dirt purge holes in the mid-chord region of a typical gas turbine blade. Six different tip ejection configurations are calculated at Reynolds numbers from 25,000 to 200,000. The detailed three-dimensional flow and heat transfer over the tip wall are presented, and the overall thermal performances are evaluated. The topological methodology, which is first applied to the flow analysis in an internal cooling passage of the blade, is used to explore the mechanisms of heat transfer enhancement on the tip wall. This study concludes that the production of the counter-rotating vortex pair in the bend region provides a strong shear force and then increases the local heat transfer. The side-mounted single hole and center-mounted double holes can further enhance tip heat transfer, which is attributed to the enhanced shear effect and disturbed low-energy fluid. The overall thermal performance of the optimum hole location is a factor of 1.13 higher than that of the smooth tip. However, if double holes are placed on the upstream of a tip wall, the tip surface cannot be well protected. The results of this study are useful for understanding the mechanism of heat transfer enhancement in a realistic gas turbine blade and for efficient designing of blade tips for engine service.
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