This paper presents a numerical investigation of an active tip-clearance control method based on cooling injection from the blade tip surface. It aims to study the influences of air injection on controlling tip clearance flow, with emphasis on the effects of the injection location on secondary flow and the potential thermal benefits from the cooling jet. The results show that injection location plays an important role in the redistribution of secondary flow within the cascade passage. Injection located much closer to the pressure-side corner performs better in reducing tip clearance massflow and its associated losses. However, it also intensifies tip passage vortex, due to less restraint deriving from the reduced tip clearance vortex. Lower plenum total pressure is required to inject equivalent amount of cooling air, but the heat transfer condition on the blade tip surface is a bit worse than that with injection from the reattachment region. Thus the optimum location of air injection should be at the tip separation vortex region.
A numerical investigation has been performed to study the influences of cooling injection from the blade tip surface on controlling tip clearance flow in an unshrouded, high-turning axial turbine cascade. Emphasis is put on the analysis of the effectiveness of tip injection when the approaching flow is at design and off-design incidences. A total of three incidence angles are investigated, 7.4°, 0°, 0°, 0°, and 7.6°, 0° relative to the design value. The results indicate that even at the off-design incidences, tip injection can also act as an obstruction to the tip clearance flow and weaken the interaction between the passage flow and the tip clearance flow. It is also found that tip injection causes the tip clearance loss to be less sensitive to the incidences. Moreover, with injection, at all these incidences the heat transfer conditions are improved significantly on the blade tip surface in the middle and aft parts of blade. Thus, tip injection is proved to be an effective method of controlling tip clearance flow, even at off-design conditions. Beside that, an indirect empirical correlation is observed to be able to perform well in predicting the losses induced by tip clearance flow at design and off-design conditions, no matter whether air injection is active or not.
This article presents a parametric investigation of an innovative tip-clearance control method. Cooling air is injected into the tip clearance to obstruct tip clearance flow, from the blade tip surface through a row of 11 equidistant holes. In order to demonstrate the influences of injection circumferential angle, four different test cases are performed and compared with the baseline case without air injection. The results indicate that injection circumferential angle plays an important role in the redistribution of secondary flow within the cascade passage. Injection at a smaller circumferential angle performs better in reducing tip clearance mass flow and its associated losses. However, it also intensifies the tip passage vortex much more, because of the reduced restraint deriving from the weaker tip clearance vortex. It can also be found that flow under-turning caused by the tip clearance vortex can be reduced greatly with a smaller circumferential angle. But the heat transfer condition is worse than that when cooling air is injected perpendicularly to the blade tip surface. Besides that, a modified tip clearance loss model is observed to be able to perform well in predicting the losses induced by tip clearance flow at all the conditions considered in this article.
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