Abstract
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.
Highlights
In axial turbines, a finite clearance is necessary to allow the relative motion between rotor tips and the casing wall, which is nominally 1% of the rotor span
This paper investigates the effectiveness of an active tip clearance control method based on tip air injection at design and off-design incidences
The main conclusions based on the numerical simulations are listed as follows
Summary
A finite clearance is necessary to allow the relative motion between rotor tips and the casing wall, which is nominally 1% of the rotor span. Morphis and Bindon [9] found that rounding of the pressure-side corner could improve totaltotal stage efficiency when the tip clearance height was in the 1∼2% rotor span range Another approach showing potential success for controlling tip clearance flow and cooling the blade tip regions should be air injection. Hamik and Willinger [14] connected the blade leading edge and the blade tip surface using an internal channel, a small part of passage flow was injected from the blade tip surface forced by the pressure difference He found that tip injection could weaken flow deviation from the design value near the casing wall, as well as the losses due to the tip clearance vortex. Various empirical correlations are presented and compared with the simulation results, to obtain a better prediction method
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