Experimental And Numerical Investigations Of Tip Clearance Flow Control In An Axial Turbine Cascade

Transformation of the energy potential of fuels into useful forms of energy has been an important aspect of civilization ever since the Industrial Revolution. The invention of the gas turbine provided great impetus to the Industrial Revolution. This machine converts chemical energy of natural gas into kinetic energy. In addition to propulsion of large and medium aircrafts, gas turbine engines are also used extensively in electrical power generation and in marine applications. Thus improving the performance of the gas turbine engine has great economic and environmental value.The energy transfer in gas turbine is achieved by a change in the angular momentum of the working fluid in a rotating blade row. The flow field of a turbine blade row contains loss regions due to the creation of secondary flow within the main flow. As these regions convect into the following blade row complex interaction mechanisms occur between the loss regions of both blade rows.In order to reduce losses and improve performance of gas turbine, it is needed to understand in detail the complicated three-dimensional flow structures within the passage. However, due to the restrict of measurement technique, it is difficult to realize, especially the tip clearance flow.Tip clearance flow is created by the fluid that passes through the radial gap between the tips of the rotor blades and the stationary rotor casing. About one third of the losses of a high-pressure stage can be due to the tip clearance flow, which deteriorates the aerodynamic and thermal performance of the axial flow turbine.In this paper, detailed flow field measurements were made downstream of the cascade using a three-hole probe. Static pressure distributions were also measured on the blade surface at 50% and 97.5% span, respectively. Besides that, numerical investigations were also conducted to study phenomena which are not easily measured in the experiments, for example, tip clearance mass flow rate, detailed flow structure inside the tiny tip clearance, distributions of velocity at entrance / exit of tip clearance and so on.First of all, we investigated three of the most important factors that affected tip clearance flow, i. e., tip clearance height, endwall relative motion and incidence angle. The results indicated that losses associated with tip clearance flow rise with increases in tip clearance height, which were showed to be linearly proportional, as well as both the size and the strength of the tip clearance vortex; The relative motion between casing wall and blade rows obstrcuted tip clearance flow in axial turbines, with remarkable reduction in tip clearance mass flow; In addition, incidence angle at the cascade inlet also influenced significantly the flow structures of the tip clearance flow. At negative incidences, tip clearance loss decreases, while it increases with positive incidences.Then a novel method was investigated that aimed to reduce the rotor tip clearance flow and hence the losses associated to the interaction of the tip clearance vortex with the main passage flow. Cooling air was injected through an array of holes on the blade tip surface that were inclined in circumferential direction, such that the injected fluid could counteract the motion of the tip clearance flow. From the measurement results at the cascade exit, it could be found that tip injection could really weaken the interaction of the tip clearance flow and the main passage flow, reducing the tip clearance mass flow and its associated losses.In order to adequately understand the physics of tip injection, we investigated experimentally and numerically the influences of injection mass flow rate, chordwise/width location of injection holes and injection circumferential/streamwise angle on the tip clearance flow. It could be concluded that effects of tip injection increased with a larger injection mass flow rate. Injection location in the blade width direction played an important role in the redistribution of secondary flow within the cascade passage. With the same amout of injection holes and injection mass flow, injection located much closer to the pressure-side corner performed better in reducing tip clearance massflow and its associated losses; Considering the chordwise distribution of injection holes, holes located in the aft part of blade could obtain better performance than that in the front part when the same amount of injection holes was applied; With holes disposed to be perpendicular to the pressure side of blade, injection at a smaller circumferential angle performed better in reducing tip clearance flow.Besides that, we also analyzed the influences of tip injection on tip clearance flow at off-design conditions, to verify the effectiveness of tip injection when the approaching flow was at off-design incidences. The results indicated that even at these off-design incidences, tip injection could also act as an obstruction to the tip clearance flow and weaken the interaction between the passage flow and the tip clearance flow. It could be also found that tip injection caused the tip clearance loss to be less sensitive to the incidences. Thus, tip injection was proved to be an effective method of controlling tip clearance flow, even at off-design conditions.At the end, we predicted the losses associated with the tip clearance flow with several empirical tip clearance loss correlations. All these correlations showed great discrepancy with experiemental data. However, a modified loss model proposed by Hamik & Willinger agreed better, which was based on the loss model of Yaras & Sjolander and takes the effects of the tip injection into consideration, even at off-design conditions.