Investigations On Tip Leakage Flow Control And Heat Transfer Characteristics Of Turbine

The existence of tip clearance in a high pressure turbine blade leads to aerodynamic performance deterioration.The tip leakage flow with high temperature heats the blade tip region and causes local overheating of blade tip.It causes tip oxidation and ablation of blade tip,resulting in performance degradation or even failure of turbine blade.Therefore,the high pressure turbine blade tip design not only requires advance tip configurations to control the tip leakage flow to improve the aerodynamic performance,but also needs efficient cooling to meet the cooling design.The tip leakage flow control and blade tip external cooling are studied in this dissertation.Conventional squealer tip has the advantages of simple structure and efficient control of tip leakage flow,and it is widely used in the unshrouded turbine blade.In order to further reduce the aerodynamic loss of the passage and reduce the slope of the efficiency to the tip clearance,this research tries to improve the tip configurations.There are differences of tip leakage flows among various tip types,which directly affect the heat transfer in the tip regions.The tip cooling based on the heat transfer of blade tip.Therefore,based on detailed blade tip heat transfer analysis,the tip cooling study is carried out to reduce temperature and heat load of blade tip and casing in this research.Firstly,the parameterization tip model is constructed.The aerodynamic optimisation of the linear turbine cascade with cavity tip is conducted by numerical method.The optimized winglet-cavity tip reduces the tip leakage mass flow rate,and weakens the tip leakage vortex and the upper passage vortex.These result in a lower cascade outlet losses and a smaller slope of aerodynamic losses to tip clearance.The pressure-side winglet tip reduces the loss of tip leakage vortex,but increases the loss of upper passage vortex.The suction-side winglet tip effectively weakens the upper passage vortex and tip leakage vortex,and pushes the tip leakage vortex away from the blade suction side.The flow field visualizations and the loss distribution of the linear cascade experiment verify the effectiveness of the winglet-cavity tip to control the leakage flow and improve the aerodynamic performance of the cascade.The study of the winglet tip key parameters,such as the winglet width and blending angle,shows that increasing the pressure-side blending angle increases the contraction coefficient above the pressure-side rim,which is beneficial to reduce the relative tip leakage mass flow rate.Increasing the suction-side winglet width reduces the total loss of the cascade outlet.The aerodynamic optimization of the blade tip is carried out in a low aspect ratio gas turbine.The results show that the winglet-cavity tip effectively improve the turbine stage efficiency and reduce the slope of turbine stage efficiency to the tip clearance.Different from the mechanism of winglet tip in the low-speed linear cascade,the optimized wingletcavity tip increases the tip leakage loss of the turbine blade.The winglet-cavity tip increases the gap internal loss.Besides,the suction-side winglet tip extension to the tip throat increases the driving pressure differences of tip leakage flow which increases the mixing velocity and loss between the tip leakage flow and mainstream.These result in a high tip leakage loss with optimized winglet-cavity tip.The research on the single side winglet tips shows that the pressure-side winglet effectively reduces the tip leakage flow,while increases the upper passage vortex loss;the suction-side winglet reduces the upper channel vortex loss,but causes the leakage vortex loss to increase.Increasing the suctionside winglet width can increase turbine stage efficiency.Three-dimensional unsteady analysis shows that the winglet tip obtains higher time-averaged efficiency,lower tip leakage flow and smaller time-variation of the total performance parameters.The turbine aerodynamic efficiency can be further improved by adopting tip combined with pressureside winglet and suction-side winglet.The aerodynamic parameters and tip configurations of the cascade directly affect the heat transfer of blade tip.Increasing the Re number enhances the heat transfer in the tip region,but has weaker influence on the tip leakage flow.Increasing the Ma number of cascade outlet strengthenes the tip leakage flow and changes the heat transfer in different regions of the flat tip.The heat transfer in the gap subsonic region enhances with increasing Ma number,and the gap supersonic region has the opposite trend.At the same time,the heat transfer characteristics of squealer tip are discussed in detail.The decomposition analysis of the blade tip shows that the difference in heat transfer between the various parts of the tip is obvious.The tip leakage flow intensity,the separation and reattachment of the leakage flow in the gap,the tip leakage vortex strength in the passage and the relative position to the blade surface can significantly affect the blade tip and casing heat load.The suction-side winglet tip makes the leakage vortex away from the blade suction-side surface,which is beneficial to reduce the blade heat load,and attenuates the tip heat load increase caused by the increase of the tip heat transfer area while realizing the reduction of the cascade outlet loss.Based on the tip heat transfer characteristics,the tip and casing cooling are performed.Blade tip coolant injection reduces blade tip temperature and heat load;pressure surface near tip coolant injeciton reduces the temperature of pressure surface near tip region,pressure-side rim and tip trailing edge;casing slot coolant injection achieves temperature and heat load reduction of casing,tip cavity surface and suction surface near tip region.The cooling of blade tip and casing cannot be achieved by using blade tip,pressure near blade tip and casing cooling separately.And the cooling of whole blade tip and casing can be realized by utilizing these three cooling methods comprehensively.Reasonable tip cooling holes arrangement,casing cooling slots design and cooling parameters setting are helpful to improve the film-cooling effectiveness of the blade tip and casing and reduce net heat load.