Flow And Heat Transfer Mechanism And Design Optimization Of Highly-loaded Turbine Blade Shroud Clearance

The shrouded blade has been widely used in highly-loaded turbines,such as high and low pressure cooling turbine of marine engine and low pressure cooling turbine of aircraft engine,due to highly-loaded turbine’s good aerodynamic and vibration characteristics.However,the introduction of the shroud structure makes the tip region of the blade form a unique leakage flow phenomenon,which makes the flow and heat transfer of the tip more complex than that of the unshrouded blade.At the same time,the existence of the blade crown also increases the stress of the blade when it works,and the front and rear edge of the blade crown is difficult to cool,which is prone to warping deformation.Under the erosion of high stress and high temperature gas,the phenomenon of blade crown block often occurs,which poses a direct threat to the safe operation of the engine.Therefore,it is necessary to study the flow and heat transfer mechanism of shrouded blade gap,and carry out the optimization design of blade crown on this basis,so as to reduce the negative effects of blade crown.Firstly,the flow field in the end region of the full crown,partial crown and optimized crown without cooling air at the tip of the blade was studied.Combined with the flow state of the leakage flow and the distribution of the heat transfer coefficient of the crown,the flow and heat transfer characteristics of the gap were discussed.At the same time,the effects of the expansion ratio of upstream and downstream of the crown and the height of the gap on the flow and heat transfer were studied.The results show that the flow and heat transfer on the surface of the shroud are enhanced by the vortex structures such as separation vortex and leakage vortex formed by the leakage flow in the shroud clearance,which leads to the high heat transfer coefficient of the shroud around it;Compared with the partial crown with the same cutting area,the optimized crown improves the heat transfer environment in the upstream area of the labyrinth,and its average heat transfer coefficient is lower;With the increase of the upstream and downstream expansion ratio or the gap height,the flow rate of the leakage flow increases,the strength of the leakage vortex increases,and the heat transfer between the leakage flow and the shroud intensifies,resulting in the increase of the heat transfer coefficient of the shroud.Secondly,the heat transfer of the tip leakage flow of different shroud structures with cold air jet is analyzed.The results show that when there is cold air jet at the tip,the cold air can not only improve the heat load environment of the endwall by cooling the shroud directly,but also reduce the heat transfer between the leakage flow and the endwall by restraining the tip leakage flow,As a result,the heat transfer coefficients of different shroud structures are reduced to some extent compared with those without cooling air;When there is a cold air jet at the top of the blade,the increase of the expansion ratio of upstream and downstream of the shroud cavity or the increase of the gap height will strengthen the leakage flow,resulting in the increase of the heat transfer coefficient of the shroud.On this basis,according to the flow and heat transfer characteristics in the end region of different shroud structures,the optimization design of the shroud was carried out.According to the different cutting depth and cutting area of the shroud surface,the aerodynamic and heat transfer performance of the partial crown and the optimized crown with different cutting schemes were compared and analyzed,The isentropic expansion efficiency of both optimized and partial shrouds decreases,and the partial shrouds decrease faster;With the same cutting area,the optimized crown has less leakage loss than partial crown;Compared with the partial crowns with the same cutting area,the isentropic efficiency increased by 0.45%,the leakage flow decreased by 0.0112 kg/s,and the average heat transfer coefficient decreased by 2%.Finally,In order to study the influence of the change of the blade structure on the flow and heat transfer in the actual cascade,a test piece with a replaceable blade crown was designed,and the numerical calculation analysis of the test scheme was carried out in advance.In this paper,the flow and heat transfer characteristics of the turbine shrouded blade gap and their influence law,and the optimization design scheme of the shroud are obtained,which can provide some reference for the development of the turbine shrouded cooling blade.