Study On Tip Leakage Flow Characteristic And Loss Mechanism Of Steam Turbine

Steam turbine is one of the main equipment of electric power production,the efficiency of steam turbine directly affects the economy of the power plant.Therefore,it is of great significance to actively improve the efficiency level of the steam turbine to speed up the development of national economy and energy conservation.In the design of steam turbine,there is a tip between the rotor blade tip and the stationary cylinder in order to avoid friction,due to the pressure difference between rotor inlet and outlet,the leakage flow is formed.The velocity of leakage flow in high parameter and large capacity units is larger,the leakage flow is more complex.Based on the computational fluid dynamics software CFX,A numerical calculation was conducted based on a high-pressure 1.5-stage with labyrinth seal in steam turbine.The main work is as follows:First,the variation of leakage vortex system in shroud exit cavity and the impact of mixing between main flow and leakage flow on the down stream stator with different tip clearances were analyzed.Besides,the related losses generated by leakage flow were calculated.Results show that the mixing loss is associated with the generation and dissipation of the backflow vortex.The mixing leads to steam flow deflection at the rotor exit and causes the incidence loss.In subsequent stator,the leakage flow near the suction side with high radical velocity causes the deflection of outflow.The main influence region of leakage flow is above 75% blade height,while the most intense region is about 95% blade height.Along with the increase of tip clearance,the related losses generated by leakage flow also increase.The main part is the mixing loss,and the mass-averaged entropy mixing loss coefficient is from 7% to 11%.Besides,the high loss zones in seal cavity and downstream stator flow field with different tip clearances were discovered.The generation mechanism of leakage losses was explored.In addition,the related viscous losses were distinguished and quantified.The results show that the viscous dissipation of leakage flow in seal cavity lies in the throttle regions under seal teeth and the impacting sites with seal teeth.In downstream stator,the direct viscous dissipation generating mechanism is not the leakage vortex,but the difference in velocity between leakage flow and main flow.As the tip clearance increases,the leakage flow has a higher turbulent kinetic energy,which mixes with main flow and leads to greater viscous dissipation.The viscous dissipation generated in upstream stator is low.However,there is a rapid increase in viscous dissipation near the trailing edge of rotor blade and upstream stator.The mixing loss in downstream stator is of about 1/3 accounts for the total viscous losses in the stage and it increases with tip clearance increasing.Furthermore,the related losses caused by the leakage flow were quantitatively calculated to break the limitation of the empirical formula and calculation formulas suitable for all types of impulse turbines are presented.The study of the distribution of high loss zone in the high-pressure 1.5-stage caused by leakage flow is of great significance to the design of high efficiency steam turbine units.