Aerodynamic Design On Multi-stage Radial Turbine And The Influence Of Gap Leakage

With the development of technology,the smaller stand-alone quality and higherperformance radial turbine has become the goal pursued by designers at home and abroad.If we want to design a high-performance radial turbine,internal flow field must be further studied owing to the complexity of internal flow sturcture of the radial turbine.At the same time,in order to reduce the stand-alone quality,reducing the number of turbine stages and increasing the single-stage expansion ratio of radial turbine has become one of the key issues for designers.In this paper,the number of stages of a four-stage radial turbine is reduced by one to shorten the axial dimensions and diminish the stand-alone quality.The aerodynamic design of a three-stage radial-inflow turbine has been finished without diminishing its power and efficiency.After reduce the number of stages,the conventional stators cannot meet the design requirements thanks to the single-stage expansion ratio of the radial turbine increased.Therefore,convergent-divergent nozzle is used to obtain supersonic airflow at first stage.On the basis of one-dimensional flow calculation,complete the three-dimensional geometry modeling of the impeller,and the internal flow field of the radial turbine is numerically simulated by the calculation software CFX.Based on the one-dimensional expected result,the three-dimensional aerodynamic performance of radial turbine was evaluated.The results showed that the Mach number at the outlet of the convergent-divergent nozzle was close to the one-dimensional expected value.The designed shaft power of this radial turbine reach 589 k W and the relative internal efficiency of more than 63%.It basically meet the design requirements and verify the rationality of the design method.The influence of the leakage flow generated by different tip clearances on the radial turbine performance is analyzed by method of numerical simulation.The simulation results show that leakage flow leaking from pressure side to the suction side induce a leakage vortex on the suction side of the blade,which causes insufficient deflection of the flow within the top 10% of the blade spanwise.With the increase of the top gap height,larger leakage flow will be generated,which leads to a reduction of blade loading especially above the middle spanwise.Higher tip clearance causes larger total pressure loss coefficient in the range above 50% spanwise,and bigger flow loss can be generated due to the mixing of leakage flow and mainstream.In addition,the study finds that radial clearance has a greater effect on performance of radial turbine than axial clearance.Therefore,the top gap with variable height distribution is set,which can help to reduce leakage loss.Numerical simulation is carried out on internal flow field of the toothed steam seal to analyse vortex structure in the chamber and study the law of leakage flow rate and friction loss with the rotational speed.The results show that flow in vapor seal is composed of several throttling processes with stepwise decline of pressure and incease of speed.Complex vortices generated in each of chambers have a dissipative effect on the flow and help prevent leakage.When the rotational speed changed from 5000r/min to 15000t/min,the leakage flow increased by 80kg/h.Overall,the changes of leakage flow is smaller.The friction loss tends to increase almost linearly with increasing speed.