Novel Guide Vane Design And Numerical Investigation Based On The Flow Characteristics In Variable Nozzle Turbine

As a common variable geometry turbine,the variable nozzle turbine has been widely studied and applied since it can achieve a better match with engine in the whole operating conditions.Currently,the investigation of the variable nozzle turbine mainly focuses on the effect of the shape of guide vane and rotor blade on turbine performance,the unsteady flow characteristic in guide vane and rotor as well as the reliability of rotor blade.While,little research has been carried out on the performance improvement and the shock weakening at turbine low-end operating condition,and the relevant theoretical model is not perfect.Moreover,the variable nozzle turbine always adopts the traditional guide vane,and there have been no studies on the gudie vane with new configuration in the related literatures.In this paper,steady and unsteady studies were carried out on a variable nozzle turbine applied in a 6.7L diesel engine.The approach to mitigate the intensity of the shock has been studied,and the effect of the new designed guide vane on the performance of the variable nozzle turbine has also been investigated.The main contents and achievements are as follows:(1)By studying the acceleration of the gas in variable nozzle turbine,an effective method that can control the intensity of the shock in the variable nozzle turbine has been proposed,and the corresponding model is established.Results show that the acceleration process of the gas is affected by the area ratio of the inlet and outlet section of the acceleration channel.The first acceleration channel is composed of two adjacent guide vane,when the acceleration channel is a converging-diverging form,there will be a shock generated on the pressure side in the critical condition;while when the acceleration channel is converging form,there will no supersonic flow is generated.The second acceleration channel is composed of the guide vane suction surface and the leading edge of the rotor.The exhaust gas acceleration process is affected by shrinkage of the acceleration channel seriously.If the acceleration channel shrinkage is large,the intensity of shock is also large;with the shrinkage degree reduces,the intensity of the shock will be decreased.(2)By studying the guide vane with different thickness and solidity with steady and unsteady methods,it is found that the nozzle leakage flow will not be changed when just changing the thickness of guide vane.If the thickness decreases and solidity increases,the nozzle leakage flow rate will be increased,while the effect of nozzle leakage flow on the non-uniform of the flow field at rotor leading edge will be mitigated.At small nozzle opening,the guide vane with small thickness and large solidity has nearly no effect on turbine efficiency,compared with the guide vane with large thickness and small solidity;while with the increase of nozzle opening,the guide vane with small thickness and large solidity can decrease the efficiency of turbine by 2%.However,the guide vane with small thickness and large solidity will mitigated the intensity of shock by nearly 75%.Furthermore,the unsteady aerodynamic interference between nozzle and rotor was also analyzed in detail.(3)Aiming at mitigating the nozzle leakage flow and shock at small nozzle opening condition,a forepart rotation guide vane has been proposed in this paper.Steady and unsteady simulations have been carried out on the base model and the forepart rotation guide vane model,the performance of the two guide vanes at three representative nozzle opening were compared,and the unsteady flow characteristic of the two models at 40%nozzle opening was also analyzed.The results show that at 10% nozzle opening,the efficiency of turbine can be improved by 8 percent while the shock was eliminated effectively by enlarging the gap between nozzle and rotor.However,restricted by the geometry of the forepart rotation guide vane,the performance of turbine shows a downward trend when the opening of guide vane is large than 80%.Thus,the use of the forepart rotation guide vane has its limits.(4)A new type guide vane,named split sliding guide vane in this paper,has been proposed and designed to improve the global performance and to mitigate the shock in variable nozzle turbine.The split sliding guide vane breaks the geometrical structure and regulation mode of the traditional guide vane.The simulation results show that the split sliding guide vane can improve turbine performance in global region,and the shock can also be mitigated effectively.At 10% nozzle opening,the efficiency of turbine can be improved by 10 percent.As the increasing of nozzle opening,the effect of nozzle leakage flow is weakened thus the degree of turbine efficiency improvement also become low by using the split sliding guide vane.At 100% nozzle opening condition,the split sliding guide vane can also increase turbine efficiency by 1%,meanwhile,the efficiency of turbine will not be decreased at other u/c values.In addation,the intensity of the shock at the leading edge of rotor blade can also be mitigated by using the split sliding guide vane.