Flow Mechanism And Control Of Radial Turbine Performance Under Swirling Inflow Conditions

Radial inflow turbines are key components of traditional turbocharged internal combustion engines and future fuel cell engines,which makes improving turbine efficiency important for energy-saving and emission reduction of vehicles.Due to the compactness of vehicle powertrains,the exhaust pipe systems are highly twisted,resulting in strong secondary flows perpendicular to the main flow direction at turbine inlet.Inlet secondary flow would lead to significant turbine efficiency reduction,which is considered to be one of the main obstacles that restrict turbine working efficiency.Improving turbine efficiency under swirling inflow conditions is a vital approach to save energy and cut down the emission of automobiles.Through studying the effect of inlet secondary flows with different directions and strengths on the flow field and performance of a radial turbine,the paper revealed that steady inlet secondary flow would lead to large circumferential velocity dissipation in the volute and strong flow distortion at the volute outlet,resulting in strong volute total pressure loss and rotor secondary flow losses,hence leading to a remarkable reduction of turbine efficiency.When the vortex core of the inlet secondary flow enters turbine volute,it will be bent due to the volute geometry,thus generating self-induced motions and leading to different flow distortion related to the vortex swirling direction.The unsteady swirling inflow effect on radial turbine performance under pulsating inflow conditions was investigated.Results showed that swirling inflow has similar influence mechanisms on turbine performance under pulsating and steady flow conditions.However,the unsteady swirling inflow would give rise to a spatial-temporal coupled flow distortion at the impeller inlet,which has a complex impact on the turbine efficiency.Based on the understanding of the secondary inflow effect,a one-dimensional(1D)design model of radial turbines was established,which could reflect the influence of key geometric parameters of the turbine and the distribution of the volute cross-section area and the centroid radius on the turbine performance under swirling inflow conditions.A novel radial turbine design method that could take the swirling inflow effect into consideration was developed.In the turbine design model,the volute is solved by a 1D flow model,in which a friction loss model is adopted to simulate the total pressure loss in the volute caused by the inlet secondary flows,and the design freedom of the volute is greatly improved.The rotor is solved by a mean-line model,and the effects of inlet secondary flows on rotor inlet flow angles and secondary flow losses are simulated by modeling of volute circumferential velocity dissipation.Based on the 1D design model of turbine,the influence of the volute key geometric parameters and the distribution of volute cross-sectional area-to-centroid-radius ratio(A/R)on the turbine performance under swirling inflow conditions,and a flow control method which involves increasing the volute centroid radius and applying a convex A/R distribution is proposed.Three-dimensional simulation results showed that by increasing the volute centroid radius the inlet secondary flow would attenuate at a higher rate in the volute and the negative influence of circumferential velocity dissipation would be alleviated.The convex distribution of volute A/R can effectively reduce the circumferential flow distortion at rotor inlet.Turbine efficiency could be improved by 4.3% due to the new volute structure under swirling inflow conditions.