Study Of Performance And Flow Mechanism Of Mix-flow With Twin-entry Volute

Turbocharger is the key enabler of the engine downsizing.Turbine is the core component in a turbocharger that can recovery the energy from the exhaust gas for the boosting of fresh intake air.Thus,it is important to improve the turbine performance to develop the high-performance turbocharger.Twin-entry turbine has two limbs which can utilize the exhaust impulse energy.Therfore,the twin-entry turbine has its prevailing advantages over single-entry turbine of better acceleration performance and better fuel economy at low load.However,during the operation of the internal combustion engine,the intermittent exhaust gas leads to the pulse inlet of the turbine.More importantly,the two limbs of the twin-entry turbine are connected to different cylinder groups,thus the flow condition in the two limbs of twin-entry volute is always out-of-phase,which results in the highly non-uniform and strong unsteady state of the twin-entry turbine.This paper investigates the turbine performance and flow field of a mixed flow turbine with twin-entry nozzled volute at different admissions,including steady and pulsating inflow conditions,via experimental,numerical and theoretical method.The main contents of the paper are showing as follows:Firstly,the experiment of turbine performance and flow filed for turbine with two volute configurations under steady and pulsating inflow conditions is carried out.And the numerical method based on the two volute configurations is applied.The results show that the unsteady hysteresis loop of the swallowing capacity is around the steady performance curve.The maximum discrepancy between the pulse averaged performance parameters and the steady performance parameters is 3.2%.At the meantime,the discrepancy between the prediction and the experimental results is less than 4.1%.Secondly,this paper proposes the variation of turbine performance and loss distribution for the twin-entry turbine under different steady admissions and pulsating inflow conditions.Results show that although the swallowing capacity of the symmetric admissions is quite similar,the turbine efficiency is obviously different.Especially,the efficiency of the case that the flow fed from the hub side is about 2.9~4.2% higher than symmetric case.With the increasing of the pulse frequency,the averaged efficiency is increasing evidently.The results of the loss breakdown show that the proportions of loss in the nozzle and the rotor are reversed under different admissions.The loss is redistributed in the nozzle and the rotor.The nozzle prefers being fed from the hub side and the rotor prefers being fed from the shroud.Compared with the loss distribution under the quasi-steady conditions,the proportion of the loss in the nozzle is evidently decreased under the pulsating inflow conditions.Finally,the paper unveils the loss mechanism of the twin-entry turbine under different admissions.The reason for loss characteristic of nozzle is front-sweep configuration of the nozzle vane which results in evident flow separation when the flow fed from the shroud.The reason for loss characteristic of rotor is the tornado-shaped vortex when the flow fed from the hub.The tornado-shaped vortex is generated by the flow separation from the leading edge attached to the suction surface near hub,developed by the imbalance of Coriolis force,pressure gradient and centrifugal force,and the combination with tip leakage flow near the shroud.Under the pulsating inflow,the entropy generation is decreased by the inertia of the secondary flow caused by the pulsating effect,thus the scale of the flow separation in the nozzle and the rotor is evidently smaller and the loss in the nozzle is lower.This paper provides the guide of the flow control and optimization of the high performance turbines.