A comparison study of loss generation between twin-entry and double-entry volutes of nozzled mixed-flow turbines

Turbochargers whose turbines contain two-entry volutes are advantageous as they help to improve the low-end torque and transient performance of internal combustion engines. Two types of two-entry turbines are applied currently in turbocharging, which are the twin-entry turbine (TE) and the double-entry turbine (DE). To develop more efficient turbocharged engines, a further understanding of the performance characteristics and loss mechanisms of two types of two-entry turbines is needed. However, few investigations have been carried out to directly compare these two types of turbines. Through an experimentally validated numerical method, this paper compares the performance and detailed flow mechanism of twin/double-entry turbines with the same nozzled mixed-flow rotor. The 3D simulation is validated against the measured performance as well as the flow field carried out on test rigs. Results show that the performance of the double-entry turbine is notably superior over that of the twin-entry counterpart although their peak swallowing capacities and efficiencies are at a similar level. In particular, the efficiency and swallowing capacity deteriorate more gently for the double-entry turbine as operational condition deviates from full admission. It was revealed via the loss breakdown of turbine components that, the volute and the nozzle are the main contributors to the performance discrepancy between the two turbines. Specifically, the loss in the twin-entry volute is higher than that of the double-entry one, especially in the case of partial admissions. Flow analysis shows that the flow is mixed strongly in the vicinity of the limb divider of the twin-entry volute in the whole circumference, while it only concentrates in the vicinity of the tongues for the double-entry turbine. Therefore, a higher entropy-rise is generated in the twin-entry volute. Moreover, the mixing flow attenuates the flow distortion in the nozzle and produces more uniform entropy rise in the nozzle for the twin-entry configuration. On the other hand, flow distortion is well preserved in nozzle for the double-entry turbine, thus regions with high entropy-rise appear in a portion of nozzle passages. Consequently, a higher loss is produced in the volute and the nozzle for the twin-entry turbine. This paper provides knowledge of performance characteristics of two two-entry turbines and sheds light on the performance improvement of two-entry turbines.