The present thesis features an understanding of secondary flows and their unsteady behaviour in radial and mixed flow turbocharger turbines. Reynolds–Averaged Navier– Stokes (RANS) equations were applied to investigate and predict flow structures under steady and pulsating conditions. To extend the understanding to engineering applications, a one-dimensional (1D) modelling is also proposed, accounting for flow field time scales in rotor.
Single passage RANS model has been applied to investigate the mechanisms of various of secondary flows and their losses in a mixed flow turbine. Apart from tip-leakage vortex, suction-surface separation, pressure-surface separation, induced and self-formed hub separation are studied by comparing evolution of flow topology at passage surfaces. Pressure-surface separation is caused by negative incidence angle. Centrifugal force leads to suction-surface separation, which later induces induced hub separation. One component of the combined force of Coriolis and centrifugal forces has is opposite to the direction of streamline, resulting in the formation of self-formed hub separation. Losses due to these separations are compared by investigating local entropy generation. Results show that the tip-leakage vortex accounts for the majority of losses. Losses due to suction-surface separation, induced hub separation are moderate. There is a competition between tip-leakage and pressure-surface separation. The presence of the self-formed hub separation has a significant detrimental effect on the turbine efficiency. To reduce secondary flows and improve turbine efficiency, a new rotor (Rotor-E) is proposed based on the analysis of Rotor-A and Rotor-B. Compared to original Rotor-A, Rotor-E has ‘constant incidence angle’ leading edge to reduce leading edge separation and increased hub radius (increases by 20%) to reduce self-formed hub separation. Consequently, losses due to leading edge and self-formed hub separations are negligible in Rotor-E. These is an efficiency improvement on the whole speed line and the improvement is around 4% at the design point.
Due to the pulsating nature of piston engines, turbines work at unsteady conditions rather than a designed steady condition. Therefore, the inertia of secondary flows in response to these unsteady conditions need to be understood. In the present work, the flow field evolution in Rotor-A was investigated by applying URANS. Results show that the time scale of the secondary flows is in the same order as the rotation period of the rotor, which is around 1=10 of engine pulses. Consequently, the turbine unsteady performance shows a ‘hysteresis’ characteristic. To describe the time scale and the ‘hysteresis’ characteristic in an engine modelling, a novel 1D modelling (TURBODYNA) for centrifugal turbomachinery is proposed. Different from classic 1D modelling, the rotor has been meshed and its unsteadiness due to the flow field time scale is considered by applying lag equation. By comparing 1D modelling with 3D CFD results, results show that rotor unsteadiness is indispensable and TURBODYNA can capture the feature correctly.