Shock, leakage flow and wake interactions in a radial turbine with variable guide vanes

The clearances of the nozzle and volute have significant influences on the unsteady flow, blade load distribution and thermodynamic performance of downstream radial turbine. The unsteady flow and blade loads are intensified due to the interactions among shock, leakage flow and nozzle wake. Few studies have been conducted to date to investigate these effects in detail. This work focuses on the tip leakage flow effects on the nozzle wake and shock. Furthermore, the blade loads of the downstream rotor are considered by evaluating the effects of shock, leakage flow and nozzle wake, and the fluctuating pressures are presented in a novel space-time diagram. To reveal the flow mechanism in a variable radial turbine, two nozzle clearance sizes were chosen for investigation via numerical simulation of the unsteady flow and the interactions among the shock, leakage flow and wake, and the turbine performance was verified with test data. The results show that the interactions between leakage flow and nozzle wake are intensified with increasing nozzle clearances, and the nozzle wake deformations also become more severe. Additionally, the gas mixing speed between the nozzle wake and leakage flow is enhanced, thus inhibiting the gas from expanding in the nozzle channel and weakening the shock at the nozzle outlet. When the nozzle clearance leakage enters the downstream rotor channel, it interacts with the rotor tip clearance leakage in the blade tip region. These interactions have significant impacts on the formation of leakage vortexes and the distribution of the inlet flow angles. With the increase in tip clearances at both sides, the load fluctuation increases at the root and the tip of rotor leading edge, whereas the fluctuation decreases due to the weakening of the upstream shock in the middle span region of the blades.