Comparative investigation of tip leakage flow and its effect on stage performance in shrouded and unshrouded turbines

Abstract

In order to provide reference for the choice of the turbine architecture in specific application, comparative investigation was conducted to investigate the shrouded and unshrouded rotor tip leakage flows by means of the commercial CFX software, which modeled the flow in a 1.5-stage turbine. Tip leakage loss production mechanisms as well as the effects of tip clearance and pitch–chord ratio on tip leakage and stage performances were discussed and compared in this article. Numerical results show that, the leakage flow at the shroud cavity inlet effectively removes the boundary layer at rotor leading edge, and then reduces the casing secondary flows inside rotor passage. There is a break-even tip clearance at which both the shrouded and unshrouded turbines have the same efficiency, and if tip clearance is less than it, the unshrouded turbine performs better than the shrouded turbine. Compared to the unshrouded turbine, the shrouded turbine can reduce the sensitivity of turbine performances to changes in tip clearance. Moreover, at the given relative change in leakage flow, the relative change in work is approximately twice the relative change in efficiency. Besides, pitch–chord ratio has the different impact on the tip leakage mixing processes in shrouded and unshrouded turbines. It is effective to achieve lower tip leakage losses by turning the leakage flows to reduce mixing losses in shrouded turbines, and by blade tip design or tip clearance treatment to reduce the tip leakage flow in unshrouded turbines.