Effect of axially non-uniform rotor tip clearance on aerodynamic performance of an unshrouded axial turbine

Abstract

Tip leakage flow is responsible for a significant amount of aerodynamic losses in a turbine stage, and the mixing of leakage flow with the rotor passage secondary flow causes losses and reduces turbine stage efficiency. This article presents a numerical investigation of the effect of axially non-uniform tip clearance on the aerodynamic performance of an unshrouded axial turbine at design and off-design conditions, in an attempt to seek an optimal tip clearance chordwise distribution to control the interaction between tip leakage and passage secondary flows, and then reduce the total losses in turbines. Different types of axially uniform and non-uniform rotor tip clearances were used in this investigation, which include uniform, expanding, shrinking, and back- and front-step tip clearances. Numerical results show that, for the axially uniform tip clearances, the interaction mechanism between tip leakage and passage secondary flows is different in the relatively small and large tip clearance heights. With the tip clearance height gradually increasing, tip passage vortex at the passage exit is first enhanced, and then it becomes weak. Axially non-uniform tip clearance changes the chordwise distribution of over tip leakage mass flow. The front-step and shrink tip clearances are able to efficiently utilize the interaction of tip leakage flow with passage secondary flows, and then reduce the total losses in turbines. For different axially non-uniform tip clearances, loading distribution near the tip has a great impact on the tip leakage flow. It will result in great losses when the maximum tip clearance and loading positions are in the same position along the blade.