Influence of Tip Clearance Size on Flow Characteristics of a Vaneless Counter-Rotating Turbine

Abstract

In this paper, three-dimensional multiblade row Navier-Stokes simulations have been performed to investigate the effects of tip clearance size on the flow characteristics of a vaneless counter-rotating turbine. The tip clearance sizes studied in this paper are 0.0mm (without rotor tip clearance), 0.5mm (0.65% high-pressure turbine rotor height, 0.52% low-pressure turbine rotor height), 1.0mm, 1.5mm, 2.0mm, 2.5mm and 3.0mm, individually. The numerical results indicate that the air mass flow rate of the vaneless counter-rotating turbine and the ratio of specific work of the high-pressure turbine to that of the low-pressure turbine increase when the rotor tip clearance size is increased. While the isentropic efficiency, the expansion ratio and the power of the vaneless counterrotating turbine decrease as the tip clearance size is increased. The results also show that the existence of the tip clearance trends to decrease the Mach number at the high-pressure turbine outlet, and increase the Mach number at the low-pressure rotor outlet, especially at near the blade tip region, the extents of the decrease and increase of the Mach number are more remarkable. When the tip clearance size is increased, the Mach number decreases at the outlet of the high-pressure turbine, while increases at the outlet of the lowpressure rotor. The numerical results also illustrate that the existence of the tip clearance trends to decrease the flow angle at the high-pressure turbine outlet, especially at near the blade tip region, the extent of the decrease of the flow angle is more prominent. And the flow angle decreases with the increase of the tip clearance size at the high-pressure turbine outlet. Compared with the results at the high-pressure turbine outlet, the effects of the tip clearance on the flow angle are more complicated at the low-pressure rotor outlet. Along the low-pressure rotor span, the outlet flow angle shows several different evolution laws with the increase of the tip clearance size.