Novel wave-shaped tip-shroud contours towards reducing turbine leakage loss

Abstract

For axial turbines, tip leakage vortex and passage vortex flow are responsible for a large part of the aerodynamic loss. To pursue higher turbine efficiency, many methods are undertaken to eliminate the tip leakage loss. In this work, by experimental and numerical methods, novel wave-shaped tip-shroud contours on the unshrouded turbine cascade were proposed. In a low-speed large-scale linear turbine cascade facility, a traditional flat tip was first conducted to comprehensively validate the numerical tool, especially the inlet conditions and the total pressure loss due to the tip leakage loss. The numerical settings were kept the same as the experimental facility. It achieved a satisfactory agreement between the CFD and experiments. It was found that in a flat tip case, a large, integral tip leakage vortex structure was identified and responded to the aerodynamic loss within the tip leakage core. Then a series of novel wave-shaped tip-shroud contours were considered in the current turbine cascade. The overtip flow exited the suction side tip gap, and the peak mass flow rate was concentrated around the troughs of the wave tip. Several small-sized, but stronger-vorticity-magnitude vortex structures were identified. By breaking down a large, integral vortex structure into several smaller-scale vortices, the vortex evolution was beneficial for the turbine efficiency. Although a slight enhancement of passage vortex was identified, the overall loss reduction of 3.0% was achieved by reducing the tip leakage loss, which dominated the loss contribution. According to the real working condition, axial displacement for the blade was considered. Within the current scope, the maximum reduction of tip leakage loss coefficient was reduced by 14.6% compared to the flat tip case. It was mainly contributed by the smaller contraction coefficient within the tip gap, which reduced the overtip leakage mass flow. Meanwhile, the vena contracta within the tip gap was altered, as well as static pressure distribution on the tip surface. As a result, the minimum static pressure was re-distributed at the throat of the tip gap. It was proven that this novel wave-shaped tip-shroud contour was an efficient way to improve the aerodynamic performance at a larger tip gap.