The impact of mainstream ingress on the leakage flow and loss in partial shrouded turbines

The vital source of the leakage loss in shrouded LP turbines is the interaction between the leakage flow and mainstream at the exit cavity and downstream of the rotor tip. This paper numerically investigates the effect of the shroud design on the leakage loss. The results show that the mainstream ingress in the exit cavity is the key flow feature affecting the intensity and the resultant loss of the interaction between the two flows, which is highly intenser in partial shrouded turbines than full shrouded turbines and causes three aspects of impact: Firstly, the abundant amount of ingested mainstream raises the pressure in the exit cavity, reducing the axial leakage flow rate through the sealing structure. However, there is circumferential leakage flow at the rotor trailing edge due to the lack of the blockage of the shroud platform. In addition, the ingested mainstream also develops into backflow and a streamwise vortex in the exit cavity, intensifying the mixing of the leakage flow and mainstream. Thirdly, the pressure raise in the cavity promotes the convergence of the cavity outflow at the suction side of the rotor blade, thus expanding the impact region of the leakage flow and increasing the mixing loss downstream. The above impacts result in an efficiency decrease of up to 0.90% in the partial shrouded turbine compared to the full shrouded turbine. Based on the loss mechanisms of mainstream ingress, this paper presents an optimized shroud design, which can effectively reduce the loss in the rotor tip by suppressing the mainstream ingress phenomenon. In this way, the optimized shroud design has improved considerably the aerodynamic efficiency by about 0.50%.