Investigation of cavity rotor tips on the flow field of a low-speed turbine under different conditions

Recent research has shown that cavity tips can mitigate turbine tip leakage flow (TLF) and related losses. However, few studies have described their performance in rotating test rigs, concentrating solely on design conditions and disregarding off-design conditions under which turbomachinery operate. Therefore, this study examines the effects of rotor cavity tips on the aerodynamic performance of a 1.5-stage, low-speed turbine at different rotating speeds and expansion ratios. Turbine performance maps are measured at different operating conditions, showing that the cavity tip improves turbine efficiency and specific work output. Meanwhile, the effects of the geometrical parameters of the cavity tip; four cavity depths (d) and two rib thicknesses (t) at a fixed tip gap of 1% of the annulus height (h); are numerically explored under two characteristic conditions; peak efficiency (PE) and maximum mass-flow (MMF). Validated three-dimensional numerical simulations are conducted with the Reynolds-averaged Navier-Stokes method. The aerodynamic performance of the turbine is enhanced using a cavity tip with the geometric parameters, and the degree of improvement initially increases then decreases as d increases. Moreover, cavity tips with thicker ribs are effective in suppressing upper passage vortices, while thinner ribs are more effective in managing tip leakage vortices. Among the investigated geometrical parameters, a cavity tip with a depth and rib thickness both equal to 2%h demonstrates the best performance in reducing TLF under varied conditions. This cavity tip, compared to a flat tip with equal clearance, improves efficiency by 0.84% at the PE and by 0.56% at MMF, respectively. Additionally, the performance of this optimized cavity tip configuration (d = t = 2%h) is further investigated at four different tip gaps under the two aforementioned characteristic operating conditions. Furthermore, the flow structure topology inside the clearance of the flat tip and cavity tip is established according to oil flow experiment and simulation results. This study elucidates the impact of cavity tips on the aerodynamic performance of the turbine, explores the mechanisms by which cavity tips reduce leakage losses, and presents a flow structure model within the cavity tip clearance. These findings provide a reference for enhancing turbine design and improving efficiency.