Research on the coupling of separation flow and tip leakage flow in low-solidity blade tip-turbine cascades

The double-layer blade configuration, consisting of upper and lower blade with an intermediate connecting section, is extensively adopted in reversible gas turbines and variable cycle engines. For upper turbine (blade tip-turbine) of this blade configuration, the coupled flow loss induced by separation flow and tip leakage flow (TLF), which arises from low-solidity and a large diameter-height ratio, constitutes a critical bottleneck restricting the improvement of turbine performance, and relevant research remains scarce. The TLF migrates over a wider range and couples with suction-side separation flow, intensifying flow blockage and turbulent mixing. In this research, combining experiments and numerical simulations, a transonic low-solidity turbine rotor linear cascade is investigated. The regulatory effects of tip clearance (0–2.5%H), solidity (1.0–1.4), Mach number (0.4–0.8), and incidence angles (−20°–20°) on the vortex coupling strength are systematically examined. High-precision measurement devices including five-hole probe and pressure-scanning valves were utilized in the experiments to acquire comprehensive flow field data, such as the total pressure, static pressure, and flow angle at the cascade inlet and outlet, as well as the static pressure distribution on the blade surface. The research results show that as the solidity increases from 1.0 to 1.2, the separation point gradually shifts from 50% axial chord to 65% on the suction surface, and separation flow disappears completely when the solidity rises to 1.4. The slowdown in the rising rate of the absolute total pressure loss coefficient when tip clearance exceeds 2%, and the mixing of TLF and separation flow is the major factor for the increase in flow loss. The total pressure loss of low-solidity cascade is more sensitive to positive incidence angles. This research reveals the coupling flow mechanism of TLF and separation flow in low-solidity blade tip-turbines, and provides experimental and theoretical support for performance improvement.