Loading-dependent leakage-flow and endwall-loss mechanisms induced by fore-vane clearance in a partially adjustable turbine vane

Variable-geometry turbines regulate flow capacity, but their benefits are often limited by endwall leakage and secondary-flow losses. A partially adjustable turbine vane, with a fixed fore-vane segment and a rotating aft-vane segment, varies effective throat area and blade loading while reducing incidence variation associated with full-vane rotation. However, the loading-dependent penalty caused by fore-vane clearance remains unclear. This study examines how fore-vane clearance, defined here as the endwall clearance of the fixed fore-vane segment, affects leakage development, vortex organization and endwall loss formation. An experimentally validated three-dimensional numerical framework compares two limiting clearance configurations, zero clearance and full clearance, under high-, design- and low-load conditions. Compared with zero clearance, full clearance increases the mass-averaged total pressure loss coefficient by 12.3%, 22.4% and 25.8%, respectively. The endwall leakage ratio rises from 1.88% to 3.43%, from 1.38% to 2.89%, and from 1.17% to 2.55%, whereas inter-segment slot leakage changes only slightly. The secondary-flow kinetic energy coefficient also increases, indicating stronger cross-passage and spanwise secondary motion. Fore-vane clearance therefore acts as a loading-dependent amplifier of endwall loss rather than a purely local leakage source. It expands the near-endwall high-loss region under all investigated conditions, but the amplification route changes with loading: leakage-vortex/passage-vortex interaction is prominent at the design condition, interaction among the leakage vortex, passage vortex and separation-related structures becomes stronger as the loading is increased, and coupling with suction-side separation appears at the lowest-load condition. These findings clarify clearance-sensitive endwall loss mechanisms in partially adjustable turbine vanes.