Flow mechanisms and loss modeling of separation-induced blockage in a high-load variable-geometry turbine cascade

Variable-geometry turbines regulate mass flow through throat-area variation, but in high-load cascades the resulting loading redistribution can strongly alter separation, blockage, and profile loss. To clarify this physics, three regulation concepts are investigated in an idealized two-dimensional midspan high-load low-pressure turbine cascade: variable stagger, variable shape with a fixed fore section and rotatable aft section, and variable thickness with a locally deformable suction-side segment. The results show that the dominant response can be interpreted through an effective-throat mechanism: geometric adjustment changes the nominal throat area, while boundary-layer separation introduces displacement blockage that reduces the effective flow area and increases loss. An experimentally validated computational fluid dynamics framework resolves flow capacity, loading, wake loss, boundary-layer parameters, skin friction, and suction-surface velocity fields over a wide adjustment range. Owing to the two-dimensional midspan formulation, the quantitative flow-capacity and loss values should be interpreted as mechanism-oriented results rather than full-span three-dimensional performance predictions. Variable stagger provides the widest flow-capacity range, but the strongest positive-turning loss sensitivity because loading shifts forward and the adverse pressure gradient intensifies. Variable shape broadens the low-loss window by confining geometric change to the aft section. Variable thickness offers limited flow modulation but perturbs loading more gently and yields an approximately linear loss increase. Compact empirical correlations are developed for flow capacity and profile loss, with coefficients calibrated for the present blade family and operating range.