Study of a locally adjustable guide vane for off-design operation of gas-turbine systems: irreversibility characteristics and adjustment guidelines

A locally adjustable guide vane (LAGV) concept is proposed as an alternative to conventional full-span rotating variable-geometry turbine (VGT) systems for off-design operation. The vane consists of a fixed fore segment and a locally adjustable aft segment, so that throat capacity can be regulated locally while the inlet geometry remains unchanged. Compared with full-span rotating VGT, this configuration confines the endwall clearance to the adjustable aft segment, reducing the estimated effective clearance-leakage area by about 60%–80% for the investigated pivot positions. An experimentally validated numerical methodology is used to examine the effects of adjustable-segment rotation angle and pivot position on mass-flow characteristics, aerodynamic performance, and irreversible losses under subsonic conditions. Entropy-generation and streamwise-vorticity analyses are applied to identify dominant endwall loss mechanisms associated with variable geometry. Under matched mass-flow conditions, the proposed LAGV consistently yields lower downstream energy-loss coefficients than the conventional full-span rotating VGT, with ξ decreasing from 0.146 to 0.107 in the increased-flow case and from 0.172 to 0.157 in the throttled case. Within the representative design range of β = ±5°, the mass-flow variation reaches −10.56% to +8.52%. The loss response is strongly asymmetric: under throttling, ξ increases by +23.84% for a/Cx = 0.6 and β = −5°, but by only +3.09% for a/Cx = 0.8 and β = −5°. A rearward pivot weakens leakage–passage-vortex interaction, mitigates endwall irreversibility, and improves exit flow-angle uniformity. Therefore, a rearward pivot with limited rotation amplitude is recommended to balance flow regulation and exergy-loss control.