Loss Evolution Characteristics Analysis of Micro Axial-Flow Turbine under Off-Design Conditions

To address the efficiency issues of micro axial-flow turbines in micro undersea vehicles operating over a wide speed range, a study on the evolution mechanism of off-design condition losses was carried out. A thermo-mechanical-fluid multiphysics coupling model was developed based on the Modelica language. Micro-scale nonlinear loss corrections were introduced, such as disk friction loss, partial admission loss, tip clearance leakage loss, and leaving velocity loss. Through transient simulations under a typical mission profile and full-speed-range parameter sweeps, the stage-wise evolution of the loss mechanism under off-design conditions was revealed. During low-speed cruise, leaving velocity loss, disk friction loss, and partial admission loss share comparable contributions, forming a co-dominant regime, while leakage loss remains relatively stable. During high-speed dash, partial admission loss and disk friction loss become the dominant loss sources. Further investigation shows that system performance is significantly influenced by both operating and geometric parameters. Higher inlet pressure and temperature can reduce the loss proportion, but proper matching between inlet parameters and aerodynamic characteristics must be ensured. Kinetic energy dissipation and micro-scale loss can be effectively suppressed by controlling tip clearance, appropriately increasing partial admission ratio, and adopting a small rotor exit angle, thereby improving the efficiency over the whole operating conditions. The findings provide a theoretical basis and simulation support for energy management, definition of high-efficiency operating intervals, and structural optimization of micro power systems.