Loss Evolution Characteristics of a Micro Axial-Flow Turbine Under Variable Operating Conditions

A study on the evolution mechanism of off-design losses is carried out to address the efficiency issues of micro axial-flow turbines in micro underwater vehicles operating over a wide speed range. A thermo-mechanical-fluid multi-physics coupling model is developed using the Modelica language, incorporating micro-scale nonlinear loss corrections such as disk friction loss, partial admission loss, tip clearance leakage loss, and exit 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 variable operating conditions is revealed: during low-speed cruise, exit 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. Increasing inlet pressure and temperature can reduce the loss proportion, but proper matching between inlet parameters and aerodynamic characteristics must be ensured. Controlling tip clearance, appropriately increasing the number of nozzles to raise the partial admission ratio, and adopting a small rotor exit angle can effectively suppress kinetic energy dissipation and micro-scale penalties, thereby improving the turbine efficiency over the whole operating range. The findings provide theoretical support and simulation-based guidance for energy management, definition of high-efficiency operating intervals, and structural optimization of micro power systems.