Tip gap recess influence on the performance of a low speed, 1.5 stage, axial turbine

In the modern aviation, the turbofan engines are widely used, particularly in the commercial sector. Constant improvements in terms of thermodynamic and propulsive efficiency are essential to reduce in-flight emissions and operational costs. To achieve this outcome, every primary component of the propulsive system must be specifically designed and optimized. With this as the primary objective, an axial turbine is analyzed in this study. These machines are involved into the extraction of energy from a flowing stream of hot gas. As the fluid proceeds, flowing through the turbine, the rotation blading extracts work in the form of mechanical energy. Since the process is not ideal, various types of energy dissipation occur which include not only thermodynamic losses due to irreversibilities in the cycle but also aerodynamic losses and mechanical friction. These combined losses reduce the overall efficiency and limit the power extraction apability of the turbine. This research examines the potential benefits in terms of performance of applying physical modifications to the rotating blade row of a shroudless, 1.5 stage axial turbine. A numerical evaluation of the flow field inside the machine is performed using the commercial Computational Fluid Dynamics (CFD) software Ansys. The validation of the obtained results is conducted by analyzing the experimental data of the Multi-stage Axial Turbine Research Facility at the ETH Zurich, which have been published by the Global Power and Propulsion Society (GPPS). Additionally, this work investigates the aerodynamics inside the turbine, focusing on the losses generated by the necessary presence of a clearance between the rotor blade and the outer casing of the rotating blisk. The resulting tip leakage mass flow is analyzed by addressing the flow structures responsible for the performance reduction. Subsequently, modifications to the geometry of the studied region are applied by introducing recessed tip on the rotor blades. A parametric investigation based on the main geometrical dimensions of the tip cavity created is conducted. The results assess a strong dependency between the aerodynamics of the recessed tip and the overall efficiency. The reduction of the tip leakage flow due to the cavity leads to a decrease of turbine internal losses which is responsible for the performance improvements as demonstrated in the current work.