Blade vibration reduction of a nozzleless radial turbine by casing treatment

High cycle fatigue (HCF) is the most common form of blade failure in nozzleless radial turbines. Current studies on blade vibration reduction primarily focus on the blade design optimization and geometry modifications, typically at the cost of increased rotor weight and reduced aerodynamic performance. This paper investigates a novel flow control method for blade vibration reduction based on casing treatment. Inspired by the generalized force method, casing treatment is proposed to offset the excitation caused by volute. Axial grooves are introduced on the casing (named casing treatment) to reduce blade excitation caused by volute, with particular emphasis on high-expansion-ratio conditions due to the large vibration amplitude. Firstly, the influence of relative position of volute tongue and grooves on blade excitation is investigated via well-validated one-way Fluid-Structure Interaction (FSI) method. Results show that the vibration amplitude can be significantly reduced without sacrificing aerodynamic performance by adjusting the relative position appropriately. Generalized force and flow field analysis reveal that the generalized forces induced by the volute and the groove offset each other when positioned correctly. Secondly, the influence of size parameters of casing treatment on blade excitation is investigated. Two of the parameters only influence the length of generalized force caused by casing treatment rather than the phase, which greatly simplifies and facilitates the optimum design of casing treatment. Numerical results suggest that the blade vibration amplitude can be reduced by up to 94 % at high pressure ratio via the optimum design of the casing treatment. The effect of optimum casing treatment under various pressure ratios is also investigated. The optimum design has satisfied effect at high pressure ratio, but further optimization is required under low pressure ratio conditions. Finally, the effectiveness of the casing treatment is validated via tip-timing experiments, demonstrating a significant reduction in blade amplitude by 48 % under the most critical operating conditions.