Numerical investigation on flow instabilities in low-pressure steam turbine last stage under different low-load conditions

The modern power generation system requires steam turbines operating at flexible operating points, and flow instabilities readily occur in the low-pressure (LP) last stage under low-load conditions, which may cause failure of the last stage moving blades. Some studies have shown that within this operating range, a shift of the operating point may lead to flow instabilities. Numerical simulation has gradually developed into a popular method for such researches, but it is expensive for a complex model, which has to be balanced between efficiency and accuracy. This work is divided into three parts: Firstly, one of the low-load conditions is selected to provide both URANS model and the Scale-Adaptive Simulation (SAS) model. The results of the two models are compared to evaluate specific flow phenomena; Secondly, through calculations of different low-load conditions, the flow structure and propagation characteristics of instabilities in the last stage are obtained; Finally, flow analysis is applied to explain the formation mechanism of flow instabilities in LP steam turbines. The results show that, the introduction of SAS model increases the randomness of flow over time, but does not fundamentally change the flow instabilities. Flow instabilities take different forms at different flow rate, from rotating instability to rotating stall. The formation of flow instabilities is related to the radial flow in the cascade passages.