In steam turbine plants, the last stages of the low pressure (LP) turbines can deliver up to 20% of the overall power of the plant. It poses lots of challenges to designers especially when the last stages are operated under low volume flow conditions. In the current paper, numerical simulations are conducted to investigate the flow features in a LP steam turbine.

In steady calculations, flows under six different mass flow rates are simulated. Performances and flow patterns in last stage rotor (LSR) in low mass flow rates are highlighted. Since the last stage is modeled as a full blade annulus, flow patterns and blade force in circumferential distribution are examined. Results show that under low mass flow rate conditions, vortices occur in the last stage and the diffuser. The LSR acts like a compressor. The periodical distributions of pressure in LSR passages are broken. High amplitude aerodynamic force fluctuations are found on LSR blades in low mass flow cases.

By conducting unsteady simulations, the time series of aerodynamic force are demonstrated to have the similar trend and magnitude of that in steady spatial sequence. The mechanism for aerodynamic force excitation is discussed in the current paper. Unsteady pressure fluctuation in tip section of LSR at low mass flow rates seems to have a significant correlation with the aerodynamic force fluctuation level rise.

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