Aerodynamic Design of LP Steam Turbine Exhaust Systems for Flexible Operation


Mr. Bowen Ding

Whittle Lab, Department of Engineering 
University of Cambridge, U.K.

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Room 7-31, Haking Wong Building, HKU

Modern large steam turbines in fossil and nuclear power plants are required to operate much more flexibly than ever before, due to the increasing use of intermittent renewable energy sources such as solar and wind. This has posed great challenges to the design of low pressure (LP) steam turbine exhaust systems, which are critical to the reduction of LP exhaust loss. In most of the previous studies, the design had been focused only on the exhaust hood. Although the interaction between the last stage and the exhaust hood has been recognised for a long time, there has been very little attention paid to the last stage blading in the design process, when the machine frequently operates at off-design conditions.

The current study focuses on the design of LP exhaust systems considering both the last stage blading and the exhaust diffuser, for a wide operating range. A 1/10th scale air test rig was built to validate the CFD tool for flow conditions similar to that of an actual machine at low load, featured by the highly swirling flow at the diffuser inlet. A numerical study was then performed to investigate the effect of inlet swirl profile on the diffuser’s capability to recover the leaving energy. The understanding of both components of the exhaust system contributed to an improved design that incorporates both blading and diffuser changes, which is predicted to increase the averaged last stage power output by 1.5% for a typical 1000MW thermal plant that mostly operates at off-design (part-load) conditions. 

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