Research on wet steam condensation flow characteristics of steam turbine last stage under zero output condition

Abstract

The zero output transformation of the low-pressure cylinder can fully exploit the peak shaving capacity of the unit and solve the problem of the mismatch between power generation peak shaving and heating supply. However, the friction between the low-pressure rotor and the internal working fluid will cause steam ventilation loss, and the flow field in the last stage is extremely unstable. It is necessary to study the condensed flow characteristics and flow field stability of wet steam under small flow rate conditions in the low-pressure stage group to determine the zero-output operating conditions and the steam ventilation limit operating conditions, which can guide the flexible transformation of cogeneration units. The flow field distribution and condensation characteristics of the last stage of the steam turbine under different flow conditions are obtained through numerical calculations by taking the last stage cascade of a 300 MW steam turbine as the research object. The results show that the vortex inside the last stage of the steam turbine is mainly distributed in the rotor cascades. As the flow rate decreases, the flow instability increases. When the flow rate is lower than 20% turbine heat acceptance (THA), vortex flow first appears at the root of the rotor cascade, and then the height of the vortex and separation increase. When the flow rate is lower than 10% THA, a vortex appears in the clearance between the rotor and stationary cascades, and finally, the vortex fills the entire space. The zero output condition appears when the flow rate is 57,000 kg/h, and the steam ventilation limit condition appears when the flow rate is 10,000 kg/h. At this time, the maximum temperature of the rotor cascade reaches 368.15 K. When the flow rate decreases, the condensation strength and the nucleation rate decrease. When the flow rate is lower than 60,000 kg/h, the Markov loss coefficient in the rotor and stationary cascades increases significantly. Based on the analysis of wet steam flow characteristics, condensation characteristics, steam ventilation conditions, and Markov loss coefficient, it is recommended that the steam turbine's last stage inlet steam flow is not less than 57,000 kg/h. The research results can provide a reference for the steam flow design of steam turbine zero-output retrofitting.