Numerical Study on Condensing Flow in Low Pressure Cylinder of a 300MW Steam Turbine

Abstract

The flow in low pressure (LP) cylinder of steam turbine is the well-known non-equilibrium condensing flow. If the LP cylinder is designed based on the equilibrium condensation theory, the flow parameters in the actual non-equilibrium condensing flow are different from the design condition, which will bring to additional ‘off-design’ losses besides the non-equilibrium losses due to spontaneous nucleation. In order to evaluate the difference between equilibrium and non-equilibrium condensing flows, as well as the ‘off-design’ losses, the hypothetical equilibrium condensing flow and the actual non-equilibrium condensing flow in the LP cylinder of a 300MW steam turbine were simulated by using the ANSYS CFX 11.0 software. The non-equilibrium condensing flow was computed in an Enlerian-Eulerian frame with the classical nucleation model. The computation domain involves all the six turbine stages of the LP cylinder, in which steam expands from superheated region to wet steam region. The non-equilibrium flow was compared with the equilibrium flow. It was found that the flow in the superheated steam stages is also affected by the ‘off-design’ operation of the wet steam stages. In the non-equilibrium flow, the thermal processes and the enthalpy drops in both superheated steam stages and wet steam stages are changed. The surface pressures and the wetness distributions in the wet steam stages are obviously varied. A rough evaluation was made to distinguish the ‘off-design’ and the non-equilibrium losses from the overall losses. For the LP cylinder investigated, the efficiency decrease caused by non-equilibrium losses in stage 5 is about 2%, and the efficiency decrease of about 0.26% due to ‘off-design’ losses is found in the wet steam stages (stage 4, 5, 6). In the superheated steam stages (stage 1, 2, 3), an efficiency increase of about 0.27% is obtained in stage 1, which indicates the positive influence of ‘off-design’ operation of the superheated steam stages in the LP cylinder investigated. The efficiencies of stage 2 and stage 3 are almost unchanged.