Assessment of the heat transfer coefficient for the prediction of ventilation power in steam turbine last stages operating at low load

During very low load operations the last stage moving blades of a low-pressure steam turbine run in ventilation condition. The flow temperature increases mainly in correspondence of the tip region and some heat flows through the inner casing. Flow and metal temperature measurements have been carried out in different locations of a low-pressure steam turbine. Steady state, non-adiabatic full annulus CFD calculations have been also carried out. Analyses allow to introduce a “new” equation for the calculation of the convective heat transfer coefficient. This is based on a modified Dittus-Bölter equation developed for internal turbulent flows. A correlation between the Reynolds number and the Nusselt number for low load calculation is also presented. Finally, an iterative method for the assessment of the performance of rear stages operating under extreme windage conditions has been considered. Results show that the inclusion of the heat loss term in the energy balance allows to improve the accuracy in the prediction of the ventilation power. By means of this method, the steam turbine can be designed to run at very low load without any mechanical issue for the rear stages, providing the best economic solution for power plant operations at low load.