Modelling of Water Film and Secondary Droplet Formation in a Steam Turbine

Abstract Low and often unstable steam parameters at the inlets and outlets of steam turbines, especially in waste-to-energy (WtE) facilities and heating plants, result in increased and often extreme demands on the flexibility and wide control range of steam turbine output. This necessitates a good prediction of machine behavior in different operating modes already at the design stage. This paper focuses on contributing to a more comprehensive computational treatment of the last stage blade erosion problem. The intensity of erosion is significantly affected by the dispersion of the coarse water phase, i.e., the size and amount of coarse water droplets formed by the breakup of water films on the blade surface. The Czech Technical University in Prague (CTU) and Doosan Škoda Power (DSPW) are currently developing a new in-house approach to predicting coarse water phase dispersion. This approach is based on empirical relationships but utilizes the Ansys commercial CFD solver in relevant steps to refine several parameters involved in these empirical relationships. By calculating the non-equilibrium condensation of steam in the turbine stages and by calculating the transport of primary droplets near the blade surface, boundary conditions for the in-house computational approach of the formation and movement of the water film on specific turbine blades were obtained. The output of the computational approach is the distribution of the coarse water phase dispersion, mainly in the trailing edge region of the stator blade of the last turbine stage along its span. Examples of the results obtained for a 34 MW DSPW turbine designed for a WtE facility are presented in the paper.