Wall Condensation in Steam Turbine Startups: Preliminary Design Assessments

Abstract The recent interest in the use of renewable energy source for power plants has led to a paradigm shift in the design of steam turbines. In particular, due to the intermitted nature of such energy sources, steam turbines are driven to operate in off-design conditions and are subject to increasingly frequent startups and shutdowns. During the startup phase, if superheated steam comes into contact with the internal walls of the machine's ducts, which, before reaching steady-state thermal conditions, may be at a temperature lower than the saturation level corresponding to the operating pressure, the phenomenon of wall condensation occurs. From the literature on evaporative cooling techniques, it is well known that heat transfer involving phase change due to latent heat leads to a large amount of heat flux, causing significant thermal gradients in the machine that can compromise its structural integrity. During the design phase, it is also necessary to estimate the condensate flowrate for the sizing of appropriate drainage channels to prevent accumulation. From a numerical point of view, although multiphysics and multiphase models have been developed to address these needs, the three-dimensional computational fluid dynamics simulation of the entire thermal transient would result in high computational costs, unsuitable for a preliminary design phase. For this reason, the aim of this work is to propose a tool for estimating the steam turbine casing thermal transient and condensate flowrate in transient thermal conditions. The code has been validated with stationary and transient experimental data, as well as a reference analytical solution, showing excellent agreement with an extremely low computational cost.