A hybrid multiphase model for simulating condensing gravity-driven flows

The recent increase in the use of renewable energy sources as the thermal input for power cycles has led to a paradigm shift in the operation of steam turbines, which are now required to offer increasingly higher operational flexibility. As a result, frequent shutdowns and start-ups have become more common. Cold start-up of steam turbines is a particularly critical phase in the overall operation of the machine, as it involves the generation of high thermal gradients and the formation of condensate due to the low metal temperatures and the high pressures involved. Simulating this phenomenon not only enables the assessment of potential damage mechanisms associated with wet steam during the start-up transient, such as thermal fatigue, erosion, and crack initiation, but also provides valuable information on the amount and distribution of liquid water produced. This information is essential for evaluating thermal imbalances and for the proper design and sizing of drainage systems, ensuring effective water removal during regular turbine operation. A novel Computational Fluid Dynamics (CFD) model is proposed and validated for the simulation of stratified condensing flows, with capabilities to predict both film condensation and pool condensation regimes. The model employs an innovative hybrid approach that couples two distinct multiphase Eulerian formulations to capture the complex physics of phase change phenomena. While originally developed to address specific requirements in steam turbine design applications, the proposed methodology represents a general-purpose computational framework applicable to gravity-driven condensing flows across a wide range of industrial and engineering contexts.