A comprehensive validation of CFD-based wet steam approach for the modelling of steam ejectors

Ejector-based system performance is closely related to the complex fluid-dynamic behavior inside ejectors, which is usually optimized through CFD simulations. The rapid expansion in ejector’s primary nozzle leads to the condensation of the working fluid, and since it strongly affects system performance, it has to be properly accounted. In this study, the built-in Wet Steam model of the CFD software Ansys Fluent is used to simulate the non-equilibrium condensation inside steam ejectors. Usually, the condensation inside steam ejectors is not considered as it is a complex phenomenon and difficult to model, hence the ideal gas model is generally used, without considering the presence of liquid droplets affecting the thermodynamic profiles inside the domain and therefore the system performances. To assess the performance of wet steam model from a local and global point of view, the entrainment ratio and pressure distribution of both wet steam and ideal gas models have been respectively compared. A set of experimental benchmarks from literature has been used to validate the model with different ejector geometries and operating conditions. The wet steam model can outperform the ideal gas model and predict entrainment ratios closer to experimental result in on-design conditions with errors lower than 10%. However, the wet steam model was found to be very dependent on the geometry of the suction chamber and the nozzle exit position, achieving the best results when the nozzle was placed near the constant area mixing section or with suction chamber with not abrupt changes in its cross-sectional area.