CFD Modelling of Two Phase Flow inside Geothermal Steam-Water Separator

Abstract

The steam-water separator is a vital component in liquid dominated geothermal steam field equipment. While various designs exist, the vertical cyclone separator dominates the design used worldwide. Most current designs are based on Bangma’s experience in Wairakei in 1961, and Lazalde-Crabtree’s (1984) empirical approach. Although the design of a vertical cyclone separator is relatively simple, understanding of the fluid behaviour within the separator is still limited. Challenges arise from the difficulty in predicting the flow regime, pressure distribution and the separation efficiency inside the separator vessel. Due to this complexity, a numerical approach from Computational Fluid Dynamics (CFD) software is needed. This paper simulates the two-phase fluid movement inside a geothermal cyclone separator using the Fluent® CFD software package. The inlet fluid characteristics were varied to see how the change in enthalpy and mass flow affected the cyclone separator performance. The effect of inlet shape design on separator performance was also studied. In order to model the swirling flow with a high degree of turbulence, as normally occurs inside the separator, the Renormalization Group (RNG) k-å turbulence model was implemented. The separator efficiency was calculated by injecting liquid droplets after a converged solution was achieved. The Harwell technique was used to get an approximate estimate of the average liquid droplet size. The CFD simulation results demonstrated a promising method for optimizing the separator design.