Numerical Predictions of Two-Phase Natural Circulation Loop Transients Using DFM- and HEM-Based Models

The concept of two-phase natural circulation loops (NCL) finds numerous conventional and non-conventional industrial applications due to its passive nature, less cost, and fewer moving parts. However, the dynamics of these systems are complex due to their regenerative feedbacks. Further, the two-phase flow patterns may change in the transient process. Therefore, the developed models should be valid and predictable in those flow patterns. In this connection, two-phase NCL models based on (Homogeneous Equilibrium Mixture Model (HEM) and Drift Flux Model (DFM) are developed using the finite volume method. Flashing phenomena, fluid and wall conduction, and water property subroutine are considered in the model. The models are validated against experimental NCL loops available in the literature. The start-up dynamics of two-phase NCLs are predicted and compared between HEM and DFM models. The drawbacks of HEM and the advantages of DFM are discussed. The changes in flow regimes during the transients are also captured using the models based on available flow regime maps. It is observed that during the transients, the flow regimes change drastically, and DFM predictions are comparatively better than HEM. In the end, the influence of input power, diameter, and height on two-phase NCL dynamics are elucidated using DFM and HEM models.