Dryout in annular flow
Overview: Dryout in annular flow is defined as the condition when there is complete removal of liquid film on the wall surface, and is commonly predicted with empirical correlations with simplified conservative assumptions. Dryout of the liquid film on the fuel rods in BWR fuel assemblies leads to an abrupt decrease in heat transfer coefficient and can result in fuel failure. Empirical correlations for prediction of dryout conditions using simplified assumptions are now insufficient to predict the phenomenon.
The challenge: Better prediction of entrainment phenomenon in annular flows and flow parameters such as void fraction, pressure gradient, interfacial friction and thus dryout would lead to a more accurate quantification of the safety margins and possibly to operating the reactor at higher core power rates. Therefore, there is a strong economic incentive and motivation for conducting further research in this field, using advanced Computational Multi-Fluid Dynamics (CMFD) approaches.
TransAT Solution: The phase-average model predicts a flow (2nd panel) where the interfaces are smeared. The CMFD approach employed (3rd panel) here involves using a level-set based interface tracking method to predict topology changes, combined with scale-resolving strategies for turbulence implemented in the code TransAT?. ITM is necessary here as entrainment occurs along the vapor-liquid interface. Turbulence is resolved using Large Eddy Simulation (LES), which, when combined with an ITM, leads to an advanced, powerful strategy referred to as Large Eddy and Interface Simulation (LEIS). The approach can accurately and more affordably predict turbulence near liquid vapor interfaces.
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Dr Chidu Narayanan
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