CFD study on flow-boiling characteristics and predicting critical heat flux under natural circulation conditions of IVR-ERVC

Advanced reactors adopt the cavity injection system (CIS) combining forced and natural circulation to implement the IVR-ERVC (in-vessel retention by external reactor vessel cooling) after severe accidents, which improves the inherent safety. The Critical heat flux (CHF) is important to assess the cooling capacity of CIS. The numerical method based on the Euler two-fluid model and the non-equilibrium wall boiling model is applied to study the subcooled flow boiling characteristics and the influence of different factors on the CHF under natural circulation conditions. It is found that the heating wall temperature is mainly affected by the heat flux and vapor migration. Vapor accumulates toward the heating wall due to the curved channel and the buoyancy, and the vapor-phase convective heat transfer dominates after the local void fraction rises drastically when the heat flux is large enough, leading to a significant reduction of the heat transfer capacity and triggering the boiling crisis. The heat transfer coefficient decreases at the 0∼45° region and increases at 45∼90°region. Increasing the subcooling and the pressure can enhance the CHF, but it will weaken the circulating driving force and reduce the mass flow rate. Raising the circulation height and shortening the channel width can improve the circulating flow rate and turbulence mixing intensity to enhance the CHF. Meanwhile, a correlation integrating multiple influencing factors is proposed to predict the CHF under natural circulation conditions. The mean relative deviation between the predicted and experimental CHF is only 9.63%, and the predictive accuracy and applicable scope are both improved remarkably compared with the existing correlations. This work can provide a deep understanding of the flow boiling characteristics of natural circulation and provide a reference for optimizing the ERVC strategy.