Effect of interfacial area concentration on one-dimensional code simulation of adiabatic two-phase flows in vertical large size channels

In one-dimensional two-fluid model-based codes, an interfacial drag force appears in a momentum conservation equation. The interfacial drag force is an essential parameter in predicting a void fraction. The accurate modeling of the interfacial drag force is indispensable for evaluating thermal-hydraulic characteristics in a nuclear reactor core. The interfacial drag force is formulated as the product of an 'overall' drag coefficient and the square of the relative velocity between gas and liquid phases. The 'overall' drag coefficient is expressed by the product of a drag coefficient, interfacial area concentration, and density of continuous phase. The rigorous model of the drag coefficient depending on a bubble shape regime has been established. The modeling of the interfacial area concentration depending on a flow regime is one of the weakest links in thermal-hydraulic analysis. The interfacial area transport equation was proposed to predict the dynamic change of the interfacial area concentration but has not reached a level accurate enough to predict the interfacial area concentration. Due to its incomplete development situation, an alternative way to predict the interfacial area concentration through a semi-theoretical interfacial area correlation has been proposed. This study aims to elucidate the effect of the interfacial area concentration on void fraction prediction in a one-dimensional thermal-hydraulic analysis. A one-dimensional two-fluid model-based code, such as TRAC-BF1, has been modified by implementing two existing constitutive equations of the interfacial area concentration into the code. This study also introduces large and small interfacial area concentration models of the interfacial area concentrations. The large and small interfacial area concentration models are designed to intentionally provide hypothetical large and small interfacial area concentrations within physically possible ranges. A total of four interfacial area concentration models is tested under adiabatic two-phase flow conditions. Code calculations with the four different models under steady-state and transient-state conditions and flow regime transitions have identified that the effect of the interfacial area concentration on the void fraction is insignificant for the adiabatic two-phase flows. The findings obtained in this study suggest that a simple interfacial area correlation is sufficient in modeling the interfacial drag force for adiabatic two-phase flows. However, robust and accurate modeling of the interfacial area concentration is still indispensable for two-phase flows with phase change because the interfacial heat transfer term in an energy conservation equation includes the interfacial area concentration.