Proposal of a new type of two-phase interchannel mixing model for application to subchannel analysis of PWR conditions

Prediction accuracy of a subchannel analysis depends strongly on modeling of the interchannel mixing effect. In the present paper, we proposed a new phenomenological two-phase interchannel mixing model in the subchannel code MATRA (Yoo et al., 1999) for application to the bubbly flow regime under PWR pressure level. The three elemental natural interchannel mixing effects (Lahey et al., 1977; Sadatomi et al., 1994), i.e., turbulent mixing (TM), diversion cross flow (DC) and void drift (VD), were separately considered in the proposed interchannel mixing model. The key constitutive relation of the new mixing model is the modeling of void drift, for which the concept of Lahey et al. (1977) that a two-phase flow approaching an equilibrium state was adopted. Based on systematic CFD simulations, correlations were proposed to describe both the void fraction distribution at equilibrium state and the effective mixing velocity due to void drift. In order to investigate the driving force of void drift, detailed examinations of the lift force acting on bubbles were conducted. We found out a close relationship between the lift force and the interchannel mixing effect of void drift. Finally, the new interchannel mixing model along with the proposed void drift model were implemented in MATRA for validation calculation with selected test cases of the ISPRA rod bundle benchmark (Herkenrath et al., 1981).