Two-group bubble interfacial area concentration correlation for upward bubbly to churn flows in vertical rod bundle flow channels

Abstract

The detailed understanding and accurate modeling of the upward two-phase flows in vertical rod bundle flow channels are significant for various industrial applications, such as performing the optimal design and evaluating the transient and accident scenarios for BWR (Boiling Water Reactor) and PWR (Pressurized Water Reactor) systems. The interfacial area concentration (IAC) describing the available interfacial area for the transfer of mass, momentum, and energy between two phases plays an important role in the two-phase flow simulation in the frequently-used thermal-hydraulics system codes. Although various IAC correlations utilizing the drag-based two-group bubble categorization, namely two-group bubble IAC correlations, have been proposed to predict the IAC changes in the two-phase flow ranging from bubbly to churn flows in simple-geometry flow channels, the two-group bubble IAC correlation for rod bundle flow channels is still underdeveloped. This study performed a survey on the existing representative two-group bubble IAC correlations for simple-geometry flow channels and collected 425 experimental IAC data taken in various vertical rod bundle flow channels. The predictive abilities of these two-group bubble IAC correlations were evaluated with the collected experimental IAC data. The evaluations showed that no existing correlations can consistently maintain satisfied predictive ability throughout the upward bubbly to churn flows in vertical rod bundle flow channels. In view of this fact, this study proposed a new two-group IAC correlation for vertical rod bundle flow channels. Specifically, the group-2 bubble void fraction model was proposed by physically analyzing the changing characteristics of the group-2 bubbles in different flow regimes. The Sauter mean diameter model of group-1 bubbles was derived by (a) selecting the distorted small bubble as the representative group-1 bubble for the bubbly, cap-bubbly and churn flows and (b) selecting the spherical small bubble as the representative group-1 bubble for the finely dispersed bubbly flow. The Sauter mean diameter model of group-2 bubbles was proposed by treating the cap-bubble as the representative group-2 bubble and taking into account the bubble deformation caused by the surrounding rods. The proposed two-group bubble IAC correlation has been checked against the collected existing experimental IAC data and the proposed correlation exhibited reliable predictive abilities in the bubbly to churn flows in various rod bundle flow channels with the mean relative deviation of 0.233, 0.212, 0.247, 0.222 and 0.234 for bubbly, finely dispersed bubbly, cap-bubbly, churn and all data, respectively.