Two-group drift-flux model for upward cap-bubbly two-phase flows in large square channels

A two-phase thermo-fluid model is required to design safe and efficient thermal-hydraulic systems. The two-fluid model is a mechanistic two-phase flow model in which the interfacial area concentration directly connects the heat and mass transfers between two phases. An approach is classifying various bubbles into two groups based on their drag coefficients. The two-group two-fluid model coupling with the two-group interfacial area transport equation can potentially provide the most accurate analysis of two-phase flows. However, the two-group approach increases the number of transport equations, inducing numerical convergence and instability problems. To mitigate the drawbacks, the two-group drift-flux model needs to be established as a constitutive equation to simplify the two-group two-fluid model by combining two gas momentum conservation equations for respective groups into a total one. The drift-flux model for large square channels has seldom been investigated, even though such channels exist in engineering systems. This study developed the two-group drift-flux model for large square channels based on existing experimental databases in such channels. The distribution parameters and drift velocities for group-one and group-two were modeled from two-group measurement data. The developed group-one and group-two distribution parameter models were consistent with the one-group distribution parameter. The experimental drift-flux plots validated the newly developed two-group drift-flux model for the respective groups, confirming its good prediction performance with-2 % and-4 % errors for group-one and group-two gas velocities, respectively.