In an effort to improve the prediction of void fraction and heat transfer characteristics in two-phase systems, the two-group interfacial area transport equation has been developed for use with the two-group two-fluid model. In the one-dimensional formulation, a closure relation is required for the group-1 and group-2 area-average local relative velocity. Furthermore, in the case of the modified two-fluid model with the gas-mixture momentum equation, the group-1 and group-2 void weighted gas velocities must be calculated with additional closure relations. Therefore, the drift-flux general expression is extended to two bubble groups in order to describe the group-1 and group-2 void weighted gas velocities and area-averaged local relative velocities. Correlations for group-1 and group-2 distribution parameters and drift velocities are proposed and evaluated with a dataset containing 57 boiling conditions taken in an internally heated annulus. The proposed distribution parameters show an agreement within +/−5%. The overall estimation of group-1 and group-2 void weighted gas velocities calculated with the newly proposed two-group drift-flux general expression shows an agreement within +/−15% of the measured value.

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