Full-range two-group drift-flux model for vertical downward two-phase flows

Accurate prediction of void fraction (VF) is essential for the safe and efficient operation of gas-liquid two-phase systems. Bubbles exhibit diverse size spectra and transport behaviors. The traditional one-dimensional drift-flux model (DFM), treating bubbles as a one-group (1G) system without accounting for the bubble-size spectrum, captures only the total VF averaged over all bubbles and cannot resolve the distinct dynamics of bubbles with different sizes. To overcome this limitation, the two-group (2G) approach has been proposed to explicitly distinguish G1 bubbles (spherical and distorted) from G2 bubbles (cap, slug, and churn-turbulent), enabling more detailed characterization of their respective transport mechanisms. Building on this framework, a 2G DFM for dispersed downward flows was developed, but a full-range 2G DFM applicable to both dispersed and separated (or annular) flows was unavailable. To address this gap, this study developed a full-range 2G DFM for downward flows, including constitutive correlations for group-wise distribution parameters and drift velocities. First, the 1G and 2G DFMs, including their basic concepts and key constitutive equations, were introduced. Then, the experimental database for both dispersed and separated downward flows was compiled. After that, the constitutive equations for the group-wise full-range distribution parameters and drift velocities were derived from theoretical analysis. As a by-product of the full-range 2G DFM, the maximum, minimum, and average liquid film thicknesses were analytically derived from the group-wise VF distributions. The proposed model was directly validated against 2G VF data in dispersed flows and was indirectly supported by average liquid film thickness in annular flows, where direct 2G VF measurements were unavailable. Good agreement was obtained, with mean absolute relative errors of 14.7% for G1 VF, 8.82% for G2 VF, and 30.6% for the average liquid film thickness. These results demonstrated that the proposed full-range 2G DFM provided a robust and physically consistent framework for predicting group-wise VFs and liquid film behavior in vertical downward flows.