  This study aims to experimentally investigate the two-group interfacial area
transport in small diameter pipes. Experimental data focusing on the bubbly to
slug transition regime, namely one-group to two-group transport region, are
collected in a 12.7 mm vertical pipe under adiabatic, air-water conditions. The
result shows the intergroup transfer in the small diameter pipe can be drastic,
especially under low superficial liquid velocities. The cause of this
phenomenon is mainly due to the large relative bubble size comparing to the
pipe cross-sectional area. The wake entrainment effect could be enhanced by the
small spherical bubbles that are acting like cap or slug bubbles in a
medium-size pipe. Based on the experimental observation, a throughout analysis
of the dependence of the drastic intergroup transfer is provided in this study.
The models predicting the initiation of drastic intergroup transfer in small
diameter pipes in terms of the bubble diameter and the void fraction are
developed. These models are compared with the two-phase data among the
different pipe sizes and the results show a good agreement. These newly
developed models are applied to the IATE wake entrainment model by developing a
transition function analogous to the sigmoid function. With the transition
function, the revised IATE model is given the new ability on predicting the
drastic intergroup transfer phenomenon.
