Two types of tube models were constructed to study the hydrodynamics of bubble-to-slug transition in vertical upward flow. First, a one-dimensional tube model was developed to compare the predictions with the existing flow regime maps. A three-dimensional model was then constructed, subdividing the whole tube into a bundle of conduits where the bubbles were moving upwards and coalesced according to the principles defined by the one-dimensional model.
The conceptual distinction between the one-and three-dimensional tube models was in the possible initial distribution of bubbles due to transverse lift and dispersion before the process of coalescence began. After the process of coalescence took place, bubble lateral migration towards the tube center was allowed whenever the boundary conditions on bubble segregation were being changed.
From the three-dimensional model, a two-dimensional version was developed to spare computing time on void fraction profile predictions. Comparison with the experimental results on convex to concave void fraction profile evolution was promising.