Experiments in Cap-Bubbly Two-Phase Flows for Two-Group IATE Development

In the two-group interfacial area transport equation (IATE) used to calculate the interfacial area concentration (a_i), bubbles are categorized into two groups. Namely, group-I consists of spherical/distorted bubbles, and group-II consists of cap/slug/churn-turbulent bubbles. Robust models for the major bubble interaction mechanisms that cause the transition from purely one-group to two-group flows are essential to the dynamic closure of the two-fluid model with the two-group IATE. Therefore, the present study seeks to establish an experimental database in cap-bubbly flows that highlights this transition to support model development. A four-sensor conductivity probe is used to obtain measurements of local time-averaged two-phase flow parameters, including the void fraction and a_i, in vertical-upward air-water two-phase flows in a 5.08-cm pipe. Four flow conditions are investigated at ⟨j_f⟩ = 2 m/s with increasing ⟨j_g⟩ to study the generation and growth of group-II bubbles. Characteristic features of the local void fraction and a_i distributions are discussed. Additionally, axial development of area-averaged void fraction and a_i that is indicative of exchange between the bubble groups is presented.