Relative Velocity in Horizontal Bubbly Two-Phase Flows: Experiments and Modeling

Two-phase flows have many important industrial applications, including in nuclear reactor coolant systems. Due to the density difference between the gas and liquid phases, buoyancy effects can have a significant impact on two-phase parameters, and thus the orientation of the flow is a major factor impacting system performance and safety. While recent work has greatly expanded the knowledge of horizontal and inclined flows, gaps still exist, especially regarding the relative motion between the two phases. There are few experiments in literature that measured both local liquid and gas velocity profiles, which are key to understanding and modeling the relative velocity.This work seeks to address this gap by performing experiments in horizontal air-water bubbly two-phase flow, specifically measuring the local gas and liquid velocities by means of a local four-sensor conductivity probe and Pitot-static probe, respectively. Eight fully developed conditions are measured and presented in the current work, with the trends in relative velocity analyzed. It was found that the relative velocity is negative throughout the pipe cross section when in a horizontal orientation. The relative velocity becomes more negative with increasing local void fraction, and remains negative as the void fraction approaches zero.With this newly established database, a model is then proposed to predict the relative velocity in horizontal bubbly flows, accounting for bubble wake interactions. The preliminary model is able to predict the void-weighted area-averaged relative velocity within 15%, and the local relative velocity with an average absolute percent difference of 38%, which is considered adequate given the experimental uncertainties.