Experiments and Modeling for Relative Motion and Local Parameter Distributions for Non-vertical Bubbly Two-phase Flows

Two-phase flows have many important industrial applications, including in nuclear reactor coolant systems. Due to the density difference between gas and liquid, 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. Despite this, most two-phase experiments and models have focused on vertical upwards flows. 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 very 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 will seek to address this gap, first by performing experiments in various non-vertical orientations, specifically measuring the local gas and liquid velocities by means of a local four-sensor conductivity probe and Pitot-static probe, respectively. Twelve fully developed horizontal bubbly flow conditions are measured and presented in the current work, with the trends in relative velocity analyzed. It is 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 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 model is able to predict the void-weighted area-average velocity within 10%, and the local relative velocity with an average absolute percent difference of 15%, which is considered adequate given the experimental uncertainties. Area-averaged and drift-velocity correlations are also developed based on this model. There is no predictive explanation given in literature for the negative relative velocity measured in horizontal bubbly flows, which this model enables. To supplement this model, a new method for estimating the local distribution of the dispersed phase in bubbly two-phase flows is proposed. This is based on the geometric packing of the bubbles and reflects the random nature of two-phase flows and bubble interactions, and preliminary qualitative comparisons show promise compared to experimental data. Finally, multiphase computational fluid dynamics simulations are improved by implementing the new relative velocity model. The peak void fraction value is improved by on average 250%, with the area-averaged relative velocity improved by about 100%.