Void fraction migration in developing bubbly flow through sudden cross-section contraction and expansion in square channels

Abstract

Precise prediction of the flow pattern and proper estimation of the void fraction distribution are needed to design and optimize multiphase reactors and bubble columns. This research presents the cross-sectional and averaged void fraction distributions of adiabatic air-water mixtures across square-to-square channel cross-section expansion and contraction, with gas superficial velocity (j_g) ranging from 0.003 m/s to 0.0054 m/s and water superficial velocity (j_l) from 0.08 m/s to 0.36 m/s. A high-speed imaging system captures the bubble information, while key liquid phase flow features are extracted using the particle image velocimetry technique. Changing j_g and j_l have different effects on the bubble size distributions. In addition to the slowing flow after the expansion plane, the higher void fraction near the walls before the expansion influences the void fraction distribution in the expansion area. Void fraction distribution in channels with contraction displays a wall and core peaking profile. Conversely, a reduction in the contraction ratio in the channel increases the wall-peaking void fraction compared to the core. The decreasing averaged void fraction across the contraction transitions from linear to polynomial as the contraction in the channel increases. A code was developed to estimate the void fraction distribution before the contraction plane in a square channel, and the output was compared with available data.