Velocity fields of N₂- and CO₂-water bubbly flows in a quasi-2D bubble column

Abstract

This study reports the distribution of gas and liquid velocity fluctuations and flow structures in N₂-water and CO₂-water bubbly flows within a pseudo-2D bubble column reactor (BCR). PIV and BIV techniques, operating at a resolution of 750 Hz, were synchronized, and ensemble averaging of the recorded velocity fields was performed to mitigate distortions caused by light interference, reflections, and shadows. Probability density functions (PDF), power spectral density (PSD), and cross-correlation values of velocity fluctuations were calculated to analyze gas bubble interaction and dissolution in dilute to dense bubbly flow regimes. The singular value decomposition (SVD) technique was applied to correlate the flow structure distribution and energy content of velocity fluctuation. The results indicate that CO₂-water bubbly flow exhibits a bimodal distribution of velocity fluctuation at low gas superficial velocity (U_g = 5.25 ± 1e-4 mm/s) due to bubble dissolution, whereas N₂-water bubbly flow at U_g = 8.67 ± 0.035 mm/s shows unimodal distribution. At high gas superficial velocities (U_g> 20.91 ± 0.075 mm/s for N₂ and U_g> 15.75 ± 0.05 mm/s for CO₂), bubbly flows exhibit unimodal distribution of velocity fluctuations. The bimodal distribution is attributed to bubble coalescence and breakup. Experimental findings also suggest that recirculation zones near the BCR walls primarily contain small flow structures with the highest energy density and rapid structural decay. In contrast, larger flow structures are found in the center of the BCR, exhibiting slower decay. Detailed insights into CO₂ bubble dissolution could enhance BCR design, thereby improving the efficiency of relevant industrial applications.