Conclusions

A systematic study of bubble size and void fraction in a batch bubble column with a porous sparger was carried out. The measurements (i.e. bubble size and void fraction) were carried out in homogenous and heterogeneous operation regimes. Bubble size measurements were performed using optical photography of large populations of bubbles (2400 and more). Void fraction was measured from the differential pressure across the bubble column height. Water and aqueous solutions of glycerin were used to test the effect of viscosity on the operation regime, bubble size, and void fraction. Gas superficial velocity was tested in the range of 6.9 mm/s < USG < 69 mm/s using compressed air. Regime transition corresponds to the change of physical behavior of the gas liquid system in bubble columns; therefore, it is appropriate to present any measurements with consideration of the operation regime. Current work uses PDF as well as probability plots to characterize the bubble size distribution in homogenous and heterogeneous operation regimes.
Results showed that in the homogenous regime, the bubble size distribution is poly-dispersed and the PDF exhibits Gaussian characteristics. In the heterogeneous regime, bubble coalescence events and shear breakage modified the bubble size distribution, results in the distribution approaching mono-dispersed as indicated by the PDF having a “spike” shape with a lognormal right leg.
Results also showed that increasing the viscosity accelerates the regime transition from homogenous to heterogeneous by allowing the formation of larger bubbles as well as bubble interaction (i.e. breakage and coalescence). Bubble size measurements were carried out in both operation regimes. In the homogenous regime, the characteristic bubble size (i.e. Sauter mean diameter) shows strong dependence on the sparger characteristics and injection condition due to the absence of breakage and coalescence. In the heterogeneous regime, experimental data exhibits a strong correlation between the Sauter mean diameter and specific input power (per unit mass). Dimensional analysis was used to propose a correlation between the scaled bubble size and the scaled specific input power. This correlation was validated against experimental data in literature both from static and vibrating bubble column studies. Void fraction was also measured in both the homogenous and heterogeneous regimes. As expected, the trend between void fraction and gas superficial velocity was dependent on the operation regime. Using dimensional analysis correlations for scaling the void fraction in homogenous and heterogeneous regimes were proposed and validated against experimental data.