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.