Figure 4 . Probability
plots of BSD with G1 in the (a) homogenous regime; 6.9 mm/s ≤USG ≤ 27.6 mm/s on a normal probability plot and
(b) heterogeneous regime; 41.4 mm/s ≥ USG ≥ 69
mm/s on a lognormal probability plot.
Figure 5 presents the PDF of the bubble size at different viscosities
(see Table 1) illustrating the sensitivity of BSD to the viscosity of
the liquid phase. Once the gas superficial velocity is sufficiently high
(in this case USG = 27.6 mm/s), the viscosity
modifies the near-Gaussian distribution (in water) to a spike shape
distribution. It was discussed that the shift in the distribution shape
is an indication of operation regime change from homogenous to
heterogeneous. Manual inspections showed that, increasing the viscosity
reduces the bubble terminal velocity due to friction drag; moreover,
increasing the viscosity effects the bubble motion by creating planar
oscillations in the bubbles trajectory, these two effects in turn
enhance the bubble coalescence, this results in the formation of larger
bubbles that are more susceptible to shear breakage. At higher gas
superficial velocities, the number of coalescence and breakage events
increases; therefore, the BSD shape shifts towards a spike (lognormal
distribution) and the standard deviation decreases (i.e. distribution
narrows). In summary, increasing the viscosity modifies the BSD, and
increases bubble-wake interactions; these effects, alter the physical
structure of the bubbly flow from homogenous to heterogeneous.