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.