So far it was discussed that the homogenous operation regime exhibits Gaussian-like characteristics and heterogeneous regime features a spike shape distribution; this was inspected using PDF plots of BSD from the range tested (see Table 1). Figure 4 shows the bubble size distribution from G1 plotted on probability coordinates. Figure 4a shows that in homogenous range (based on gas superficial velocity) the BSD exhibits linearity on a normal distribution probability plot. The near-Gaussian behavior in the homogenous regime was also discussed above by means of the PDF shape and higher order statistics. BSD from heterogeneous operation cases (41.4 mm/s ≥ USG ≥ 69 mm/s) in G1 was plotted on a lognormal probability plot (see Figure 4b). It is interesting to see the strong linear behavior of BSD on a lognormal probability plot. Figure 4b also shows that the range at which the BSD exhibit linear trend on a lognormal probability plot starts at the mode of the PDF (thick solid vertical line) and ends at the Sauter mean diameter (d32 , vertical lines). The mode of the PDF corresponds to the most frequent bubble size,dmf , interested reader can refer to Mohagheghian and Elbing47 for analysis ofdmf . The present study shows that only a portion of the right leg of the BSD PDF within heterogeneous regime is lognormal. The PDF exhibits a linear trend betweendmin and dmf ; furthermore, the PDF can be modeled with a second order polynomial curve betweend32 and dmax .