Results and Discussion
3.1 Regime
identification
The objective was to find a threshold for the transient gas superficial
velocity and study the effect of viscosity on this threshold. BSD
characteristics and higher order statistics (skewness and kurtosis) were
used to identify the operation regimes. Figure 3 shows the probability
density function (PDF) of bubble size in aqueous solutions of glycerin
(G1 and G3, see Table 1) at various superficial gas velocities. Figure 3
shows that as the gas superficial velocity exceeds 27.6 mm/s in both
cases the PDF shape changes from a bell shape into a spike shape; bubble
column literature24,40 attributed the aforementioned
shift in PDF shape to regime alternation from homogenous to
heterogeneous regime. In addition, Figure 3a shows that the lower gas
superficial velocities (USG ≤ 27.6 mm/s) exhibit
a bimodal shape in the PDF, this feature has been reported in studies of
the homogenous operation regime in the bubble column
literature.15,21,36,37 In homogenous bubbly flow with
no bubble breakage and coalescence events, the injection condition (i.e.
pore size and gas superficial velocity) determines the bubble size. The
pore size distribution on the sparger is discrete and non-monodisperse,
which explains the deviation of bubble size distribution from a truly
Gaussian distribution. Higher order statistics (skewness and kurtosis)
of the BSD were obtained for further inspection of the operation regime
shift. Table 2 presents the Sauter mean diameter
(d32 ), (arithmetic) mean diameter
(d10 ), standard deviation (RMS), as well as
higher order statistics of BSD in G1 at the gas superficial velocities
tested. Skewness (S ) and kurtosis (κ ) of the bubble size
distribution in Table 2 show a significant deviation from a
Gaussian-distribution (S ~0, κ~3)
when the gas superficial velocity exceeds USG =
27.6 mm/s.