**4.1 Effect of viscosity on the rise velocity (structure velocity)**

The effect of liquid viscosity on the rise velocity (structure velocity) has been studied by making a comparison between the respective structure velocities obtained from the ECT for the range of viscosities considered (i.e. 5, 100, 1000 and 5000 mPa s). The physical properties of the liquids used are given in **Table 1**.

A plot of structure velocity versus superficial gas velocity for all the viscosities considered is given in **Figure 9** which shows that structure velocity increases with an increase in superficial gas velocity which is in agreement with the observations of Abdulkaldir et al. [39] and decreases with increase in viscosity as shown in **Figure 10** (obtained from ECT Plot3d Image reconstruction software). The structure velocity of 5 and 100 mPa s is found to be approximately the same due to similar void fraction data values. The variation from small to bigger spherical cap and developing slug in 5 and 100 mPa s, and the slug flow in 1000 and 5000 mPa s (as shown in **Figure 9**) has been discussed by Kajero et al. [33].

This can be explained using the slug Reynolds number, a dimensionless parameter (Eq. 16).

A plot of slug Reynolds number versus superficial gas velocity is made at various viscosities as shown in **Figure 11**, with an indication of laminar flow.

**Figure 11** reveals that as viscosity increases, slug Reynolds number decreases tending towards zero. This can be explained as follow:

i. Occurrence and prevalence of laminar flow as viscosity increases: According to Bendiksen [21], Reynolds number in the range 5000–110,000 (for low viscous fluids) give turbulent flow. As the Reynolds numbers of the viscosities considered are less than 5000, laminar flow prevails. For large slug Reynolds number, viscous effect will be negligible, while for small slug Reynolds number, viscous effect will be dominant [40]. So, since

