**Dedication**

high-speed camera at low superficial gas velocity for 1000 and 5000 mPa s, while

The variations observed in the agreement could be due to the viscosities of the liquids used. Viana et al. [43] used silicone oil of viscosity range 1–3900 mPa s. De Cachard and Delhaye [44] and Nicklin et al. [7] used water. Also, from the structure velocity plots using cross-correlation, the distribution coefficient is in the range 1.07 to 1.6, while that of modified Viana et al. [43] and De Cachard and Delhaye [44] is approximately 2.25. A distribution coefficient of 2.0 was used for Nicklin et al. [7] as

The video technique of determining the rise velocity of bubbles gave errors of 4.3, 4.6, 7.3 and 11.5% for 5, 100, 1000 and 5000 mPa s, respectively when com-

1.The forces acting on a Taylor bubble in a 50 mm column diameter include inertia force, surface tension force, viscous force and gravitational force. Surface tension force can be neglected based on Eotvos number greater than 70. The remaining forces have an influence on the rise velocity of Taylor bubble. Furthermore, viscous and gravitational forces were observed to have dominant effect over inertia forces, hence causing the rise velocity of Taylor

2.The dimensionless parameters: Froude number, Reynolds number and inverse dimensionless viscosity all played vital roles in affecting the rise velocity of Taylor bubbles in various viscosities for a 50 mm diameter column. The dimensionless parameters being functions of the fluid properties and column diameter. Froude number helped to categorize the flow in the four viscosities considered into subcritical (slow and tranquil flow, *Fr* < 1), with lower rise velocity of Taylor bubbles, critical (*Fr* = 1) and supercritical flow (fast rapid

3.The rise of large bubbles through the liquid in the column agrees with Stokes law where drag force is directly proportional to viscosity and an inverse relationship exists between drag coefficient and Reynolds number, as superficial gas velocity increases. The drag force is also exponentially proportional to the void fraction and it retards the motion of the Taylor bubbles through the liquid. An inverse relationship in the form of Power law expression also exists between the drag force and drag coefficient. The drag coefficient was high at low Reynolds number but low at high Reynolds number, which contributed to the rise velocity of Taylor bubbles decreasing

4.The rise velocity of Taylor bubbles increases with an increase in superficial gas velocity for each viscosity considered (i.e. 5, 100, 1000, 5000 mPa s).

5.The comparison between the rise velocity of Taylor bubbles obtained from the ECT, high-speed camera, cross correlation, manual time series, Nickel et al. [7] model, modified models of De Cachard and Delhaye [44] and Viana et al. [43]

From the foregoing, the following conclusions can be drawn:

flow, *Fr* > 1), with higher rise velocity of Taylor bubbles.

bubbles to decrease as viscosity increases.

from low to higher viscosity liquids.

gave reasonably fair agreement.

**218**

Nicklin et al. [7] over predicts it.

*Vortex Dynamics Theories and Applications*

proposed for laminar flow.

**5. Conclusions**

pared with cross-correlation technique.

This publication is in loving memory of Late Prof. Barry Azzopardi.
