Abstract

The rise velocity of Taylor bubbles in small diameter bubble column was measured via cross-correlation between two planes of time-averaged void fraction data obtained from the electrical capacitance tomography (ECT). This was subsequently compared with the rise velocity obtained from the high-speed camera, manual time series analysis and likewise empirical models. The inertia, viscous and gravitational forces were identified as forces, which could influence the rise velocity. Fluid flow analysis was carried out using slug Reynolds number, Froude number and inverse dimensionless viscosity, which are important dimensionless parameters influencing the rise velocity of Taylor bubbles in different liquid viscosities, with the parameters being functions of the fluid properties and column diameter. It was found that the Froude number decreases with an increase in viscosity with a variation in flow as superficial gas velocity increases with reduction in rise velocity. A dominant effect of viscous and gravitational forces over inertia forces was obtained, which showed an agreement with Stokes law, where drag force is directly proportional to viscosity. Hence, the drag force increases as viscosity increases (5 < 100 < 1000 < 5000 mPa s), leading to a decrease in the rise velocity of Taylor bubbles. It was concluded that the rise velocity of Taylor bubbles decreases with an increase in liquid viscosity and, on the other hand, increases with an increase in superficial gas velocity.

Highlights

  • Slug flow is characterized by Taylor bubbles, which has large pockets of bullet shaped bubbles occupying almost the entire cross-section of the column

  • The effect of liquid viscosity on the rise velocity has been studied by making a comparison between the respective structure velocities obtained from the electrical capacitance tomography (ECT) for the range of viscosities considered (i.e. 5, 100, 1000 and 5000 mPa s)

  • From the foregoing, the following conclusions can be drawn: 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

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Summary

Introduction

Slug flow is characterized by Taylor bubbles, which has large pockets of bullet shaped bubbles occupying almost the entire cross-section of the column. The Taylor bubble is surrounded by a thin film of liquid, and below, it is the liquid slugs, which are agglomerate of small bubbles. The rise velocity of a single isolated Taylor bubble is dependent on inertia and drag forces [4]. A number of parameters affect the rise velocity of Taylor bubbles through a stagnant liquid; such. Mao and Dukler [6] explained that in a situation whereby the liquid is flowing, the rise velocity of a Taylor bubble must depend on the velocity of the liquid flowing upstream as well as the rise due to buoyancy.

Background
Fluid flow studies using dimensionless numbers
Reynolds number, Re
Inverse dimensionless viscosity, N f
Froude number
Experimental arrangements
Rise velocity of Taylor bubbles
Cross-correlation
Manual time series analysis
High-speed camera estimate
Effect of viscosity on the rise velocity (structure velocity)
Drag force, FD
Comparison of structure velocity computation methods
Conclusions
The dimensionless parameters
Full Text
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