Concentration Measurements In Pipelines Containing Flowing Two-Phase Mixtures
Authors: Sumner R.J. , McKibben M. , Shook C.A. 
Published: March 2010

Satisfactory interpretation of pipeline flow behaviour for dispersed two-phase systems cannot be obtained from pressure drops and mean velocities alone. The concentration distribution of the dispersed (droplet or particle) phase in an emulsion or slurry must be known so that an appropriate model can be selected. Measurements with oil-in-water emulsion flows have shown that their pipeline flow behaviour is similar to that which has been observed with solid particles as the dispersed phase.

This investigation used intrusive probes to determine velocity and concentration distributions within pipes and a surface mounted probe for measurements at the pipe wall. Most of the measurements were made with solid particles so that the dimension of the dispersed phase was constant and known.

The measurements show that in turbulent flow the velocity and concentration distributions are nearly independent of mean velocity as long as gravitational effects are small. A region of low concentrationwas observed at the pipe wall in most cases. The inward migration tendency increases with panicle diameter and concentration.

Introduction

Pipeline flow of single-phase fluids is well understood and the concept of viscosity is often extended to characterize two-phase mixtures. However, there is evidence that the continuum model, which is implied in the use of a viscosity, is inappropriate for emulsions of heavy oil in water. Wyslouzil et al.(1) found indirect evidence of radial droplet migration away from the pipe wall in both laminar and turbulent flow. The resulting layer of reduced concentration explained their observations of significantly lower flow resistance or, in terms of a continuum model, lower effective mixture viscosities for emulsions in pipeline flow. Although theoretical explanation or this migration tendency can be found for infinitely dilute dispersions, there is no existing theory for highly concentrated mixtures.

To a first approximation, slurries and emulsions have many properties in common and the "viscosity" vs concentration relationships for the two types of systems are often assumed to be similar(2).or heavy oil in water emulsions, the high viscosity of the oil inhibits circulation within the drops. This increases the similarity of the drops to solid particles dispersed in liquids, The deformable nature of a fluid does increase the maximum concentration of the dispersed phase which can be achieved in liquid-liquid mixtures, however.

In view of the importance of radial migration, an investigation of its magnitude has been undertaken. Because droplet or particle diameter was considered likely to be important and because droplet size had been shown to increase with time by previous workers, this investigation was undertaken using slurries rather than emulsions. Because the previous evidence was indirect, the present investigation was concerned with measuring concentration as a function of position in the pipe. Simultaneous measurements of the velocity distribution were also co be undertaken.

Experimental Procedure

To remove the effects of gravity upon the distributions, vertical flow was employed in the experiments. Figure 1 shows the test loop which was used. Because the volume of the system was known precisely, slurries were prepared by adding a weighted quantity of solids to the reservoir tank while the pump circulated the flow slowly.