Quantitative analysis of Drag Reduction in Horizontal Slug Flow
Authors: Daas M. , Kang C. , Jepson W.P. 
Published: December 2007

Abstract
Calculations have been done to predict the components of pressure drop in slug flow. This analysis is aimed to understand, in quantitative manner, the contributions of both frictional and accelerational components to total pressure drop in horizontal slug flow and the effect of DRA's on each component. Experimental results were in good agreement with predicted values.
The DRA used in this study was effective in reducing both components of the pressure drop. The accelerational component was found to be dominant and formed over 80% of the total pressure drop. It increased dramatically with increasing superficial gas velocity. With the addition of 20 ppm DRA, the accelerational component was noticed to decrease by a factor of 67% as well as the frictional component. At DRA concentration of 50 ppm, they decreased by a factor of 78%. Total drag reduction was found to generally decrease at higher superficial gas velocities.
In sharp contrast with expectations, the drag reduction was recovered mainly from the accelerational component indicating that the DRA worked not only in the buffer zone but also in the mixing zone in the slug body. The accelerational drag reduction reached values as high as 88% out of total drag reduction.

Introduction
Since the discovery of darg reduction phenomenon in 1947 by Toms, extensive work has been carried out in horizontal and inclined pipelines to examine the effect of the addition of drag reducing agents on pressure drop. Drag reducing agents were found to have significant influence in decreasing the frictional pressure drop in single-phase flow. Since that time, drag reducing agents were believed to work only on the frictional component of the pressure drop of multiphase flow, and not influencing the other contributions to total pressure drop, e.g. accelerational and gravitational components.
Although several theories were introduced regarding drag reduction phenomenon, a precise and exact understanding of the mechanism of drag reduction is not established yet especially in multiphase flow. It is believed that DRA's work mostly in the region near the wall, namely buffer zone, by reducing the friction factor of the flow through diminishing the turbulent structures and changing the velocity profile there. This study may help to recognize where drag reduction takes place in a quantitative manner so as developing current theories about how and where DRA's work. A new proposed mechanism may be achieved with the help of such analysis.
Several models were examined, but few were applied in the above study to break down the pressure drop in slug flow in a horizontal system into its components. Hubbard and Dukler (1975) introduced equations to calculate the contributions of both frictional and accelerational components. In their model, they assumed that with in the slug body the two phases are homogeneously mixed with negligible slip and the frictional contributor could be calculated using an equation similar to ones in single phase flow after modifying the density of the mixture and the friction factor. The accelerational contribution was calculated under the assumption that the stabilized slug can be considered as a body receiving and losing mass at equal rates. The velocity of the liquid in the film just before pickup is lower than that in the slug and a force is therefore necessary to accelerate this liquid to slug velocity (Hubbard & Dukler, 1975).
Fan, Ruder, and Hanratty (1993) introduced a new model to predict the pressure drop across a stable slug. In their model, they assumed the slug as a hydraulic jump. Further more, they assumed that pressure change occurs in the rear of the slug. This pressure change could be positive or negative.