Flow of Dilute Solid-Liquid Mixtures in Horizontal Concentric and Eccentric Annuli
Authors: Kelessidis V.C , Bandelis G.E. , Li J. 
Published: March 2010

Abstract
The phenomena occurring during the flow of dilute solid-liquid mixtures in a horizontal concentric and 100% eccentric annulus are presented, analyzed and discussed. The experiments were performed in a 5 m long, 7 cm by 4 cm transparent annulus. A range of superficial liquid velocities between 0 and 2.32 m/s were studied, with water and two aqueous Carboxyl-Methyl-Cellulose (CMC) solutions; one with Newtonian rheology (with a viscosity of 6.79 cP) and one with non-Newtonian rheology (with apparent viscosity of 8.3 cP at a shear rate of 170 s−1). The solids used were 2 mm glass spheres with sphericity close to one, while the tested solid loading was 2% and 4% w/v. The flow patterns at various experimental conditions were observed visually by video monitoring equipment and were analyzed to produce flow pattern maps. The main parameters affecting the transitions to the particular flow pattern are presented and the conditions for the transition are discussed. A method is presented for flow pattern detection using pressure drop measurements and the positive implications for real-life applications are discussed.

Introduction
Flow of solid-liquid mixtures in conduits is encountered in several situations of industrial significance like solid transportation with long pipelines, oil well and geothermal drilling, oil and gas production and mineral and waste-water processing. The flow geometry may be pipe or annulus in a vertical, inclined or horizontal orientation. While the issues dealing with vertical configurations have been mostly resolved, there are several unresolved issues for the flow of two-phase solid-liquid mixtures in horizontal and inclined conduits(1–10).
During the flow, the liquid and solid phases may distribute in a number of geometrical configurations or flow patterns. The main parameters determining a particular flow pattern are the liquid velocity, the solid loading, the physical properties of the liquid and of the solids, the inclination angle and the conduit shape and size(2, 11,12). A detailed description of the flow patterns in a concentric annulus has been given by Kelessidis and Bandelis(13). They are the suspended symmetric flow, the suspended asymmetric flow, the moving bed flow and the stationary/moving bed flow (see Figure 1). At even lower liquid superficial velocities, the solids pile up in the annulus and full blockage may occur. Experimental evidence and theoretical analysis indicate that this may occur at relatively high solids concentrations.
There have been several theoretical and experimental studies of solid-liquid flow in conduits(1–10, 14, 15) to describe the phenomenon, with many covering the situations of high solids loading (&gt;20% w/v) targeting primarily slurry transport. However, there are situations of industrial significance, like oil-well drilling, where the volumetric solids concentration rarely exceeds 5% w/v(13). Solid suspension by liquid turbulent forces has also received considerable attention(13, 16, 17).
FIGURE 1: Schematic representation of flow patterns in a horizontal concentric annulus(13) (Available in full paper)
Flow pattern maps for the flow of two-phase solid-liquid mixtures flowing in horizontal pipes have been presented previously(5, 10, 11) but flow pattern maps for the flow in horizontal concentric and eccentric annuli have not been presented.