Drag Reducing Flows by Polymer Solutions in Annular Spaces

We analyze the use of water solutions of Xanthan Gum (XG) for drag reduction (DR) in annular spaces. We provide a direct quantitative comparison between the DR in an annulus and that in straight tubes. We can fairly compare the data from the two geometries by using the general definition of the Reynolds number, which is independent of the geometry. With such a definition, the product of the friction factor by Re is a constant in laminar flows. Moreover, the friction factor for a turbulent flow of Newtonian fluids in an annulus fits Colebrook's correlation. Our main results show that the DR is more pronounced in annular pipes than tubes. We believe this is due to the relative increase of the buffer zone in an annular geometry.