Pseudo-laminarization effect of dilute and ultra-dilute polymer solutions on flows in narrow pipes

The properties of dilute and ultra-dilute polymer solutions flowing in a narrow pipe (inner diameter: 680, 400, and 125 μm) were evaluated experimentally by measuring the pressure drop at constant flow rate. Deionized water, silicone oil, and several aqueous solutions of polyacrylamide (PAA), polyethylene oxide (PEO), and xanthan gum (X-Gum) were used. In viscosity measurements, 100- and 10-ppm solutions exhibited non-Newtonian viscosities whereas 1-ppm solutions exhibited Newtonian viscosities. We applied a power-law model to the non-Newtonian viscosities and estimated a generalized Reynolds number. Moreover, we focused on the transition and turbulent regions because the flows being observed were through narrow pipes. For water and silicone oil, good agreement was obtained between the resultant pressure drop and the predicted value. Furthermore, the critical Reynolds number for water and silicone oil was approximately 1.8×103. In contrast, laminar flow was maintained in the transition region for the dilute and ultra-dilute polymer solutions. In the 125- μm capillary flows, the maximum critical Reynolds numbers for the ultra-dilute polymer solutions were 2.4×103 (PAA), 3.9×103 (PEO), and 3.0×103 (X-Gum). Even though ultra-dilute solutions were used, pseudo-laminarization was obtained. To understand the experimental results, we estimated the first normal stress difference. All such values were correlated with the wall shear rate. We conclude that the pseudo-laminarization can be associated with the elasticity of the polymer solutions and is typified by the appearance of strong elastic properties in small-scale flows.