ENTRANCE EFFECT IN CAPILLARY-FLOW OF DILUTE AND SEMI-DILUTE POLYMER-SOLUTIONS

The total extra pressure losses (end effect) in capillary tubes of very small diameters (from 0.1 to 0.5 mm) was studied for dilute polymer solutions of hydrolyzed and non-hydrolyzed polyacrylamide. The onset of the end effect is related to a critical wall shear rate {A figure is presented}. The visualization of the flow patterns in the entrance of the capillary shows that for {A figure is presented} the flow converges sharply into the die bounded by large secondary flow. The end effect is observed whatever the diameter of the capillary used, but the results indicate that in order to avoid a perturbation of the entrance flow by inertia effects a very small diameter capillary must be used. The total extra pressure loss ΔPend is proportional to the polymer concentration and increases with the shear rate, according to power-laws, in which the exponent is a function of the shear rate range. The end effect is interpreted in terms of the elongational flow in the entrance of the capillary tube; when the inertia effects are negligible, the entrance converging flow is essentially governed by the elastic properties of the polymer solutions, and can produce large extension of polymer molecules; the energy dissipated by viscous friction on the stretched molecules can be represented by ΔPend. © 1979.