Entry Flow of Polymer Solutions through Multi-Hole Abrupt Contractions

Abstract

Flow visualizations and velocity measurements for entry flows of shear-thinning aqueous solutions of polyacrylamide through three kinds of multi-hole contractions have been carried out using the tracer method and LDV technique. In these experiments effects of the number of contractions, distance between contractions and rheology of solutions on the flow structures have been studied. For the multi-hole contractions square arrays of 9 holes and 25 holes were used. In the steady flow regime dimensionless lip vortex length is almost the same for all multi-hole contractions. Swirling-like vortices are also observed for both three-by-three and five-by-five contractions with dimensionless pitch 4, while two-dimensional lip vortices are observed for the three-by-three contraction with dimensionless pitch 8. It is found from LDV measurements that periodic oscillations of the main flow occur at the onset of unsteady flow for all multi-hole contractions and the frequency of the oscillation increases as the Weissenberg number increases. For multi-hole contractions, flow interaction through each contraction is very important factor which decides both steady and unsteady flow structures.