Axisymmetric entry flow of semi-dilute xanthan gum solutions: prediction and experiment

Experimental observations of the flow patterns in axisymmetric entry for rigid rod molecules are presented. The features of the flow are a large secondary flow vortex whose size is relatively independent of the flowrate, a vortex which increases in size with the rigid rod polymer concentration, decreases in size as the solvent viscosity increases, and increases as the contraction ratio increases. The secondary flow vortex sizes predicted and observed for dilute and semi-dilute rigid fibre suspensions are similar to those observed for the semi-dilute rigid rod xanthan gum solutions. Also, rounding a corner in the contraction had no effect on the observed asymmetric entry flow of the rigid rod molecular solutions. Good agreement was observed between the dimensionless re-attachment length for a 0.04% w/w xanthan gum solution and the predicted secondary re-attachment length from Binding [J. Non-Newtonian Fluid Mech. 27 (1988) 173-189] which used the independently measured extensional viscosity in the prediction. The agreement between experiment and observation confirmed Binding's assertion that the fluid will show vortex enhancement if its steady shear viscosity undergoes shear thinning more rapidly than its extensional viscosity undergoes elongational thinning. Vortex enhancement for fluids with constant extensional viscosity and steady shear viscosity, as observed, agreed with predictions by Debbaut ct al. [J. Non-Newtonian Fluid Mech. 29 (1988) 119-146] and Debbaut and Crochet [J. Non-Newtonian Fluid Mech. 30 (1988) 169-184]. From the data presented it would be fair to speculate that normal stresses impede vortex growth while elongational thickening increases vortex growth. (C) 1998 Elsevier Science B.V. All rights reserved.