Unstable entry flow of polymer solutions

Flow visualization experiments of shear-thinning aqueous polyacrylamide solutions through a tubular contraction were carried out using the tracer method for studies on the transition from the steady flow regime to the unstable now regime and on the structure of the unstable flow. It has been found that two basic transition patterns can be identified according to flow properties of polymer solutions and contraction ratio. The one is observed in rather dilute solution flows through a tube with a small contraction ratio. In this case, the secondary flow vortex decays with an increase in flow rate, then the unstable flow becomes observable when the secondary flow is almost completely diminished. In the unstable flow there are several stream bundles, which consist of two counterrotating vortex tubes, i.e. Goertler-like vortices, and three dimensional lip vortices between them. The other is observed in rather concentrated solution flows through a tube with a large contraction ratio. In this case, the secondary flow vortex grows with an increase in flow rate. The unstable flow is characterized by the fact that the secondary flow vortex grows and decays alternately. Furthermore, it has been confirmed that flow observations near the entrance face are useful and fundamental means for understanding of three dimensional flow structure as well as detailed structure of the secondary flow vortex.