LOW REYNOLDS-NUMBER FLOW VISUALIZATION OF LINEAR AND BRANCHED SILICONES UPSTREAM OF ORIFICE DIES

The results obtained from a series of experiments dealing with the flow of four polydimethylsiloxanes (PDMS) having different molecular characteristics through an orifice are reported. The use of adequate methods of visualization has permitted us to obtain the structure of both stable and unstable recirculating vortices developing upstream of the orifice, and to measure the velocity profiles on the axis by analyzing the photonegatives. The whole set of experimental results, including rheometrical data and the measurement of the total pressure loss, permits the evaluation of non-dimensional parameters typical of the flow. The effect of shear-thinning, elongation, and elasticity have been quantified and discussed by using current results available in the literature. Finally, this study provides a fundamental experimental basis, which may be quite useful for the validation of numerical models using appropriate constitutive laws relative to mixed flows of viscoelastic materials with axisymmetrical boundaries.