LASER DOPPLER MEASUREMENT OF THE AXIAL VELOCITY DISTRIBUTION IN THE CONVERGENT FLOW OF A POLYMERIC FLUID INTO A RECTANGULAR SLIT

Abstract

The center line velocity distributions in the convergent flow of a silicone oil as a Newtonian fluid and a solution of silicone rubber ia silicone oil as a non-Newtonian fluid into a rectangular slit of various entrance angles have been measured by means of laser Doppler velocimetry. The non-Newtonian fluid used conformed to the behavior of a power-law fluid at shear rates greater than 20 s -1 with a non-Newtonian index n =0.76. The convergent flow studied had a contraction ratio of 12, with slit height of 0.83mm, slit length to height ratio of 18.1, slit width to height ratio of 14.3 and the width to height ratio before entering the slit of 1.19. Flow experiments were done at room temperature with shear rates at the slit wall of (1.7—11)·10 3 s -1 and generalized Reynolds number 0.21—2.2. No elastic turbulence was observed in front of the slit entrance. The center line velocity gradient along the flow direction was appreciably reduced in the flow cells of small entrance angles. Two new phenomena have been observed for the convergent flow of the non-Newtonian fluid into a rectangular slit of 180 entrance angle: (ⅰ) a deceleration region in front of entrance of the slit which is more pronounced at higher flow rates and shows a minimum velocity at four slit heights in front of the slit entrance; (ⅱ) an increase of the center line velocity in the slit up to 5% at three half slit heights before exiting from the slit. These abnormal behavior might be an up-stream phenomenon pro-indicating the oncoming of a stepwise change of flow velocity, peculiar to the convergent flow of a visco-elastie non-Newtonian fluid.