Fiber Orientation in Fiber Suspension Flow

Abstract

We investigate experimentally the orientation of the fibers in the flow of both Newtonian and viscoelastic fluids through a parallel plate channel and a channel with an abrupt contraction. We obtain quantitatively the development of the fiber orientation from initial orientation to stable equilibrium orientation along the channel and the variation of orientation in the direction of the channel width. We also study, in the flow through a channel with an abrupt contraction, the fiber orientation along the stream lines which can be found as the path lines of small tracer particles suspended in the fluid. The relation between the fiber orientation and the velocity field is examined. The effects of shear thinning and elasticity on the fiber orientation are also discussed by comparing the results in the viscoelastic flow with those in the Newtonian flow. The conclusions are summarized as follows : (1) The fibers orient well along the stream lines with an increase in viscoelasticity in both a parallel plate channel and a channel with an abrupt contraction. This phenomenon is essentially equivalent to the phe nomenon that a slender body, namely a straight circular cylinder with a large length to diameter ratio, rotates towards a vertical orientation when falling through quiescent polymer solutions. Both shear thinning and elas ticity, thus, strongly contribute to the orientation process of fibers ; however, elasticity plays a dominant role. (2) The drastic change of the velocity field is due to viscoelasticity and flow rate in a channel with an abrupt contraction. The fiber orientation state, thus, is strongly influenced by these factors ; the fibers align in a manner of wine-glass shape as the contraction is approached, and the fiber orientation abruptly expands in the immediate downstream region of the contraction in a highly elastic fluid flow.