Flow properties of wood pulp-fiber suspensions in circular pipes

Abstract

The flow characteristics of wood pulp suspensions in circular pipes have been investigated experimentally. In studying the flow mechanism, we mainly consider the yield shear stress, which represents the fiber-network strength in the flocculation of pulp fibers. The experimental results for five regimes, into which the flow was classified on the basis of the behavior of pulp fibers and the flow characteristics, as reported in the author's recent work (2010), were correlated with the fiber concentration Cs in equation of the form τ=aCs^b where τ is the shear stress on the pipe wall and a and b are constant. The yield shear stresses were determined by the measurement of the pressure loss. They are not dependent so much on the pipe diameter and become large with increase of the pulp-fiber concentration. The flocculation of pulp fibers starts to become loose near the pipe wall when a shear stress exceeding about four times the yield shear stress acts on the suspension. The values of the disruptive and dispersive shear stresses are formulated as simple expressions depending on only the fiber concentration. Furthermore, the corresponding critical and turbulent Reynolds numbers are presented. The pressure loss of the pulp suspension in the turbulent flow becomes smaller than that for water, and the ratio of both can be expressed by a simple empirical equation.