Fiber-Concentration Distributions of Papermaking Pulp Suspension Flowing in a Channel with a Sudden Expansion

Abstract

In this study, we analyze the flow characteristics of pulp suspensions in a sudden expansion channel, which is used as a tube bank in papermaking machines. Flow visualization using a light section method and measurements of time-averaged and fluctuating fiber concentrations, Ca and C’, and pressure, were performed. The experiments were carried out on pulp suspensions with a bulk fiber concentration (Cs) of 0–2.0 wt% in a channel with an expansion ratio of   2 at varied average velocities (Ua) of 0.046.6 m/s. We examined the effect of flow velocity on the changes in concentration and pressure distributions along the channel axis. The distribution of the pulp fiber concentration behind the sudden expansion significantly changes depending on the flow state in the upstream channel before the expansion. In particular, for low and moderate velocities, the flow field is characterized in the main flow region consisting of flocculated plug fibers and a recirculation region with a low concentration near the expansion corner by a type of streamline originated from the thin water-layer in the upstream channel. The distribution of Ca shows a tongue-shaped structure in the central part of the cross section, and C’ is large near the annulus streamline. As the velocity is further increased, the flocculated fibers gradually disperse, and the distributions of Ca and C’ become almost uniform. In addition, the uniformity of fiber concentration does not change much in the section downstream from a distance 10 times the step height from the sudden expansion plane.