1957 Volume 1

This paper summarises three related investigations the flow characteristics of pulp suspensions, the strength of dilute fibre networks and the flexibility of individual fibres.

When pulp flows through a circular pipe, three regimes of flow (designated as laminar plug, mixed and turbulent flow) may exist, separated by two transition velocities, which, with relatively long-fibred pulps, are sharply defined. Plug flow is characterised by the development of a fibre free wall layer whose existence can be demonstrated in several ways. Mixed and turbulent flow involve the breakdown of the fibre network by the turbulent stresses . Relationships between flow properties and concentration and types of pulp are discussed. Pulp suspensions can exhibit the Weissenberg effect, which may be taken as evidence of their visco-elastic nature.

The tensile strength of pulp suspensions at concentrations down to 0.2 percent. and less is measured by a simple experimental technique. It increases with fibre concentration, fibre length, extent of delignification and of beating. A close correlation is found between the network strength at 0.8 percent. fibre concentration and the conventional wet-web strength at 15 20 percent. It is concluded that the networks and wet webs are held together mainly by interfibre friction. The predicted correlation between the static network strength and the friction factor in turbulent flow is shown to exist.

A method of measuring the flexibility o)f individual fibres is described. The method is based on a classification of rotational orbits of the fibres in laminar shear and provides a measure of the distribution of fibre flexibility in a given sample. It is shown that flexibility increases with increased fibre length, decreased amount of residual lignin and with beating. By means of a series of measurements on pulps cooked to various yields a correlation is shown between fibre flexibility, network strength and the ability of the fibres to suppress turbulence in water.