Fluid mechanics of fibre suspensions related to papermaking - DiVA

CHAPTER 4
Summary of the papers
Paper 1
The first paper summarizes the pressure and wire position measurements performed
in an experimental facility called the KTH-Former. The KTH-Former
intends to model mechanisms in the roll forming zone of a paper machine. It
includes a headbox, a roll and a wire, which can reproduce a one-sided dewatering
event. Measurements were carried out with pure water for three different
wires: non-permeable, semi-permeable and conventional open wire. Although
not useful in papermaking, the non-permeable wire is relevant when trying to
understand the mechanisms of roll forming. In the study, the pressure profile
and the distance between the solid roll and the wire was measured along the
wrap angle on the roll. The wrap angle is measured from the position were
the free-jet from the headbox impinges on the wire to the point where the wire
separates from the roll, Figure 4.1. Depending on the wrap angle the forming
roll can result in partial or complete roll dewatering. For the case of partial
dewatering the web is still saturated at the point where it separates from the
roll. The measured pressure distribution showed a more complex pattern than
the simple model p = T/R, which normally is referred to as the nominal pressure
(T is wire tension and R roll radius). An example of an obtained pressure
distribution for the non-permeable wire is shown in Figure 4.3.
It is shown that an increase in wire tension has a similar effect as a decrease
in flow-rate (jet flow rate from the headbox) on the shape of the pressure
distribution. This is a consequence of that the flow to a large extent is governed
by the relation between the dynamic pressure and the nominal pressure. For
the case of partial dewatering the suction peak that appears at the roll-wire
separation point has a strong influence on the pressure distribution upstream.
Finally, it is shown that the drainage has a stabilizing effect on the dewatering
pressure.
Paper 2
This paper formulates a physical model that is intended to explain fundamental
mechanisms that control partial dewatering in roll forming in papermaking.
The flow around the forming roll is modelled in a cylindrical coordinate system
and the wire is assumed to be impermeable. The governing equations are
reduced based on a discussion where the magnitude of the different terms is
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