Project Details
Figure 1: Schematic of the simulated headbox.
Since sheet formation in paper production requires a surface density that is as homogeneous as possible, the effects of previously introduced turbulence should be uniform across the entire width. The rapid immobilization of the fibre network during dewatering means that measures such as additional static formers and vibrating the fabric are only able to improve homogeneity to a very limited extent. In particular, the formation of grooves after the settling lip (see diagram in Figure 1), which is caused by turbulence tubes in the headbox, leads to major problems that increase with the production rate.
Every rebuild and stop of a paper machine means a loss for the manufacturer. Here, our flow simulations offer a cost-effective way of checking several operating points or configurations in a time and material-saving manner before the most promising ones are then tested on the machine.
Shear Rate
Viscosity
To prevent early immobilization of the fiber mesh, the shear rate must not yet approach zero when it hits the screen, as the viscosity increases and the fiber movement decreases as the shear rate decreases. Accordingly, area (1) in the diagram in Figure 3 represents a state with a high shear rate, for example directly after the diffuser tubes, while area (2) represents increasing immobilization with increasing distance from the turbulence source.
Figure 2: Images of the pulp mixtures at various magnifications.
Figure 2 shows the fibers used.
Figure 3: Shear rate-dependent viscosity of different fiber blends.
For this project, the shear rate-dependent viscosity of the pulp suspension and a transient large eddy simulation were used to correctly map the turbulence. The improved knowledge of the flow conditions at the operating point (video from PPP ?), which is accessible through our simulations, gives you a deeper insight into the flow behavior of the fiber suspension, which measurements alone do not allow. Furthermore, it can be seen that so far all turbulence introduced has already dissipated upstream of the breast roll, while turbulence is still visible at the optimized design point when it hits the breast roll. (Video ) Our flow simulations can therefore not only serve as a decision-making aid for planned changes, but also contribute to a better understanding of the processes in your system, which can reveal weak points and be helpful for future decisions.
Our simulations as part of the project show that changing the flow rate and adjusting the consistency can already bring significant improvements. To optimize future paper machines, geometric optimization of the diffuser pipes can also be considered, provided this is structurally possible.