Project Details
Figure 1 (left): Basic geometry of the headbox. Figure 2 (right): Geometry optimization of the headbox according to potential theory.
As part of this project, possible geometry optimizations of a simple headbox were investigated. This specific headbox consists of two symmetrical segments, therefore only one segment and thus half the width was actually simulated, but with the correctly set boundary conditions it is possible to exrapolate to the full paper web width. The deviation from the average basis weight of cross-sections of the resulting paper web is used to assess the quality of the simulation results. Figure 1 shows the basic geometry, which is used to determine the distance to the run-out lip of the evaluation plane.
Figure 3: Speed distribution basis.
A particular challenge in simulation validation arises in this project, as the actual cross profile is only accessible for measurements when dry, while the simulation ends in the screen section. Particularly with such orders, where the measurement results cannot be read directly in the simulation, the background knowledge and process understanding of our engineers is required in order to draw the right conclusions and still carry out a successful optimization. It is not only important to take into account the shear rate-dependent viscosity of the pulp used, but also to adjust the remaining boundary conditions according to the operating point in order to identify the real problem areas. Based on the basic simulation, an evaluation level was selected that could be correlated with the measurement data in the best possible way. In the flow optimization phase, the simple initial geometry means that there is only the option of adjusting the external geometry or inserting internals. Our engineers made this decision on the condition that the most uniform cross profile possible is achieved by a uniform mass flow at the outlet lip.
Figure 4: Velocity distribution optimized geometry.
One of the most promising solutions was obtained from an analytical design of the outer contour based on freedom from friction. This geometry based on potential theory is shown in Figure 2. A comparison of the velocity distributions in the middle of the outlet lip height between the base (Figure 3) and optimized geometry (Figure 4) shows an earlier uniformity with fewer peaks at the outlet lip. Our engineers were able to achieve further good results with suitable internals which, in combination with slight external geometry optimization, deliver the optimum result.
