Coherent Structures of Suspension Flow and their Inheritance in Paper

Abstract

Normally, the main purpose of vanes installed in the slice chamber of a hydraulic headbox is to control the tensile strength ratio by affecting the mean fiber orientation of the suspension. The use
of vanes inherently leads to the formation of peculiar vortex structures, referred to here as coherent flow structures (CS), in the downstream flow. These CS are believed to produce non-homogeneity in paper. Although the CS are geometrically three dimensional, their machine direction (MD) and cross direction (CD) components are dominant, cf. Kármán vortex street, and have distinctive characters of their own. The CD component of the vortex maintains its coherent nature better than the MD component and, therefore, its appearance in a form of MD spatial scale is used to express the inheritance of the flow structures in the paper. By using bluff vanes of different thicknesses, it is shown that the CD component of the vortex street maintains its characteristic appearance from the slice chamber to the slice jet. This made it possible to study the inheritance of flow structures in paper with more realistic vanes. By marking particular parts of the flow in the slice chamber with dye streamers the CS were also made visible in the paper. An analysis of these samples reveals that the average scale of ink spots is related to the MD spatial scale of flow structures. Finally, a correlation between the CS of flow and the structural cockling tendency was found. Thus, important complementary information has been created to serve the goal of finding a general linkage between structures in flow and paper. The knowledge of this link would not only allow research and analysis of wet end operations without actually producing paper, but would also provide a means of evaluating wet end conditions in a mill environment by paper analysis.