Generation of streamwise vortices in a slice knife model: can streaks be generated at the slice exit?

APPLICATION: The slice knife can generate streaks by producing streamwise vortices in flow entering the wire section of a paper machine. Modern headbox designers are eliminating the slice knife.

How do headboxes generate streaks? In answering this question, these researchers used digital particle image velocimetry (DPIV) to study the way a fluid flows past a channel terminated by a partial blockage. The blockage represents the slice knife, and the experiments were carried out in a water tunnel under conditions similar to those of actual headboxes.

Mean and instantaneous DPIV vector fields showed an unsteady spanwise (CD) vortex forming in the corner formed by the top nozzle wall and knife. More importantly, turbulent streamwise vortices are also generated. In size and spacing, the streamwise vortices were found to correlate closely to streak spacings found in paper. Consequently, the slice knife probably does contribute to streak formation. Fortunately, headboxes are now being designed without slice knives View this paper online at http://www.tappi.org/index.asp?pid=28861

Tsun-Ya Hsu and Timothy Wei are with the Dept, of Mechanical and Aerospace Engineering, Butgers, The State University of New Jersey, Piscataway, New Jersey 088548058. Email Wei at [e-mail].