Concentric mixing of hardwood pulp and water.

APPLICATION:This study demonstrates the importance of high velocity ratio in producing a uniform stock mixture in concentric thick stock dilution before the fan pump.

Concentric mixing before the fan pump, if not done properly, can significantly affect the spatial and temporal consistency and chemical uniformity of the stock leaving the approach flow area, leading to severe MD and CD nonuniformities in the final sheet. TAPPI recently published approach flow guidelines for concentric mixing used for thick stock dilution before the fan pump. However, in view of the importance of thick stock dilution, a rigorous analysis of concentric mixing was in order, along with a reexamination of the velocity ratio criteria.

We completed concentric mixing experiments with velocity ratios of up to 6 using hardwood pulp of 1.0%, 1.9%, and 2.9% consistency and water. By increasing the velocity ratio (ratio of inner:outer jet velocity), the inner jet spread angle is found to be larger and the downstream mixing region uniform. Furthermore, local consistency measurements show a flattening of the concentration profile with increasing velocity ratio, confirming mixing improves as velocity ratio increases.

For the fiber stock tested, mixing is significantly dependent on the stock consistency when the velocity ratio is small (Rv @ 1). This result indicates that shear stress and turbulence required to fully dislodge the fiber network are not delivered by the fluid streams. Mixing results from hydrodynamic instabilities and macroscale variations. which lead to downstream nonuniformities.

At higher velocity ratios, when the flow is turbulent, mixing is significantly affected by the velocity ratio, but there is no clear indication that mixing is affected by the stock consistency. These trends are evidence that once the fiber network strength is overcome by shear stress and turbulence, the mixture behaves as a conventional Newtonian fluid in turbulent flow. Mixing at high velocity ratio results from microscale turbulence that leads to a relatively uniform downstream mixture. View this paper online at http://www.tappi.org/index.asp?pid=29255

Aklilu T.G. Giorges is with Georgia Institute of Technology, Atlanta, GA 30332, USA; David E. White is with Institute of Paper Science and Technology, Atlanta, GA 30332, USA; Theodore J. Heindel is with lowa State University, Ames, lowa 50011, USA. Email White at [e-mail].