Date of Award
Bachelor of Science
Dr. Dewei Qi
The objectives of this thesis are 1) to investigate the physical reasons of flocculation, which contribute to nonuniformity in the CD through interactions between fibers in a turbulent flow and 2) to study the effects of velocity profiles in the headbox on variations in the CD by utilizing a computer simulation.
Fluid velocities emerging from the headbox vary greatly due to the turbulence generating tube banks. As the fluid exits the tube bank and enters the converging portion of the headbox, patterns formed from the bottom tubes are extended throughout this convergence. This velocity profile is caused by fluid flow characteristics consisting of maximum flow at the center of the tube and velocity at the tube wall that approaches zero, with the bottom tube bank being dominant. With the addition of-fibers to this flow, the velocity gradients tend to rotate fibers to the MD. Upon densification these fiber alignment streaks in the MD contribute greatly to CD variations as localized variations in density.
Fiber flocculation is another phenomena that is important to the reduction of CD variations in paper. Local mechanisms extracted from fluidization with water of spherical and cylindrical particles falling against gravity are applied to micro-fiber flocculation. There are essentially three mechanisms to this phenomena: drafting, kissing and tumbling, where interaction in the wake causes flocculation. When fluidized, one fiber is drawn into the wake of another, termed drafting. The second fiber quickly accelerates to make intimate contact with the first, called kissing. Once fibers kiss, they quickly become intertwined due to vortex shedding and create a micro-floe that will continue to grow until turbulence is encountered. This interaction is responsible for the agglomeration that leads to micro-fiber flocculation in the converging portion of the headbox where turbulence is at a minimum.
Turbulence is generated by a strip placed on the floor of the headbox perpendicular to the direction of flow. The addition of this strip would add turbulence to the lower portion of the headbox creating a more uniform velocity profile and reducing fiber agglomeration and micro-fiber flocculation. The turbulence distributions are numerically simulated utilizing FLUENT™.
Standerfer, Erik M., "New Mechanisms of Micro-Fiber Flocculation and Computer Simulation of Headbox Flow" (1997). Paper Engineering Senior Theses. 513.