Characterization of Pulp Suspensions, 1996 Engineering Conference Proceedings

Robert L. Powell, Matthew Weldon, Sitaram Ramaswamy, Michael J. McCarthy

University of California

Nuclear magnetic resonance imaging (NMRI) is used to study the flow of pulp suspensions undergoing steady flow in a tube. Phase flow encoded NMRI is used to determine average velocity profiles across the tube and these are coupled with pressure drop measurements. NMRI allows non-invasive velocity measurements in opaque pulp suspensions and thereby overcomes the drawbacks of other techniques such as laser Doppler and hot film anemometry. At the flow rates and concentrations studied, the suspensions undergo mixed flow in which there is plug flow in the center of the tube, and a high shear region near the wall. The size of the plug decreases as the mean flow rate increases. Using the pressure drop measurements, the shear stress distribution can be calculated and its value at the plug surface can be estimated which in turn can be considered a measure of the suspension yield stress. Additional information can be gained from the NMRI signal intensity measurements at each point in the velocity profile, which are related to the local turbulence intensity. Both the effects of fiber concentration and fiber type (which relates to aspect ratio, stiffness, etc.) will be discussed. Independent determinations of the pulp yield stress using a Haake Constant Stress Rheometer will be presented.