The apparent yield stress of pulp fiber suspensions

The apparent yield stress is one of the most important rheological properties of pulp suspensions in designing process equipment for the pulp and paper industry. Therefore, determining a reliable apparent yield stress measurement technique is of importance, not only for pulp suspensions but for any fluid exhibiting yielding behavior. In this work two established and extensively used methods for determining apparent yield stress are compared with a velocity profile determination technique using ultrasonic Doppler velocimetry. The apparent yield stresses are determined for various commercial pulp suspensions at fiber mass concentrations ranging from 0.5 to 5 wt %. The results are compared and models are proposed to represent the apparent yield stress of pulp suspensions as a function of fiber mass concentration. It is concluded that the apparent yield stress measurements obtained using the local velocity profile determination technique are the most reliable. (c) 2010 The Society of Rheology. [DOI: 10.1122/1.3473923]