Motion of pulp fibre suspensions in rotary devices

Suspensions of semi‐bleached kraft pulp at low and medium mass concentrations were sheared in a rotary shear tester at angular velocities up to 524 rad/s (5000 rpm). The use of baffled housings, with rotors having lugs, imposed shear within the body of the suspensions and prevented slip at the vessel walls. The torque was measured as a function of angular velocity, and high speed photographs were taken through a transparent plate to aid in interpretation of the findings. The torque vs. angular velocity curves and flow phenomena were found to depend on the mass concentration of the suspension, the gap width between the rotor and housing, and the air content of the suspension. With little gas present, the flow initially involved tangential flow in an active cavity whose radial extent increased with increasing angular velocity. When the flow reached the outer baffles, a marked flow transition occurred, probably explaining what has been called “fluidization” by Gullichsen and Harkonen (1981). As the gas content of the suspension increased, the gas congregated around the rotor, impeding the transfer of momentum to the suspension. The apparent viscosity of a 10% by mass suspension was of the order of 16 Pa · s under the test conditions.