The development of a near-wall boundary layer over a flat plate in concentrated pulp fiber suspensions

The development of a boundary layer over a flat plate placed in a square pipe was studied in pulp suspensions at concentrations of 0.5, 1 and 1.8% at flow velocities of 0.24-0.96 m/s. Laser Doppler Anemometry (LDA) was used to measure velocities in the boundary layer from y=0-3 mm at x=0, 5, 10, 20 and 30 mm, where x=0 mm was the position where the flow met the plate. During the measurement time the pressure drop over the plate was also measured and it was concluded that the flow was in the rolling friction regime. The results of boundary layer thickness show that an increase in velocity or a decrease in concentration gave a thicker boundary layer in contrast to a laminar Newtonian boundary layer. In a power law fluid, boundary layer thickness decreases with an increase in shear thinning or velocity. Furthermore, in the pulp suspensions, a boundary layer with a constant thickness was reached 20 mm from the plate edge in contrast to a Newtonian and a shear thinning boundary layer which continued to increase along the plate. The difference in boundary layer development is believed to depend on the two-phase effects in pulp suspensions.