The focus of the present work is an experimental study of the transition to turbulent ow of papermaking fibre suspensions in a cylindrical pipe. The suspensions used in this study possess yield stress. With this class of fluid the axial profile in fully developed slow ow is characterized by an unyielded or plug zone. With increasing ow rates the size of the plug diminishes. One of the remaining open questions with these suspensions is the role of the plug during transition.
In this work we characterize the size of the plug using ultrasound Doppler velocimetry (UDV) as a function of ow rate for dilute, i.e. less than 2% consistency, papermaking suspensions in a 50 mm diameter, 10 m long cylindrical pipe. The plug size was determined through analysis of local spatial and temporal variations of the velocity, strain- rate and the fluctuating component of velocity. With this, we were able to estimate the yield stress of the suspension through knowledge of the applied pressure gradient and find the yield stress to be in the range of 2–10 Pa, depending upon the consistency and Reynolds number Re. We observe complex behavior with the plug in which we see initially that with increasing velocity, the plug diminishes through a densification-type mechanism in a response to an increase frictional pressure drop. At higher Re, it diminishes through an erosion-type behavior. We estimate the critical Reynolds number Rec for the disappearance of the plug to be Rec ~ 105.