| Summary: | The focus of the present work is an experimental study of the transition to turbulent flow of papermaking fibres in a cylindrical pipe. The suspensions used in this study possess a yield stress. With this class of fluid the radial profile in fully developed slow flow is characterized by an unyielded or plug zone. With increasing flow rates the size of the plug diminishes. One of the open remaining 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 as a function of flowrate for dilute, i.e. less than 2% consistency papermaking suspensions in a 50mm diameter pipe. The plug size was determined through analysis of local spatial and temporal variations in velocity, strain-rate, and the fluctuating component of velocity. With these 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 2-60 Pa, depending upon consistency, fibre type and manufacturing methodology. The yield stress measurements were benchmarked against measurement methodologies reported in the literature.
During flow, we observe complex behavior with the plug in which we found that with increasing velocity the plug diminishes through a densification mechanism in response to increasing frictional pressure drop. At higher Re, it diminishes through an erosion type mechanism. We estimate the critical Reynolds number for the disappearance of the plug to be Rec 10⁵.
In perhaps the most unique measurements in this work we find that drag reduction begins when rp/R < 0.8, even when dilute long chain polymers are added to the system. For papermaking fibres, drag reduction displays non-monotonic behavior with plug size. === Applied Science, Faculty of === Mechanical Engineering, Department of === Graduate
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