New Experimental Results on the Flow Regimes in Closed Channel Flows of Wood Fibre Suspensions

Abstract

We consider here the behaviour of wood fibre suspension with fibre concentration above that of sedimentation in a pressure driven flow in a straight pipe with smooth walls. The flow behaviour can be roughly divided in two main regimes: the plug flow regime that occurs at low flow rates and the drag reduction regime that occurs at high flow rates. We utilized new experimental methods in order to gain more detailed understanding on the flow behaviour of wood fibre suspensions, and especially on the relevant physical phenomena inducing such behaviour. In addition to carrying out conventional loss experiment, the velocity profiles across the pipe were measured using pulsed ultrasound velocimetry (PUDV) techniques, and the thickness of the lubrication layer in fully developed flow was measured using a laser optical device. Based on our direct measurements, we were able to identify five different flow regimes in suspension flows. In addition, we refined the qualitative picture of these flows in relation to the forming of fibre plug and to the physical phenomena taking place in transition from one flow regime to another one.