FLOC RUPTURE AND RE-FLOCCULATION IN TURBULENT SHEAR FLOW

The dynamics of fibre suspension flow, especially breakup and reformation of fibre flocs in a closed channel flow past a forward facing step was studied experimentally using fast CCD camera imaging and image analysis techniques that allow for simultaneous measurement of floc size and turbulent flow field of fibres. The recirculation eddy downstream of the expansion step was found to exists only if the step height exceeds mean fibre length. When existing, the behavior of the eddy is similar to that of Newtonian fluid flows. Experimental correlations between floc size and turbulent flow quantities were found indicating that the ratio of the minimum floc size found at the end of the recirculation region and the floc size upstream of the step is strongly correlated with the size of the largest scales of the turbulent field and less directly with the total turbulent intensity of the flow immediately after the step. In addition, an approximate power law scaling behavior of the floc size with the turbulent intensity was found within the decaying turbulence region downstream of the recirculation eddy.