Flow of concentrated fiber suspensions over a backward facing step studied using LDA

Laser Doppler Anemometry (LDA) was used to study the flow structures of pulp suspensions over a backward facing step at two step heights (5 and 10 mm). Experiments were conducted at four consistencies (1, 1.5, 2 and 3%) and two pre-determined free stream velocities (1.3 and 1.8 m/s). The flow structures in the suspensions were compared with measurements made in water. The experimental results showed that it was possible to measure recirculation flow, mixing layers, reattaching flows as well as different flow regimes with LDA. Further measurements in turbulent concentrated pulp flows are needed to distinguish and characterize turbulent structures. Results show that the reattachment length decreased along with a decrease in velocity or an increase in consistency in pulp suspensions with a consistency of at least 1.5%. At 1% a decrease in velocity gave an increase in the reattachment length probably due to a change in the flow regime from laminar to transition or turbulent.