FINE-PARTICLE DEPOSITION IN LAMINAR AND TURBULENT FLOWS

The deposition of fine silica and polystyrene spheres was measured for conditions of laminar and turbulent flow (960 less than or equal to R e less than or equal to 16040) in a rectangular channel using image an alysis. The plate glass deposition surfaces were rendered positively c harged by coating them with a cationic copolymer while, under the wate r chemistry conditions employed, both types of particles were negative ly charged. It was found that, contrary to the results for laminar flo w, the initial deposition rates in turbulent flow decreased with incre asing Re, indicating that deposition was no longer mass-transfer contr olled and that particle attachment played an increasingly important ro le as Re was raised. Attachment was modelled as a rate process in seri es with mass transfer in which the attachment rate varies inversely as the square of the friction velocity. Under the conditions of the pres ent experiments, no particle re-entrainment was observed, so that the declining rate of particle accumulation on the wall recorded in each r un could only be attributed to a declining deposition rate. Even where asymptotic accumulations were reached, particle coverages never excee ded 3.5%.