COLLISION FREQUENCIES BETWEEN FRACTAL AGGREGATES ACID SMALL PARTICLESIN A TURBULENTLY SHEARED FLUID

Three groups of aggregates with fractal dimensions of 1.89 +/- 0.06, 2 .21 +/- 0.06, and 2.47 +/- 0.10 were generated by coagulation of latex microspheres (2.85 mu m) in a Jar-test (paddle-mixing) device. The co llision rates between these fractal aggregates (200-1000 mu m) and sma ll (1.48 mu m) particles were measured in the turbulent shear environm ent of the paddle mixer at mean shear rates of 2.1, 7.3, and 14.7 s(-1 ). Collision frequencies were 5 orders of magnitude higher than predic ted by a curvilinear model but 2 orders of magnitude lower than predic ted by a rectilinear model. Collision frequencies much higher than pre dicted by the curvilinear collision kernel were attributed to signific ant flow through the interior of the fractal aggregates. The fluid she er rate (G) and the aggregate fractal dimension (D) affected the colli sion frequency function (beta) between fractal aggregates and small pa rticles, resulting in beta similar to G(1-0.33D). According to this re lationship, as D --> O, the aggregates become infinitely porous and P becomes proportional to G(1) as described by a rectilinear collision m odel based on aggregates sweeping out all fluid within their pathway. As aggregates become less fractal and D --> 3, beta becomes relatively insensitive to the magnitude of G as predicted by a curvilinear model .