OBSERVATIONS OF COAGULATION IN ISOTROPIC TURBULENCE

Turbulent-shear-induced coagulation of monodisperse particles was exam ined experimentally in the nearly isotropic, spatially decaying turbul ence generated by an oscillating grid. The 3.9 mu m polystyrene micros pheres used in the experiments were made neutrally buoyant and unstabl e by suspending them in a density-matched saline solution. In this way , particle settling, double-layer repulsion and particle inertia were negligible and the effect of turbulent shear was isolated. The coagula tion rate was measured by monitoring the loss of singlet particles as a function of time and reactor turbulence intensity. By restricting co nsideration to experimental conditions where the singlet concentration was in excess, the effect of higher-order aggregate (i.e. triplet) fo rmation was negligible and nonlinear regression using an integral rate expression that included terms for doublet formation and breakup was used to obtain the turbulent coagulation rate constant. The strength o f the van der Waals attractions was characterized with the Hamaker con stant obtained from Brownian coagulation experiments. Since particle b ulk mixing was fast compared to the coagulation rate, the observed coa gulation rate constants were averages over the local coagulation rates within the grid-stirred reactor. Knowledge of the spatial variation o f turbulence within the reactor was necessary for quantitative predict ion of the experiments because model predictions for the coagulation r ate are nonlinear functions of shear rate. The investigation was condu cted with particles smaller than the length scales of turbulence and s ince the smallest turbulent length scales, the Kolmogorov scales, have the highest shear rate they controlled the rate of particle aggregati on. The distribution of the Kolmogorov shear rate at various grid osci llation frequencies was obtained by measuring the turbulent kinetic en ergy (E) using acoustic Doppler velocimetry and relating E to the Kolm ogorov shear rate using scaling arguments. The experimentally measured turbulent coagulation rate constants were significantly lower than th eoretical predictions that neglect interparticle interactions; however , simulations that included particle interactions showed excellent agr eement with the experimental results. The favourable comparison provid es evidence that the computer simulations capture the important physic s of turbulent coagulation. That is, particle transport on length scal es comparable to the particle radius controls the rate of turbulent sh ear coagulation and particle interactions are significant.