Evolution of aggregate size and fractal dimension during Brownian coagulation

Fractal aggregate coagulation is described within a general framework of mu ltivariate population dynamics. The effect of aggregate morphology on the c oagulation rate, is taken into account explicitly, introducing in addition to aggregate particle size, the aggregate fractal dimension, as a second in dependent variable. A simple constitutive law is derived for determining th e fractal dimension of an aggregate, resulting from a coagulation event bet ween aggregates with different fractal dimensions. An efficient Monte Carlo method was implemented to solve the resulting bivariate Brownian coagulati on equation, in the limits of continuum and free molecular flow regimes. Th e results indicate that as the population mean fractal dimension goes from its initial value towards its asymptotic value, the distribution of fractal dimension remains narrow for both flow regimes. The evolution of the mean aggregate size in the continuum regime is found to be nearly independent of aggregate morphology. In the free molecular regime however, the effects of aggregate morphology, as embodied in its fractal dimension, become more im portant. In this case the evolution of the aggregate size distribution cann ot be described by the traditional approach, that employs a constant fracta l dimension. (C) 2001 Elsevier Science Ltd. All rights reserved.