Solving the Coagulation Equation by the Moments Method

TL;DR

This paper introduces a moments-based method for efficiently solving the coagulation equation, reducing computational costs while accurately tracking the evolution of particle size distributions in various regimes.

Contribution

The paper presents a novel moments method that simplifies solving the coagulation equation for arbitrary collision kernels, enabling faster computations in particle evolution models.

Findings

Good agreement between moment solutions and direct integration results.

Method effectively handles different collision kernels and particle porosities.

Significant reduction in computational time compared to full distribution tracking.

Abstract

We demonstrate an approach to solving the coagulation equation that involves using a finite number of moments of the particle size distribution. This approach is particularly useful when only general properties of the distribution, and their time evolution, are needed. The numerical solution to the integro-differential Smoluchowski coagulation equation at every time step, for every particle size, and at every spatial location is computationally expensive, and serves as the primary bottleneck in running evolutionary models over long periods of time. The advantage of using the moments method comes in the computational time savings gained from only tracking the time rate of change of the moments, as opposed to tracking the entire mass histogram which can contain hundreds or thousands of bins depending on the desired accuracy. The collision kernels of the coagulation equation contain all the necessary information about particle relative velocities, cross-sections, and sticking coefficients. We show how arbitrary collision kernels may be treated. We discuss particle relative velocities in both turbulent and non-turbulent regimes. We present examples of this approach that utilize different collision kernels and find good agreement between the moment solutions and the moments as calculated from direct integration of the coagulation equation. As practical applications, we demonstrate how the moments method can be used to track the evolving opacity, and also indicate how one may incorporate porous particles.