Lagrangian Based Analysis of Particle Concentration Decay in Chamber and Room Environments

Abstract

In indoor aerosol transport models as well as aerosol chamber reactor models, a common assumption is that the particles are well-mixed spatially, and deposition of particles on walls and surfaces is modeled via a first order kinetic approach wherein the deposition rate is equal to the product of the aerosol concentration and a deposition rate (loss) coefficient. The loss coefficient is frequently measured experimentally by loading a system with particles of interest, and monitoring particle decay over time. Though common practice, previous research studies point towards ambiguity and time varying particle loss coefficients. While this is often attributed to experimental uncertainty, the aim of this thesis is to reexamine some of the baseline implicit assumptions in these experimental protocols by introducing a new model to determine these coefficients. Specifically, a Lagrangian point of view is adopted which assumes that each particle follows a unique trajectory in space. The model presents a more rigorous analysis of the numerical technique used to determine particle deposition rates and accounts for spatial variation in particle concentration and time dependence and non-linearity of particle deposition rates. A combination of Eulerian-Lagrangian approaches is used to test the applicability of this model in a test scenario by simulating the fluid phase using Computational Fluid Dynamics (CFD) and the discrete particle phase by using particle equations of motion. Upon reexamination of some of the baseline implicit assumptions in the techniques reported in literature, it was observed that, the particle deposition rate is time dependent, does not follow a semi-log linear relationship, is non-constant, is affected by variation in spatial concentrations of particles and the local characteristics of the fluid phase affect the nature of the particle concentration decay curve. It was observed that varying the spatial particle concentration values lead to a 13% variation in the value of the particle deposition rate.