Aerosol dynamics in porous media

In this thesis, a computational model was developed for the simulation of aerosol formation through nucleation, followed by condensation and evaporation and filtration by porous material. Understanding aerosol dynamics in porous media can help improving engineering models that are used in various industrial, medical and environmental applications. Within the Euler-Lagrange framework of modeling two-phase flow, the trajectories of individual aerosol droplets, as well as the heat and mass transfer with their surroundings, were evaluated in velocity, temperature and species concentration fields that were computed by applying the immersed boundary (IB) method to flow in complex domains. Focusing mainly on rather dilute situations the so-called ‘one-way’ coupling approximation was adopted that allows to separate the problem of determining the flow field from the problem of tracking the motion of inertial droplets in that flow field. Following this approach, in Chapters 2 and 3 we concentrated on the problem of filtration of droplets by porous filters. First, we focused our attention on particle deposition on the solid filter surface due to inertial impaction. A numerical approach was described to simulate the motion of a large number of particles suspended in a gas flow that avoids numerical filtration of massless/passive particles. We considered two structured porous media in 3D, composed of in-line and staggered arrangements of square rods. It was established that the inner structure of a porous medium strongest influences the deposition of particles. In staggered geometries filtration appeared to depend strongly on particle inertia suggesting that the staggered geometry can be used to separate particles according to their Stokes number. The ‘no-slip consistent’ particle tracking described in Chapter 2 is formulated entirely in terms of the phase-indicator function related to the inner structure of the filter. This enables adapting this method directly to more complex filter geometries which was done in Chapter 3, where we considered dynamics of droplets in a realistic porous filter. In this chapter, the dynamics of droplets was governed both by Stokes drag and Brownian motion. The effects of inertial motion and Brownian diffusion on the filtration characteristics were first illustrated for flow through a straight pipe. Subsequently, the filtration characteristics of a steady flow through a realistic porous material were determined, illustrating the potential of the approach in terms of predicting such macroscopic aspects based on pore-resolved flow. High filtration efficiency was observed in case of dominant Brownian motion or dominant inertial motion and a reduced filtration efficiency was found for droplets of intermediate size. Filtration of already generated aerosol droplets can be regarded as an indirect way of controlling the aerosol that eventually emanates from a process. A more direct way implies control over the conditions at which the aerosol actually forms. This involves coupling of the fluid flow with the process of nucleation and subsequent evolution of the aerosol properties due to evaporation and condensation. We restricted ourselves to single-species aerosols and adapted the classical nucleation theory (CNT) which links locally supersaturated vapor state to the nucleation of so-called ‘critical clusters’. The nucleation rate from CNT is adopted in the Euler-Lagrange framework as the probability per unit of time and volume to generate such critical clusters. Subsequent growth of a newly formed droplet can arise from further condensation of vapor molecules onto the droplet, thereby influencing the local vapor concentration and temperature fields. This computational model was applied to a laminar flow in a channel between two parallel plates. Nucleation was initiated by rapid cooling of air saturated with dibutylphthalate vapor at the inflow of the channel. Due to a sharp temperature drop at some location along the channel a supersaturated state is achieved, thereby inducing droplet nucleation. This approach illustrates a first application of the Euler-Lagrange framework to aerosol formation and presents aspects such as the evolving droplet size distribution and characteristics of the aerosol as it emanates from the end of the channel. It is a basis for studying the dependence of the aerosol formation process on important process parameters such as the temperature, the cooling rate and the flow velocity.