Near-wall dispersion, deposition and transformation of particles in automotive exhaust gas aftertreatment systems
Conference contribution, 2017
Particles are present in exhaust gas aftertreatment systems in many shapes and sizes. Combustion-generated nanoparticles span a size range from the nanoscale to the microscale, and the droplets introduced in urea-based selective catalytic reduction systems are one to two orders of magnitude larger still. Successful optimization of aftertreatment systems for pollution control relies on the existence of numerical tools to predict the momentum, heat and mass transfer between these types of particles and the surrounding gas phase. Such tools can only be readily obtained if our fundamental understanding of the phenomena pertaining to the particle behavior is correct. The main aim of the current lecture is to discuss our present understanding of these phenomena and the consequences recent advances in this field have had on the way we model and measure particles in these systems.
The small nano-particulates present in automotive exhaust gas aftertreatment systems are typically described as spherical and inert, closely following the gas phase streamlines apart from a superimposed Brownian motion. We will review when this picture provides a sound basis for the development of mathematical models of particle behavior, and when the underlying assumptions break down. More specifically, significant emphasis will be placed on the behavior of real particulate matter, produced by an internal combustion engine. We will show that the deposition of such particles in an automotive catalyst substrate cannot in general be well described by the aforementioned modelling approach, as particle transformations become active inside the substrate channels, altering the apparent deposition efficiency. A conceptual model is proposed that is able to explain the initially observed discrepancies between measurements and simulations, by describing the particulate matter as a mixture of three different types of particles: truly inter particles, semivolatile particles and completely volatile particles. The conceptual model is corroborated by experimental and numerical investigations into the behavior of truly inert particles in automotive catalyst substrates.
Finally, the role of particle inertia on the particle dispersion and deposition is also evaluated by experimental and numerical investigations of more complex substrate channel designs. In connection to this discussion, the role of turbulence on the particle motion and transformation in reactive droplet flows will also be briefly assessed.