10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Aerosol Impaction under High Knudsen Number, High Mach Number Conditions for Applications in Additive Manufacturing
CHENXI LI, Bernard Olson, Christopher Hogan Jr., University of Minnesota
Abstract Number: 528 Working Group: Aerosol Modeling
Abstract Aerosol deposition has emerged as a novel additive manufacturing (AM) method for surface coating and thin film production. Though inertial impaction is typically the deposition mechanism, AM based deposition functions in a very different regime from traditional sampling based impactors. Specifically, the AM process involves accelerating supermicrometer particles (rarely submicrometer particles) with a high-speed carrier gas leading to particle at supersonic speeds with the potential plastic particle deformation on the substrate. It is generally acknowledged that the instantaneous particle speed upon impaction is critical to controlling coating/film properties such as porosity, and mitigating particle bounce during deposition. Therefore, to achieve desirable particle impaction speed, converging-diverging nozzles (i.e. de Laval nozzles) are frequently used to accelerate particles.
Though considerable effort has been devoted to simulation of particle trajectories as well as impaction speed, some critical aspects of using de Laval nozzles for particle acceleration are yet to be fully understood. This presentation discusses two issues related to this process. The first issue concerns drag force exerted on particles by carrier gas. As particles traverse through the nozzle, rapid gas rarefication and densification (due to a shock) can lead to particle Knudsen number (Kn) variations by as much as ~100, ranging from the free molecular to continuum regimes. In addition, due to gas stagnation close to substrate or insufficient particle acceleration, particle Mach numbers (Ma, based on relative speed of gas and particles) can exceed unity with potential shockwave formation close to the particle surface. Simulation of particle trajectories, therefore, requires a drag law that is valid for a wide range of of Kn and Ma. The first part of this presentation consists of summary and comparison of the existing drag laws with a focus on range of validity, and proposal of a new drag law based on direct simulation Monte Carlo valid for the range 0 < Kn < infinity and 0 < Ma < 3.
The second issue relates to how operating conditions of the converging/diverging nozzle influences particle impaction speed. Specifically, two factors are discussed: (1) particle size, (2) upstream/downstream pressure of the nozzle. Given other operating conditions, we show that impaction speed is a function of particle size, with an optimum particle size with the maximum impaction speed for a given nozzle geometry. As to upstream/downstream pressure, our simulations demonstrate they change the shock wave structure and have a significant effect on deceleration of nanosize particles by changing the gas pressure close to the substrate.