Simulation of low-density gas droplet supersonic flows expanding into vacuum
Abstract
Simulation of low-density supersonic free jets is considered in application to vacuum technology processes. In particular, results have been obtained for thin film deposition processes for copper. The vapor phase of monatomic copper is simulated using the direct simulation Monte Carlo (DSMC) technique. The growth rates obtained from the DSMC simulations are compared with measured growth rates. It is shown that, by simultaneously calculating the mass flow rate and the Knudsen number accurately, excellent agreement is obtained between simulated and measured growth rates. Based on this comparison, parameters of a variable hard sphere molecular model for copper vapor have been determined. Three different compact models for the dependence of the normalized growth rate profiles on the Knudsen number are considered and compared with each other. Droplet transport including momentum and energy coupling between the gas and droplet phase are used to compute the trajectories of droplets originating at the throat and lip of two different nozzles. The maximum droplet radius reaching the substrate and the variation of droplet size with angle predicted by the trajectory computations agrees well with the measurements using Optical Microscopy. Droplet formation models using the Classical Nucleation Theory (CNT) and droplet growth using the Hertz-Knudsen equations are also considered. It was found that these models do not predict the formation of micron-sized droplets that are observed in the deposition experiments.
Degree
M.S.A.A.
Advisors
Alexeenko, Purdue University.
Subject Area
Physical chemistry|Aerospace engineering
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