Direct Molecular Dynamics Simulation of Nucleation during Supersonic Expansion of Gas to a Vacuum

We develop a methodology for direct molecular-level simulation of adiabatic expansion of gas through a small orifice to a vacuum. The gas attains supersonic speeds, cools, and nucleates. The proposed approach combines equations of frictionless fluid dynamics with molecular dynamics simulation in an...

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Bibliographic Details
Published in:Journal of chemical theory and computation 2018-05, Vol.14 (5), p.2332-2340
Main Authors: Klíma, Martin, Kolafa, Jiří
Format: Article
Language:English
Subjects:
Computer simulation
Conical flow
Conical nozzles
Fluid dynamics
Fluid flow
Molecular dynamics
Nozzles
Nucleation
Orifices
Simulation
Sonic booms
Steady flow
Water vapor
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Summary:We develop a methodology for direct molecular-level simulation of adiabatic expansion of gas through a small orifice to a vacuum. The gas attains supersonic speeds, cools, and nucleates. The proposed approach combines equations of frictionless fluid dynamics with molecular dynamics simulation in an expanding periodic box. There are two key components of the proposed algorithm: (i) a time-reversible integrator tailored to an expanding system, and (ii) an iterative procedure employed to satisfy the condition of steady flow. For a conical nozzle (opening angle of 60°), the simulations with argon and water vapor predict cluster sizes in agreement with the experiment. Clusters of irregular shapes observed in the experiment [J. Lengyel et al. Phys. Rev. Lett. 2014, 112, 113401] are not reproduced. The role of friction, turbulence, and sonic boom originating at the sharp nozzle edge is discussed.
ISSN:1549-9618
1549-9626
DOI:10.1021/acs.jctc.8b00066