Nanodroplet Evaporation

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Nanodroplet Evaporation

• By Manish Shrivastava
• 04/24/06

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Motivation

• In combustion systems using sprays and atomizers
  nano-droplets generated
• Understanding nano- droplet behavior is important
  for a variety of applications (lifetimes and
  evaporation rates)
• Assumptions of the D2 law are questionable for
  nano-droplets
• Quasi-steady evaporation
• Uniform droplet temperature
• Constant droplet and vapor properties
• Spherical droplet
• MD simulations useful to predict behavior of
  nano-droplets

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Previous Work

• Long et al. 1996 and Walther et al. 2001 MD
  simulation of Ar droplet in Ar vapor
  (subcritical)
• Showed agreement with Knudsen aerosol theory
• MD results approached D2 law with increasing
  system size
• Kaltz et al. 1997
• O2 droplet in He and H2 vapors
• Subritical and Supercritical conditions
  (different results)
• Consolini et al. 2003
• Xe droplet in N2 vapor
• Subcritical and supercritical conditions

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Problem Definition

• Look at nanodroplet evaporation using MD
• Effects of variables
• Cell size
• Temperature
• Pure argon and a mixture of 2 components

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Approach

• Start with FCC argon crystal with 256 atoms
• Generate a sphere knocking off atoms at radius
  greater than 4 LJ units from the centre 234
  atoms
• MD simulation NVT
• Initial Temperature 100K
• Droplet evaporation Increasing the simulation
  cell size
• Temperature effects100-140K

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Calculation of droplet diameter

• The whole simulation cell cut into equal slices
  of length 0.1 LJ units each in the X-direction
• Number of atoms in each slice counted
• Droplet length calculated as maximum number of
  corresponding cells with number of atoms greater
  than 2 in each slice.
• Number 2 chosen by trial and error
• Droplet diameter calculated after 25000 time
  steps

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Results Cell size
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Results Temperature
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2 components

• Goal To look at effects of differences in well
  depth of the two components on droplet
  equilibrium size
• Assumed uniformly mixed solution
• ?1 ?2
• ?29?1
• ?25?1
• ?22?1
• ?1-2 (?1 ?2)0.5

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Results 2components
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Conclusions

• Droplet diameter decreased with increasing cell
  size
• Increase in temperature caused decrease in
  droplet size.
• At 115 K (cell size15.1LJ units) many clusters
  of 4 to 5 atoms were seen (hard to distinguish a
  droplet)
• For a 2-component solution
• Increase in ?2/?1 ratio causes lower evaporation
  of droplet (higher droplet size)
• Expected as the 2 component solution tends to be
  more stable compared to a single component