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Gas-Condensed Phase Interactions: Gas-Condensed Phase Interactions: Flame-Surface Heat ExchangeFlame-Surface Heat Exchange

John E. Adams, Tamas Szabo, and Ali Siavosh-Haghighi

Department of ChemistryUniversity of Missouri-Columbia

Columbia, MO 65211-7600

MURI Review, Picatinny Arsenal, October 27, 2004

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Context: Burning Rates

• Continuum modeling of two-phase and three-phase combustion processes– One-dimensional– Homogeneous– Conserve mass, atomic species, energy within each phase– Match species, energy fluxes at interfaces– Surface regression through single-component evaporation– Multi-ingredient mixtures treated using a

phenomenological pyrolysis law (CYCLOPS code)

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Simplifications, Difficiencies

• Multi-component evaporation– Adsorbed/absorbed combustion products– Products of condensed-phase reactions

• Liquid-phase diffusion• Real gases• Complex liquid-phase behavior in a mixture• Direct gas-surface reactions• Missing experimental data

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Simulations

• Gas-liquid surface collisions– Equilibrate liquid sample– Create interface(s) by expanding the

simulation cell

• Analysis– Energy transfer to the surface– Trapping of colliding species– Evaporation at the liquid surface

– Ts dependences

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Structureless Model System

• Lennard-Jones potential model– Generalizable via corresponding states

• “Light” gas, “heavy” surface species(ratio = 0.35)

• Hot gas, “cold” surface

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Surface Contours

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

-35 -25 -15 -5 5 15 25 35

z ()

Nu

mb

er D

ensi

ty

T*=0.923

T*=0.756

T*=0.719

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Final Energies (Ei = 92 kJ/mol)

5 15 25 35 45 55 66 75 85 95 105

Scattered

All

0

200

400

600

800

1000

Fre

qu

ency

Final Energy (kJ/mol)

ScatteredAll

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Ts-Dependence of Ef

5 15 25 35 45 55 65 75 85 95 105

T*=0.719

T*=0.923

0

200

400

600

800

1000

1200

1400

1600

Fre

quen

cy

Final Energy (kJ/mol)

T*=0.719T*=0.923

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Ts-Dependence of Trapping

15

20

25

30

35

40

45

0.7 0.75 0.8 0.85 0.9 0.95

Reduced Surface Temperature (T* )

Per

cen

t tr

app

ed s

pec

ies

55

30

0

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Real Energetic Material: Nitromethane

• Prototypical CHNO material• Potential model from Agrawal, Rice, and

Thompson• Structureless impinging species (2/3 of

CH3NO2 mass)

• High-energy incident species, Ts = 360 K

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Surface Contour

Nitromethane, 360K

0

0.002

0.004

0.006

0.008

0.01

0.012

-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35

z (angstroms)

Num

ber

dens

ity

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Final Energies (Ei = 92 kJ/mol, i = 55°)

515

2535

4555

6575

8595

105non-trapped

all

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Fra

ctio

n

Final Energy (kJ/mol)

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Next Steps

• Extend database of vapor densities as a function of T

• Evaluate dependence of energy transfer and trapping on the inclusion of internal degrees of freedom

• Describe system in terms of reactive potentials• Investigate dynamics in the presence of

temperature gradients