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Application of ReaxFF-Reactive Molecular Dynamics and Continuum Methods in High-Temperature/Pressure Pyrolysis of Fuel Mixtures

Summary

Book chapter on using ReaxFF reactive molecular dynamics together with continuum modeling to study high-temperature and high-pressure pyrolysis of fuel mixtures (including toluene and n-dodecane), motivated by combustion devices that operate above the critical pressure of fuel or oxidizer. The discussion connects Arrhenius-type rate parameters from atomistic simulations to continuum treatments and examines when simple kinetic pictures break down.

Methods

Sourced from the chapter PDF (pdf_path).

  • Reactive MD (ReaxFF): Simulations use the CHO-2016 combustion ReaxFF parameterization (Ashraf et al., as cited in the chapter) for toluene and n-dodecane pyrolysis. All atomistic runs are NVT (fixed N, V, T) in periodic cubic cells; temperature is controlled with a Berendsen thermostat (damping 100.0 ps). The integration timestep is 0.1 fs throughout. Bond-order cutoff 0.3 is used only for species identification (not to alter dynamics).
  • Single-component benchmarks: 40 molecules per species in cubic boxes chosen to give overall densities 0.2 and 0.4 kg per dm^3 (e.g. toluene boxes 31.20 A and 25.00 A; n-dodecane 38.39 A and 30.47 A). After minimization, NVT equilibration 10 ps at 1500 K, then 10 distinct initial samples; NVT production at 2000-2600 K in 100 K steps with average initial pressures in the ~26-75 MPa range over the first 5 ps of each run. n-Dodecane trajectories 50 ps; toluene 200 ps (less reactive). Mixtures: compositions and cubic box sizes per Table 7.1 (e.g. 1:40 through 40:40 n-dodecane:toluene at 0.2 and 0.4 kg per dm^3); 10 ps equilibration at 1500 K, then 200 ps NVT-MD at 2000-2600 K (100 K intervals) at 0.1 fs timestep.
  • Continuum (0D): Matching zero-dimensional pyrolysis simulations use the same initial temperature, density, and mole fractions as the MD cases; constant-volume, constant-temperature integration parallels the NVT atomistic setup. A cubic equation of state handles real-gas effects; a 179-species, 1895-reaction Arrhenius mechanism (as referenced in the chapter) supplies continuum kinetics.

Findings

  • ReaxFF-MD is used to examine how a highly reactive fuel alters the behavior of a less reactive fuel as concentration, temperature, and density/pressure change.
  • Activation energies from Arrhenius-type analyses are compared between ReaxFF-based MD and continuum simulations, and limitations of the continuum side are discussed.
  • The work identifies pressure/temperature and mixing conditions where simple first-order Arrhenius relations are not applicable, linked to different initial reaction mechanisms and product distributions for the two fuels.
  • The chapter argues that ReaxFF MD can yield atomistic insight into fuel-mixture combustion properties under supercritical conditions where experiments are difficult.

Limitations

  • Continuum mechanism (179 species) is validated in the chapter mainly at lower P/T than the ReaxFF windows; extrapolation caveats apply. Atomistic runs use elevated T/P to accelerate chemistry (as discussed in Sec. 7.3).

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