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Pyrolysis of binary fuel mixtures at supercritical conditions: A ReaxFF molecular dynamics study

Evidence and attribution

Authority of statements

Prose sections below (Summary, Methods, Findings, etc.) are curated summaries of the publication identified by doi, title, and pdf_path in the front matter above. They are not new primary claims by this wiki.

For definitive numerical values, reaction schemes, and interpretations, use the peer-reviewed article (and optional records under normalized/papers/ when present)—not this page alone.

Summary

Supercritical pressures in propulsion/combustion hardware invalidate low-P Arrhenius kinetic models that neglect pressure effects on pathways. This work uses ReaxFF MD to study binary fuel pyrolysis at supercritical conditions for JP-10/toluene and n-dodecane/toluene mixtures, comparing to detailed continuum chemistry where the continuum model fails to capture atomistic trends. Mixing ratio and density change cooperative decomposition: a more reactive component can accelerate breakdown of a less reactive partner beyond naive “fastest reaction dominates” intuition; pyrolysis products from one species can promote chemistry in others. The study maps regimes where first-order kinetics and simple Arrhenius behavior break down. Rocket injector and diesel relevant fluids often exist as multicomponent liquids at elevated P; the paper uses mixtures to expose synergistic cracking that single-fuel models omit.

Methods

Force-field lineage (ReaxFF, CHO-2016)

The study uses the CHO-2016 combustion ReaxFF parameterization of Ashraf et al. (reference [42] in Fuel), developed as an update to CHO-2008 (Chenoweth et al.) to improve C1 chemistry and hydrocarbon mechanical properties. Nonbonded interactions taper to zero by 10 Å as summarized in §2, with EEM-based charge equilibration consistent with standard ReaxFF practice.

MD application — supercritical binary pyrolysis

Engine / code. Reactive molecular dynamics with ReaxFF follows the Penn State pyrolysis protocol of prior ReaxFF studies [41,42] (implementation details align with the LAMMPS workflows documented on [[2018ashraf-venue-paper]]).

System size and composition. Single-component benchmarks place 40 molecules of n-dodecane, JP-10, or toluene in cubic cells sized to \(\rho = 0.2\) and \(0.4\ \mathrm{kg\,dm^{-3}}\). Binary sweeps fix 40 toluene molecules and vary the co-reactant count to realize mixing ratios \(\alpha\) from 0.025 to 1.0; Table 1 lists total atom counts up to 2120 (n-dodecane/toluene) and 1640 (JP-10/toluene) at the highest loading.

Boundaries / periodicity. Bulk liquid cells are cubic boxes with three-dimensional periodic boundary conditions (PBC) as in [41,42].

Ensemble. All production trajectories use constant-volume NVT MD.

Timestep. \(\Delta t = 0.1\ \mathrm{fs}\).

Duration / stages. After energy minimization, systems are equilibrated with NVT MD at 1500 K for 10 ps (no pyrolysis during this window). Ten independent samples then undergo NVT production runs at 2000–2600 K (100 K steps). n-Dodecane/JP-10-rich cases use 50 ps production; toluene-rich and mixture cases use 200 ps to accumulate sufficient toluene decomposition statistics (§3).

Thermostat. Berendsen thermostat with 100 fs damping constant (§3).

Barostat. N/A — the protocol fixes volume and diagnoses pressure from the NVT state; no NPT barostat coupling is used in the MD stages described.

Temperature. 1500 K equilibration; 2000–2600 K production grid (Table 1).

Pressure. Instantaneous pressures at 0.2 and \(0.4\ \mathrm{kg\,dm^{-3}}\) reach tens to hundreds of MPa at 2000–2600 K (tabulated \(P,T,Z\) in Fuel).

Electric field. N/A — no applied field.

Replica / enhanced sampling. N/A — brute-force NVT trajectories.

Continuum reference (0D detailed chemistry)

Zero-dimensional constant-volume isothermal reactor integrations mirror the MD \(T,\rho\) state points using a 179-species, 1895-reaction Arrhenius mechanism (§3).

Experiments

N/A — computational study; experiments appear only as literature context.

Findings

Outcomes and mechanisms

Binary JP-10/toluene and n-dodecane/toluene mixtures pyrolyze differently with composition and density, because radical/product pools from one component accelerate chemistry in the partner beyond a single fastest-reaction picture. ReaxFF MD resolves atomistic pathways at supercritical \(T,\rho\) where continuum models trained at low \(P\) miss pressure-sensitive channels.

Comparisons

Zero-dimensional detailed kinetics at matched \(T,\rho\) shows where continuum Arrhenius networks disagree with MD-observed decomposition trends; the abstract argues this highlights regimes where first-order Arrhenius pictures fail.

Sensitivity and design levers

Mixing ratio \(\alpha\), density (0.2 vs 0.4 kg dm\(^{-3}\)), and temperature (2000–2600 K) control reaction timing and product slates; Table 1 enumerates the atom budgets used for each state point.

Limitations and outlook (as authored)

The article notes CHO-2016 is less extensively tested on very large hydrocarbons than on smaller training targets, and that closed-cell MD omits flow/mixing physics present in engines.

Corpus honesty

Numerical settings above are taken from papers/Ashraf_Shabnam_Fuel_2018.pdf §3 and Table 1; the Elsevier imprint lists Fuel 235 (2019) 194–207 while the DOI landing metadata may read 2018—use the PDF’s issue line for citations.

Limitations

  • Force-field uncertainty for large oxygenated product spaces; validation against experiment at matching P/T remains essential.
  • Jet fuel surrogate compositions in engines include additives and aromatic fractions beyond the binary blends highlighted here; extrapolation should be done cautiously.
  • Reactor residence time and mixing nonuniformities can shift product slates relative to closed MD cells at fixed density.

Relevance to group

Application-forward combustion/fuels + ReaxFF paper for high-pressure pyrolysis of realistic mixtures.

Citations and evidence anchors

  • reaxff-family
  • Supercritical fuel pyrolysis and mixture effects
  • ReaxFF for hydrocarbon cracking at extreme conditions