A reactive molecular dynamics study of the effects of an electric field on n-dodecane combustion
Summary¶
ReaxFF combustion simulations of n-dodecane include external electrostatic fields using a local charge-equilibration treatment that permits charge transfer until orbital overlap limits and captures field-induced polarization. Isolated n-dodecane charges under field are first compared to DFT. Reactive boxes then scan temperature and density with varying E-field strength. Arrhenius-like parameters for selected channels are probed via NEB on minimum-energy paths and collision-frequency arguments. Polarization from close contacts remains weak unless strong external fields are applied. Fuel consumption slows under strong fields; weak fields show no clear monotonic trend. O₂ consumption can rise with strong fields at high T and ρ, but decreases when T and ρ drop. NEB finds activation-energy shifts up to about 2.3 kcal mol⁻¹ for oxygen-containing reactions vs. field strength. Oxygen chemistry is rate-controlling under field because it couples directly to oxidation and indirectly modulates fuel pyrolysis.
Methods¶
A — ReaxFF + electrostatics (field response)¶
- Lineage: ReaxFF combustion parameterization with charge equilibration extended so external E-fields polarize molecules with overlap-limited charge transfer (local QEq-style response as described in Combust. Flame).
B — Reactive MD (combustion under field)¶
- Engine: LAMMPS ReaxFF reactive boxes for n-dodecane oxidation; scan temperature T, mass density ρ, and electric field magnitude/orientation per article.
- Species / kinetics: Time series for fuel and O\(_2\) consumption; Arrhenius-style interpretation of selected channels.
- Collision / energy diagnostics: Kinetic-energy partitioning and collision frequency analyses as stated in Methods.
C — Quantum chemistry¶
- DFT validation of charges and polarization for isolated n-dodecane in an applied field vs ReaxFF.
D — Nudged elastic band (NEB)¶
- NEB on minimum-energy paths for oxygen-containing elementary reactions; reported activation-energy shifts up to ~2.3 kcal mol⁻¹ with field strength.
Reactive MD (combustion box): Engine: LAMMPS; 3D PBC reacting mixtures of n-dodecane and O₂ (atom counts and stoichiometry in the PDF); NVT or NVE (see article) with Nose–Hoover-type thermostat for hot combustion temperatures; ~0.1–0.25 fs timestep (typical ReaxFF; PDF); ps production; barostat N/A if fixed volume; mass density ρ sweeps and N/A for isotropic hydrostatic pressure as an independent thermodynamic control in the same sense as NPT (the article uses (T, ρ) state points); externally applied electrostatic field E (strength sweeps in Combust. Flame); N/A metadynamics; NEB is a separate static path refinement, not on-the-fly replica exchange.
Findings¶
- Need strong fields to materially alter kinetics; otherwise polarization remains minor.
- n-Dodecane consumption decreases under strong fields.
- O₂ response to field flips sign between high and low T/ρ conditions.
- NEB: up to ~2.3 kcal mol⁻¹ barrier shifts for O-bearing elementary steps.
- O₂ dynamics gate overall reactivity under E-field.
- The authors highlight that weak fields leave polarization small unless close contacts or high external E are present, so flame-relevant field strengths remain a practical limiting case for observable kinetic shifts. Comparisons / sensitivity: versus DFT for isolated molecule charges; T and ρ sweeps with E-field; NEB barrier shifts (≤~2.3 kcal/mol). Limitations (turbulent flames, practical E): see authored discussion. Corpus / PDF: use version-of-record for field magnitudes and reaction network IDs.
Limitations¶
Field strengths required for strong effects may exceed practical combustion devices; transferability to turbulent flames is not claimed. The NEB subset and Arrhenius diagnostics emphasize elementary channels that may not isolate coupled turbulence–chemistry interactions present in engines; readers should treat field effects as proof-of-principle for ReaxFF electrostatic extensions rather than device-ready kinetics without further validation. Species budgets should be read against the equation-of-state and grid definitions in the full article when mapping to reduced chemistry models.
Relevance to group¶
Demonstrates ReaxFF + field + QEq-style electrostatics for hydrocarbon oxidation with van Duin parametrization lineage.