Nonequilibrium internal energy distributions during dissociation.

Area of Science:

• Chemical Physics
• Aerospace Engineering
• Computational Chemistry

Background:

• Non-equilibrium energy distributions in gases significantly impact chemical reaction rates.
• Understanding these distributions is critical for modeling high-enthalpy flows, such as those encountered in hypersonic flight.
• Previous models often struggle to capture the complex molecular dynamics under such extreme conditions.

Purpose of the Study:

• To develop a predictive model for nonequilibrium vibrational and rotational energy distributions in nitrogen gas.
• To utilize surprisal analysis to accurately represent molecular energy level populations during rapid heating and dissociation.
• To investigate the influence of these non-Boltzmann effects on the dissociation rate in nitrogen.

Main Methods:

• Direct molecular simulations (DMSs) of rapidly heated nitrogen gas were performed.
• An ab initio potential energy surface (PES) was employed for accurate molecular interactions.
• Surprisal analysis was used to construct a model based on simulation data.

Main Results:

• The developed surprisal-based model successfully captured the overpopulation of high internal energy levels during the excitation phase.
• The model also accurately predicted the depletion of high internal energy levels during the quasi-steady-state (QSS) dissociation phase.
• Non-Boltzmann effects were shown to influence the overall dissociation rate due to strong coupling between internal energy and dissociation chemistry.

Conclusions:

• The surprisal-based model provides a robust method for capturing molecular-level nonequilibrium physics in nitrogen.
• The model's simple functional form yields a continuum-level expression that accounts for energy distributions and their coupling to dissociation.
• This approach is relevant for analyzing conditions behind strong shockwaves and improving models for hypersonic flight.