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Bond energy is the energy required to break a bond homolytically. These values are usually expressed in units of kcal/mol or kJ/mol and are referred to as bond dissociation energies when given for specific bonds or average bond energies when indicated for a given type of bond over many compounds. Firstly, the bond dissociation energy for a single bond is weaker than that of a double bond, which in turn is weaker than that of a triple bond. Secondly, hydrogen forms relatively strong bonds with...
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Nonequilibrium internal energy distributions during dissociation.

Narendra Singh1, Thomas Schwartzentruber2

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|December 20, 2017
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This study introduces a new model for nitrogen energy distribution during rapid heating, capturing non-equilibrium effects crucial for understanding high-speed gas dynamics and hypersonic flight. The model accurately predicts energy level populations during excitation and dissociation phases.

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high-temperature thermochemistryhypersonic flowsnonequilibrium distributionshock wavessurprisal analysis

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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.