Non-Boltzmann Vibrational Energy Distributions and Coupling to Dissociation Rate

TL;DR

This paper introduces a generalized model for nonequilibrium vibrational energy distributions in high-temperature flows, capturing non-Boltzmann effects that influence dissociation rates, based on ab initio calculations of air chemistry behind shock waves.

Contribution

The paper presents a new simple model for non-Boltzmann vibrational distributions that accurately reproduces ab initio simulation results for high-temperature air chemistry.

Findings

Non-Boltzmann distributions show overpopulation of high-energy vibrational states.

Dissociation rates are significantly affected by non-Boltzmann vibrational populations.

The model quantitatively matches ab initio simulation data.

Abstract

In this article, we propose a generalized model for nonequilibrium vibrational energy distribution functions. The model can be used, in place of equilibrium (Boltzmann) distribution functions, when deriving reaction rate constants for high-temperature nonequilibrium flows. The distribution model is derived based on recent \textit{ab initio} calculations, carried out using potential energy surfaces developed using accurate computational quantum chemistry techniques for the purpose of studying air chemistry at high temperatures. Immediately behind a strong shock wave, the vibrational energy distribution is non-Boltzmann. Specifically, as the gas internal energy rapidly excites to a high temperature, overpopulation of the high-energy tail (relative to a corresponding Boltzmann distribution) is observed in \textit{ab initio} simulations. As the gas excites further and begins to dissociate, a depletion of the high-energy tail is observed, during a time-invariant quasi-steady state (QSS). Since the probability of dissociation is exponentially related to the vibrational energy of the dissociating molecule, the overall dissociation rate is sensitive to the populations of these high vibrational energy states. The non-Boltzmann effects captured by the new model either enhance or reduce the dissociation rate relative to that obtained assuming a Boltzmann distribution. This article proposes a simple model that is demonstrated to reproduce these non-Boltzmann effects quantitatively when compared to \textit{ab initio} simulations.