Direct molecular simulation and quasi-classical trajectory calculation studies of 5-species air mixtures

Abstract

We present Direct Molecular Simulation (DMS) and Quasi-Classical Trajectory (QCT) calculation studies for chemically reacting 5-species air mixtures (N2,N,O2,O,NO) over a range of gas temperatures T = 8000 K - 30000 K. For this, we rely exclusively on a set of recent ab initio potential energy surfaces (PESs) generated by the Theoretical and Computational Chemistry group at the University of Minnesota. Our DMS calculations allow us to follow the coupled internal energy excitation of a ro-vibrationally cold nitrogen-oxygen mixture toward the much higher heat bath temperature and study simultaneously occurring chemical reactions (dissociation, exchange). We extract the internal energy (rotation + vibration), as well as vibration-specific population distributions of the diatomic species during the quasi-steady-state (QSS) dissociation phase and observe that for all three their high-energy tails are depleted relative to the corresponding Boltzmann distributions at the heat bath temperature. A comparison of reaction rate coefficients extracted under thermo-chemical equilibrium (using QCT) and during the QSS dissociation phase (using DMS) then allows to us to quantify the macroscopic effect that these vibrationally depleted distributions have on the dissociation reactions. By contrast, a similar comparison reveals that the Zeldovich exchange reactions are almost unaffected by the vibrationally depleted distributions. Unlike dissociation, the exchange reactions do not exhibit any significant vibrational bias and take place at near-thermal rates at all of the temperatures studied. Furthermore, we quantify the amount of vibrational and rotational energy removed and gained in the exchange- and a subset of the dissociation reactions. Such macroscopic quantities are of interest for enhancing the fidelity of multi-temperature nonequilibrium chemistry models used in computational fluid dynamics (CFD) codes.