This work aims to construct a reduced order model for energy transfer and dissociation in non-equilibrium nitrogen mixtures. The objective is twofold: to present the Coarse-Grain Quasi-Classical Trajectory (CG-QCT) method, a novel framework for constructing a reduced order model for diatom-diatom systems; and to analyze the physics of non-equilibrium relaxation of the nitrogen molecules undergoing dissociation in an ideal chemical reactor. The CG-QCT method couples the construction of the reduced order model under the coarse-grain model framework with the quasi-classical trajectory calculations to directly construct the reduced model without the need for computing the individual rovibrational specific kinetic data. In the coarse-grain model, the energy states are lumped together into groups containing states with similar properties, and the distribution of states within each of these groups is prescribed by a Boltzmann distribution at the local translational temperature. The required grouped kinetic properties are obtained directly by the QCT calculations. Two grouping strategies are considered: energy-based grouping, in which states of similar internal energy are lumped together, and vibrational grouping, in which states with the same vibrational quantum number are grouped together. A zero-dimensional chemical reactor simulation, in which the molecules are instantaneously heated, forcing the system into strong non-equilibrium, is used to study the differences between the two grouping strategies. The comparison of the numerical results against available experimental data demonstrates that the energy-based grouping is more suitable to capture dissociation, while the energy transfer process is better described with a vibrational grouping scheme. The dissociation process is found to be strongly dependent on the behavior of the high energy states, which contribute up to 50% of the dissociating molecules. Furthermore, up to 40% of the energy required to dissociate the molecules comes from the rotational mode, underscoring the importance of accounting for this mode when constructing non-equilibrium kinetic models. In contrast, the relaxation process is governed primarily by low energy states, which exhibit significantly slower transitions in the vibrational binning model due to the prevalence of mode separation in these states.

Construction of a coarse-grain quasi-classical trajectory method. I. Theory and application to N2-N2 system.

We report the results of detailed calculations of reactive, inelastic, and dissociative processes in collisions of atomic oxygen with molecular nitrogen in their respective electronic ground states. Cross sections are calculated as a function of collision energy in the range 0.001–10 eV, considering the whole rovibrational ladder. Some problems related to the vibrational energy levels of the asymptotes of 3A″ and 3A′ potential energy surfaces used in this work are solved by an appropriate scaling at the level of cross sections. The results are compared with data in the literature, obtaining excellent agreement with experimental thermal data for reactive processes on a very large temperature range, and reasonable agreement with indirect dissociative data. Significant discrepancies are observed with previous reactive state-to-state results calculated on less detailed potential energy surfaces. Inelastic results are compatible with extrapolation of experimental thermal rate coefficient for temperatures highe...

Reactive, Inelastic, and Dissociation Processes in Collisions of Atomic Oxygen with Molecular Nitrogen

Understanding gas–phase chemical kinetics is important for modeling hypersonic flows. This paper discusses quasiclassical trajectory analysis, in which gas–phase interactions are simulated using ab...

Quasiclassical Trajectory Analysis of Nitrogen for High-Temperature Chemical Kinetics

A conventional and high-fidelity direct simulation Monte Carlo approach is applied to a number of hypersonic cases where validation data is available. Changes in the collision model are found to have a small effect on surface properties, with nitrogen recombination being the only notable exception.

https://link.aps.org/accepted/10.1103/PhysRevFluids.4.033405

Nonequilibrium air flow predictions with a high-fidelity direct simulation Monte Carlo approach

Kinetics of nonequilibrium processes in air plasma formed behind shock waves: state-to-state consideration

Quasi-classical trajectory (QCT) calculations are used in this work to calculate state-specific N2(X1Σ)+O(3P)→2N(4S)+O(3P) dissociation and N2(X1Σ)+O(3P)→NO(X2Π)+N(4S) exchange cross sections and rates based on the 13A″ and 13A′ ab initio potential energy surface by Gamallo et al. [J. Chem. Phys. 119, 2545–2556 (2003)]. The calculations consider translational energies up to 23 eV and temperatures between 1000 K and 20 000 K. Vibrational favoring is observed for dissociation reaction at the whole range of collision energies and for exchange reaction around the dissociation limit. For the same collision energy, cross sections for v = 30 are 4 to 6 times larger than those for the ground state. The exchange reaction has an effective activation energy that is dependent on the initial rovibrational level, which is different from dissociation reaction. In addition, the exchange cross sections have a maximum when the total collision energy (TCE) approaches dissociation energy. The calculations are used to generate compact QCT-derived state-specific dissociation (QCT-SSD) and QCT-derived state-specific exchange (QCT-SSE) models, which describe over 1 × 106 cross sections with about 150 model parameters. The models can be used directly within direct simulation Monte Carlo and computational fluid dynamics simulations. Rate constants predicted by the new models are compared to the experimental measurements, direct QCT calculations and predictions by other models that include: TCE model, Bose-Candler QCT-based exchange model, Macheret-Fridman dissociation model, Macheret’s exchange model, and Park’s two-temperature model. The new models match QCT-calculated and experimental rates within 30% under nonequilibrium conditions while other models under predict by over an order of magnitude under vibrationally-cold conditions.

Ab initio state-specific N2 + O dissociation and exchange modeling for molecular simulations.

In strong shock waves in rarefied gases, chemical processes such as dissociation and exchange reactions occur in conditions of thermodynamic nonequilibrium, and the rate coefficients are functions ...

Assessment of Classical Impulsive Models of Dissociation in Thermochemical Nonequilibrium

Classical impulsive model for dissociation of diatomic molecules in direct simulation Monte Carlo

A previously proposed classical impulsive model for dissociation of diatomic molecules in direct simulation Monte Carlo (DSMC), the Macheret-Fridman for direct simulation Monte Carlo (MF-DSMC) model [Luo et al., “Classical impulsive model for dissociation of diatomic molecules in direct simulation Monte Carlo,” Phys. Rev. Fluids 3, 113401 (2018)], is extended in this work. To improve the prediction of state-specific rates at high vibrational energy, the anharmonic vibrational phase angle distribution function is first incorporated into the model. Then, to improve the prediction of thermal equilibrium dissociation rates, the general concept of calculating total collision cross sections with the MF-DSMC model is discussed and the framework of implementing a collision model based on exponential potential is constructed. The improved model is validated by comparisons with quasiclassical trajectory calculations, empirical estimations, and experimental measurements. In general, better agreement compared with the original version of the model is obtained. The improved model is also evaluated by simulating O2 reacting shock experiment.

Han Luo

Papers

Ab initio state-specific N2 + O dissociation and exchange modeling for molecular simulations.

Assessment of Classical Impulsive Models of Dissociation in Thermochemical Nonequilibrium

Classical impulsive model for dissociation of diatomic molecules in direct simulation Monte Carlo

Development of an impulsive model of dissociation in direct simulation Monte Carlo

Verification and Improvement of Impulsive Model for Dissociation of Diatomic Molecules in DSMC