Direct molecular simulation of nitrogen dissociation based on an ab initio potential energy surface

Abstract

The direct molecular simulation (DMS) approach is used to predict the internal energy relaxation and dissociation dynamics of high-temperature nitrogen. An ab initio potential energy surface (PES) is used to calculate the dynamics of two interacting nitrogen molecules by providing forces between the four atoms. In the near-equilibrium limit, it is shown that DMS reproduces the results obtained from well-established quasiclassical trajectory (QCT) analysis, verifying the validity of the approach. DMS is used to predict the vibrational relaxation time constant for N2–N2 collisions and its temperature dependence, which are in close agreement with existing experiments and theory. Using both QCT and DMS with the same PES, we find that dissociation significantly depletes the upper vibrational energy levels. As a result, across a wide temperature range, the dissociation rate is found to be approximately 4–5 times lower compared to the rates computed using QCT with Boltzmann energy distributions. DMS calculations predict a quasi-steady-state distribution of rotational and vibrational energies in which the rate of depletion of high-energy states due to dissociation is balanced by their rate of repopulation due to collisional processes. The DMS approach simulates the evolution of internal energy distributions and their coupling to dissociation without the need to precompute rates or cross sections for all possible energy transitions. These benchmark results could be used to develop new computational fluid dynamics models for high-enthalpy flow applications.

A Coupled Vibration-Dissociation Model for Nitrogen from Direct Molecular Simulation

“…This is another way of stating that the tail of the distribution undergoes depletion. The most interesting feature not evident from [40] and [41] have been plotted.…”

“…[30,29,32,31,33] for more details. DMS was used to investigate dissociation of nitrogen in two studies, first accounting for only diatom-diatom interactions [40] and later including both atom-diatom interactions and diatom-diatom interactions [41] in a full reacting nitrogen system. In these studies, the gas system reaches a quasi-steady-state (QSS) characterized by time-invariant molecular vibrational energy distributions, where the populations of high v-levels are depleted compared to the corresponding equilibrium Boltzmann distribution.…”

“…The finding that nonequilibrium vibrational energy distributions account for a variation in the dissociation rate off 3−4 times compared to the assumption of Boltzmann distributions, is accurately captured. Model predictions are compared with both DMS [14,15,16] and QCT [38] results.…”

“…At high temperatures, strong thermo-chemical nonequilibrium produces non-Boltzmann internal energy distributions [14,15,16]. For modeling, either this nonequilibrium behavior of the distributions is ignored, or modeled empirically by comparing to dissociation rates inferred from experiment by assuming a computational model [2,17,18] or through computations [19,1,20].…”

“…Such DMS simulations include all relevant physics for rovibrational excitation and coupling to dissociation. We utilize the statistics computed from recent DMS studies [14,15,16], which used the ab-initio PES developed by Paukku et al [34] for nitrogen.…”