Coarse grain modeling and direct molecular simulation of nitrogen dissociation

This work presents the analysis of non-equilibrium energy transfer and dissociation of nitrogen molecules (N₂(¹Σ⁺ _g)) using two different approaches: the direct molecular simulation (DMS) method and the maximum-entropy quasiclassical trajectory (ME-QCT) method. The two methods are used to study the thermochemical relaxation in a zero-dimensional isochoric and isothermal reactor. In the reactor, the nitrogen molecules, initially at room temperature, are heated to several thousand degrees Kelvin, driving the system toward a strong non-equilibrium condition. The analysis considers the thermochemical relaxation for temperatures ranging from 10,000 to 25,000 [K]. Both methods make use of the same potential energy surface (PES) for the N₂(¹Σ⁺ _g) − N₂(¹Σ⁺ _g) system taken from the NASA Ames quantum chemistry database. Within the ME-QCT method, the rovibrational energy levels of the electronic ground state of the nitrogen molecule are lumped into a reduced number of bins. Two different grouping strategies are used: the more conventional vibrational based grouping, widely used in literature, and the energy based grouping. The analysis of the temporal evolution of the population densities of the integral energy level and the concentration profiles shows excellent agreement between the predictions obtained with ME-QCT energy based grouping and the DMS solutions. Minor discrepancies are observed during the early stages of the relaxation process, with the ME-QCT method overpredicting the energy transfer rate. The reasons of the disagreement are traced back to the inability of this grouping strategy in capturing the significant mode separation of the low lying states. On the contrary, the vibrational grouping, traditionally considered state-of-the-art, well captures the macroscopic energy transfer of the vibrational manifold, but clearly fails to reproduce both the evolution of the concentration profiles, and the microscopic distribution of the internal energy levels. The reasons of the inaccuracy of the vibrational grouping are traced back to the underlying assumption of equilibrium distribution of the rotational energy levels.