Direct molecular simulation of nitrogen dissociation under adiabatic postshock conditions

Direct molecular simulations (DMS) of nitrogen dissociation in adiabatic reservoirs covering a wide enthalpy range are presented. These conditions more realistically reproduce the gas state in a hypersonic shock layer than prior isothermal DMS studies. The Minnesota ab initio potential energy surface is used for nitrogen molecule-molecule (N₂ - N₂) and nitrogen atom-molecule (N - N₂) classical trajectory calculations. Profiles of gas parameters (mixture composition, gas temperature, etc.) are reported, and the molecules' internal energy population distributions during dissociation examined. It is observed that the evolution of the gas mixture under adiabatic conditions can be divided into two phases. Early on, excess translational energy is available for molecules to dissociate, regardless of their internal energy. This causes a sudden drop in the reservoir kinetic temperature and slows down subsequent dissociation. At later stages, the gas settles into a quasi-steady-state (QSS) regime, where dissociation proceeds primarily from higher-lying rovibrational levels near and above the dissociation threshold energy. The dissociation rate becomes limited by depletion of high-lying internal energy levels. Subsequently, adiabatic simulations are compared with prior isothermal DMS calculations performed at similar reservoir temperatures. The same shape is effectively observed for population distributions in both cases. This retroactively justifies the use of isothermal conditions to obtain QSS dissociation rate coefficients.