Nonequilibrium effects in high enthalpy gas flows expanding through nozzles

An approach based on the direct simulation Monte Carlo method is proposed to model a core flow in a converging-diverging nozzle. The area of applicability of this approach is defined by the Boltzmann equation, which allows fully kinetic models that accurately capture thermal and chemical nonequilibrium to be applied to gas flows where the flow regime rapidly changes from continuum to transitional. The approach is validated through comparison with available experimental data. The examination of nonequilibrium and reaction rate effects for Caltech's T5 shock tunnel condition has shown little impact of nonequilibrium but demonstrated significant sensitivity of nitric oxide (NO) density to all exchange reaction and NO recombination rates. The use of the most recent theoretical and experimental rates results in a factor of two lower NO density at the nozzle exit as compared to the conventional Park rates, which indicates that re-visiting of the latter may be necessary. Multi-parametric sensitivity study of T5 conditions has not provided an explanation for a large drop in free-stream temperature and NO density over time, under constant flow velocity, observed recently in T5. Modeling of High Enthalpy Shock Tunnel Gottingen conditions has demonstrated considerable nonequilibrium between vibrational modes of N-2, NO, and O-2; it has also shown that the vibration-dissociation coupling strongly influences mole fractions of NO and O-2. Published under an exclusive license by AIP Publishing.

Automated summary

A new method based on the direct simulation Monte Carlo method is proposed to model core flow in a converging-diverging nozzle, with validation through comparison with experimental data. The study shows that nitric oxide (NO) density is highly sensitive to exchange reactions and NO recombination rates under specific conditions, while being less affected by nonequilibrium. Additionally, vibration-dissociation coupling has a significant influence on mole fractions of NO and O-2 under different shock tunnel conditions.