Nonequilibrium radiative heat flux modeling for the Huygens entry probe

An electronic collisional-radiative model is proposed to predict the nonequilibrium populations and the radiation of the excited electronic states CN(A, B) and N₂(A, B, C) during the entry of the Huygens probe into the atmosphere of Titan. The model is loosely coupled with flow solvers using a Lagrangian method. First, the model was tested against measurements obtained with the shock-tube of NASA Ames Research Center. Then, the model was applied to the simulation of Huygen's entry. Our simulations predict that the population of the CN(B) state is lower than the Boltzmann population by a factor 40 at trajectory time t = 165 s and by a factor 2 at t = 187 s and that the population of the CN(A) state remains close to the Boltzmann population for both trajectory points. The radiative heat fluxes, driven by the CN(A, B) states, are lower than predictions based on the Boltzmann populations by a factor 15 at t = 165 s and a factor 2 at t = 187 s.