Analysis of chemical kinetic parameters for hydrogen atmospheres

A reliable chemical kinetic model for the hydrogen-helium system is essential to simulate hypersonic entries into gas and ice giants. Several chemical kinetic models exist in the literature for finite-rate chemistry of H₂ dissociation and ionization. A systematic study of these chemical kinetic models is performed to quantify the similarities and differences among the models. Under the conditions of interest, ionization of He occurs weakly and the flow thermochemistry is also insensitive to the rate of reaction with He as the colliding partner. The comparison of various models reveals that the electron impact ionization of H atoms is the dominant mechanism that dictates the shock stand-off distance as well as the shock layer densities and temperature. A decrease in one order of magnitude in the pre-exponential factor of the ionization reaction causes a significant shift in the shock stand-off distance and similar changes in the density of individual species in the system. The number density and temperatures along the stagnation line are imported into a line-by-line radiation solver to calculate the radiance with both the quasi steady state and the Boltzmann approximation. Comparison of the radiance profiles against experimental results shows that none of the models existing in literature captured the qualitative trends. Experimental data indicates slow ionization processes, while the finite-rate chemical kinetic models predict fast ionization chemistry. It is suggested that improvements to the modeling of electron impact ionization of hydrogen atoms has the most potential for increasing accuracy.