The four giant planets, Jupiter, Saturn, Uranus, and Neptune, contain most of the mass and angular momentum of our planetary system and thus are believed to have played a critical role in the formation and evolution of our solar system. Additionally, the giant planets represent the most frequently observed class of exoplanets, and therefore present a relatively accessible analog for exoplanetary research. Despite this, the giant planets are the least studied in our solar system. Our knowledge of them is primarily based on remote sensing from Earth-based observatories and space telescopes, which have inherent limitations when compared to in situ probe measurements. For these reasons, future probe missions to the giant planets have been identified as top priorities for the planetary science community. The upper atmospheres of the giant planets are primarily composed of gaseous hydrogen (H₂) and helium (He). During atmospheric entry, a shock wave forms in front of the probe. Under the high temperatures found in the post-shock region, H₂ molecules dissociate and H atoms become electronically excited, eventually ionizing to form protons and electrons. To predict heat loads and design mass-efficient thermal protection systems, it is necessary to model each of these non-equilibrium thermochemical processes accurately. The primary objective of this thesis is to investigate the thermochemistry of H₂/ He shock layers and to develop accurate yet computationally-efficient kinetic models for future giant planet probe missions. First, a novel one-temperature diatomic dissociation model is developed to capture thermal non-equilibrium and non-Boltzmann effects for H₂ dissociation. In particular, the rovibrational state-specific master equations are used to derive macroscopic chemical source term and rovibrational energy expressions that are valid in all three key limits/ regimes of dissociation-dominated flows, i.e., the thermal equilibrium limit, the quasi-steady-state (QSS) regime, and the pre-QSS regime. Next, optimal fits for H₂ dissociation rate constants are developed through a comprehensive literature review of available experimental and computational data. This includes data from high-temperature shock tube experiments (2,000 to 8,000 K), low-temperature discharge-flow tube experiments (below 350 K), and ab initio computational studies. Then, a detailed literature review of the electronic excitation and ionization rate constants of atomic H (by both electron and heavy-particle impact) is performed. Using the best estimates of these rate constants along with the newly developed H₂ dissociation model, an 11-species thermochemical model with state-specific kinetics for atomic H is developed. To validate the kinetic model, 1-D steady shocks are simulated using a space-marching code that explicitly accounts for shock tube boundary layer effects. The resulting radiance profiles are compared to experimental data from the NASA Ames Electric Arc Shock Tube (EAST) facility, and are found to reproduce the measured values reasonably accurately while capturing the distinct induction zone behavior observed in the experiments. Finally, a reduced-order non-Boltzmann kinetic model for H ionization is constructed using an analogous QSS framework to the one developed for diatomic dissociation. This model reproduces the majority of the results of the state-specific H ionization model, despite treating H as a single bulk species in the flowfield calculations.
Alex Takuya Mori Carroll (Thu,) studied this question.