Computational study of nonequilibrium chemistry in high temperature flows.

Abstract

Recent experimental measurements in the reflected shock tunnel CUBRC LENS-I facility raise questions about our ability to correctly model the recombination processes in high enthalpy flows. In the carbon dioxide flow, the computed shock standoff distance over the Mars Science Laboratory (MSL) shape was less than half of the experimental result. For the oxygen flows, both pressure and heat transfer data on the double cone geometry were not correctly predicted. The objective of this work is to investigate possible reasons for these discrepancies. This process involves systematically addressing different factors that could possibly explain the differences. These factors include vibrational modeling, role of electronic states and chemistry-vibrational coupling in high enthalpy flows. A state-specific vibrational model for CO2, CO, O2 and O system is devised by taking into account the first few vibrational states of each species. All vibrational states with energies at or below 1 eV are included in the present work. Of the three modes of vibration in CO2, the antisymmetric mode is considered separately from the symmetric stretching mode and the doubly degenerate bending modes. The symmetric and the bending modes are grouped together since the energy transfer rates between the two modes are very large due to Fermi resonance. The symmetric and bending modes are assumed to be in equilibrium with the translational and rotational modes. The kinetic rates for the vibrational-translation energy exchange reactions, and the intermolecular and intramolecular vibrational-vibrational energy exchange reactions are based on experimental data to the maximum extent possible. Extrapolation methods are employed when necessary. This vibrational model is then coupled with an axisymmetric computational fluid dynamics code to study the expansion of CO2 in a nozzle. The potential role of low lying electronic states is also investigated. Carbon dioxide has a single excited state just below the dissociation limit. CO and O recombine exclusively to this excited state and then relaxes to the ground electronic state. A simple model is proposed to represent the effect of this intermediate state in the recombination process. Preliminary results show that this excited electronic state is a potential reason for increased shock standoff distance observed in LENS facility.The general role of chemistry-vibrational coupling in modeling recombination dominated flows is also investigated. A state-specific model is developed to analyze the complex chemistry-vibration coupling present in high enthalpy nozzle flows. A basic model is formulated assuming molecules are formed at a specific vibrational level and then allowed to relax through a series of vibration-vibration and vibration-translation processes. This is carried out assuming that the molecules behave as either harmonic or anharmonic oscillators. The results are compared with the standard vibration-chemistry model for high enthalpy nozzle flows. Next, a prior recombination model that accounts for the rotational-vibrational coupling is used to obtain prior recombination distribution. A distribution of recombining states is obtained as a function of the total energy available to the system. The results of this model are compared with recent experiments. Additionally, a reduced model is formulated using the concepts of the state-specific model. The results of this reduced model is compared with the state specific model.