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
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.
University of Minnesota Ph.D. dissertation. November 2010. Major: Aerospace Engineering and Mechanics. Advisor: Dr.Graham V. Candler. 1 computer file (PDF); xii, 97 pages, appendices A-B. Ill. (some col.)
Computational study of nonequilibrium chemistry in high temperature flows..
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