Physics-based molecular modeling of recombination reactions

Kondur, Chaithanya

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https://hdl.handle.net/2142/122191 Copy

Description

Title

Physics-based molecular modeling of recombination reactions

Author(s)

Kondur, Chaithanya

Issue Date

2023-08-03

Director of Research (if dissertation) or Advisor (if thesis)

Stephani, Kelly A

Doctoral Committee Chair(s)

Stephani, Kelly A

Committee Member(s)

• Panesi, Marco
• Panerai, Francesco
• Glumac, Nick
• Haskins, Justin A

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Gas-phase recombination
• Surface Catalytic Recombination
• Desorption
• Recombination Reactions
• Molecular Dynamics
• QCT
• Hypersonics

Abstract

The dissertation presented herein focuses on enhancing the computational modeling of recombination reactions that transpire between atoms and molecules in both the gas phase and during gas-surface interactions. These reactions exhibit diverse behaviors contingent upon the specific regimes in which they take place. Consequently, the scope of this study is limited to their applications within hypersonic re-entry conditions, typically characterized by elevated temperatures and diminished collision frequencies. Such reactions exert a notable influence on the chemical composition of the gases within the flowfield surrounding the spacecraft, thereby impacting the heat loads encountered by the vehicle. Thus, the development of accurate and predictive models assumes paramount significance in the design of critical spacecraft components, specifically the thermal protection systems that serve to shield the vehicle from the formidable aerothermodynamic forces experienced during re-entry. Recombination reactions often form a part of an extensive set of coupled processes, and hence the competing nature of the various reactions are also of interest in this work. The gas-phase recombination involves the collision between three species and two collision time scales. The recombination mechanisms considered include the Direct mechanism where the three bodies enter the zone of interaction simultaneously and the Two-Step Binary Collision (TSBC) mechanism where two bodies first interact to form an intermediate complex with a finite lifetime. The collision of this intermediate complex with the third body may lead to a recombination event. A binary collision model is utilized to identify the intermediate complexes and quantify their properties. However, the model's predictions raise questions about the validity of certain assumptions commonly made in simple gas-phase recombination models, necessitating further investigation. To address these limitations, a new three-body collisional framework that leverages the theory of binary collisions is developed, which is then used to classically simulate the recombination dynamics. The classical trajectory simulations are used to investigate the recombination dynamics such as the recombination mechanisms and the internal energy distributions of the recombined molecules. A novel rate constant expression is developed to account for the presence of multiple time scales in the collisional process. This framework is used to investigate the recombination of atomic oxygen and atomic nitrogen, and efforts are undertaken to investigate the effects of the third body in the latter system. A good agreement is observed in the rate constants with those obtained from detailed balance, the dynamical information such as the distribution of internal energies is consistent with expectations from investigations of other processes. The results challenge the common assumptions made in simplified models and provide insights into the relative importance of the recombination mechanisms. In contrast to the gas-phase recombination where all the participants are gaseous species, the surface catalytic recombination occurs as a result of gas-surface interactions. These exothermic reactions directly impact the heat flux experienced by thermal protection systems. Molecular Dynamics (MD) simulations are employed to examine the atomic oxygen-graphene surface interactions, elucidating the various reaction mechanisms that arise from such interactions. Notable mechanisms include catalytic pathways leading to oxygen molecule production, adsorption reactions, and oxidation reactions resulting in the formation of CO and CO2 molecules, along with minor processes. Quantification of the relative importance of the different oxygen molecule production mechanisms is conducted, highlighting the Eley-Rideal (ER) mechanism as the overwhelmingly dominant process. The ER mechanism involves collisions between gas-phase atoms or molecules and surface-adsorbed atoms, leading to the creation of new molecules. To analyze the significance of the ER mechanism and its competing processes across a wide range of gas and surface conditions, a novel collision framework is developed, specifically tailored for low collision frequency scenarios. The study reveals that ER mechanisms exhibit a strong dependence on gas temperature, whereas oxidation and adsorption reactions are largely una ected by gas temperature variations. Interestingly, two distinct ER mechanisms are observed based on the residence time of the oxygen molecules formed as a result of the catalytic process. The interaction between oxygen atoms and a carbon  fiber (CF) microstructure is simulated to investigate the influence of surface morphology on reaction kinetics. The study demonstrates that the probability of ER mechanisms decreases on the more complex CF microstructure surface. However, this decrease in catalytic reactions is counterbalanced by a notable increase in non-catalytic reactions including oxidation and chemisorption. The competing nature of the catalytic and non-catalytic reactions is a result of the limited number of active sites on the carbon surface, and is important since it impacts the overall surface heat transfer experienced by the thermal protection systems. Additionally, the graphene surface is observed to generate hyperthermal oxygen molecules, characterized by high kinetic energy, in comparison to the CF surface. An analysis of the differences in the two ER mechanisms observed revealed that the O2 molecules formed at edge sites tend to exhibit large residence time on the surface. This, therefore, reveals the importance of evaluating site-specific desorption rate constants. However, the large time scales involved are inaccessible by MD simulations, and hence this work presents an alternative approach to model the site-specific desorption process. Desorption of adsorbates from surfaces occurs through the interaction of the adsorbate with the surface phonons. A phonon master equation model is used to study the desorption of adsorbates from carbon surfaces. The adsorbate-phonon interaction is modeled as a surface-adsorbed anharmonic oscillator undergoing a random walk driven by thermal fluctuations in the lattice. Desorption results from a random walk that leads to a continuum state of the oscillator. A new semi-classical model is introduced, in which molecular dynamics is used to quantify the thermal fluctuations, which are then used to compute the transition rates using the density matrix formulation. The rate constants for oxygen atom desorption from the top site, bridge site, and edge site configurations of the graphite surfaces are presented. The importance of representing the atomic interactions accurately is demonstrated by including two interaction models that differ in their treatment of adsorbate-induced surface relaxation. Desorption rate constants are found to be the smallest at edge sites, and the largest at top sites, which aligns well with theoretical predictions and observations from the analysis of the surface catalytic processes.

Graduation Semester

2023-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/122191

Copyright and License Information

Copyright 2023 Chaithanya Kondur

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