Modeling of boundary layer thermochemistry in hypersonic flows

Sharma Priyadarshini, Maitreyee

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https://hdl.handle.net/2142/117763 Copy

Description

Title

Modeling of boundary layer thermochemistry in hypersonic flows

Author(s)

Sharma Priyadarshini, Maitreyee

Issue Date

2022-11-27

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

Panesi, Marco

Doctoral Committee Chair(s)

Panesi, Marco

Committee Member(s)

• Dutton, J. Craig
• Hirata, So
• Jaffe, Richard

Department of Study

Aerospace Engineering

Discipline

Aerospace Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Hypersonics, Non-equilibrium chemistry, computational chemistry, deep learning

Abstract

High planetary (re-entry) speeds of spacecraft often encountered in space exploration mission designs lead to high temperatures in the surrounding gas due to the formation of a strong bow shock. Consequently, molecules surrounding the spacecraft deviate from their equilibrium state, start chemically reacting and become electronically excited at these elevated temperatures. The exothermicity of the chemical reactions and spontaneous emission from electronically excited atoms and molecules result in high surface heat fluxes and intense radiation in the shock layer. One key factor in keeping the contents of the spacecraft safe from this incoming radiation is the design and characterization of the Thermal Protection System (TPS). This includes identifying suitable materials, computing the required thickness of the TPS, and choosing the type of TPS (ablative/reusable) to be used. These designs are currently very conservative, resulting in a decreased available payload for the mission. One reason is the lack of predictive models for heat flux computations. This research aims to develop a computational framework to accurately predict and mitigate heating on the spacecraft's surface. This is achieved through model development of chemical processes occurring during reentry and designing techniques for reducing the heating on spacecraft surfaces. Past efforts in the field focused on the formulation of reduced-order models that leverage statistical mechanics to account for the chemical non-equilibrium state of the atmosphere to reduce computational costs. A drawback of these models is that they rely on available kinetics data for all possible reaction mechanisms. Furthermore, even for mechanically simple molecular systems like nitrogen-nitrogen molecular collisions, hundreds of millions of possible reactions need to be considered for the characterization of the state of the gas, making the computational task highly intractable. This work approaches these challenges using machine learning techniques and the development of surrogate models for molecular collision or scattering calculations, specifically for the quasi-classical trajectory method. The machine learning tools are employed in an innovative way to reconstruct all possible reaction probabilities ($\mathcal{O}(10^8)$) using as few as 100,000 reaction mechanisms. This work achieves an order of magnitude reduction in computational costs of scattering calculations while maintaining accuracy in the predicted reaction probabilities. Following the second objective of this research, accurate thermodynamic and kinetics data of the gases present in the boundary layer of the spacecraft have been computed. The described procedure herein entails the application of quantum chemistry methods to estimate molecular properties and using this data to compute accurate thermodynamics for ablation products and pyrolysis gases. In addition, excited electronic states of these molecules are studied to recognize absorption features in the VUV spectral region that can lead to attenuation of radiation incoming from the shock layer due to the N(I) 174.29 nm line. Among other findings, it is for the first time that C$_{\mathrm{4}}$ and C$_{\mathrm{4}}$H are recognized as potential radiation absorbing molecules in the boundary layer. It is also found that the collisions of the HCN chemical system in the hypersonic boundary layers are understudied. This knowledge gap in the literature is addressed through the development of potential energy surfaces and detailed scattering calculations for the HCN molecule. The study highlights the importance of exchange channels in governing the overall kinetics behavior of the chemical system. Finally, a new and innovative technique for surface heat flux mitigation is detailed by a novel approach to doping chemical compounds into the TPS material in its synthesis. The thesis presents the first database of favorable molecules that can be used for TPS doping by studying their electronic absorption spectrum and thermochemical properties. Among numerous favorable molecules included as part of this research, silica, already a minor constituent in some TPS materials and coatings, proves to be an excellent doping agent, which can be shown to reduce the radiative heating by up to 40\% in the backshell region.

Graduation Semester

2022-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Maitreyee Sharma Priyadarshini

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