Development of modeling tools for mesoscopic simulation of reacting high-temperature flows in phenolic-based thermal protection systems

Gosma, Mitchell

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https://hdl.handle.net/2142/130012 Copy

Description

Title

Development of modeling tools for mesoscopic simulation of reacting high-temperature flows in phenolic-based thermal protection systems

Author(s)

Gosma, Mitchell

Issue Date

2025-07-07

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

Stephani, Kelly

Doctoral Committee Chair(s)

Stephani, Kelly

Committee Member(s)

• Panerai, Francesco
• Panesi, Marco
• Lee, Tonghun
• Swaminathan Gopalan, Krishnan
• Stewart, Benedicte

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Thermal Protection Systems
• TPS
• Direct Simulation Monte Carlo
• DSMC
• Ablation
• Pyrolysis
• Rarified Flow
• Material Response
• Transport
• Hypersonics

Abstract

Carbon-based, low-density ablating Thermal Protection Systems (TPS), typically a composite of phenolic resin and porous carbon-fiber preform, have become an essential component of various spacecraft. As such, a high level of importance has been placed on developing accurate simulations of these heat shields as they disintegrate during atmospheric entry. One area currently needing additional study is the composition and thermo-chemical effects of pyrolysis gases, formed from thermally degrading phenolic resin as they advect through the charring TPS and into the boundary layer. This dissertation focuses on characterizing the thermal, chemical, and gas transport properties of carbon-phenolic composites under high-temperature, entry-relevant conditions. The overarching goal is to advance predictive modeling capabilities, thereby facilitating the design of next-generation TPS for future space exploration. First, continuum-scale material response models and chemical kinetic solvers are employed to study the composition and thermal behavior of pyrolysis gases. Legacy and modern-day ablation codes typically assume equilibrium pyrolysis gas chemistry. Yet, experimental data suggest that speciation from resin decomposition is far from equilibrium. A thermal and chemical kinetic study was performed on pyrolysis gas advection through a porous char, using the Theoretical Ablative Composite for Open Testing (TACOT) as a demonstrator material. The finite-element tool SIERRA/Aria simulated the ablation of TACOT under various conditions. Temperature and phenolic decomposition rates generated from Aria were applied as inputs to a simulated network of perfectly stirred reactors (PSRs) in the chemical solver Cantera. A high-fidelity combustion mechanism computed the gas composition and thermal properties of the advecting pyrolyzate. The results indicate that pyrolysis gases do not rapidly achieve chemical equilibrium while traveling through the simulated material. Instead, a chemically reactive zone exists in the ablator between 1400 and 2500 K, wherein the modeled pyrolysis gases transition from a chemically frozen state to chemical equilibrium. These finite-rate results demonstrate a significant departure in computed pyrolysis gas properties from those derived from equilibrium solvers. Under the same conditions, finite-rate-derived gas is estimated to provide up to 50% less heat absorption than equilibrium-derived gas. This discrepancy suggests that nonequilibrium pyrolysis gas chemistry could substantially impact ablator material response models. Building on these findings, a Direct Simulation Monte Carlo (DSMC) framework has been constructed to incorporate detailed gas-phase chemistry and transport processes, enabling accurate mesoscale simulations of pyrolysis gas dynamics both within the TPS and in the boundary layer during atmospheric entry. Focusing first on the latter, the accuracy of transport processes in DSMC simulations depends on the collision cross-section parameters used to model the particle interactions. In this work, we provide a comprehensive collision-specific Variable Soft Sphere (VSS) parameter database for accurate simulation of transport properties in DSMC. A Nelder-Mead optimization scheme is used to find optimized VSS parameters from collision integrals, which are acquired either from high-fidelity literature sources or computed herein using a phenomenological potential model. The final collision parameter database contains over 200 neutral and ionized species, encompassing the compositions of all planetary atmospheres in the solar system, as well as the ablation and pyrolysis products of common spacecraft thermal protection systems. Best-fit parameters are provided over a range of 1000 - 20000 K. A secondary database, fitted from 300 - 4000 K, is provided for use in other applications, such as plume impingement, porous media flow, or combustion phenomena. The accuracy of the database is evaluated by comparing gas transport properties predicted by the database with those obtained from the ab initio collision integral data. The average error across the high and low temperature-range databases was found to be less than 1% and 3%, respectively. The provided parameter sets can be readily applied to model any neutral or weakly ionized gas mixture containing the included species, thus providing a comprehensive database that will be of great interest to the DSMC community. Next, enhanced chemistry models, capable of simulating complex chemically reacting mixtures of polyatomic gases, have been implemented in the direct simulation Monte Carlo (DSMC) code SPARTA. SPARTA’s Total Collision Energy (TCE) is modified to allow vibrational-energy-dependent reaction probabilities for dissociation and exchange, converting Bird’s original formulation to one that respects quantized vibrational levels. Recombination is reformulated through a modified Quantum-Kinetic (QK) reaction model combined with a partition function-based equilibrium constant methodology to maintain detailed balance, while post-reaction energies were redistributed using the Larsen–Borgnakke scheme and a double-relaxation-prohibitive relaxation model to achieve proper thermalization of translational, rotational, and vibrational modes. Verification efforts covered a range of dissociation, exchange, and recombination reactions over temperatures from 1000 - 7000 K. High-temperature rates generally agreed with Arrhenius targets within 5%, and a multivariate fitting procedure proved capable of reducing low-temperature errors to below 10% for high activation energy dissociation reactions. Recombination probabilities based on equilibrium constants and two literature expressions matched reference values within 6%. Full forward–reverse reservoir tests confirmed that both chemical composition and internal energy distributions relax to equilibrium as expected. The framework’s capabilities are further demonstrated in a two-dimensional axisymmetric DSMC simulation of NASA’s planned Dragonfly mission during its planned descent into Titan's atmosphere. Notable results include post-shock translational temperatures of up to 27000 K, formation of significant CN concentrations near the ablator shoulder, and a maximum stagnation-point surface temperature of 1160 K. The dissertation concludes by building on the results of previous chapters to construct a holistic modeling framework in the DSMC code SPARTA to simulate pyrolysis gas flow inside of a porous TPS microstructure. Taking advantage of the unique non-continuum flow environment inside the TPS, a set of modifications is made to the previously developed chemistry model to further improve agreement with Arrhenius rates, and a new equilibrium model is added to provide consistency with continuum chemistry solvers. The modified chemistry model is verified in SPARTA using a reduced 35-species, 125-reaction pyrolysis gas chemistry mechanism, showing near-perfect agreement with Cantera. A new subsonic boundary condition is additionally developed and verified to allow for the accurate simulation of multi-species flow. Finally, a gas-surface chemistry model is designed based on the classical Motz-Wise equation to allow for the simulation of heterogeneous reactions, with modifications made to the existing ablation capability in SPARTA to allow for the modeling of surface growth. The final holistic framework is used to simulate ongoing experiments being performed by collaborators at Sandia National Laboratories and the Center for Hypersonics and Entry Systems Studies. These experiments seek to study the deposition of carbon from pyrolysis gas species onto carbon fibers, a phenomenon known as coking. The final performed DSMC simulations show good qualitative agreement with currently available experimental results. Namely, aromatic species are seen to deposit significantly deeper inside the TPS, and in greater quantities, than light hydrocarbons such as acetylene. Growth of carbon fiber geometry over time is seen to result in a set of competing effects on the evolution of the internal flow: the growth of fiber surface area enhances reactivity, while the decline in available flow volume decreases gas residence time. Collectively, these advances position SPARTA as a versatile open-source tool for multiscale studies of reacting, rarefied, and porous-media flows in any planetary atmosphere. The goal is that the holistic framework can be effectively coupled with dedicated experiments to produce high-fidelity, validated heterogeneous models for the internal carbon-phenolic TPS environment. DSMC simulation results can then be further utilized to produce improved physical models for modern material response codes, leading to increased accuracy and fidelity. This serves the ultimate aim of improving TPS design, reducing launch and material costs, and further advancing the cause of human space exploration.

Graduation Semester

2025-08

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/130012

Copyright and License Information

Copyright 2025 Mitchell Gosma

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