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 highly 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.
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 highly 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.
To understand the gas-surface chemistry above the thermal protection system of a hypersonic vehicle, it is necessary to map out the kinetics of key elementary reaction steps. In this work, extensive periodic density functional theory (DFT) calculations are performed to elucidate the interaction of atomic oxygen and nitrogen with both the basal plane and edge sites of highly oriented pyrolytic graphite (HOPG). Reaction energies and barriers are determined for adsorption, desorption, diffusion, recombination, and several reactions. These DFT results are compared with the most recent finite-rate model for air-carbon ablation. Our DFT results corroborated some of the parameters used in the model but suggest that further refinement may be necessary for others. The calculations reported here will help to establish a predictive kinetic model for the complex reaction network present under hypersonic flight conditions.
High heat flux (>500 kW/m2) ignitions occur in scenarios involving metal fires, propellants, lightning strikes, above ground nuclear weapon use, etc. Data for material response in such environments is primarily limited to experimental programs in the 1950s and 1960s. We have recently obtained new data in this environment using concentrated solar energy. A portion of the experimental data were taken with the objective that the data be useful for model validation. To maximize the utility of the data for validation of predictive codes, additional focus is placed on repeatability of the data, reduction of uncertainties, and characterization of the environment. We illustrate here a portion of the data and methods used to assess environmental and response parameters. The data we present are novel in the flux range and materials tested, and these data constitute progress in the ability to characterize fires from high flux events.
A variety of energy sources produce intense radiative flux (»100 kW/m2) well beyond those typical of fire environments. Such energy sources include directed energy, nuclear weapons, and propellant fires. Studies of material response to irradiation typically focus on much lower heat flux; characterization of materials at extreme flux is limited. Various common cellulosic and synthetic-polymer materials were exposed to intense irradiation (up to 3 MW/m2) using the Solar Furnace at Sandia National Laboratories. When irradiated, these materials typically pyrolyzed and ignited after a short time (<1 s). The mass loss for each sample was recorded; the topology of the pyrolysis crater was reconstructed using a commercial three-dimensional scanner. The scans spatially resolved the volumetric displacement, mapping this response to the radially varying flux and fluence. These experimental data better characterize material properties and responses, such as the pyrolysis efflux rate, aiding the development of pyrolysis and ignition models at extreme heat flux.
Heat release that leads to thermal runaway of lithium-ion batteries begins with decomposition reactions associated with lithiated graphite. We broadly review the observed phenomena related to lithiated graphite electrodes and develop a comprehensive model that predicts with a single parameter set and with reasonable accuracy measurements over the available temperature range with a range of graphite particle sizes. The model developed in this work uses a standardized total heat release and takes advantage of a revised dependence of reaction rates and the tunneling barrier on specific surface area. The reaction extent is limited by inadequate electrolyte or lithium. Calorimetry measurements show that heat release from the reaction between lithiated graphite and electrolyte accelerates above ~200°C, and the model addresses this without introducing additional chemical reactions. This method assumes that the electron-tunneling barrier through the solid electrolyte interphase (SEI) grows initially and then becomes constant at some critical magnitude, which allows the reaction to accelerate as the temperature rises by means of its activation energy. Phenomena that could result in the upper limit on the tunneling barrier are discussed. The model predictions with two candidate activation energies are evaluated through comparisons to calorimetry data, and recommendations are made for optimal parameters.
The thermal environment generated during an intense radiation event like a nuclear weapon airburst, lightning strike, or directed energy weaponry has a devastating effect on many exposed materials. Natural and engineered materials can be damaged and ignite from the intense thermal radiation, potentially resulting in sustained fires. Understanding material behavior in such an event is essential for mitigating the damage to a variety of defense systems, such as aircraft and weaponry. Flammability and ignition studies in this regime (very high heat flux, short duration) are less plentiful than in the heat flux regimes representative of typical fires. The flammability and ignition behavior of a material may differ at extreme heat flux due to the balance of the heat conduction into the material compared to other processes. Length scale effects may also be important in flammability and ignition behavior, especially in the high heat flux regime. A variety of materials have recently been subjected to intense thermal loads (~100–1000 kW/m2) in testing at both the Solar Furnace and the Solar Tower at the National Solar Thermal Test Facility at Sandia National Laboratories. The Solar Furnace, operating at a smaller scale (≈30 cm2 area), provides the ability to test a wide range of materials under controlled radiative flux conditions. The Solar Tower exposes objects and materials to the same flux on a much larger scale (≈4 m2 area), integrating complex geometry and scale effects. Results for a variety of materials tested in both facilities are presented and compared. Material response often differs depending on scale, suggesting a significant scale effect. Mass loss per unit energy tends to go down as scale increases, and ignition probability tends to increase with scale.
The surface area dependence of the decomposition reaction between lithiated graphites and electrolytes for temperatures above 100◦C up to ~200◦C is explored through comparison of model predictions to published calorimetry data. The initial rate of the reaction is found to scale super-linearly with the particle surface area. Initial reaction rates are suggested to scale with edge area, which has also been measured to scale super-linearly with particle area. As in previous modeling studies, this work assumes that electron tunneling through the solid electrolyte interphase (SEI) limits the rate of the reaction between lithium and electrolyte. Comparison of model predictions to calorimetry data indicates that the development of the tunneling barrier is not linear with BET surface area; rather, the tunneling barrier correlates best with the square root of specific surface area. This result suggests that tunneling though the SEI may be controlled by defects with linear characteristics. The effect of activation energy on the tunneling-limited reaction is also investigated. The modified area dependence results in a model that predicts with reasonable accuracy the range of observed heat-release rates in the important temperature range from 100◦C to 200◦C where transition to thermal runaway typically occurs at the cell level.
The thermal environment generated during an intense radiation event like a nuclear weapon airburst, lightning strike, or directed energy weaponry has a devastating effect on many exposed materials. Natural and engineered materials can be damaged and ignite from the intense thermal radiation, potentially resulting in sustained fires. Understanding material behavior in such an event is essential for mitigating the damage to a variety of defense systems, such as aircraft and weaponry. Flammability and ignition studies in this regime (very high heat flux, short duration) are less plentiful than in the heat flux regimes representative of typical fires. The flammability and ignition behavior of a material may differ at extreme heat flux due to the balance of the heat conduction into the material compared to other processes. Length scale effects may also be important in flammability and ignition behavior, especially in the high heat flux regime. A variety of materials have recently been subjected to intense thermal loads (~100–1000 kW/m2) in testing at both the Solar Furnace and the Solar Tower at the National Solar Thermal Test Facility at Sandia National Laboratories. The Solar Furnace, operating at a smaller scale (≈30 cm2 area), provides the ability to test a wide range of materials under controlled radiative flux conditions. The Solar Tower exposes objects and materials to the same flux on a much larger scale (≈4 m2 area), integrating complex geometry and scale effects. Results for a variety of materials tested in both facilities are presented and compared. Material response often differs depending on scale, suggesting a significant scale effect. Mass loss per unit energy tends to go down as scale increases, and ignition probability tends to increase with scale.
Intense, dynamic radiant heat loads damage and ignite many common materials, but are outside the scope of typical fire studies. Explosive, directed-energy, and nuclear-weapon environments subject materials to this regime of extreme heating. The Solar Furnace at the National Solar Test Facility simulated this environment for an extensive experimental study on the response of many natural and engineered materials. Solar energy was focused onto a spot (∼10 cm2 area) in the center of the tested materials, generating an intense radiant load (∼100 kW m−2 –1000 kW m−2) for approximately 3 seconds. Using video photography, the response of the material to the extreme heat flux was carefully monitored. The initiation time of various events was monitored, including charring, pyrolysis, ignition, and melting. These ignition and damage thresholds are compared to historical ignition results predominantly for black, α-cellulose papers. Reexamination of the historical data indicates ignition behavior is predicted from simplified empirical models based on thermal diffusion. When normalized by the thickness and the thermal properties, ignition and damage thresholds exhibit comparable trends across a wide range of materials. This technique substantially reduces the complexity of the ignition problem, improving ignition models and experimental validation.