Publications

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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.

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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.

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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.

The surface topology of a solid subjected to destructive environments is often difficult to quantify. In thermal environments, the size and shape of the solid changes as it pyrolyzes, ablates, warps, or chars. Quantitative descriptions of such responses are valuable for data reporting and model validation. In this work, a three-dimensional scanner is evaluated for non-destructive material analysis. The scans spatially resolve the response of materials to a high-heat-flux environment. To account for the effect of distortion induced in thin materials, back-side scans of the sample are used to characterize the displacement of the bulk material. Data spanning the area of the sample, rather than using a net or average quantity, enhances the evaluation of the crater formed by the incident flux. The 3D reconstruction of the sample also provides the ability to perform volumetric calculations. The data obtained from this methodology may be useful for characterizing materials exposed to a variety of destructive environments.

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When exposed to a strong radiant heat source (>1,000 kW/m2), combustible materials pyrolyze and ignite under certain conditions. Studies of this nature are scarce, yet important for some applications. Pyrolysis models derived at lower flux conditions do not necessarily extrapolate well to high-heat-flux conditions. The material response is determined by a complex interplay of thermal and chemical transport phenomena, which are often difficult to model. To obtain model validation data at high-heat-flux conditions (up to 2500 kW/m2), experiments on a variety of organic and engineered materials were performed at the National Solar Thermal Test Facility at Sandia National Laboratories. Mass loss during the short duration (2-4 sec) heat pulse was determined using the pre- and post-test weight. The mass-loss data were fairly linear in the fluence range of 200-6000 kJ/m2. When divided into subsets based on material types, the mass loss was similar at the peak flux/fluence condition for engineered polymers (≈1 g) and organic materials (≈2.5 g), although some exceptions exist (PMMA, dry pine needles). Statistical correlations were generated and used to evaluate the significance of the observed trends. These results contribute to the validation data for simulating fires and ignition resulting from very high incident heat flux.