Publications

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Predicting the behavior of solid fuels in response to a fire is a complex endeavor. Heterogeneity, charring, and intumescence are a few examples of the many challenges presented by some common materials. If one desires to employ a 3-dimensional computational fluid dynamics (CFD) model for fire, an accurate solid combustion model for materials at the domain boundary is often desirable. Methods for such modeling are not currently mature, and this is a current topic of research. For some practical problems, it may be acceptable to abstract the surface combustible material as a 1-dimensional reacting boundary condition. This approach has the advantage of being a relatively simple model, and may provide acceptably accurate predictions for problems of interest. Such a model has recently been implemented in Sandia's low-Mach number CFD code for reacting flows, the SIERRA/FUEGO code. Theory for the implemented model is presented. The thermal transport component of the model is verified by approximating a 1-D conduction problem with a closed form solution. The code is further demonstrated by predicting the fire behavior of a block of burning plexiglas (PMMA). The predictions are compared to the reported data from a corresponding experimental program. The predictions are also used to evaluate the sensitivity of model parameters through a sensitivity study using the same test configuration.

Spent nuclear fuel reprocessing may involve some hazardous liquids that may explode under accident conditions. Explosive accidents may result in energetic dispersion of the liquid. The atomized liquid represents a major hazard of this class of event. The magnitude of the aerosol source term is difficult to predict, and historically has been estimated from correlations based on marginally relevant data. A technique employing a coupled finite element structural dynamics and control volume computational fluid dynamics has been demonstrated previously for a similar class of problems. The technique was subsequently evaluated for detonation events. Key to the calculations is the use of a Taylor Analogy Break-up (TAB) based model for predicting the aerodynamic break-up of the liquid drops in the air environment, and a dimensionless parameter for defining the chronology of the mass and momentum coupling. This paper presents results of liquid aerosolization from an explosive event.

New aircraft are being designed with increasing quantities of composite materials used in their construction. Different from the more traditional metals, composites have a higher propensity to burn. This presents a challenge to transportation safety analyses, as the aircraft structure now represents an additional fuel source involved in the fire scenario. Most of the historical fire testing of composite materials is aimed at studying decomposition, flammability or yield strength under fire conditions. The majority of this testing has been performed on a small-scale. Heterogeneous reactions are often length-scale dependent, and this is thought to be particularly true for composites which exhibit significant microscopic dynamics that can affect macro-scale behavior. A series of discovery tests has been designed to evaluate composite materials under various structural loading conditions with a consistent applied thermal boundary condition. Mass-loss, heat flux, and temperature response have been measured throughout the experiment. Several panels have been tested, including simple composite panels, and sandwich panels. A major objective of the testing was to understand the importance of the structural loading on a composite material during exposure to firelike conditions. During flaming combustion at early times, there are features of the panel decomposition that are unique to the type of structural loading imposed on the panels. At load levels tested, fiber reaction rates at later times appear to be independent of the structural loading.

Carbon fibers are being increasingly used in composites for aircraft. They are bound together with a binder, often an epoxy. There are many grades of binders, and many different types of composites sold on the market. They are expensive. We have some donated materials of unknown type, and would like to be able to be cost-effective and use them without incurring a large cost to analyze the materials using laboratory methods. Visual inspection is not normally sufficiently accurate to be able to tell one composite from another. Optical methods that involve a broader spectrum have commonly been used to discriminate organic materials. A five-band spectral reflectometer is used to measure reflectivity of the surfaces, and is a simple way of extracting data into the infrared bands. The instrument used in these tests is less resolved than a narrow band spectrometer, but is easier to deploy because it is a hand-held device that only requires a flat surface of approximately 3 cm diameter. Reflectivity of many different composite materials, including a bismaleimide, several thermoset epoxies, and some low temperature epoxies from various manufacturers is measured. Other materials are also included to demonstrate that non-composites can be rejected by the methods. Analysis shows that the reflectometer measurements are capable of discriminating some materials, but have difficulty with discriminating others. The raw reflectivity data are likely to be helpful for future radiation modeling of composite surfaces. Copyright © 2013 by ASME.

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Carbon fiber composite materials are increasingly being used in the design and fabrication of transportation vehicles. In particular, the aviation industry is increasing transitioning from metals to this class of composites due to the high strength and low weight of the materials. Most aviation structural composites are thermoset, meaning they require thermal processing to harden the epoxy. In the event of a fire, they will behave significantly different than the metals they replace. Because they are not homogeneous, they also differ significantly from homogeneous solid combustibles. Sandia National Laboratories is motivated to study burning composites because we maintain experimental and modeling capabilities for assessing transportation safety. Understanding the thermal environment created by transportation fires is therefore paramount. This type of focus is not typical of the general literature on these materials in the fire environment. A serious issue with the majority of fire performance data found in the open literature is that the length and mass scales are generally orders of magnitude below those used in vehicle design. With a non-traditional perspective on composite fires, Sandia has performed several test series. Together with a review of the work from other institutions as found in the literature, this report presents a phenomenological overview of the relevant work on the behavior of composite materials in a fire environment.

Carbon fiber composite materials are increasingly being used in the design and fabrication of transportation vehicles. In particular, the aviation industry is increasing transitioning from metals to this class of composites due to the high strength and low weight of the materials. Most aviation structural composites are thermoset, meaning they require thermal processing to harden the epoxy. In the event of a fire, they will behave significantly different than the metals they replace. Because they are not homogeneous, they also differ significantly from homogeneous solid combustibles. Sandia National Laboratories is motivated to study burning composites because we maintain experimental and modeling capabilities for assessing transportation safety. Understanding the thermal environment created by transportation fires is therefore paramount. This type of focus is not typical of the general literature on these materials in the fire environment. A serious issue with the majority of fire performance data found in the open literature is that the length and mass scales are generally orders of magnitude below those used in vehicle design. With a non-traditional perspective on composite fires, Sandia has performed several test series. Together with a review of the work from other institutions as found in the literature, this report presents a phenomenological overview of the relevant work on the behavior of composite materials in a fire environment.

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In the interest of providing an economically sensible use for the copious small-diameter wood in Northern New Mexico, an economic study is performed focused on mobile pyrolysis. Mobile pyrolysis was selected for the study because transportation costs limit the viability of a dedicated pyrolysis plant, and the relative simplicity of pyrolysis compared to other technology solutions lends itself to mobile reactor design. A bench-scale pyrolysis system was used to study the wood pyrolysis process and to obtain performance data that was otherwise unavailable under conditions theorized to be optimal given the regional problem. Pyrolysis can convert wood to three main products: fixed gases, liquid pyrolysis oil and char. The fixed gases are useful as low-quality fuel, and may have sufficient chemical energy to power a mobile system, eliminating the need for an external power source. The majority of the energy content of the pyrolysis gas is associated with carbon monoxide, followed by light hydrocarbons. The liquids are well characterized in the historical literature, and have slightly lower heating values comparable to the feedstock. They consist of water and a mix of hundreds of hydrocarbons, and are acidic. They are also unstable, increasing in viscosity with time stored. Up to 60% of the biomass in bench-scale testing was converted to liquids. Lower ({approx}550 C) furnace temperatures are preferred because of the decreased propensity for deposits and the high liquid yields. A mobile pyrolysis system would be designed with low maintenance requirements, should be able to access wilderness areas, and should not require more than one or two people to operate the system. The techno-economic analysis assesses fixed and variable costs. It suggests that the economy of scale is an important factor, as higher throughput directly leads to improved system economic viability. Labor and capital equipment are the driving factors in the viability of the system. The break-even selling price for the baseline assumption is about $11/GJ, however it may be possible to reduce this value by 20-30% depending on other factors evaluated in the non-baseline scenarios. Assuming a value for the char co-product improves the analysis. Significantly lower break-even costs are possible in an international setting, as labor is the dominant production cost.

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The behavior of carbon fiber aircraft composites was studied in adverse thermal environments. The effects of resin composition and fiber orientation were measured in two test configurations: 102 by 127 millimeter (mm) test coupons were irradiated at approximately 22.5 kW/m{sup 2} to measure thermal response, and 102 by 254 mm test coupons were irradiated at approximately 30.7 kW/m{sup 2} to characterize piloted flame spread in the vertically upward direction. Carbon-fiber composite materials with epoxy and bismaleimide resins, and uni-directional and woven fiber orientations, were tested. Bismaleimide samples produced less smoke, and were more resistant to flame spread, as expected for high temperature thermoset resins with characteristically lower heat release rates. All materials lost approximately 20-25% of their mass regardless of resin type, fiber orientation, or test configuration. Woven fiber composites displayed localized smoke jetting whereas uni-directional composites developed cracks parallel to the fibers from which smoke and flames emanated. Swelling and delamination were observed with volumetric expansion on the order of 100% to 200%. The purpose of this work was to provide validation data for SNL's foundational thermal and combustion modeling capabilities.

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Transportation accidents frequently involve liquids dispersing in the atmosphere. An example is that of aircraft impacts, which often result in spreading fuel and a subsequent fire. Predicting the resulting environment is of interest for design, safety, and forensic applications. This environment is challenging for many reasons, one among them being the disparate time and length scales that must be resolved for an accurate physical representation of the problem. A recent computational method appropriate for this class of problems has been developed for modeling the impact and subsequent liquid spread. This involves coupling a structural dynamics code to a turbulent computational fluid mechanics reacting flow code. Because the environment intended to be simulated with this capability is difficult to instrument and costly to test, the existing validation data are of limited scope, relevance, and quality. A rocket sled test is being performed where a scoop moving through a water channel is being used to brake a pusher sled. We plan to instrument this test to provide appropriate scale data for validating the new modeling capability. The intent is to get high fidelity data on the break-up and evaporation of the water that is ejected from the channel as the sled is braking. These two elements are critical to fireball formation for this type of event involving fuel in the place of water. We demonstrate our capability in this paper by describing the pre-test predictions which are used to locate instrumentation for the actual test. We also present a sensitivity analysis to understand the implications of length scale assumptions on the prediction results. Copyright © 2011 by ASME.

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