Jet A Explosion Experiments: Laboratory Testing

This report describes a series of experiments and analyses on the flammability of Jet A (aviation kerosene) in air. This is a progress report on ongoing work. The emphasis so far has been on measuring basic explosion parameters as a function of fuel amount and temperature. These parameters include vapor pressure, flammability limits, peak explosion pressure and pressure as a function of time during the explosion. These measurements were undertaken in order to clear up some fundamental issues with the existing data. The report is organized as follows: First, we give some background with data from previous studies and discuss the fuel weathering issues. Second, we describe the facility used to do combustion experiments, the combustion test procedures and the results of the combustion experiments. Third, we give estimates of peak pressure, review the standard analysis of pressure histories and discuss the application to the present data. Fourth, we review the standard approach to flammability limits and the issues in determining Jet A flammability. Fifth, we discuss the problems associated with measuring vapor pressure and describe our results for Jet A. Sixth, we present a model for Jet A which illustrates the issues in analyzing multicomponent fuels. Finally, we apply these results to TWA 800 and summarize our conclusions to date

Spark Ignition Energy Measurements in Jet A

Experiments have been carried out to measure the spark ignition energy of Jet A vapor in air. A range of ignition energies from 1 mJ to 100 J was examined in these tests. The test method was validated by first measuring ignition energies for lean mixtures of the fuels hexane (C6H6) and propane (C3H8) in air at normal temperature (295 K) and pressure (1 atm). These results agree with existing data and provide new results for compositions between the lean flame limit and stoichiometric mixtures. Jet A (from LAX, flashpoint 45–48 [degress] C) vapor mixtures with air have been tested at temperatures between 30 and 60 [degrees] C at two fuel mass loadings, 3 and 200 kg/m3, in an explosion test vessel with a volume of 1.8 liter. Tests at 40, 50, and 60 [degrees] C have been performed at a mass loading of 3 kg/m3 in an 1180-liter vessel. Experiments with Jet A have been carried out with initial conditions of 0.585 bar pressure to simulate altitude conditions appropriate to the TWA 800 explosion. Ignition energies and peak pressures vary strongly as a function of initial temperature, but are a weak function of mass loading. The minimum ignition energy varies from less than 1 mJ at 60 [degrees] C to over 100 J at 30 [degrees] C. At temperatures less than 30 [degrees] C, ignition was not possible with 100 J or even a neon sign transformer (continuous discharge). The peak pressure between 40 and 55 [degrees] C was approximately 4 bar. Peak pressures in the 1180-liter vessel were slightly lower and the ignition energy was higher than in the 1.8-liter vessel. The following conclusions were reached relative to the TWA 800 crash: (a) spark ignition sources with energies between 5 mJ and 1 J are sufficient to ignite Jet A vapor, resulting in a propagating flame; (b) the peak pressure rise was between 1.5 and 4 bar (20 and 60 psi). (c) a thermal ignition source consisting of a hot filament created by discharging electrical energy into a metal wire is also sufficient to ignite Jet A vapor, resulting in a propagating flame; (d) laminar burning speeds are between 15 and 45 cm/s; and (e) the limited amount of fuel available in the CWT (about 50 gal) did not significantly increase the flammability limit. The rapid decrease in spark ignition energy with increasing temperature demonstrates that hot fuel tanks are significantly more hazardous than cool ones with respect to spark ignition sources. A systematic effort is now needed in order to utilize these results and apply spark ignition energy measurements to future analyses of fuel tank flammability. Some key issues that need to be addressed in future testing are: (a) effect of flashpoint on the ignition energy-temperature relationship; (b) ignition energy vs. temperature as a function of altitude; (c) effect of fuel weathering on ignition energy; and (d) the effect of ignition source type on ignition limits

Electronic band structure and exchange coupling constants in ACr2X4 spinels

We present the results of band structure calculations for ACr2X4 (A=Zn, Cd, Hg and X=O, S, Se) spinels. Effective exchange coupling constants between Cr spins are determined by fitting the energy of spin spirals to a classical Heisenberg model. The calculations reproduce the change of the sign of the dominant nearest-neighbor exchange interaction J1 from antiferromagnetic in oxides to ferromagnetic in sulfides and selenides. It is verified that the ferromagnetic contribution to J1 is due to indirect hopping between Cr t2g and eg states via X p states. Antiferromagnetic coupling between 3-rd Cr neighbors is found to be important in all the ACr2X4 spinels studied, whereas other interactions are much weaker. The results are compared to predictions based on the Goodenough-Kanamori rules of superexchange.Comment: 15 pages, 10 figures, 3 table

Curved quasi-steady detonations: Asymptotic analysis and detailed chemical kinetics

We consider the problem of slightly-curved, quasi-steady diverging detonation waves. For sufficiently small curvature, the reaction zone structure equations can be formulated as a two point boundary value problem for solutions containing a sonic point. Analytical solutions to this boundary value problem are obtained for the case of a one-step Arrhenius reaction in the limit of high activation energy. The analytic solution results in a nonlinear relationship between detonation velocity and curvature. For extremely small curvature, this relationship is consistent with previous linear analyses, whereas in the nonlinear regime it predicts a critical maximum curvature, beyond which no quasi-steady solutions with a sonic point can be found. We also analyse the curved detonation structure for the gaseous fuel-oxidizer combination of H_2 and O_2 with various diluents. Using a standard shooting method we generate numerical solutions of the two-point boundary value problem based on realistic thermochemistry and a detailed chemical reaction mechanism. Similar to the large activation energy results, the numerical solutions reveal a nonlinear detonation speed-curvature relation and a critical maximum curvature for the existence of quasi-steady solutions. The relation of this critical curvature to the critical scales of multi-dimensional detonation is discussed

Results of 1/4-Scale Experiments. Vapor Simulant And Liquid Jet A Tests

A quarter-scale engineering model of the center wing tank (CWT) of a 747-100 was constructed. This engineering model replicated the compartmentalization, passageways, and venting to the atmosphere. The model was designed to scale the fluid dynamical and combustion aspects of the explosion, not the structural failure of the beams or spars. The effect of structural failure on combustion was examined by using model beams and spars with deliberately engineered weak connections to the main tank structure. The model was filled with a simulant fuel (a mixture of propane and hydrogen) and ignited with a hot wire. The simulant fuel was chosen on the basis of laboratory testing to model the combustion characteristics (pressure rise and flame speed) of Jet A vapor created by a Jet A liquid layer at 50C at an altitude of 13.8 kft. A series of experiments was carried out in this model in order to: (a) investigate combustion in a CWT geometry; and (b) provide guidance to the TWA 800 crash investigation. The results of the experiments were observed with high-speed film, video, and still cameras, fast and slow pressure sensors, thermocouples, photodetectors, and motion sensors. A special pseudo-schlieren system was used to visualize flame propagation within the tank. This report describes the test program, facility, instrumentation, the first 30 experiments, comparisons between experiments, and performance of the instrumentation; then examines the significance of these results to the TWA 800 crash investigation. The key results of this study are: Flame Motion: The motion of flame was dominated by the effects of turbulence created by jetting through the passageways and vent stringers. A very rapid combustion event (lasting 10 to 20 ms) occurred once the flame traveled outside of the ignition bay and interacted with the turbulent flow. Most of the gas within the tank was burned during this rapid event. Compartments: The combustion time decreased with an increasing number of compartments (bays) within the tank. With six bays, combustion took only 100 to 150 ms to be completed from the time of ignition until the end of the rapid combustion phase. The total combustion event was three to four times shorter with compartments than without. Venting: Venting to the outside of the tank through the model vent stringers had a negligible effect on the combustion progress or on the peak pressure reached at the end of the burn. Ignition Location: Variation of the ignition location produced distinctive pressure loads on the structural components. Liquid Fuel: Lofting of a cold liquid fuel layer was produced by the combustion-induced gas motion. Although this spray of liquid eventually ignited and burned, it did not contribute to the pressure loading. Structural Failure: Structural failure resulted in flame acceleration, decreasing the overall combustion time. TWA 800 Investigation: The pressure loads were sufficiently high, up to 4 bar, and the combustion events were sufficiently short, that the forward portion (spanwise beam 3, front spar) of the CWT structure would fail as a direct consequence of the explosion. A combination of pressure loads was produced in some tests consistent with the TWA 800 wreckage. Replica tests, structural modeling, and sensitivity studies on fuel concentration are needed before any conclusions can be drawn about probable ignition locations. Cargo Bay: Tests with a simplified model of a half-full cargo bay indicated that repeated pressure waves with an amplitude of 1 bar or less are produced when an explosion scenario similar to TWA 800 is tested. Future Testing: Future studies should include replica tests, tests with Jet A vapor and warm liquid Jet A layers, and sensitivity tests to examine ignition location, fuel concentration, and vent area perturbations. Summary: Explosion tests in a 747-100 CWT model reveal that a very complex pattern of combustion occurs due the interaction of the flame and the flow-generated turbulence. A wide range of structural load patterns occur, depending on the location of the ignition source. Some of these load patterns are consistent with damage believed to be associated with the initial explosion event in TWA 800. Sensitivity of the loading to the ignition location indicates that narrowing down the ignition location in TWA 800 may be possible. However, the complexity of the combustion and structural failure processes in the actual center wing tank mandates extremely careful consideration of the uncertainties that enter into this process

O Vínculo Constitucional Entre o Exército e As Polícias Militares: Reflexos na Estrutura Organizacional, Formação e Prática Profissional (1934 1988)

As polícias militares (PMs) estão vinculadas constitucionalmente ao Exército Brasileiro na condição de força auxiliar e reserva desde a Constituição de 1934. A atribuição constitucional de polícia responsável pela ordem pública, ao mesmo tempo em que força auxiliar e reserva do Exército, impõe duas lógicas distintas do uso da força, que redundam na prontidão para o combate e no emprego de atividades propriamente policiais. Esse vínculo tem possibilitado a instrumentalização das PMs pelo Governo Federal para a repressão política nos estados em períodos de exceção, como a ditadura Vargas (1937-1945) e a ditadura militar (1964-1985). Após o restabelecimento da democracia, devido à fragilidade dos mecanismos de controle democrático sobre as polícias, a autonomia conferida às instituições policiais durante os regimes autoritários permaneceu, manifestando-se em práticas arbitrárias e violentas, incompatíveis com o ambiente democrático. A ação policial na repressão aos crimes de ordem social e política a partir de 1935 foi balizada pela Lei de Segurança Nacional (LSN), uma legislação especial cujas condições políticas determinavam o seu rigor e o seu alcance. Na prática, o atrelamento constitucional com o Exército tem influenciado na estrutura organizacional, na formação e na interação dos policiais militares com a sociedade. Verifica-se ainda a presença da doutrina de segurança nacional por meio de análise de grades curriculares, documentos, legislação e manuais do Exército que serviram de orientação para a formação desses profissionais nas décadas de 70, 80 e 90. A Polícia Militar se apresenta resistente a reformas e a qualquer controle externo; e ela persiste, mesmo após a Constituição de 1988, com os comportamentos e práticas observados durante o regime militar. E essa resistência ocorre devido à alta institucionalização dessas polícias, sendo o vínculo constitucional com o Exército um grande empecilho institucional para a realização das necessárias reformas. Palavras-Chave: Polícia Militar. Exército. Democracia. Constituição. Política. Institucionalização

Joint genomic and proteomic analysis identifies meta-trait characteristics of virulent and non-virulent Staphylococcus aureus strains

Staphylococcus aureus is an opportunistic pathogen of humans and warm-blooded animals and presents a growing threat in terms of multi-drug resistance. Despite numerous studies, the basis of staphylococcal virulence and switching between commensal and pathogenic phenotypes is not fully understood. Using genomics, we show here that S. aureus strains exhibiting virulent (VIR) and non-virulent (NVIR) phenotypes in a chicken embryo infection model genetically fall into two separate groups, with the VIR group being much more cohesive than the NVIR group. Significantly, the genes encoding known staphylococcal virulence factors, such as clumping factors, are either found in different allelic variants in the genomes of NVIR strains (compared to VIR strains) or are inactive pseudogenes. Moreover, the pyruvate carboxylase and gamma-aminobutyrate permease genes, which were previously linked with virulence, are pseudogenized in NVIR strain ch22. Further, we use comprehensive proteomics tools to characterize strains that show opposing phenotypes in a chicken embryo virulence model. VIR strain CH21 had an elevated level of diapolycopene oxygenase involved in staphyloxanthin production (protection against free radicals) and expressed a higher level of immunoglobulin-binding protein Sbi on its surface compared to NVIR strain ch22. Furthermore, joint genomic and proteomic approaches linked the elevated production of superoxide dismutase and DNA-binding protein by NVIR strain ch22 with gene duplications

Dopant clustering and vacancy ordering in neodymium doped ceria

Lanthanide doped cerias, show fast oxide ion conduction and have applications as electrolytes in intermediate temperature solid oxide fuel cells. Here, we examine the long- and short-range structures of Ce1−xNdxO2−x/2 (0.05 ≤ x ≤ 0.30, NDC) using reverse Monte Carlo modelling of total neutron scattering data, supported by measurements of electrical behaviour using a.c. impedance spectroscopy. Three distinct features are evident in the local structure of NDC, viz.: clustering of Nd3+ cations, preferred Nd3+-oxide ion vacancy association and oxide ion vacancy clustering with preferential alignment in the 〈100〉 direction. Interestingly, the presence of preferential dopant cation-oxide ion vacancy association is also observed at 600 °C, although diminished compared to the level at room temperature. This suggests a continued contribution of defect association enthalpy to activation energy at elevated temperatures and is reflected in similar compositional variation of high- and low-temperature activation energies