Spark Ignition Measurements in Jet A: part II

An improved system for measuring the ignition energy of liquid fuel was built to perform experiments on aviation kerosene (Jet A). Compared to a previously used system (Shepherd et al. 1998), the present vessel has a more uniform temperature which can be held constant for long periods of time. This ensures thermal equilibrium of the liquid fuel and the vapor inside the vessel. A capacitive spark discharge circuit was used to generate damped sparks and an arrangement of resistors and measurement probes recorded the voltage and current histories during the discharge. This permitted measurement of the energy dissipated in the spark, providing a more reliable, quantitative measure of the ignition spark strength. With this improved system, the ignition energy of Jet A was measured at temperatures from 35C to 50C pressures from 0.300 bar (ambient pressure at 30 kft) to 0.986 bar (ambient pressure near sea level), mass-volume ratios down to 3 kg/m^3, with sparks ranging from 10 mJ to 0.3 J. Special fuel blends with flash points (Tfp) from 29C to 73.5C were also tested. The statistical properties of the ignition threshold energy were investigated using techniques developed for high-explosive testing. Ignition energy measurements at 0.585 bar with high mass-volume ratios (also referred to as mass loadings) showed that the trend of the dependence of ignition energy on temperature was similar for tests using the stored capacitive energy and the measured spark energy. The ignition energy was generally lower with the measured spark energy than with the stored spark energy. The present ignition energy system was capable of clearly resolving the difference in ignition energy between low and high mass-volume ratios. The ignition energy vs. temperature curve for 3 kg/m^3 was shifted approximately 5C higher than the curve for high mass-volume ratios of 35 kg/m^3 or 200 kg/m^3. The ignition energy was subsequently found to depend primarily on the fuel-air mass ratio of the mixture, although systematic effects of the vapor composition are also evident. As expected, the ignition energy increased when the initial pressure was raised from 0.585 bar to 0.986 bar, and decreased when the pressure was decreased to 0.3 bar. Finally, tests on special fuels having flash points different from that of commercial Jet A showed that the minimum ignition temperature at a spark energy of about 0.3 J and a pressure of 0.986 bar depends linearly on the flash point of the fuel

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

Collapse transition of a square-lattice polymer with next nearest-neighbor interaction

We study the collapse transition of a polymer on a square lattice with both nearest-neighbor and next nearest-neighbor interactions, by calculating the exact partition function zeros up to chain length 36. The transition behavior is much more pronounced than that of the model with nearest-neighbor interactions only. The crossover exponent and the transition temperature are estimated from the scaling behavior of the first zeros with increasing chain length. The results suggest that the model is of the same universality class as the usual theta point described by the model with only nearest-neighbor interaction.Comment: 14 pages, 5 figure

Exact Partition Function Zeros of a Polymer on a Simple-Cubic Lattice

We study conformational transitions of a polymer on a simple-cubic lattice by calculating the zeros of the exact partition function, up to chain length 24. In the complex temperature plane, two loci of the partition function zeros are found for longer chains, suggesting the existence of both the coil-globule collapse transition and the melting-freezing transition. The locus corresponding to coil-globule transition clearly approaches the real axis as the chain length increases, and the transition temperature could be estimated by finite-size scaling. The form of the logarithmic correction to the scaling of the partition function zeros could also be obtained. The other locus does not show clear scaling behavior, but a supplementary analysis of the specific heat reveals a first-order-like pseudo-transition.Comment: 21 pages, 4 figure

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

Identification of a rhythmic firing pattern in the enteric nervous system that generates rhythmic electrical activity in smooth muscle

The enteric nervous system (ENS) contains millions of neurons essential for organization of motor behavior of the intestine. It is well established that the large intestine requires ENS activity to drive propulsive motor behaviors. However, the firing pattern of the ENS underlying propagating neurogenic contractions of the large intestine remains unknown. To identify this, we used high-resolution neuronal imaging with electrophysiology from neighboring smooth muscle. Myoelectric activity underlying propagating neurogenic contractions along murine large intestine [also referred to as colonic migrating motor complexes, (CMMCs)] consisted of prolonged bursts of rhythmic depolarizations at a frequency of ∼2 Hz. Temporal coordination of this activity in the smooth muscle over large spatial fields (∼7 mm, longitudinally) was dependent on the ENS. During quiescent periods between neurogenic contractions, recordings from large populations of enteric neurons, in mice of either sex, revealed ongoing activity. The onset of neurogenic contractions was characterized by the emergence of temporally synchronized activity across large populations of excitatory and inhibitory neurons. This neuronal firing pattern was rhythmic and temporally synchronized across large numbers of ganglia at ∼2 Hz. ENS activation preceded smooth muscle depolarization, indicating rhythmic depolarizations in smooth muscle were controlled by firing of enteric neurons. The cyclical emergence of temporally coordinated firing of large populations of enteric neurons represents a unique neural motor pattern outside the CNS. This is the first direct observation of rhythmic firing in the ENS underlying rhythmic electrical depolarizations in smooth muscle. The pattern of neuronal activity we identified underlies the generation of CMMCs

Few-body hydrodynamic interactions probed by optical trap pulling experiment

We study the hydrodynamic coupling of neighboring micro-beads placed in a dual optical trap setup allowing us to precisely control the degree of coupling and directly measure time-dependent trajectories of the entrained beads. Average experimental trajectories of a probe bead entrained by the motion of a neighboring scan bead are compared with theoretical computation, illustrating the role of viscous coupling and setting timescales for probe bead relaxation. The findings provide direct experimental corroborations of hydrodynamic coupling at larger, micron spatial scales and millisecond timescales, of relevance to hydrodynamic-assisted colloidal assembly as well as improving the resolution of optical tweezers. We repeat the experiments for three bead setups