Ignition-to-spread transition of externally heated electrical wire

Ignition of electrical wires by external heating is investigated in order to gain a better understanding of the initiation of electrical-wire fires. An ignition-to-spread model is developed to systematically explain ignition and the following transition to spread. The model predicts that for a higher-conductance wire it is more difficult to achieve ignition and the weak flame may extinguish during the transition phase because of a large conductive heat loss along the wire core. An experimental study was performed using several sample wires with different core metals, diameters and coating thicknesses of polyethylene. A coil heater was adopted as the ignition source, and both the heat flux and heating time were selected as the main parameters to identify the flashpoint and spread point of wire fires. Experiments show that additional heating times after flash are required in order to fully pass the transition and achieve a spreading flame, agreeing with model predictions. Finally, the effects of different heating lengths, environmental pressures, and oxygen concentration on wire ignition are discussed, which may be useful for upgrading the design and standards of future fire-safe wires.     

Auger studies comparing the surface concentration of barium on tungsten impregnated and M-cathodes

A study comparing the surface concentration of barium on both an impregnated tungsten cathode surface and M-type cathode surface was made to determine whether the higher electron emission capability of the latter can be attributed to a higher average surface barium concentration. Relative quantitative data was obtained by the use of a computerized Auger microprobe spectrometer on the surface of the cathode, half of whose area was M-type, the other half being the tungsten impregnated type. The results indicate that both cathode surfaces have equivalent barium concentrations and that they are approximately equal to a barium monolayer after 3000 h of cathode life.     

Improving the signal to noise of FIR polarimetry onconstrained-access plasma experiments

Experimental results are presented that demonstrate the pronounced effects produced on the polarization state of an FIR (far infrared) laser beam when a portion of the beam intersects a metallic tubulation at a glancing angle. The effects produced by this interaction are sufficient to totally obscure the desired measurement of Faraday rotation. A model is developed which qualitatively predicts the observations. A method of suppressing this effect, which is viable for the heterodyne method of measuring Faraday rotation, is presented. It is further shown that reflections off the mechanically vibrating tubulations are the source of a variable effective birefringence, which is perceived as an anomalous Faraday rotation. Minimizing these reflections by threading the inside of the tubulations overcame this problem and allowed measurements to be obtained     

Asymptotic analysis of quasi-steady n-heptane droplet combustion supported by cool-flame chemistry

A skeletal chemical-kinetic mechanism for n-heptane cool flames is simplified to the maximum extent possible by introduction of steady-state approximations for intermediaries, following procedures employed previously in addressing two-stage ignition. A pair of ordinary differential equations in mixture-fraction space is thereby obtained, describing the quasi-steady structures of the temperature and heptylketohydroperoxide fields. Application of activation-energy asymptotics for the partial-burning regime to this pair of equations is shown to provide convenient expressions for flame structures and the extinction condition associated with maximally reduced chemistry. With the mixture-fraction co-ordinate related to radius, these results are used to address droplet-combustion experiments that have been performed in the International Space Station. Droplet diameters at extinction are predicted as functions of the oxygen concentration in the atmosphere and are compared with experiment. While the results are encouraging concerning qualitative predictions of dependences of extinction diameters on atmospheric conditions, there are noticeable quantitative differences that point to deficiencies in the analysis, likely resulting from a number of oversimplifications. Further investigation is therefore recommended.     

A simple model for a two-stage chemical reaction with diffusion

We analyse the evolution of a two-stage chemical reaction betweentwo neighbouring plumes of reactants. Under the assumption thatthe plumes are approximately Gaussian we derive a system ofordinary differential equations for the total amount, the centroidand the variance of each reactant. We compare the solution ofthese equations with full numerical simulation of the reaction.Excellent agreement is obtained, with solution of the near-Gaussianmodel requiring considerably less computational effort thanthe full simulations. Of key importance is the yield of thereaction, and we discuss this feature in particular.     

Spectral sequences and adiabatic limits

A Taylor series analysis of the Laplacian as the underlying manifold is demormed leads to a Hodge theoretic derivation of the Leray spectral sequence.Partially supported by an NSF postdoctoral fellowship     

Gas-phase deprotonation of arylalkylamines. A collision-induced dissociation study

The collision-induced dissociation (CID) of deprotonated arylalkylamines of general formula R(1)C(6)H(4)CHR(2)CH(2)NR(3)(2) (where R(1) = H, OH, F or NO(2); R(2) = H or OH; R(3) = H or CH(3)) generated by negative chemical ionization with H(2)O and D(2)O as ionizing reagents, is discussed. The negative chemical ionization mass spectra show that, in the absence of a hydroxy group in the aromatic ring, deprotonation takes place at the benzylic position whereas the proton is lost from the OH group when present. The nitro compound forms only M(-.) ions. The CID spectra of the deprotonated molecules show that fragmentations are strongly dependent on the structural features of the molecules, namely the presence or absence of substituents in the aromatic ring or aliphatic chain. Copyright 1999 John Wiley & Sons, Ltd.