Free Burning Research Articles

Temperature measurement of nonsteady arcs

The temporal change of the radial temperature profile of a free burning argon arc at power frequency and a flashover arc near the surface of a polluted plate is experimentally determined with the help of a spectroscopic system, the detail of which is described in this paper. The image of the arc is scanned by a rotating 30-facet mirror drum over the entrance slit of a grating spectrometer. The intensity of up to four spectral lines can be simultaneously measured. The temperature results for the argon arc are derived by employing the absolute intensity method and the relative intensity method. This preliminary experimental investigation shows that the system is capable of measuring and monitoring the temperature development of nonsteady arcs which may be randomly moving in a known space.

Simulating Wildfire Containment with Realistic Tactics

Abstract Existing simulation models for fire protection planning rely on a containment algorithm which fails to account for the interaction between the production of containment line and a fire's capacity to spread. This paper describes a technique for simulating wildland fire containment which explicitly accounts for this interaction by extending, generalizing, and, in some cases, simplifying methods reported earlier (Anderson 1983, Albini et al. 1978, Mees 1985, Anderson 1989). Representing the containment boundary in parametric form [i.e., expressing its Cartesian (x, y) or polar (r, ) coordinates as functions of a dimensionless parameter instead of specifying y as a function of x or r as a function of ], leads to a formalism with two significant advantages: (1) it allows a free choice for the shape of the free burning fire boundary, and (2) it is appropriate for simulations of parallel (indirect) attack as well as direct head and tail attack. In general, the technique requires solution of a first-order, nonlinear, differential equation, a task easily accomplished using numerical methods. The dependence of final fire size and containment time on the line-building rate and eccentricity of the free burning fire boundary are illustrated with the special case of an expanding ellipse and constant line-building rate, for which the containment boundary can be obtained by a simple quadrature. Our General Formulation algorithm is demonstrated with a series of simulations that span the range of likely parameters for fire spread and fireline production. Comparisons made on representative fires between predictions of the General Formulation and models currently in use indicate that current models almost always overestimate fire size, sometimes by as much as an order of magnitude. For. Sci. 42(3):267-281.

Mechanisms of anode power deposition in a low pressure free burning arc

Anode power deposition is a dominant power loss mechanism for arcjets and magnetoplasmadynamic (MPD) thrusters. In this study, a free burning arc experiment was operated at pressures and current densities similar to those in arcjets and MPD thrusters in an attempt to identify the physics controlling this loss mechanism. Use of a free burning arc allowed for the isolation of independent variables controlling anode power deposition and provided a convenient and flexible way to cover a broad range of currents, anode surface pressures, and applied magnetic field strengths and orientations using an argon gas. Test results showed that anode power deposition decreased with increasing anode surface pressure up to 6.7 Pa and then became insensitive to pressure. Anode power increased with increasing arc current, while the electron number density near the anode surface increased linearly. Anode power also increased with increasing applied magnetic field strength due to an increasing anode fall voltage. Applied magnetic field orientation had an effect only at high currents and low anode surface pressures, where anode power decreased when applied-field lines intercepted the anode surface. The results demonstrated that anode power deposition was dominated by the kinetic energy of the current-carrying electrons acquired over the anode fall region. Furthermore, the results showed that anode power deposition can be reduced by operating at increased anode pressures, reduced arc currents, anode current densities, and applied magnetic field strengths, and with magnetic field lines intercepting the anode.

Local thermodynamic equilibrium in the cathode region of a free burning arc in argon

Measurements of normalized plasma emission coefficients for the 696.55 nm Ar I line for free burning argon arcs at 1 atm are presented for various arc parameters. These coefficients show variations in the cathode region of the arc, depending upon the arc current, the cathode composition and the mode of operation of the arc, and suggest departures from local thermodynamic equilibrium in the plasma near the cathode tip. In this paper, these results are discussed and an explanation is proposed based on the overpopulation of the ground state level of the neutral atoms. This overpopulation is due to the injection of a large mass flow of cold gas into the arc core under the influence of pressure gradients resulting from the magnetic pinch force in the cathode region.

Prediction of anode temperatures of free burning arcs

Predictions of the temperatures of the anode of free burning arcs are made using a theory that treats an arc and its electrodes as a unified system. The theory predicts the temperature distributions of the arc column, the cathode and the anode as a function of the arc operating conditions. Temperature profiles on the surface of copper anodes for arcs in argon are calculated for various arc operating conditions. Theoretical predictions of the threshold current for melting a water-cooled copper anode range from 1260 to 480 A for anode thicknesses ranging from 2 to 10 mm and are in good agreement with experimental results.

Optical tomography of plasmas by spectral interferometry

A white light illuminated Mach-Zehnder interferometer and a spectrograph are used to obtain spectral side-on interferograms of a steady state free burning arc for different observation directions by rotating the arc between taking the records. The vertical plasma column contains chromium atoms being evaporated by plasma-wall interactions from an electrically insulated and cascaded cylindrical chromium oxide wall surrounding the discharge in the bottom electrode region. The phase change of light waves passing the plasma column is determined highly resolved in the spectral neighbourhood of resonance lines of neutral chromium atoms using FFT algorithms. The angularly dependent integral phase changes corresponding to different observation directions are subject to a computerized tomographic reconstruction procedure yielding the spatially resolved distribution of ground state chromium atoms in the observed cross section of the slightly asymmetric plasma column.

Numerical simulation of the dynamics of free-burning flames, fire whirls, and firestorms.

The paper presents the results of testing a promising package of mathematical models for predicting the dynamics of certain formations in free burning fires, based on a generalized system for time-dependent two-dimensional compressible viscous turbulent gas flows taking combustion into account. The dynamics of flames and fire whirls is investigated using the control volume method. For the first time, firestorms occurring during large fires are numerically simulated; this phenomenon is initiated by the convection of the background vertical component of the atmospheric circulation in the fire influence zone.

Free burning laser-sustained plasma in a forced flow

The boundaries of the existence of a laser-sustained argon plasma in forced convective flow were investigated experimentally and numerically. The plasma was maintained by a cw CO2 laser with an output power of 2.5 kW and burnt in free space at atmospheric pressure. Flow velocities changed from 2.3 m s-1 to 8 m s-1. The f number was 8.8. The results show that a quasi-2D model in which the axial flow is given by the relation rho u= rho 0u0( rho / rho 0)12/, where rho 0 and u0 are the density and velocity of the cold gas respectively, describes the observed phenomena fairly well.

Theoretical study of the melting of the cathode tip of a free burning arc in argon for various conical angles

Theoretical predictions, based on a unified arc-cathode theory, are made regarding the melting of the tips of thoriated tungsten cathodes for free burning arcs in argon. Theoretical results with cathode conical angles of 30 degrees , 45 degrees and 60 degrees are in fair agreement with experimental results. The current density, and thus the degree of melting, increases strongly with the sharpness of the cathode at its extreme tip. The effect of melting is to modify the sharpness of the tip so that, for 200 A arcs, the tip shape for conical angles of 60 degrees and less becomes approximately the same with a radius of curvature of approximately 0.25 mm.

Plume rise of smoke coming from free burning fires

In this paper, attention is focussed on the plume rise of smoke arising from free burning fires. Free burning fires have both geometric and emission ch

Self-organized criticality in a forest-fire model

A forest-fire model is introduced which contains a lightning probability f. This leads to a self-organized critical state in the limit f→0 provided that the time scales of free growth and burning down of forest clusters are separated. We derive scaling laws and calculate all critical exponents. The values of the critical exponents are confirmed by computer simulations. For a two-dimensional system, there is evidence that the forest density in the critical state assumes its minimum possible value, i.e. that energy dissipation is maximum.

A unified theory of free burning arcs, cathode sheaths and cathodes

A theoretical method of predicting properties of free burning arcs and their cathodes is presented in a unified treatment. The method combines a one-dimensional model of the non-equilibrium plasma sheath adjacent to the cathode and a two-dimensional model for the arc column and the solid cathode. Two internal boundaries divide the arc-cathode domain into an arc region, a sheath region and a cathode region. The internal boundary conditions are adjusted during the iteration procedure to satisfy the energy conservation and current continuity equations. The effective resistance of the cathode sheath region is obtained from the sheath calculation assuming charge transport using an ambipolar diffusion approximation. No assumptions are made as to the distributions of, current density and temperature at the cathode surface. The model accounts for cathode surface effects and assumes that the cathode is a thermionic emitter. Material functions such as the thermal and electrical conductivities of the arc plasma and cathode are required as input parameters. Predictions are made, for any given arc current and cathode configuration, of the temperature and current density distributions in the arc and the cathode. Information is also provided about sheath properties. The results from a calculation for 200 A arc burning in argon with a thoriated tungsten cathode are in good agreement with the experimental measurements of the arc column and cathode surface temperatures and the arc voltage.

Excitation de niveaux à haute énergie pendant l'extinction d'un arc à bas courant

The observation of voltage in low current free burning arcs with fast recorders shows that instabilities and spontaneous extinctions appear as unipolar pulses of the arc voltage which are time related with a sharp spike of emitted light. To understand the physical basis of these instabilities and of the spontaneous extinctions, we use the time resolved spectroscopy of the light emitted by the arc. An estimate of the energy of the electrons just at the extinction is given

Boron, calcium and silicon in an arc plasma in air with chlorine: calculation of the plasma composition

The composition of an air plasma in the presence of chlorine (0.1%), and boron, calcium or silicon (at concentrations in the plasma of 0.0001%), was calculated assuming thermal equilibrium, in the temperature range 298–6000 K. The calculated plasma composition reproduces the composition of the d.c. arc plasma free burning in air, as the plasma is assumed to be in (local) thermal equilibrium. Assuming that an arc is usually operated between graphite electrodes, the presence of carbon in the plasma (1%) was also taken into account. It was found that the presence of chlorine in the plasma caused a decrease in the atomic, ionic and molecular forms of calcium, boron and silicon and their spectral line and band intensities (compared with observations in the absence of chlorine) only at temperatures <3000 K. The presence of carbon in the plasma (under the conditions investigated) had almost no influence on the partial pressures of calcium, boron and silicon species and on the partial pressures of compounds of chlorine with oxygen and nitrogen. Also, the change in partial pressures of CN, C2etc.(the radiation of these molecules produces the background band spectra in the graphite arc), in the presence of chlorine, cannot be observed.

Small-scale compartment fire experiments with PMMA cribs

Small-scale compartment fire experiments and free burning experiments were carried out with single poly(methyl methacrylate) (PMMA) slabs and three different PMMA crib geometries. In the compartment experiments, two different ventilation opening were used. The results show that the cribs do not melt down into a pool fire in either the compartment fire tests or the free burning tests. In addition, the results show that, in the compartment fire tests, the outer area of the cribs controls the fire behaviour whereas, in the free burning case, the total surface area of the crib controls the fire. The results provide valuable information on what fuel geometry to use in compartment fire experiments and what fuel area to use in mathematical fire modelling.

Thomson scattering diagnostics of an argon arc plasma and spectral resolution of the ion feature with a narrow bandwidth dye laser

Thomson scattering diagnostics of a free burning argon arc are reported. The electronic plasma parameters are measured with the aid of a periodically pulsed Nd:YAG laser with second harmonic generation. With arc currents in the 100 A range, the electron temperature and density on the axis are Te approximately=1.7 eV and ne approximately=5*1016 cm-3, respectively; and the scattering parameter alpha is close to 1.5. The ion feature is resolved with the aid of a narrow-bandwidth dye laser tuned relative to a spectrally fixed detection channel. Electron and ion temperatures are in satisfactory agreement.

Heat Loss to the Interior of a Free Burning Vertical Wall and its Influence on Estimation of Effective Heat of Gasification

Abstract Conduction heat transfer into a vertical free burning slab was measured as a function of time and distance from the leading edge, and its contribution to the known values of effective heat of gasification was analyzed. The present set of experiments was performed using clear and black PMMA (polymethylmethacrylate) samples. The heat conduction into the pyrolyzing surface was deduced from temperature measurements in the interior of burning slabs. The surface conduction heat flux decreased from 14 kW/m2 for clear PMMA and 13kW/m for black PMMA at t = 300s to almost 8kW/m2 at around l = 1000s and then reached a plateau, indicating a substantial degree of unsteadiness in the early part of combustion of slabs. It is concluded that, when estimating burning rates in numerical calculations one has to be careful in selecting a value for the effective heat of gasification because of the generally unsteady nature of heat conduction into the solid interior.

Quantum theory of laser-field effects on relaxation in solids: Slow versus fast stochastic modulation and applications to optical transients.

A consistent explanation of recent studies by Szabo and Muramoto [Phys. Rev. A 39, 3992 (1989); 38, 5928 (1988)] on the experimental test of modified optical Bloch equations is presented. It is shown that the observed peculiarities of optical transients in solids are connected with the mechanism of relaxation in systems with slow stochastic modulation. A theory of laser-modified relaxation and analysis of free induction decay (FID), hole burning, and rotary echo (RE) in such systems are presented. The universal character of the FID time dependence and field-independent RE decay is predicted.

MODELLING OF HIGH INTENSITY ARCS INCLUDING A NON-EQUILIBRIUM DESCRIPTION OF THE CATHODE SHEATH

An axisymetric model has been developed for prediction of high intensity electric arcs under atmospheric pressure. A set of thermodynamic and electromagnetic equations, including interaction terms (JOULE effect, LORENTZ forces) is solved using the orthogonal finite difference numerical code Melodie. Conservation equations in the laminar arc column (mass, momentum, energy and current) are written according to the local thermodynamic equilibrium (L.T.E.) assumption and the OHM law approximation. The proper boundary condition at the cathode is derived from a one-dimensional description of non-equilibrium electrode boundary layer, coupled with full computation of heat transfer in solid region (conic cathode in tungsten). Numerical calculations performed for free burning and transferred argon arcs show a good agreement with experimental data.

Combustion efficiency, radiation, co and soot yield from a variety of gaseous, liquid, and solid fueled buoyant diffusion flames

The following compilation of data describes combustion conditions for the free burning of propane, heptane, 3 crude oils, wood and polyurethane cribs in fuel package sizes less than a meter, having heat release rates up to 1/2 MW. Measurements include heat release rate from O2 depletion calorimetry, mass loss rate, incident radiative flux to nearby targets, flame and liquid interface temperatures and overall major gaseous species yield including soot. The information contained herein provides a data base for buoyant diffusion flame modelling regarding radiation and elementary chemical characterization. There appears to be a one-to-one correspondence between a fuels radiative behavior and its soot and CO yield. Urethanes and crude oil have significantly higher values for all these parameters as compared to propane, heptane and wood. No correlation was found between these measurements and time-averaged thermocouple readings of temperature.