Argon Burns Research Articles

ВЛИЯНИЕ ФИЗИЧЕСКИХ ПРОЦЕССОВ В ПРЕДПЛАМЕННОЙ ОБЛАСТИ НА ГОРЕНИЕ ЧАСТИЦ АЛЮМИНИЯ В КИСЛОРОДСОДЕРЖАЩИХ СРЕДАХ

The processes in preflame region while combustion of aluminum particles in media «79% Ar + 21% O2» и «79% Не + 21% O2» were analyzed. The necessity of oxygen dissociation taking into account for explanation of effect of physical processes on particles rate of combustion is proved. The calculations are implemented for particles of diameter 220 m and ambient pres-sure p=0,1 MPa provided that quasistationarity and symmetry of processes. Values of concentration of dissociating oxidizer on the flame boundary are established to be lower than those calculated without dissociation taking into account (in 2,5 times lower for combustion in helium and by 30% lower for combustion in argon). Distributions of medium temperature and concentrations of oxidizing components are obtained depending on relation of coordinates R of inner points of region to current particle radius R0. The preflame region is revealed to be in the range of 2,4R/R05,1 for burning in helium and to be in the range of 15R/R06,9 for burning in argon. The results of analysis show necessity of molecular oxygen dissociation taking into account in the region under consideration. The results explain differencies of burning rates obtained in experiments from calculated values. Also re-sults of this work explain particles combustion rates ratio in media posessing essentially different transport coefficients.

Modelling of a heavy-current intensively blasted electric arc at atmospheric pressure

The paper deals with intensively blasted electric arc burning in argon in a cylindrical channel of a designed experimental arc heater. The measured values of the arc current, the total voltage between a tungsten cathode and a grounded copper anode, the argon flow rate, and the water flow rates and temperature increments in individual parts of the arc heater characterize the operational conditions, but the parameters of the arc inside remain hidden. The simplified mathematical model has been designed to reveal the arc behavior in the arc heater channel, namely the arc radius and arc temperature development. The model is based on the energy and mass conservation laws and Ohm’s law and utilizes theoretically calculated transport and thermodynamic properties of argon altogether with the measured results as input data. Some simplifying assumptions are applied. A two-zone arrangement with a hot arc zone in the middle and a cold zone near the channel wall is supposed. The arc radius development along the arc heater’s channel is described by Schlichting’s formula for a free beam. Such combinations of three characteristic values of exponent n r, current density j 0 at the cathode tip, and arc temperature T A(s) at the end of the near-cathode layer are sought to enable as close as possible accordance between the computed and experimental data. Thereafter, a slight tuning of these parameters is needed to avoid extreme values of electric field intensity and temperature oscillations near the cathode while not to substantially worsen the reached agreement of measured and computed values. The functionality of the model is demonstrated using four typical sets of experimental data.

Estimation of the Intensively Blasted Electric Arc Model Sensitivity to Selected Variables

Results of measurements carried out on the fabricated experimental modular-type arc heater serve as input data for the designed simplified model of the intensively blasted electric arc burning in argon inside the cylindrical arc heater's anode channel. The axial dependence of the arc temperature and radius is expressed using the exponent, the current density on the cathode tip and the arc temperature at the end of the near-cathode boundary layer. These quantities form the vector of state variables that is sought to minimize the value of the objective function expressing the deviations between measured and computed values. On a typical example, the paper demonstrates the sensitivity of the modelling to individual state variables.

Propane burning in argon, carbon dioxide, and water vapor at increased pressure

The paper presents investigation results on propane combustion in argon, carbon dioxide, and water vapor under the conditions of O2 deficiency and high reagent density when they are uniformly heated up to 620 K. Based on time dependences of reaction mixture temperature, the self-ignition temperature of propane was determined. It has been established that the oxidation of propane in an Ar and H2O medium proceeds according to the mechanism of chain-thermal explosion. The results of mass spectrometric analysis showed that oxidation in a CO2 medium is characterized by the lowest degree of propane conversion. It was also found that at a low density of water vapor, the oxidation of propane is accompanied by a high yield of H2. The mechanisms of CO2 and H2O molecule participation in the oxidation of propane are discussed in the paper.

UPDATED VERSION OF THE SIMPLIFIED MODEL OF INTENSIVELY BLASTED ELECTRIC ARC

The article deals with an improvement of a simplified model of intensively blasted electric arc burning in argon in an anode channel of a modular-type arc heater. Gradually gained experience in the application of the model with various input data has revealed that some refinement is needed. The contribution focuses especially on the process of searching for an optimum combination of the state variables, which are the exponent determining the arc radius development along the anode channel axis, the current density at the cathode, and the arc temperature at the end of the near-cathode layer. The limiting relations for the state variables are set and discussed. Three constituent parts of the used objective function are analysed and compared. The procedure is demonstrated on a chosen set of experimental data. Further computations with many sets of measured data are necessary to test the designed procedure and to confirm the determination of the state variables’ range.

Digital visualisation of the process of heating and melting of metal in arc discharge with a non-consumable electrode

Images of an electric arc, burning in argon, were produced using a digital camera in different wavelength ranges of the visible spectrum. The charts of the thermal fields of the heating spot were constructed and the plasma temperature in the anode section of the arc was calculated. It is shown that the digital technology of producing images in the visible range of the wavelength can be used for analysis of the processes taking place directly in the anode region of the arc and for evaluating the dimensions of the arc column.

Calculation of Arc Power Losses in the Simplified Model of Intensively Blasted Electric Arc

In previous versions of the simplified model of intensively blasted electric arc burning in argon in the arc heater's anode channel, the authors used the constant total power loss coefficient for estimation of arc power losses in all anode channel individual parts. Using this approach, the model with relatively low computational complexity has led to very good agreement between the total computed and experimentally obtained values, but when the computed and measured power losses of individual anode channel segments have been compared, considerable differences have been revealed. In the modified model, theoretically computed net emission coefficient of argon is used in the energy equation to express the arc power losses. This way, satisfactory accordance is achieved between not only the total, but also partial measured and computed values. Exemplary results are given in figures and tables and discussed.

Visualization of the process of metal heating and melting in the anode zone in an arc discharge with a tungsten electrode

Pictures of an electric arc burning in argon, obtained by means of a digital camera, within the different domains of wavelengths of the visible spectrum are presented. Maps of the thermal fields of the heating spot are plotted, and the plasma temperature in the anode arc zone is calculated. The application potential of the digital image technology in the visible wavelength domain for analysis of the processes directly in the anode arc zone and for estimation of the arc column parameters is shown.

Особенности электродинамики высокочастотного факельного разряда, горящего в аргоне

Особенности электродинамики высокочастотного факельного разряда, горящего в аргоне

Features of the Distribution Process of the Electromagnetic Field Frequency Components in the High-Frequency Torch Discharge Plasma

The measurements of axial harmonics distribution of the radial component of the electric field have been made for a torch discharge burning in argon and argon-air mixture. The resonant character of the electric field fourth harmonic attenuation has been found. Using the Saha formula for the two-temperature plasma the electrons concentration and the electrons Langmuir frequency of the investigated plasma discharge have been evaluated.

Nonlinear Electrodynamic Effects of the Torch Discharge Argon Plasma

The measurements of the amplitude-frequency characteristics were done for the high-frequency torch discharge burning in argon. The resonant character of the electric field fourth harmonic attenuation was found. The axial harmonics distribution measurements of the electric field were done for the torch discharge burning in argon-air mixture. The electron concentration in the investigated plasma discharge was evaluated. The selective influence of the change in the electron concentration on the propagation process of the fourth harmonic of the electric field was found.

Comparing deposition of organic and inorganic siloxane films by the atmospheric pressure glow discharge

The atmospheric pressure glow discharge burning in argon with small admixture of oxygen and hexamethyldisiloxane is used for the deposition of organosilicon and SiO x -like films on the stainless steel surface. The organosilicon film has high growth rate (85 nm/min) and extremely low surface roughness (0.8 nm), while the SiO x -like film shows relatively low growth rate (40 nm/min) and higher surface roughness (9 nm). The surface wettability, microhardness and the corrosion resistance of the deposited siloxane films are detected. The organosilicon film is hydrophobic (contact angle: 98°) and soft (microhardness: 0.68 GPa), while the SiO x -like film is hydrophilic (contact angle: 45°) and hard (microhardness: 7.85 GPa). The potentiodynamic polarization tests show that by depositing siloxane films on the surface, the corrosion resistance of the stainless steel in the Hanks solution can be largely improved. In particular, the SiO x -like film exhibits better corrosion resistance than the organosilicon film.

Radiation of electric arc burning in argon

Using real experimentally obtained integral values, the paper deals with modelling of electric arc stabilised by flowing gas. Attention is focused namely on approximate estimation of radiation coefficient of argon. A designed model of electric arc burning in argon of atmospheric pressure inside arc heater’s anode channel is described. The model makes it possible to compute axial and/or radial dependencies of some quantities of interest (temperature, velocity, electric field intensity, arc radius, etc.), and subsequently to judge energy exchange between the arc and its surroundings. Sets of model’s input data, including arc voltage, arc current, argon flow-rate, and flow-rates and temperatures of water cooling individual parts of the arc heater, have been measured during numerous experiments. In a studied case with relatively high argon flow-rate, radiation has been found to be prevailing mechanism of energy transfer from arc to anode channel walls. Based on this finding, techniques have been designed for simple approximate estimation of radiation coefficient of argon in a limited extent of temperatures. As an example, they have been tested on a particular set of measured and computed data. Argon radiation coefficient estimated in this way has been compared with the results of theoretical computations carried out by other authors. Considering simplifications used and differences between a real situation and an ideal theoretical model, agreement of the results is within satisfactory limits.

Holographic Interferometry of Barrier-Torch Discharge in Argon and Helium

Holographic interferometry of atmospheric RF barrier-torch discharge burning in argon and helium is reported. Holographic interferometer of Mach–Zehnder type with He–Ne laser was used for temperature measurement of neutral particles of working gas. Interference pattern was registered by a high-speed charge-coupled devices (CCD) camera. Resulting temperature ranged from 330 to 450 K. A slight difference between temperatures measured close to the glass nozzle outlet and close to the substrate was observed.

Power balance at the cathode of an electric arc burning in argon – A spectroscopic approach and a thermal model

The evaluation of the power brought by an electric arc to the surface of the electrodes still remains an important problem. In this paper we propose a method allowing a macroscopic approach of the power balance at the surface of a copper cathode submitted to a non stationary electric arc in argon. This method is based on a spectroscopic measurement of the instant of apparition of copper vapour in the arc and on the use of a numerical modelling of thermal phenomena occurring in the electrode. Under some simplifying assumptions, we reach an interval of possible values for the power and the surface density brought to the electrode by the arc.

Characterization of a dc plasma torch through its light and voltage fluctuations

The Fast Fourier Transform method has been applied to light and voltage fluctuations in a low power (<5 kW) dc plasma torch burning in argon. The influence of the gas flow on the development of turbulence in the plasma jet has been studied. The temporal and spatial non-homogeneities observed in the plasma flow have been linked to the dynamic processes in a dc plasma torch.

Influence of the anode material on an argon arc

The interaction between the arc and the anode was experimentally studied by means of a transferred arc burning in argon with copper, iron, or steel anodes. Depending on the rate of anode cooling, a stable plasma was obtained just above the anode, established either in pure argon (strong cooling) or in a mixture of argon with metal vapor. Temperature and metal concentration fields were deduced from spectroscopic measurements. Two important results were reached: the arc radius near the anode depends on the nature of the electrode, even without anode erosion; and the presence of metal vapor leads to a cooling of the plasma. The same arc configurations were theoretically simulated by a two-dimensional model. The comparison between experimental and numerical results allows a large proportion of the observed phenomena to be interpreted, in spite of partial discrepancies between predicted and measured values. The dimension of the arc root at the anode depends on the thermal conductivity of the solid metal, whereas the cooling effect due to metal vapor in the plasma is explained by the increases of electrical conductivity and of radiative losses in the presence of the vapor.

Anode parameters of short arcs at low current

A theoretical model of a low-current electric arc at the anode is presented. This method is based on the numerical resolution of a closed system of equations which makes it possible to determine the magnitude of a few anode parameters. The input parameters required are the arc current and physical constants of the anode material. We restrict ourselves to three anode parameters: power lost by conduction, spot radius and current density. The results from our calculations are compared with experimental measurements based on a calorimetric method for an arc burning in argon and arc current between 2 - 6 A. A comparison of our results and those reported in the literature is also made.

Three-dimensional modelling of unsteady high-pressure arcs in argon

Three-dimensional, unsteady behaviour of high-pressure electric arcs in argon is studied by means of numerical simulations. Attention is paid to argon arcs whose steady structure is fairly well understood. Results are reported for the case of a one centimetre long arc, burning in argon and driven by a total current of 200 A and 300 A. The influence of the boundary conditions for the electrical potential on the macroscopic structure of the arc is studied and it is found that the distribution of the current density near the cathode is one of the critical parameters which can significantly modify both the distribution of mean quantities and the stability of the arc. All mean quantities found from three-dimensional calculations are compared with two-dimensional axi-symmetric fields obtained by a previously used code for arc simulations. The results indicate good agreement between the results from three-dimensional calculations, two-dimensional axi-symmetric simulations and results from physical experiments documented in the literature.

Surface temperature measurements for tungsten-based cathodes of high-current free-burning arcs

Measured temperature profiles for various oxide-tungsten cathodes and for pure tungsten cathodes are presented for high-current arcs burning in argon at atmospheric pressure. Temperature profiles are also presented for thoriated tungsten cathodes with different cathode cone angles, are currents and composition of the gas provided to the arc. Evidence is also presented that the temperature and the behaviour of the cathode are sensitive to the oxygen concentration in the argon.