Cross-flow Arc Research Articles

Evaluation of a nonlinear variational multiscale method for fluid transport problems

Diverse transport problems, especially those based on fluid flow models, are intrinsically multiscale and nonlinear, characteristics that often lead to intricate dynamics such as the development of instabilities and turbulence. Computational simulations that resolve all scales in these problems are often unfeasible, prompting to coarse-grained simulation strategies in which small-scale features are modeled instead of resolved. Variational Multiscale (VMS) methods, and particularly residual-based Large-Eddy Simulation (LES) approaches, have proven effective and robust for the coarse-grained simulation of complex transport problems. VMS methods avoid the assumption of separable nonlinearity and the reliance on empirical small-scale models by using a variational decomposition of scales together with a residual-based approximation of the small-scales. Evaluation of a nonlinear VMS approach, denoted as VMSn, is presented for the coarse-grained simulation of transient-advective-diffusive-reactive (TADR) transport problems arising from fluid flow models. In contrast to classical VMS approaches that neglect the effect of the small scales on the transport operator, VMSn treats the inter-dependence between large- and small-scales upfront. The treatment of inter-scale coupling involves the solution of a local algebraic nonlinear system describing the evolution of the small-scales. The VMSn approach is complemented with two algebraic approximations of the small-scales: one based on the main diagonal of the transport matrices and another that preserves transport fluxes and is suitable for generic TADR systems. The suitability of the VMSn approach for handling general TADR problems and regimes is evaluated with benchmark incompressible, compressible, and magnetohydrodynamic laminar flow problems, the incompressible Taylor-Green vortex flow, the turbulent free jet, and the two-temperature arc in crossflow. Simulation results show that VMSn leads to minor improvements in accuracy with respect to the classical VMS for the laminar flow problems, but to significantly greater accuracy for the turbulent flows and the unsteady plasma flow problems, while using the same cohesive numerical formulation.

Investigation of flow regimes in arc plasma–gas interactions using a two-temperature arc in crossflow model

The perpendicular impingement of a gas stream on an electric arc, a configuration known as the arc in crossflow, is of primary relevance in the study of plasma–gas interactions as well as in industrial applications such as circuit breakers and wire-arc spraying. The flow dynamics in the arc in crossflow are the result of coupled fluid-thermal-electromagnetic phenomena accompanied by large property gradients, which can produce significant deviations from Local Thermodynamic Equilibrium (LTE) among electrons and gas species. These characteristics can lead to the establishment of distinct flow regimes depending on the relative values of the controlling parameters of the system, such as inflow velocity, arc current, and inter-electrode spacing. A two-temperature non-LTE model is used to investigate the arc dynamics and the establishment of flow regimes in the arc in crossflow. The plasma flow model is implemented within a nonlinear Variational Multiscale (VMS) numerical discretization approach that is less dissipative and, hence, better suited to capture unstable behavior than traditional VMS methods commonly used in computational fluid dynamics simulations. The Reynolds and the Enthalpy dimensionless numbers, characterizing the relative flow strength and arc strength, respectively, are chosen as the controlling parameters of the system. Simulation results reveal the onset of dynamic behavior and the establishment of steady, periodic, quasi-periodic, and chaotic or potentially turbulent regimes, as identified by distinct spatiotemporal fluctuations. The computational results reveal the role of increasing the relative arc strength on enhancing flow stability by delaying the growth of fluctuating and unstable flow behavior.

Characterization of the arc in crossflow using a two-temperature nonequilibrium plasma flow model

Diverse industrial applications such as circuit breakers and wire arc spraying involve the interaction between an electric arc and a stream of gas impinging perpendicular to it, a configuration commonly referred to as the arc in crossflow. The arc in crossflow is simulated using a three-dimensional time-dependent two-temperature (heavy-species and electrons) plasma flow model to better capture plasma-gas interactions and deviations from Local Thermodynamic Equilibrium (LTE). The coupled fluid-electromagnetic flow model is solved in a monolithic manner using variational multiscale finite element method. Simulation results are validated with experimental findings and contrasted against results obtained with a LTE model. Results from the two-temperature model corroborate experimental observations while providing quantification of the deviation between heavy-species and electron temperatures. The model is used to characterize the arc in crossflow as a function of the Reynolds and Enthalpy dimensionless numbers, which encapsulate the inter-dependence among the main parameters total current, inflow velocity, and inter-electrode spacing. The characterization revealed the behavior of arc shape, voltage drop, arc power, the degree of nonequilibrium, as well as the characteristic plasma front thickness, with varying controlling parameters.

Three-Dimensional Modelling of a DC Arc in Cross-Flow

A commercial CFD (computational fluid dynamics) code FLUENT was usedand modified to model an atmospheric pressure argon arc in a lowcross flow by solving the fully coupled conservation equations.Numerical experiments, with an arc current of 100 A to 200 A,an arcing distance of 3 mm to 6 mm, and a cross-flow velocityof 10 m/s to 30 m/s, were carried out. The modelling resultsshow that the arc tends to take the shortest path to the anode whendeflected by the cross flow; its anode attachment is fartherdownstream than the cathode one. Furthermore, due to the low inputgas flow imposed in this study, the effect of electromagnetic forceis important and it influences the crosscut shape of the arcsignificantly.

Physics of an arc in cross flow

Arcs in cross flow are encountered in a number of applications, forexample in wire arc spraying. Optimization of such processes has relied inthe past on an empirical approach and on intuition based on a qualitativeunderstanding of the process. A theoretical treatment of an arc exposed to acold gas flow perpendicular to its axis requires a three-dimensional (3D)formulation. A computer code has been developed to solve the 3D conservationequations for an atmospheric pressure argon arc in cross flow, and this codehas been validated by comparison with previously published experimentalresults. Results have been obtained for typical conditions encountered in awire arc spray situation, arc current 100 to 200 A, and arcing gaps of 1 and2 mm, and varying cross-flow velocities. Results are presented in the form oftemperature and velocity fields, and current density and potentialdistributions. It is apparent that the location of the highest temperaturedoes not coincide with that of the highest electric power dissipation because oftransverse convective effects, and that the anode attachment is fartherdownstream than the cathode attachment. The results clearly indicate that 3Dmodelling is required to capture the physical effects of the arc in crossflow, and that the code can be used to illustrate the parametric dependencesof the plasma flow in various cross-flow situations.

Measurement of the temperature and flow fields of the magnetically stabilized cross-flow N2 arc

A straight, steady-state cross-flow arc is burning in an N2 wind tunnel. The arc is held in position by the balance of the Lorentz forces produced by an external magnetic field perpendicular to the arc axis and by the viscous forces of the gas flow acting on the arc column. The temperature field in the discharge is determined spectroscopically using the radiation of N I lines. Because of the lack of rotational symmetry an inversion method developed by Maldonado was used to determine the local emission coefficient from the measured integrated spectral intensity distributions across the arc in various directions. For known local temperature the mass flow field inside the arc may be evaluated from the convective term of the energy equation and the continuity equation. This is done by expanding the terms of these two equations around the point of the temperature maximum into Fourier-Taylor series and determining coefficients of the same order and power. The solution of the resulting set of algebraic equations yields the unknown coefficients of the mass flow. The flow field obtained by these calculations shows a relatively strong counterflow through the arc core. In the region for which the series expansion holds a partial structure pertaining to a closed double vortex can be recognized.

Temperature Measurements in Alternating Current Cross-Flow Arcs

Using optical techniques appropriate to columns having non-circular cross-section, temperature distributions within an alternating current cross-flow arc column have been obtained. Experiments were conducted upon a 61.1 A rms, 840 Torr argon AC plasma (having 11.2 mm electrode spacing) in a crossflow of 1.1 m/s. Isotherm distributions, obtained in horizontal planes, were generally non-circular with major axis in the direction of flow. Column deflections increased as arc current was reduced. The (characteristic) temperature-time history showed that temperature reached a minimum following current zero.

Temperature measurements in rail electrode cross-flow arcs

Temperature measurements in rail electrode cross-flow arcs

Analysis of Cross-Flow Arcs in Transverse Magnetic Fields

Through the specification of two independent variables (e. g., arc current and mainstream velocity), and using appropriate phenomenological relations, the flow fields, temperature fields, and arc boundary shapes of magnetically balanced cross-flow arcs have been determined. Flow through the arc occurs at lower Reynolds numbers, Reo ~ 1; at higher Reynolds numbers a dividing streamline is formed, inside of which double vortex flow is found. Isotherms are nearly circular for the case of flow through the arc. At the higher Reynolds numbers the isotherms are non-circular throughout the cross-section with major axis transverse to the direction of flow.

Magnetically balanced cross-flow arcs.

Magnetically balanced cross-flow arcs.

Transverse magnetic field effects on a cross- flow arc

Experiments were conducted upon an 80 amp, 1.1 atm argon cross-flow arc in a constant velocity mainstream flow of 182.8 cm/sec; electrode spacing was held constant at 6.3 mm. Applied transverse magnetic fields ranged from the zero-balanced mode (B = 0) to the balanced configuration (B = 11.2 gauss). Isotherm distributions were obtained in several horizontal planes; cross-sectional shapes and profiles were thereby inferred. The arc cross-sectional shape was found to be noncircular in all cases. For the zero-balanced mode, the major axis of the cross-sectional shape was in the direction of flow; for the balanced configuration, the major axis was in the direction transverse to the flow. The experimental results indicated that the influence of the magnetic field was perceived throughout the plasma. For the zero-balanced mode, the effects of forced convection were found principally in the outer region; the inner core was approximately circular in cross section.

Effects of forced convection upon the characteristics of a steady-state cross-flow arc in the presence of an applied transverse magnetic field

Experiments were conducted upon a 130-A 1.1-atm balanced argon arc over a range of velocities from 5.09 to 13.13 m/s; electrode spacing was 12.7 mm. Two stable modes of operation were found: the colinear mode, with arc attachments fix at the apexes of the electrodes, and the upstream mode, with arc attachments on the upstream sides of the electrodes. Below a critical velocity of about 5 m/s, the colinear mode only was found. For the colinear mode, arc isotherm distributions, cross-sectional shape, and profile varied markedly with distance along its length; these were observed to be relatively uniform for the upstream mode. Significant lateral broadening of both configurations was found. The effect of velocity upon the upstream mode was to decrease the relative broadening of the plasma.

Effects of Velocity and Current on Temperature Distribution Within Crossflow (Blown) Electric Arcs

The effects of both velocity and current upon the temperature distribution within, and the cross-sectional shape of, steady-state 1.1 atm argon crossflow arcs have been determined experimentally. The tests were conducted over one range at constant current (I = 60.3 amp), U = 0, 41.8 ≤ Ucm/sec ≤ 127.0 and another range at constant velocity (U = 41.8 cm/sec), 42.5 ≤ Iamp ≤ 80.8. Forced convection (at constant arc current) exerts a profound influence upon the crossflow arc. At higher velocities, forced convection appears to completely penetrate the plasma. The effect of increasing current (at constant velocity) is to shield a central core region from the flow field. As a result of the effects of velocity, electrode design, interactions of the electrode jets, and slight misalignment of the jets, neither the isotherms nor the cross sections can be considered to be generally circular, even at the higher currents.

Stability and shape of magnetically balanced cross-flow arcs

References '%-M 1 Garden, R., An Instrument for the Direct Measurement of Thermal Radiation, Review of Scientific Instrumeni^XA. 24, May 1953, pp. 366-370. , v-^ . 2 Malone, E. W., Design and Calibration of Thin Foifteat Flux Sensors, Paper P6-2-PHYMMID^67, 1967, Instrument Society of America. llfi 3 Woodruff, L. W., Hearne, L. F., and Keliher, T. J., Interpretation of Asymptotic Calorimeter Measurements, AIAA Journal, Vol. 5, No. 4, April 1967, pp. 795-797.

Diagnostics on Steady-State Cross-Flow Arcs

A novel cross-flow arc system has been developed which permits the attainment of repeatable arc configurations, oriented in the vertical plane containing the horizontal upstream flow vector, symmetrical in cross section, and symmetrical with respect to the midlocation between the electrodes. A bow-shaped arc is found at lower currents (less than about 20 amperes); a cusp-shaped arc is obtained at higher currents (greater than about 40 amperes). A transition regime is observed at intermediate levels. A unique optical system has been developed in order to observe the arc simultaneously at many different azimuthal locations. With this system, and with the aid of an inversion method developed in [8], isotherm distributions within the arc have now been obtained for the first time. These isotherms, obtained in a horizontal plane near midlocation between the electrodes, indicate that the arc may be divided into a central core region, nearly circular in cross section, and an outer region wherein convective exchange probably predominates. For a given average through-flow velocity, the extent of the central core becomes greater as the current is increased. The cross-flow arc appears noncircular in cross section, with the major axis oriented in the direction of flow. The experiments were conducted in argon (at atmospheric pressure), at constant speed (43.8 cm/s), and constant electrode spacing (6.64 mm); arc current was varied in the range between 17.2 and 34.1 amperes.