Pascale Gillon

Uses of intense d.c. magnetic fields in materials processing

Apart from classical MHD effects on liquid metals such as braking of motions, stabilization of convection or shaping of free surfaces, the intense d.c. magnetic fields available from superconducting coils open the way to new phenomena which could lead to new methods in materials processing. Homogeneous magnetic field applied during solidification produce a texturation of the solid by magnetic orientation of crystallites present in the liquid. This effect is illustrated by examples of magnetic processing of high Tc superconductors, permanent magnets and biological materials. In the inhomogeneous parts of the field, a magnetic force develops related to the field gradient intensity and the material susceptibility. We present three specific applications developed recently in our research laboratory: levitation, thermomagnetic convection and phase separation.

Magnetic levitation stabilized by eddy currents

An experimental apparatus has been set up that allows stable levitation of a melted paramagnetic body. The appropriate geometry for both a static and an alternating magnetic field that has been used for this purpose is described. This new technique is a promising new development in the area of contactless material elaboration and gravity-free experimentation.

Stabilization of Lifted Laminar Co-Flow Flames by Oxygen-Enriched Air

The effects of a co-flow of oxygen-enriched air on the characteristics of a laminar jet diffusion flame of methane are experimentally investigated using a coaxial burner positioned vertically. The volume content of oxygen varies from 21% (air) to 30%. Flame structure is observed through the heights of flame lift-off and flame length based on initial rates of CH4, air, and oxygen percentages. Stability diagrams showing the states of the flame and transitions between these states are drawn. The reduction of the lift-off height observed during the addition of oxygen is explained by the increase of the laminar flame speed and heat release due to a higher flame temperature.

Methane/Air-Lifted Flames in Magnetic Gradients

The authors analyzed the behavior of a laminar CH4/air flame with a central methane jet and surrounding air jet in a large range of fuel and air flow rates. Different regimes of flame stability are observed from an anchored flame to a stable lifted flame, which is destabilized before extinction. It is shown that the flame instabilities are coupled to the production of vortices on the air jet/ambient air side and is depending on the injected air velocity. Influence of an upward increasing magnetic field is studied by setting the flame inside the bore of an electromagnet. Measurements at different values of methane and air flow rates show that the flame liftoff height is decreased by the magnetic gradient. It is attributed to the magnetic force, which develops on air via its action with paramagnetic oxygen. The magnetic force interacts with the air jet structures and influences the flame stability.

Magnetic field influence on coflow laminar diffusion flames

The behavior of a laminar methane air flame with a central methane jet and a surrounding air coflow is analyzed in a large range of fuel and air flow rates. Different regimes of flame stability are described from an anchored flame to a stable lifted flame which is destabilized before extinction. Influence of an upward increasing magnetic field generated by an electromagnet is then studied. Experimental measurements at different values of methane and air flow rates show that the flame lift-off height is decreased by the magnetic gradient. These effects are attributed to the magnetic force which develops on air via its action on the paramagnetic oxygen molecules. The magnetic force interacts with the air jet structures upstream of the flame and then influences the flame stability.

Experimental Study of A Flickering Methane Diffusion Flame with Coflow of Oxygen-Enriched Air

In this study, the effect of a coflow of oxygen enriched air on the flickering of methane laminar diffusion flame is investigated using two visualization methods. Characteristics of the flame and instabilities of the shear layer between the hot combustion products and ambient air are extracted from flame images at various oxidizer flow rates and oxygen enrichments. These data are linked to the origins of the flame tip oscillations and to the presence or not of a detachment of the top of the flame. We observed that, when the flame is lifted, bulk flickering is always present because the flame remains in the zone of instabilities development. However, an increase of the oxygen enrichment decreases the flame length and the lift height and the instabilities observed in the external shear layer are pushed downstream modifying the behavior of the flame oscillation. Above a given oxygen content, the flame oscillations are observed to disappear.

Electric field influence on the stability and the soot particles emission of a laminar diffusion flame

ABSTRACT Influence of electric fields on flames has been studied for many years and the ionic wind constitutes the main explanation of the observed effects on the flame structure and pollutant emissions. However, previous works have been limited to small flames. The interaction mechanisms of an electric field with longer flames, involving both ionic wind and buoyancy are not fully identified. In the present paper, the effects of a D.C. electric field on a laminar 88-mm-long ethylene diffusion flame burning in ambient air are investigated. Based on the calculated electric field configuration, the influence of both downward and upward electric field is compared via imaging, electrical diagnostic and soot measurements. The application of a negative (directed downstream) electric field triggers a flickering instability and an electric instability at higher field strength, in which self-sustained flame oscillations of flame length directly affect ion current. Conversely, the flame is stabilized by a positive electric field. In-situ soot volume fraction measurements show that the electric field decreases the average soot volume fraction measured on a stable flame axis, whereas flame oscillations lead to a sooting flame.

Magnetic Effects on Flickering Methane/Air Laminar Jet Diffusion Flames

ABSTRACT The article investigates the responses of buoyant jet diffusion flames to the application of magnetic field gradients. In a magnetic gradient, the paramagnetic oxygen is submitted to a magnetic force of attraction directed to the center of the magnet. Positive and negative magnetic gradients effects were compared to the case with no applied magnetic field. Measurements in methane/air flames from a coaxial injector show that over a range of air coflow velocity, the magnetic gradients affect both the oxygen supply at the flame edge and the displacement of the vortices in the air side of the high temperature reaction zone. Upward increasing magnetic field attracts paramagnetic oxygen upwards leading to variations of the lift height and the flame length and counteracts the gravity convective motion attested by a noticeable decrease of the flickering frequency, whereas the upward decreasing magnetic field generates a downward magnetic force on oxygen, depriving the flame edge of oxygen attested by a higher lift height and enhancing the gravity convection in air along the flame evidenced by an increase of the flickering frequency. The flame visible luminosity is shown to be impacted by the magnetic field gradient effect that is related to the soot production through modifications of local temperature, stoichiometry, and residence time.

Heat Transfer from A Laminar Jet Methane Flame in A Co-Annular Jet of Oxygen-Enriched Air

The influence of oxygen enrichment of the air co-jet on the combustion of a laminar methane jet diffusion flame is studied experimentally by the measurement of the heat flux transferred to an assembly of heat exchangers with oxygen concentration ranging from 21% to 41% and firing rates ranging from 0.5 to 1.2 kW. It is observed that, as expected, heat flux increases with oxygen concentration; less methane is necessary to produce the same heating effect as oxygen index is increased. The radiant fraction is found to increase with the oxygen concentration. Deducing the convection part, it is shown that convection is highly dependent on the flame regime with a lesser rate of increase when the flame is reattached to the burner. Evolution of the convective coefficient may be used as a detection of the transition from a lifted flame to a flame attached to the burner.

Lengths of Lifted Laminar Flames Under Vertical Magnetic Field Gradient

The effect of a vertical magnetic field gradient on stoichiometric lengths of methane-air diffusion lifted flames was numerically investigated. The study was developed for understanding the evolution of these lengths when coaxial jet air velocity was varied keeping constant fuel velocity. As the air jet velocity is increased, gradient in mixture fraction and liftoff height increase and flame length decreases. Calculated reacting flow results of liftoff and flame length are presented for two cases, with and without magnetic field. For the same initial flow conditions, the application of the negative magnetic field gradient results in an increase of the stoichiometric flame lengths and a reduction of liftoff heights. For the lifted flame under magnetic field gradient, the Froude number was modified to take into account the magnetic buoyancy effect and it is shown that the correlation proposed by Altenkirch et al. (1976) can still be applied to represent lengths of lifted flames.