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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.

Influence of magnetic fields on the liftoff and blow out properties of the methane/air co-flow diffusion flame has been investigated experimentally. The present experiments showed that magnetic gradients are able to minimize the liftoff height and increase the flow rate above which the flame blows out. This observed increase of stability of lifted flames is attributed to the magnetic force that develops on paramagnetic oxygen in magnetic gradients. Two mechanisms are provided to explain the extension of the lifted flame stability. By decreasing locally the air velocity upstream of the flame, the magnetic force, resulting from an upward decreasing magnetic field, modifies the fuel mixture fraction at the flame edge, reducing the liftoff height of the flame. The second mechanism is related to the variation of the magnetic susceptibility with temperature and oxygen mass fraction in which originates a magnetic convection phenomenon in air in the flame vicinity.

Resume La rencontre d'un jet et d'un ecoulement perpendiculaire engendre des structures tourbillonnaires instationnaires, a la frontiere du jet. Cette note decrit, a partir de visualisations quantitatives, l'evolution de ces structures. Une des evolutions possibles est particulierement detaillee: le processus d'appariement entre deux tourbillons consecutifs.

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.