научная статья по теме MAGNETIC FIELD INFLUENCE ON COFLOW LAMINAR DIFFUSION FLAMES Химия

Текст научной статьи на тему «MAGNETIC FIELD INFLUENCE ON COFLOW LAMINAR DIFFUSION FLAMES»

ХИМИЧЕСКАЯ ФИЗИКА, 2010, том 29, № 4, с. 26-32

ГОРЕНИЕ, ВЗРЫВ ^^^^^^^^^^^^ И УДАРНЫЕ ВОЛНЫ

УДК 541.126

MAGNETIC FIELD INFLUENCE ON COFLOW LAMINAR DIFFUSION FLAMES

© 2010 P. Gillon1, J. N. Blanchard12, V. Gilard12

1ICARE-ST2I-CNRS, 1С avenue de la Recherche Scientifique, 45071 Orléans Cedex 2, France 2IUT, Université d'Orléans France E-mail: gillon@cnrs-orleans.fr Received 19.12.2008

The behaviour 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.

INTRODUCTION

Efficiency of a combustion system strongly depends on the injecting device in its ability to realize a uniform mixture. In the present paper, we focus on the flow field upstream of a laminar lifted flame from non premixed methane and air jets. From the two initially separated streams of fuel and oxidizer, the level of homogeneity of the reactants mixing determines the flame position at a specific distance of the injector outlet and the flame stability. The lifted methane/air flame is produced at the exit of a coaxial burner: the internal methane jet is surrounded by an annular air jet; the coaxial jets issuing in a quiescent air. Coaxial jet flows are characterized by a spacial variation of density: the density ratio between air and methane is 1.8. Strong velocity gradients develop between the two jets and between the air jet and external quiescent air. In these transverse gradients of both velocity and density originate aerodynamic instabilities. Coherent structures have been observed in the shear layers at the jets interfaces (Kelvin—Helmholtz instabilities). The mixing is largely due to the shear layers destabilizing the two jets.

When a non uniform magnetic field is applied to a medium, a magnetic force develops due to the magnetic properties — paramagnetic or diamagnetic — of the medium. The magnetic force per unit volume Fi generated by a non uniform magnetic field on species i, is given by eq. (1):

Ft = (1/2^o№;V0B2). (1)

The magnetic force is proportional to the mass density p (kg • m-3) and the magnetic susceptibility

(massic magnetic susceptibility m3 • kg 1) of the ith chemical species of mass fraction Yj and to the gradient of the square magnetic flux density (B2, T2 • m-1). The magnetic susceptibility x, ratio of the magnetization to the magnetic field strength, is positive and depends on temperature for a paramagnetic substance whereas it is negative, with no dependence on temperature for diamagnetics.

In diffusion flames, hydrocarbon fuels, nitrogen, carbon monoxide, carbon dioxide are diamagnetic; oxygen is the principal paramagnetic gas. As the oxygen paramagnetic susceptibility is orders of magnitude larger, the diamagnetic behaviour is considered as negligible.

Many papers referred to the magnetic influence on combustion. Recently, Wakayama [1] investigated methane diffusion flames within magnetic field gradients. It was observed that a decreasing magnetic field along the flame caused its shape more elongated and slender while an increasing magnetic field produced shorter and thicker flames. These effects were attributed to the oxygen strong paramagnetic property and the diamagnetic property of the combustion products. The influence of magnetic gradients on partially pre-mixed and diffusion flames in air are presented by Wakayama [2]. It was found that decreasing magnetic fields increased the combustion rate for diffusion flames while the magnetic fields had little effect on the premixed flame. No effect was observed in uniform magnetic field. It is concluded that the dominant magnetic action is on the oxygen flow in increasing magnetic fields. In [3] the magnetic support of combustion using butane diffusion flame in microgravity is investigated. It is shown that magnetically induced

convection driven by the magnetic force on air sustains the combustion in microgravity.

Yamada et al. [4] investigated numerically the action of magnetic field on OH radical distribution in a H2/O2 diffusion flame. It was observed that the magnetic gradient induced changes in the repartition of OH density in the flame. The effect related to the magnetic force on oxygen is more efficient because the mass density of O2 and then the magnetic susceptibility is much larger in the peripheral region of the flame. Yamada et al. [5] completed their first study by experiments. Using a co-axial type burner set between pole pieces of a permanent magnet, they measured the distribution of the OH radical by a PLIF method. The radial migration of the OH towards the central axis of the flame by the magnetic field predicted numerically was obtained in experiments. Numerical simulations made by solving the equations of gas dynamics and magnetism showed that the magnetic effect is essentially due to the magnetic force acting on O2 and not directly on OH itself.

Baker et al. [6] studied the characteristics of slotted laminar jet diffusion flames in an upward decreasing magnetic field using an assembly of prisms in permanent magnets. The magnetic field is found to decrease the flame height, prevent the flames from attaching to the prisms, increase the intensity of the flame, reduce the flow rate limit at which soot inception is visible and decrease the flow rate limit for extinction. They provide a general relationship between the experimental data and dimensionless parameters.

Khaldi [7] studied experimentally and numerically the behaviour of a small propane diffusion flame in ambient air submitted to strong vertical magnetic gradients. Both increasing and decreasing magnetic fields were investigated. It is shown that the flame adopts a behaviour strongly related to the thermoconvection induced in air in the vicinity of the flame. The parametric analysis taking into account the combination of gravity and magnetic buoyancy confirms the experimental data. The thermomagnetic convection phenomena due the O2 magnetic susceptibility dependence on temperature are detailed in [8].

Stability ofjet diffusion flames is largely referred in literature. When the fuel mass flow rate exceeds a critical value, the base of the diffusion flame quits the burner tip and remains suspended above at a certain distance of the burner. The phenomenon is known as lift-off. When the mass flow rate increases further, the lift-off height increases until the flame becomes flat and then blows out. Extensive experimental and numerical studies have been carried out to define the characteristics of the lift-off phenomenon of round laminar diffusion flames. A lifted flame is stabilized through the triple flame mechanism. A triple flame is a flame structure that has been observed in partially premixed regimes. Just upstream the flame, premixing occurs and the flame front is formed of two branches,

a fuel rich branch develops in the direction of the fuel stream and a fuel lean branch on the air side. Behind these two branches, hot fuel and oxidizer burn in a trailing diffusion flame along a stoichiometric surface forming the third part of the triple flame. The flame is said to be stabilized where the flame propagation speed with respect to the fuel/oxidizer matches the local flow velocity on the stoichiometric line. Phillips [9] made the first observation of a triple flame. Focusing on an axisymetric configuration, Plessing and co workers [10] presented an experimental and numerical analysis of a laminar triple flame. Their work is particularly documented in results and measurements but limited to low lift-off heights. Chung and Lee [11] investigated experimentally and analytically the structure and propagation of laminar lifted flames formed by axisymetric burner. Ghosal et Vervisch [12] revisiting the Chung's analysis, proposed a detailed theory of the lift phenomenon. They also reported on hysteresis where lift-off and reattachment do not happen at the same value of the fuel mass flow rate and time dependent instabilities occur near the critical conditions.

In this paper, the lift-off of a diffusion flame produced by two coaxial jets of methane and air in the presence of an upward increasing magnetic field is studied experimentally and results are compared to the same cases without magnetic field. By perturbing the air flow by the magnetic force, we aim at destabilizing the air mixing layers in order to modify the mechanisms involved in the mixing of the reactants upstream of the flame. We will then be able to predict the possibilities offered by a magnetic field to stabilize or destabilize a lifted flame.

EXPERIMENTS

A schematic representation of the burner with a picture of the experimental set-up is shown in Fig. 1. The burner consists of a 4 mm inner diameter fuel surrounded by a concentric 10 mm inner diameter coflow tube. Methane is delivered through the fuel jet while air passes through the coflow tube. Methane and air flow rates are regulated by using volumetric flow meters.

In the experiments, an electromagnet is used to generate a horizontal magnetic field. The magnetic field induction was measured along the vertical z-axis position using a gaussmeter with a transverse probe. The magnetic induction (normalized with the max value of 1.4 T) and the product of the field gradient to the field (normalized with the max value of 14 T2/m) distributions along the z-axis are

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