Lean blowoff of bluff body stabilized flames: Scaling and dynamics

Stabilization of lifted tribrachial flames in a laminar nonpremixed jet

The limit of large activation energy is studied for the process of simultaneous mixing and chemical reaction of two reactants undergoing a one-step irreversible Arrhenius reaction. Consideration is restricted to problems of the evolution type—like unsteady mixing and boundary-layer combustion—for which the solution is uniquely determined in terms of the initial conditions. The continuous transition from the nearly-frozen to the near-equilibrium regimes is described. The analysis uncovers the existence of: (i) An ignition regime, in which a mixing layer develops with only minor effects of the chemical reaction, until a thermal runaway occurs somewhere within the mixing region; at this location chemical equilibrium then is established rapidly, (ii) A deflagration regime, in which premixed flames originate from the ignition point and move through the mixing region to burn completely the reactant not in excess. And (iii) a diffusion-flame regime, in which a thin diffusion flame, that is established w...

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An Asymptotic Analysis of Unsteady Diffusion Flames for Large Activation Energies

This paper reviews recent advances in understanding the structure of turbulent partially premixed and stratified flames. The term “partially premixed” refers here to compositionally inhomogeneous mixtures that include flammable and non-flammable fluid while “stratified” combustion refers to a reacting front propagating through a range of compositions within the flammable limits. An overview of relevant laminar flame concepts is first introduced. In laminar partially premixed flames, the interaction between rich and lean mixtures is significant leading to improvement in the flame’s resistance to extinction by straining. In lean back-supported laminar stratified flames, the flux of excess heat and radicals into the lean fluid results in higher flame speeds, broader reaction zones, and extended flammability limits compared to homogeneous counterparts. Rich stratified flames are more complex due to the combined fluxes of heat as well as reactive species such as H2 and CO. Recent research in turbulent partially premixed as well as stratified flames is reviewed. Detailed measurements in burners representative of those found in gas turbine combustors show that partial premixing at the lifted flame base increases with instability. Well-characterised laboratory burners where different fuel concentration gradients may be imposed at the jet exit plane show improved flame stability due to mixed-mode combustion. Maximum stability is reached at some optimum level of compositional inhomogeneity. Highly resolved measurements in turbulent stratified flames show that the mass fractions of CO and H2 increase with stratification; a result that is consistent with laminar flame studies. Such experiments are, however, very difficult and require multi-level conditioning of the data. The paper concludes with a brief review of potential numerical approaches employed in the calculations of turbulent flames with inhomogeneous inlet conditions. A key challenge here is to reproduce the effects of increasing levels of stratification and/or inhomogeneity on the compositional structure of turbulent flames.

Partial premixing and stratification in turbulent flames

Flame spreading across liquid fuels

A study was made of flame propagation along methane-air roof layers using a laboratory scale gallery, 8 feet long by 3 inches wide by 6 to 10 inches high. The effects of various environmental and gas layer factors on the speed of the propagating flame were determined and dimensional analysis techniques were used to develop a relationship between these factors and flame speed. The model was designed to study the early stages of flame propagation along a methane-air layer ignited at the closed end of a tunnel.

Flame Propagation in Layered Methane-Air Systems

The ignition of single magnesium particles (28 to 120 micron diameter) suspended in cold oxidant atmospheres by means of an electrostatic levitation device, and the incendivity of these particles when suspended in a stoichiometric methane-air mixture, were investigated using pulsed laser radiation to heat the particles. Highspeed Schlieren and direct photographic techniques were used in the study. The critical radiant power density required for particle ignition was found to increase with particle size. Approximately equal radiant power densities were required for ignition of identical particles suspended in air and pure oxygen, although an increase in radiant power density was necessary for particle ignition in a helium-20 percent oxygen mixture. Results indicate that particle ignition occurred in the magnesium vapor shroud following surface vaporization. The incendivity experiments indicate that the critical radiant power densities necessary for the particle to ignite methane-air mixtures incre...

Ignition and Incendivity of Laser Irradiated Single Micron-Size Magnesium Particles

Characteristics of flames propagating in vertical tubes in the direction of steep methane-air concentration gradients of about 1.3 volume-percent methane per cm were investigated using interferometric techniques. The maximum flame velocity for upward and downward propagation in both 2.5- and 5.0-cm diameter tubes occurred in the fuel-rich region; lower flame speeds were observed with the smaller tube diameter at corresponding methane concentrations. Following flame extinguishment in upward propagation, the hot product gas region behind the flame front continued to rise in a predictable fashion as a coherent column into the nonflammable fuel-rich region. This hot column rose at a constant velocity, approximately equal to the flame speed at the time of flame extinguishment. Flame extinguishment limits in the layered mixtures were found not to be strongly dependent on tube diameter. Interferometric techniques developed in this study appear to be useful for the rapid measurements of flammability limi...

Dynamics of Flame Propagation through Layered Methane-Air Mixtures

The rupture mechanism of edge-supported liquid films appears to involve the viscous and drag energies as well as previously postulated kinetic and surface energies. Although details are obscure, the mechanism appears to involve a liquid-gathering process at the free edge, followed by fragmentation of this thickened edge into drops whose radii are approximately 50 times the film's original thickness.

Rupture Mechanism of a Liquid Film

A passive explosion barrier for suppressing the effects of an explosion in a mine gallery, or the like, comprises a receptacle containing an explosion suppressing substance adapted to be supported by framework adjacent the mine roof. In one embodiment, a corrugated face plate is hinged to the base of the receptacle on one side thereof. First and second support flanges are formed respectively on the upper end of the face plate and on the rim of the receptacle on the side thereof opposite the plate. The receptacle is mounted to the framework with the support flanges resting on first and second beams and with the face plate viewing the explosion source and oblique to a horizontal plane. The weight of the receptacle, and friction between the face plate flange and first beam, prevent the receptacle from releasing due to accidental disturbances. However, the force of wind accompanying an explosion acts against the face plate, causing it to both lift from the first beam and pivot toward the receptacle. As this occurs, the receptacle slips from the first beam and spills the suppressing substance into the path of the explosion. Wind force also maintains the receptacle flange temporarily in contact with the second beam. This imparts rotation to the receptacle as it falls to the ground for thorough dispersion of the suppressing substance. In another embodiment, a pair of face plates are hinged to opposite sides of the receptacle for bidirectional operation.

John Corry

Papers

Flame Propagation in Layered Methane-Air Systems

Ignition and Incendivity of Laser Irradiated Single Micron-Size Magnesium Particles

Dynamics of Flame Propagation through Layered Methane-Air Mixtures

Rupture Mechanism of a Liquid Film

Passive explosion barrier