Ignition diffusion-flame

The Beckstead-Derr-Price model (Fig. 1) considers both the gas-phase and condensed-phase reactions. It assumes heat release from the condensed phase, an oxidizer flame, a primary diffusion flame between the fuel and oxidizer decomposition products, and a final diffusion flame between the fuel decomposition products and the products of the oxidizer flame. Examination of the physical phenomena reveals an irregular surface on top of the unheated bulk of the propellant that consists of the binder undergoing pyrolysis, decomposing oxidizer particles, and an agglomeration of metallic particles. The oxidizer and fuel decomposition products mix and react exothermically in the three-dimensional zone above the surface for a distance that depends on the propellant composition, its microstmcture, and the ambient pressure and gas velocity. If aluminum is present, additional heat is subsequently produced at a comparatively large distance from the surface. Only small aluminum particles ignite and bum close enough to the surface to influence the propellant bum rate. The temperature of the surface is ca 500 to 1000°C compared to ca 300°C for double-base propellants. 

Turbulent Diffusion FDmes. Laminar diffusion flames become turbulent with increasing Reynolds number (1,2). Some of the parameters that are affected by turbulence include flame speed, minimum ignition energy, flame stabilization, and rates of pollutant formation. Changes in flame stmcture are beHeved to be controlled entirely by fluid mechanics and physical transport processes (1,2,9). 

Various kinds of information can be expected from the high pressure combustion and flame experiments Reaction kinetics data for conditions of very high collision rates. Results about combustion products obtained at high density and with the quenching action of supercritical water, without or with flame formation. Flame ignition temperatures in the high pressure aqueous phases and the ranges of stability can be determined as well as flame size, shape and perhaps temperature. Stationary diffusion flames at elevated pressures to 10 bar and to 40 bar are described in the literature [12 — 14]. 

Sanchez, A. L., A. Linan, and F. A. Williams. 1992. A bifurcation analysis of high-temperature ignition of H2-O2 diffusion flames. 24th Symposium (International) on Combustion Proceedings. Pittsburgh, PA The Combustion Institute. 1529-37. 

DIFFUSION FLAMES The flames seen in pyrotechnics are classified as vapour phase diffusion flames. The principal characteristic is that the fuel and oxidiser are initially separate (as finely powdered components). Combustion takes place when fuel and oxidiser crystals are subjected to high temperatures (from an igniter flame) the finely powdered components decompose, giving off columns of gas which mix by diffusion. The diffusion flame takes place in the zone where the gases mix. 

This burner, a butane lighter, a candle, a burning log, and a match (after ignition) are aU examples of diffusion flames where one generally provides the fuel and relies on natural convection of air to provide the oxidant... 

Diffusion Flame. When a slow stream of fuel g s flows from a tube into the atmosphere, air diffuses across the boundary of the stream and Brms an envelope of expl mixture around a core of gas. The core decreases in height until it disappears at some distance above the tube. It thus assumes the shape of a cone. On ignition, a flame front spreads thru the mixture and stabilizes itself around the cooe of fuel gas. The hydrocarbons in common fuel gases crack to form free C H. The shell of carbon-bearing gas so formed gives such flames their luminosity Turbulent Jet Flame. When a gas stream issues from an orifice above a certain critical velocity, it breaks up into a turbulent jet that entrains the surrounding air. The flame of such a jet consists of random patches of combustion and no cohesive combustion surface exists... 

Consider the opposed stagnation flow between two parallel porous disks that are separated by L = 2 cm. (e.g., Fig. 13.12). Air flows from one disk and methane flows from the other. Both inlet streams are at a temperature of T = 300 K and a velocity of u = 100 cm/s. If a flame is ignited, a strained diffusion flame is established between the... 

Assuming an appropriate ignition source and using GRI-Mech for the reaction mechanism, compute the structure of the steady-state strained diffusion flame. 

In a mixture at rest, a diffusive flame would take the form of a sphere convective motion of the gas leads to the flame taking the form of a bent cap. When a lean mixture is ignited with an electric spark, one or several such caps form and rise slowly upward to the end of the tube. An elementary calculation of diffusive combustion in a gas at rest yields a combustion temperature which depends only on the ratio i9/k, and not on the radius... 

In the analysis considered in the preceding section, it was assumed that a uniform spray had been established initially and that once ignited, each droplet burned with an envelope flame around it. These conditions have been achieved reasonably well in the laboratory for various fuel-lean sprays [65]. However, in practical systems the sprays are not uniform, the manner in which the spray penetrates the oxidizing gas is important, and a cloudburning mode of combustion (in which diffusion flames surround groups of droplets, see the last paragraph of Section 3.3.6) may occur [2], [79]. These realities motivate studies of spray penetration and cloud combustion. 

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