Flame diffusion

An important development of iiie diffusion flame technique was introduced [33] in 1964 in which a collecting system for the removal of the alkali metal halide products was incorporated. The halide products are the result of the reaction of two separate organic halides with the chosen alkali metal and one of the organic halides is labelled with chlorine-36. This technique, which is of considerable potential, has received relatively little attention [34—37]. It has been extended to the case of two different halides that are not labelled, using a conductimetric analysis of the alkali metal halide mbctures [36]. 

When two reacting gases A and B interpenetrate each other in the region of Knudsen molecular flow, the rate coefficient k is given [2] by the equation 

The great majority of the rate coefficients determined by the sodium flame technique have been calculated [17] in accordance with the equation 

A critical examination of the possible errors of the diffusion flame method was carried out by Heller [38]. This holds for reactions of collision yields between 1 in 50 and 1 in 5000 and permits the determination of rate coefficients within a 100% error limit provided that the following experimental conditions apply (a) i 3 cm (preferably 

(b) 5 x 10 v/8 12 x 10 , (c) the effect of slip in the capillaries must be taken into account in determining Cnai) (d) the influence of the halogen compound on 5 has to be minimised by keeping the mixture ratio of halide to carrier gas within well-defined limits. Further modifications of these conditions were proposed by Haresnape et al. [26] to allow the method to apply to collision yields of 1 in 5. The majority of diffusion flame studies published after the appearance of Polanyi s book [2] follow the experimental conditions suggested by Heller. It has been suggested [39] that intercomparability of results in a series of compounds can best be achieved by keeping all variables constant with the exception of the halide pressure itself. 

For the hydrogen-diffusion air flame the same burners are used as for the air- or dinitrogen oxide-acetylene flames. An inert gas (argon or nitrogen) is mixed with hydrogen to improve the analytical characteristics of the flame. Then the sample is sprayed into the spray chamber with the help of the inert gas. Diffusion flames are employed for determination of easily atomizing elements, such as arsenic or selenium. 

---

Gaseous fuels. Gas burners can be diffusion flame burners or pre-aeraled burners. Diffusion flame burners may be relatively simple, with fuel gas burning at an orifice in the presence of... 

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. 

Fig. 1. The postulated flame stmcture for an AP composite propellant, showing A, the primary flame, where gases are from AP decomposition and fuel pyrolysis, the temperature is presumably the propellant flame temperature, and heat transfer is three-dimensional followed by B, the final diffusion flame, where gases are O2 from the AP flame reacting with products from fuel pyrolysis, the temperature is the propellant flame temperature, and heat transfer is three-dimensional and C, the AP monopropellant flame where gases are products from the AP surface decomposition, the temperature is the adiabatic flame temperature for pure AP, and heat transfer is approximately one-dimensional. AP = ammonium perchlorate.

Validation and Application. VaUdated CFD examples are emerging (30) as are examples of limitations and misappHcations (31). ReaUsm depends on the adequacy of the physical and chemical representations, the scale of resolution for the appHcation, numerical accuracy of the solution algorithms, and skills appHed in execution. Data are available on performance characteristics of industrial furnaces and gas turbines systems operating with turbulent diffusion flames have been studied for simple two-dimensional geometries and selected conditions (32). Turbulent diffusion flames are produced when fuel and air are injected separately into the reactor. Second-order and infinitely fast reactions coupled with mixing have been analyzed with the k—Z model to describe the macromixing process. 

The vapor cloud of evaporated droplets bums like a diffusion flame in the turbulent state rather than as individual droplets. In the core of the spray, where droplets are evaporating, a rich mixture exists and soot formation occurs. Surrounding this core is a rich mixture zone where CO production is high and a flame front exists. Air entrainment completes the combustion, oxidizing CO to CO2 and burning the soot. Soot bumup releases radiant energy and controls flame emissivity. The relatively slow rate of soot burning compared with the rate of oxidation of CO and unbumed hydrocarbons leads to smoke formation. This model of a diffusion-controlled primary flame zone makes it possible to relate fuel chemistry to the behavior of fuels in combustors (7). 

The formation of carbon black in a candle flame was the subject of a series of lectures in the 1860s by Michael Faraday at the Royal Institution in London (23). Faraday described the nature of the diffusion flame, the products of combustion, the decomposition of the paraffin wax to form hydrogen and carbon, the luminosity of the flame because of incandescent carbon particles, and the destmctive oxidation of the carbon by the air surrounding the flame. Since Faraday s time, many theories have been proposed to account for carbon formation in a diffusion flame, but controversy still exists regarding the mechanism (24). 

Flame Types and Their Characteristics. There are two main types of flames diffusion and premixed. In diffusion flames, the fuel and oxidant are separately introduced and the rate of the overall process is determined by the mixing rate. Examples of diffusion flames include the flames associated with candles, matches, gaseous fuel jets, oil sprays, and large fires, whether accidental or otherwise. In premixed flames, fuel and oxidant are mixed thoroughly prior to combustion. A fundamental understanding of both flame types and their stmcture involves the determination of the dimensions of the various zones in the flame and the temperature, velocity, and species concentrations throughout the system. 

Many different configurations of diffusion flames exist in practice (Fig. 4). Laminar jets of fuel and oxidant are the simplest and most well understood diffusion flames. They have been studied exclusively in the laboratory, although a complete description of both the transport and chemical processes does not yet exist (2). 

Fig. 4. Various configurations (a—e) used to obtain gaseous diffusion flames where a = air and / = fuel (33).

The discussion of laminar diffusion flame theory addresses both the gaseous diffusion flames and the single-drop evaporation and combustion, as there are some similarities between gaseous and Hquid diffusion flame theories (2). A frequentiy used model of diffusion flames has been developed (34), and despite some of the restrictive assumptions of the model, it gives a good description of diffusion flame behavior. 

In addition to the Burke and Schumann model (34) and the Displacement Distance theory, a comprehensive laminar diffusion flame theory can be written using the equations of conservation of species, energy, and momentum, including diffusion, heat transfer, and chemical reaction. 

Evaporation and burning of Hquid droplets are of particular interest in furnace and propulsion appHcations and by applying a part of the Burke and Schumann approach it is possible to obtain a simple model for diffusion flames. 

Combustion chemistry in diffusion flames is not as simple as is assumed in most theoretical models. Evidence obtained by adsorption and emission spectroscopy (37) and by sampling (38) shows that hydrocarbon fuels undergo appreciable pyrolysis in the fuel jet before oxidation occurs. Eurther evidence for the existence of pyrolysis is provided by sampling of diffusion flames (39). In general, the preflame pyrolysis reactions may not be very important in terms of the gross features of the flame, particularly flame height, but they may account for the formation of carbon while the presence of OH radicals may provide a path for NO formation, particularly on the oxidant side of the flame (39). 

Diffusion Flames in the Transition Region. As the velocity of the fuel jet increases in the laminar to turbulent transition region, an instabihty develops at the top of the flame and spreads down to its base. This is caused by the shear forces at the boundaries of the fuel jet. The flame length in the transition region is usually calculated by means of empirical formulas of the form (eq. 13) where I = length of the flame, m r = radius of the fuel jet, m v = fuel flow velocity, m/s and and are empirical constants. 

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). 

Consider the case of the simple Bunsen burner. As the tube diameter decreases, at a critical flow velocity and at a Reynolds number of about 2000, flame height no longer depends on the jet diameter and the relationship between flame height and volumetric flow ceases to exist (2). Some of the characteristics of diffusion flames are illustrated in Eigure 5. 

The physics and modeling of turbulent flows are affected by combustion through the production of density variations, buoyancy effects, dilation due to heat release, molecular transport, and instabiUty (1,2,3,5,8). Consequently, the conservation equations need to be modified to take these effects into account. This modification is achieved by the use of statistical quantities in the conservation equations. For example, because of the variations and fluctuations in the density that occur in turbulent combustion flows, density weighted mean values, or Favre mean values, are used for velocity components, mass fractions, enthalpy, and temperature. The turbulent diffusion flame can also be treated in terms of a probabiUty distribution function (pdf), the shape of which is assumed to be known a priori (1). 

The various studies attempting to increase our understanding of turbulent flows comprise five classes moment methods disregarding probabiUty density functions, approximation of probabiUty density functions using moments, calculation of evolution of probabiUty density functions, perturbation methods beginning with known stmctures, and methods identifying coherent stmctures. For a thorough review of turbulent diffusion flames see References 41—48. 

When gas concentrations are high, burning is characterized by the presence of a tall, turbulent-diffusion, flame plume. At points where the cloud s vapor had already mixed sufficiently with air, the vertical depth of the visible burning zone is about equal to the initial, visible depth of the cloud. 

Orifice flames can be cliaracterized as either prenii.xed or diffusion flames. In a prenii.xed flame, tlie air for combustion is already nii.xed witli the fuel gas before it leaves the orifice or pipe. In a diffusion flame, fuel e.xiting tlie orifice is piu c and the air needed for combustion diffuses into tlie fuel gas from the surroundings. Orifice flames can also be cliaracterized by tlie fame Reynolds number. Tlie flame lengtli of a diffusion flame can be calculated by Jost s equation. ... 

T. Baron, "Reactions in Turbulent Free Jets-The Turbulent Diffusion Flame," Cbem. Eng. Prog., 50, 73 (1954),... 

Diffusion Flame. Wlien the fuel and oxidizer are initially unmixed and then mix in a thin region where the flame is located, the flame is called a diffusion flame (Figure 2). The word diffusion is used to describe the flame because the fuel and oxidizer are mixed on the molecular level by the random thermal motion of the molecules. An example of a diffusion flame is a candle flame or flares at an oil refinei y. 

Laminar Versus Turbulent Flames. Premixed and diffusion flames can be either laminar or turbulent gaseous flames. Laminar flames are those in which the gas flow is well behaved in the sense that the flow is unchanging in time at a given point (steady) and smooth without sudden disturbances. Laminar flow is often associated with slow flow from small diameter tubular burners. Turbulent flames are associated with highly time dependent flow patterns, often random, and are often associated with high velocity flows from large diameter tubular burners. Either type of flow—laminar or turbulent—can occur with both premixed and diffusion flames. 

Shapes of diffusion flames surrounding a burning spherical fuel droplet. 

Liquid Pool Flames. Liquid fuel or flammable spills often lead to fires involving a flame at the surface of the liquid. This type of diffusion flame moves across the surface of the liquid driven by evaporation of the fuel through heat transfer ahead of the flame. If the liquid pool or spill is formed at ambient conditions sufficient to vaporize enough fuel to form a flammable air/fuel mixture, then a flame can propagate through the mixture above the spill as a premixed flame. 

I lere, too, computer based predictions about the nature of these flames require information about the chemicals and science of diffusion flames for the predictions to be accurate. The predictions are made accurate by comparison with measured chemical species concentrations, measured temperatures, and flow characteristics. 

Otherwise known as diffusion flame or post-aerated, these normally comprise a simple nozzle supplying gas at a controlled pressure into a chamber where air is made available via entrainment into the flame by natural draft. Common types are the Bray jet, Aeromatic and Drew jet (Figure 24.8). 

---