Diffusion flames heat release

In flames only the net heat release is measured. This datum can be used in two different ways—in simple systems such as the hydrogen bromine flame chemical kinetic information can be inferred from thermal measurements, but in more complex flames heat release rates are useful primarily only as consistency checks. From the standpoint of chemistry, the most important physical process in the flame is diffusion, since it affects the composition. This will be discussed in more detail in the following section. 

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. 

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). 

Transient computations of methane, ethane, and propane gas-jet diffusion flames in Ig and Oy have been performed using the numerical code developed by Katta [30,46], with a detailed reaction mechanism [47,48] (33 species and 112 elementary steps) for these fuels and a simple radiation heat-loss model [49], for the high fuel-flow condition. The results for methane and ethane can be obtained from earlier studies [44,45]. For propane. Figure 8.1.5 shows the calculated flame structure in Ig and Og. The variables on the right half include, velocity vectors (v), isotherms (T), total heat-release rate ( j), and the local equivalence ratio (( locai) while on the left half the total molar flux vectors of atomic hydrogen (M ), oxygen mole fraction oxygen consumption rate... 

Calctilated species mole fractions, temperature, and heat-release rate across propane jet diffusion flames in "still" air at a height of 3 mm in... 

Figures 4.6—4.8 are the results for the stoichiometric propane-air flame. Figure 4.6 reports the variance of the major species, temperature, and heat release Figure 4.7 reports the major stable propane fragment distribution due to the proceeding reactions and Figure 4.8 shows the radical and formaldehyde distributions—all as a function of a spatial distance through the flame wave. As stated, the total wave thickness is chosen from the point at which one of the reactant mole fractions begins to decay to the point at which the heat release rate begins to taper off sharply. Since the point of initial reactant decay corresponds closely to the initial perceptive rise in temperature, the initial thermoneutral period is quite short. The heat release rate curve would ordinarily drop to zero sharply except that the recombination of the radicals in the burned gas zone contribute some energy. The choice of the position that separates the preheat zone and the reaction zone has been made to account for the slight exothermicity of the fuel attack reactions by radicals which have diffused into...

Many practical industrial processes are diffusion limited (i.e., have a high Damkohler number), and the assumption that the chemistry is fast is often sufficient to predict the overall characteristics of the process. For instance, in turbulent diffusion flames, the rates of fuel oxidation and heat release are often governed by the turbulent transport and mixing. 

If we are dealing with mutual diffusion of gases which are close in molecular weight (e.g., carbon monoxide and air), it may be shown that the temperature of the flame pellet will prove to be equal to the theoretical combustion temperature of the mixture. This equality depends on the existence in the kinetic theory of gases of a simple relation between the diffusion coefficient (on which the supply of reagents and heat release rate depend) and the thermal conductivity (on which the heat evacuation depends). 

In the present case, the flame front is stabilized by the CRZ (1) but the heat release magnitude is reduced in the evaporation zone because of both effects (2) and (3). To determine the flame regime (premixed and/or diffusion), the Takeno index T = VYjp.VYq and an indexed reaction rate... 

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