Gaseous fuel-oxidant mixture

Table 19.14. Adiabatic flame temperature of selected gaseous fuel-oxidant mixture...

Flammable Limits The minimum and maximum concentration of fuel vapor or gas in a fuel vapor or gas/gaseous oxidant mixture (usually expressed in percent hy volume) defining the concentration range (flammable or explosive range) over which propagation of flame will occur on contact with an ignition source. See also Lower Flammable Limit and Upper Flammable Limit. 

Crystalline particles that produce gaseous oxidizer fragments are used as oxidizer components and hydrocarbon polymers that produce gaseous fuel fragments are used as fuel components. Mixtures of these crystalline particles and hydrocarbon polymers form energetic materials that are termed composite propellants . The oxidizer and fuel components produced at the burning surface of each component mix together to form a stoichiometrically balanced reactive gas in the gas phase. 

For gaseous flames, the LES/FMDF can be implemented via two combustion models (1) a finite-rate, reduced-chemistry model for nonequilibrium flames and (2) a near-equilibrium model employing detailed kinetics. In (1), a system of nonlinear ordinary differential equations (ODEs) is solved together with the FMDF equation for all the scalars (mass fractions and enthalpy). Finite-rate chemistry effects are explicitly and exactly" included in this procedure since the chemistry is closed in the formulation. In (2). the LES/FMDF is employed in conjunction with the equilibrium fuel-oxidation model. This model is enacted via fiamelet simulations, which consider a laminar counterflow (opposed jet) flame configuration. At low strain rates, the flame is usually close to equilibrium. Thus, the thermochemical variables are determined completely by the mixture fraction variable. A fiamelet library is coupled with the LES/FMDF solver in which transport of the mixture fraction is considered. It is useful to emphasize here that the PDF of the mixture fraction is not assumed a priori (as done in almost all other flamelet-based models), but is calculated explicitly via the FMDF. The LES/FMDF/flamelet solver is computationally less expensive than that described in (1) thus, it can be used for more complex flow configurations. 

From the point of view of chemical composition of gaseous fuels, it concerns with the exception of the mixtures of gases. Among some of the essential properties of gaseous fuels that depend on their chemical composition and that affect burner design there are LHV, amount of oxidizer necessary for stoichiometric combustion of the mixture, temperature and pressure of gas in gas distribution lines before burner inlet. If gaseous fuels contain shares of liquid carbohydrates or an amount of solid particles, it can cause, in some cases, significant problems. 

The addition point of gaseous fuels requires careful consideration to avoid homogeneous reactions upstream of the reformer vith autothermal reforming and partial oxidation. Commercial flame arresters are normally not capable of operating under the elevated temperatures of the fuel processor. Microchannels are known to act as flame arresters (see Section 6.3.2) and may be inserted into the tubing system to avoid uncontrolled reaction of the fuel/air mixture. For liquid fuels, which are usually injected into the pre-heated steam feed or even into the air/steam feed mixture, either cooled injection nozzles [567] or the application of steam jackets may be used to ensure stable operation of the nozzle. 

A variety of materials can be used as fuels in energetic mixtures, and the choice of material will depend on a variety of factors—the amount of heat output required, ignitability, rate of heat release needed, cost of the materials, stability of the fuel and fuel-oxidizer pair, and amount of gaseous product desired. Fuels can be divided into three main categories metals, nonmetallic elements, and organic compounds. 

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