Vapor-phase combustion

Free Radical Trap Theories. Combustion vapor-phase reactions have been studied using premixed gas flames such as methane. Considerable information concerning the mechanism of flame propagation has resulted from this work 40, 49, 50). Basically the process occurs predominantly by branching chain reactions among free radicals. The major chain branching reactions are... 

Flame Retardants. Flame retardants are added to nylon to eliminate burning drips and to obtain short self-extinguishing times. Halogenated organics, together with catalysts such as antimony trioxide, are commonly used to give free-radical suppression in the vapor phase, thus inhibiting the combustion process. Some common additives are decabromodiphenyl oxide, brominated polystyrene, and chlorinated... 

Vanadium-Sodium Compounds Most Corrosive. Physical property data for vanadates, phase diagrams, laboratory experiments, and numerous field investigations have shown that the sodium vanadates are the lowest melting compounds and are the most corrosive to metals and refractories. These compounds are thought to form by either the vapor phase reaction of NaCI and V2O5 or by the combination of fine droplets of these materials upon the cooler parts of combustion equipment. 

Vapor-phase fuel-distribution image converted to an equivalence-ratio field downstream of the maximum liquid-phase fuel penetration. Quantitative planar images are obtained in the optical engine using PLRS. (From Espey, C., Dec, J.E., Litzinger, T.A., and Santavicca, D.A., Combust. Flame, 109,65,1997.)... 

Fire, or burning, is the rapid exothermic oxidation of an ignited fuel. The fuel can be in solid, liquid, or vapor form, but vapor and liquid fuels are generally easier to ignite. The combustion always occurs in the vapor phase liquids are volatized and solids are decomposed into vapor before combustion. 

Consider possible vapor-phase reactions. These might include combustion reactions, other vapor-phase reactions such as the reaction of organic vapors with a chlorine atmosphere, and vapor-phase decomposition of materials such as ethylene oxide or organic peroxide. 

The description of ISAT presented above follows closely the presentation in Pope (1997), and has been employed successfully in transported PDF studies of combusting systems (Saxena and Pope 1998 Saxena and Pope 1999 Xu and Pope 2000) and vapor-phase chlorination (Shah and Fox 1999 Raman et al. 2001 Raman et al. 2003). A commercial version of ISAT is described in Masri et al. (2003) and has the following additional features ... 

As mentioned, the addition of a small amount of water to the bomb ensures that the vapor phase remains saturated throughout the experiment, so that liquid water is produced in the combustion reaction. It also ensures that the mixture of nitric oxides formed by the oxidation of the N2 will be converted to NOjT(aq), which is simple to determine. 

Table 9.1 and Fig. 9.11 also depict vaporization temperatures of the metals in each product composition and give a graphical representation of Glassman s criterion. When Vvo (or Td, as the case dictates) of the refractory compound formed is greater than the vaporization temperature, Tb, of the metal reactant, small metal particles will vaporize during combustion and bum in the vapor phase. When the contra condition holds, much slower surface reactions will... 

As mentioned in the previous section, the condition for vapor phase combustion versus heterogeneous combustion may be influenced by pressure by its effect on the flame temperature (Tvol or Td) as well as by its effect on the vaporization temperature of the metal reactant (Th). For aluminum combustion in pure oxygen, combustion for all practical conditions occurs in the vapor phase. In air, this transition would be expected to occur near 200 atm as shown in Fig. 9.15 where for pressures greater than —200 atm, the vaporization temperature of pure aluminum exceeds the adiabatic flame temperature. This condition is only indicative of that which will occur in real particle combustion systems as some reactant vaporization will occur at temperatures below the boiling point... 

In certain respects, the combustion of boron is different from that of carbon because, under normal temperature and pressure conditions, the product oxide, B203, is not a gas. Thus, a boron particle normally has an oxide coat that thickens as the particle is heated in an oxidizing atmosphere. This condition is characteristic of most metals, even those that will bum in the vapor phase. For the efficient combustion of the boron particle, the oxide coat must be removed. The practical means for removing the coat is to undertake the oxidation at temperatures greater than the saturation temperature of the boron oxide B203. This temperature is about 2300 K at 1 atm. 

Boron does not meet Glassman s criterion for vapor-phase combustion of the metal. Thus, the boron surface remains coated with a vitreous B203 layer and boron consumption becomes extremely slow consequently, boron is not burned efficiently in propulsion devices. 

Gomez et al. (1988) studied the vapor-phase pyrolysis and combustion of 2,4-D in the temperature range 200-1,000 °C. 2,4-D began to decompose at 200-250 °C. In the presence of air, 2,4-D completely degraded when the temperature exceeded 900 °C. HCl and chlorine were identified as products of thermal degradation. 

For combustion applications and for accurate modeling of the processes involving melting, decomposition, and subsequent oxidation, it is necessary to determine whether or not the fuel undergoes chemical decomposition when it is liquefied and, subsequently, converted to the vapor state. Mass spectral analyses were performed to provide information about the substances in the vapor phase as the fuel is heated. 

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