Mixture composition range, flame

The ranges of mixture composition pins operating temperature and pressure are important. Unless a detonation flame arrester is used, it is essential to ensure that DDT will not occur after a flame enters the system. Even if a detonation flame arrester is used, it is important to use the correct type of arrester for the service. For example, if it is possible for a flame to approach from either direction, a bidirectional flame arrester must be selected. 

In the present work, therefore, a comparative study of the production of O-heterocycles during the cool-flame combustion of three consecutive n-alkanes—viz., n-butane, n-pentane, and n-hexane—was carried out under a wide range of reaction conditions in a static system. The importance of carbon chain length, mixture composition, pressure, temperature, and time of reaction was assessed. In addition, the optimum conditions for the formation of O-heterocycles and the maximum yields of these products were determined. The results are discussed in the light of currently accepted oxidation mechanisms. 

There exist two values of the mole percent of fuel in a fuel-air mixture—the lower or lean flammability limit and the upper or rich flammability limit—that define a range within which self-sustaining combustion can occur. A fuel-air mixture whose composition falls outside these limits is incapable of igniting or exploding, even if exposed to a spark or flame. The composition range between the two flammability limits is called the explosive range of the mixture. 

Cool Flames. An intriguing phenomenon known as "cool" flames or oscillations appears to be intimately associated with NTC relationships. A cool flame occurs in static systems at certain compositions of hydrocarbon and oxygen mixtures over certain ranges of temperature and pressure. After an induction period of a few minutes, a pale blue flame may propagate slowly outward from the center of the reaction vessel. Depending on conditions, several such flames may be seen in succession. As many as five have been reported for propane (75) and for methyl ethyl ketone (76) six have been reported for butane (77). As many as 10 cool flames have been reported for some alkanes (60). The relationships of cool flames to other VPO domains are depicted in Figure 6. 

Similarly, Figure 6.3.9b depicts the situation in which partial quenching of the flame results from unequal composition of the reactant mixtures issued from the inner and outer tubes, while keeping the mean velocities constant. If the equivalence ratio in the inner tube is excessively leaner or richer to exceed a typically flammable range, it would result in local extinction, thereby exhibiting a hole in the center of the premixed flame. 

One of the most characteristic features of flame propagation is the existence of flammability limits. For any given fuel-oxidizer mixture, there exists a range of compositions, which usually centers about the... 

A combustion aerosol differs from a premixed, combustible gaseous system in that it is not uniform in composition. The fuel is present in the form of discrete particles, which may have a range of sizes and may move in different directions with different velocities than the main stream of gas. This lack of uniformity in the unburned mixture results in irregularities in the propagation of the flame through the spray and, thus, the combustion zone is geometrically poorly defined. 

Flammability limits The range of gas or vapor compositions in air that will bum or explode if a flame or other ignition source is present. Important The range represents an unsafe gas or vapor mixture with air that may ignite or explode. Generally, the wider the range the greater the fire potential. 

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