Fuel oxidizer mixture, ignition

Two properties of gases and vapors that may determine when an ignition can occur are the minimum ignition energy (MIL) and the antoignition temperature (AIT). These are discussed in Section 4.1.2 above. The MIL is a function of the pressure, temperature, and composition of a fuel-oxidant mixture. 

The point c represents an ignition temperature Tt (or Tc) and from the conditions there, Semenov showed that a relationship could be obtained between this ignition temperature and the initial temperature of the mixture—that is, the temperature of the wall (T0). Recall that the initial temperature of the mixture and the temperature at which the vessel s wall is maintained are the same (T()). It is important to emphasize that T0 is a wall temperature that may cause a fuel-oxidizer mixture to ignite. This temperature can be hundreds of degrees greater than ambient, and T0 should not be confused with the reference temperature taken as the ambient (298 K) in Chapter 1. 

Ignition of a fuel-oxidizer mixture occurs when an external source of energy initiates interactions among the controlling convective, transport and chemical processes. Whether the process results in deflagration, detonation, or is simply quenched depends on the intensity, duration, and volume affected by an external heat source. Ignition also will depend on the initial ambient properties of the mixture which determine the chemical induction... 

Flammability limits are limits of composition or pressure beyond which a fuel-oxidizer mixture cannot be made to burn. They are of practical interest especially in connection with safety considerations because mixtures outside the limits of flammability can be handled without concern about ignition. For this reason, extensive tabulations of limits of flammability have been prepared [1], [2]. Meanings of these tabulations and their relationships to ignition and extinction phenomena will be considered here in Section 8.2. 

It can be assumed that a flame unit, or burner jet, will produce a serviceable flame if the burning velocity of the fuel-oxidant mixture is approximately the same as the velocity of the mixture at the burner. Should the burning velocity be too high the flame will flash back, i.e. the mixture within the flame unit will ignite. Should the burning velocity be too low the flame will blow off, i.e. the mixture will not ignite at the burner port but will bum at some distance from it. Such flames are unsatisfactory. 

Continuing the characterization of materials by measurable attributes, the ignitibility of single substances exposed to air or other gases will be described, as well as the behavior of fuel-oxidizer mixtures under various conditions leading to initiation—heat, static ela -tricity, impact, and friction. Destructive influences, especially of moisture and other external factors comprising the broad field of surveillance and surveillance testing, will be described. 

Constant, reliable, and rapid ignition of the fuel-oxidant mixture so the mixture can bum quickly and release the heat of combustion into the melt. 

When a flame front moves, it travels at less than the speed of soimd (sub-sonically), whereas a detonation occuurs when a combustion wave moves at supersonic speeds. Botii are sustained by Q, but in a detonation, a compressive shock wave is created. When a potentially explosive mixture exists (conditions to be addressed shortly), tiie factors that determine whether a detonation or deflagration occurs are tiie fuel-oxidant mixture, the degree of confinement, and the source of tiie ignition. For now, note that an explosion does not require a chemical reaction to sustain it. For example, when the concrete roof was blown off the reactor building during the Chernobyl accident in 1986, the explosion was tiie result of the rapid vaporization of water (a steam explosion). However, it was not a chemical detonation, since no chemical reaction drove the expansion, pressure buildup, and catastrophic structural failure. 

Auto-ignition temperature The lowest temperature at which a fuel/oxidant mixture will spontaneously ignite. 

Surface Temperatures. At low temperatures, the oxidation reaetions on the eatalyst are kinetieally eontrolled, and the eatalyst aetivity is an important parameter. As the temperature inereases, the build-up of heat on the eatalyst surfaee due to the exothermie surfaee reaetions produees ignition and the eatalyst surfaee temperature jumps rapidly to the adiabatie flame temperature of the fuel/air mixture on ignition. Figure 10-26 shows a... 

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. 

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