Flammable limits for

Flammability Limits There are both upper (or rich) and lower (or lean) limits of flammability of fuel-air or fuel-oxygen mixtures. Outside these hmits, a self-sustaining flame cannot form. Flammability limits for common fuels are listea in Table 27-18. 

Flammability limits for pure components and selected mixtures have been used to generate mixing rules. These apply to mixtures of methane, ethane, propane, butane. 

In these examples, the inerts CO2 and Nj are combined with the combustibles H2 and CO and the small amount of CH4 is taken alone. Next, the ratio of inert to combustible is obtained for each group, as shown in Table 5, and the flammable limits for each such mixture are obtained from Figure 14. The mixture rule formula is now applied, using the data as just obtained, and the limits are calculated as shown in Table 5. 

Figure 14. Flammable limits for hydrogen, carbon monoxide, methane, with nitrogen, carbon dioxide and water vapor.

Figure 15. Flammable limits for paraffin hydrocarbons, with nitrogen and carbon dioxide.

Lower flammability limit for toluene in air is 12,000 ppm (Growl and Louvar, 1990). 

The effect of natural gravity on flammability limits has been known for a long time. The difference between flammability limits for downward and upward flame propagation was first observed by White [26], for hydrogen/air mixtures. Subsequently, similar effects were also found for other mixtures. For propane flames, the lean flammability limit for both downward and upward propagation was observed to be = 0.53. The rich limits were = 1.64 for downward and = 2.62 for upward propagation. Such wide gap between the flammability limits for rich mixtures is explained in... 

The creation of a steady flame hole was previously carried out by Fiou et al. [36]. In their experiments, a steady-annular premixed edge flame was formed by diluting the inner mixture below the flammability limit, for both methane/air and propane/air mixtures. They found that a stable flame hole was established when the outer mixture composition was near stoichiometry. Their focus, however, was on the premixed flame interaction, rather than on the edge-flame formation, extinction, or propagation. 

The Flammability limits for a number of materials are given in Table 9.2. 

Sandia National Laboratory (SNL) completed a literature survey on flammability limits for mixtures of hydrogen and air. Nearly 80 investigations of hydrogen flammability... 

Flammability limits for methane, hydrogen, and gasoline. (Courtesy of Air Products and Chemicals, Inc., Allentown, PA.)... 

Flammability limits for vapors are determined experimentally in a specially designed closed vessel apparatus (see Figure 6-14 on page 255). Vapor-air mixtures of known concentration are added and then ignited. The maximum explosion pressure is measured. This test is repeated with different concentrations to establish the range of flammability for the specific gas. Figure 6-5 shows the results for methane. 

LFL, is the lower flammable limit for component i (in volume %) of component i in fuel and air,... 

Using the assumption of a minimum flame temperature needed for ignition of the mixture, determine the minimum fuel mass loss rate per unit surface area (m l) to cause flame propagation through the boundary layer. The heat of combustion that the volatile wood produces (Ahc) is 15 kJ/g. (Hint the adiabatic flame temperature at the lower flammable limit for the mixture in the boundary layer must be at least 1300 °C.)... 

Macek [27] examined the flammability limits for premixed fuel-air systems and small diffusion flames under natural convection conditions, and computed the equilibrium flame temperature for these flame systems. Data were considered for the alkanes and alcohols at their measured premixed lower flammability limits, and at their measured... 

Vapor Mixtures Frequently, flammability data are required for vapor mixtures. The flammability limits for the mixture are estimated by using LeChatelier s rule [LeChatelier, Estimation of Firedamp by Flammability Limits, Ann. Mines (1891), ser. 8, 19 388-395, with translation in Process Safety Progress, 23(3) 172]. 

As the temperature of a liquid increases, the partial pressure of the vapor above the liquid increases hence, the percentage vapor composition above the pool increases. Liquid boils when its partial vapor pressure reaches the external pressure and the percentage of vapor reaches 100%. Flash point is when the vapor pressure of a substance is such that the concentration of vapor in air above the substance corresponds to the lower flammable limit. For flammable liquids, the term flammable is typically used for liquids with a flash point below 100°F (37.8°C), and the term combustible is used for liquids with a flash point above 100°F (37.8°C) (NFPA, 1997), but this in no way reflects the intensity of an ensuing fire. 

Use laminar premixed free-flame calculations with a detailed reaction mechanism for hydrocarbon oxidation (e.g., GRI-Mech (GRIM30. mec)) to estimate the lean flammability limit for this gas composition in air, assuming that the mixture is flammable if the predicted flame speed is equal to or above 5 cm/s. For comparison, the lean flammability limits for methane and ethane are fuel-air equivalence ratios of 0.46 and 0.50, respectively. 

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