Flammability limits downward propagating

Jarosinski ]., Strehlow R.A., and Azarbarzin A., The mechanisms of lean limit extinguishment of an upward and downward propagating flame in a standard flammability tube, Proc. Combust. Inst., 19 1549-1557,1982. 

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... 

For propane flames, fhe quenching distance (for downward propagating flames) is limited by the distance between the walls of about 10 mm. In larger channels, the flame is quenched at the flammability limits. 

An accurate application of equation (24) to the calculation of flammability limits entails accounting for variations other than that of El ROT a f Relevant parameters are exhibited explicitly in equation (24). That limits usually are somewhat wider for flames propagating upward in tubes than for flames propagating downward [1], [2] may be attributed to a larger value of L(Tf) for downward propagation. The evaluation of L Tf) will be considered in Section 8.2.5, where questions will be addressed relating to the physics of losses and also to the accuracy with which equation (24) follows from equation (21). It has already been indicated in Section 5.3.7... 

The lower and upper flammabihty limits are the boundary-line equilibrium mixtures of vapor or gas with air, which if ignited will just propagate a flame away from the ignition sonrce. Each of these limits has a temperature at which the flammabihty Emits are reached. The lower flammability limit temperature corresponds approximately to the flash point, but since the flash point is determined with downward flame propagation and nonnniform mixtnres and the lower flammability temperature is determined with npward flame propagation and nnifrom vapor mixtures, the measured lower flammability temperature is often somewhat lower than the flash point. 

FIGURE 9.5 Limits of flammability methane in air as (downward propagation), showing influence of temperature. (From Coward, H. F. and Jones, G. W., Bulletin 503, U.S. Department of the Interior, Bureau of Mines, 1952.)... 

Many factors enter into the experimental determination of flammable limits of gas mixtures, including the diameter and length of the tube or vessel used for the test, the temperature and pressure of the gases, and the direction of flame propagation—upward or downward. For these and other reasons, great care must be used in the application of the data. In monitoring closed spaces where small amounts of gases enter the atmosphere, often the maximum concentration of the combustible gas is limited to one-fifth of the concentration of the gas at the lower limit of flammability of the gas-air mixture. 

In reactor accidents the conditions inside containment prior to hydrogen combustion may include elevated temperature, elevated pressure, and the presence of steam. The flammability limits widen with increasing temperature. For example, at 212 F (lOO C) the lower limit for downward propagation is approximately 8.8% (see Figure 4.6-3). 

Figure 4.6-3 Effect of initial temperature on downward propagating flammability limits in hydrogen air mixtures...

---