A premixed fuel-air mixture will only burn as long as the fuel concentration is between the upper and
lower flammability limits, i.e. UFL and LFL. The flammability limits are experimentally determined data. The flammability limits in air depend on initial temperature and pressure. Standard test conditions are 25 oC and 1 atm.
The figure shows the flammable range for some fuel-air mixtures:
Changes in initial pressure will for hydrocarbons in air not change the LFL significantly, but the UFL will increase. As shown in next figure, the flammable range will widen when the initial temperature is increasing:
This calculator calculates Flammability limits for fuel mixtures by Le Chatelier's law:
where C1,C2....Ci [vol.%] is proportion of each gas in the fuel mixture without air (Kuchta,1985).
