To determine the boundaries of flammability, two quantities are defined in terms of volume fraction of refrigerant in air: Lower Flammability Limit
(LFL) and Upper Flammability Limit
For the flammability risk the helium is treated as if it were hydrogen and the flammability limit
depicts ambient conditions, i.e., 24[degrees]C and 1 bar according Shapiro and Moffette .
Wallesten and Chomiak  explored the possibility of extending the lean flammability limit
by changing the spark position inside the pre-chamber.
The boundaries between Class 2 and Class 3 refrigerants are defined as a lower flammability limit
(LFL) of greater than 100 g/[m.sup.3] (6.2 lb/1,000 [ft.sup.3]) and heat of combustions (HOC) of less than 19 000 kJ/kg (8,169 Btu/lb).
The term "flammability limit
" is most often used to describe the range of fuel/air concentrations that will support flame propagation.
There is nonhomogeneous fuel-air mixture; the closer the observed point to the N side the higher the fuel concentration, even higher than the flammability limit
. Hence, the combustion reaction could not occur at the N side.
As the temperature rises a temperature is reached where ignition and sustained combustion is possible and this is termed the Lower Flammability Limit
In general, a flame at the flammability limit
should have the lowest flammability intensity.
As the fuel-to-air ratio is increased from the lower flammability limit
, the flame temperature will increase.
Great importance must be given to the ignition source that can affect the flammability limit
results or even the flame propagation regime.
The ratio of flammability limits
is defined as the upper flammability limit
divided by the lower flammability limit