Flammability limits estimation for fuel–air–diluent mixtures tested in a constant volume vessel

A model for estimating the flammability envelope of mixtures containing inert gases for a constant volume system was derived based on the energy balance equation and thermal radiation loss. The combustion products along the flammability boundaries were analyzed by Fourier Transform Infrared Spectroscopy (FTIR) to verify the assumptions of the proposed model. The model was validated for acetone, methyl formate, methanol, and isopropanol with either steam or nitrogen as the inert gas. The heat loss effects on the estimated flammability envelopes were small. The predicted lower flammability limits (LFLs) closely matched the measured limits, excluding those at the region approaching the limiting oxygen concentration (LOC). The estimated upper flammability limits also agreed well with the measured limits. The FTIR spectra indicated that the increase in the LFL around the LOC is attributed to the existence of unburned fuel, not the formation of CO as previously reported.