Flammability limits for exhaust gas diluted flames

Abstract

Flammability limits of propane/air (=1.0) flames diluted by exhaust gases have been determined by extrapolating the calculated laminar flame speeds to a minimum flame speed limit as a function of pressure, dilution and unburned gas temperature. The flame speed calculations show an approximately linear decrease in flame speed with dilution. This result compares well with the two available experimental determinations of the functional behavior of the flame speed with dilution. The flammability limits for these diluted flames decrease monotonically, but not linearly with pressure. The limits do, however, increase with unburned gas temperature. A typical dilution limit is 38% by volume of exhaust gases for an unburned mixture temperature of 500 K and a pressure of 4 atmospheres. Length scales based on these laminar flame calculations were also determined. These length scales are an indicator of the turbulent flame structure. The results presented here show that dilution causes a large increase in the flame widths. This large increase in the flame width is sufficient to change the turbulent flame structure from a well defined “wrinkled” flame to a regime which is somewhere between the extremes of a wrinkled flame and the distributed reaction. This means that the turbulent flame structure in a homogeneous-charge, S.I. engine operating with a sufficiently diluted mixture may be much different than that which has been previously assumed.