Effect of inhibitors on flammability limits of dimethyl ether/air mixtures

Abstract

As dimethyl ether is being extensively used as an environmentally friendly motor fuel, the issue of its fire and explosion safety is becoming increasingly urgent. In this work, the action of trimethylphosphate, bromotrifluoromethane (CF3Br), trifluoroiodomethane (CF3I) and iron pentacarbonyl (Fe(CO)5) on the upper and lower concentration flammability limits (CFLs) of dimethyl ether/air mixture was studied using the counterflow burner technique and chemical kinetic modeling. Iron pentacarbonyl was shown to extend the lower limit and to have a small inhibition effect on the upper limit. The other additives, even in small concentrations, were shown experimentally to narrow the flammability limits with the effect on the rich limit being greater than that on the lean one. The kinetic mechanisms for flame inhibition by the abovementioned compounds used in the numerical simulations unsatisfactorily predicted the effect of inhibitors on the lower CFL. In addition, modeling greatly overpredicted the inhibiting effect of Fe(CO)5 on the upper limit compared to experimental data. As for the rich limit, the modeling predicted the effect of CF3I and TMP well and underpredicted the inhibition effect of CF3Br. As compared to other inhibitors, CF3I was found to narrow the upper CFL most effectively, whereas TMP is known to have the greatest effect on the speed of freely propagating flames. A possible explanation of the different inhibiting effects of chemically active inhibitors on freely propagating and counterflow flames has been proposed. As, in counterflow flames, the residence time can be less than the characteristic time of chemical reactions, the influence of some reactions on the flame parameters can be different in stretched (counterflow) flames and non-stretched (laminar freely propagating) flames. For TMP, this suggestion has been validated by analysis of the contributions of reactions involving phosphorus species to the rates of production and consumption of H atoms.