Experimental and numerical characterization of freely propagating ozone-activated dimethyl ether cool flames

Abstract

Experimental flame characterization is necessary for the development and validation of chemical kinetics models. Low temperature oxidation produces a cool flame, which is a combustion phenomenon resulting from negative temperature coefficient (NTC) behavior. Kinematic stabilization of premixed, freely propagating, ozone-activated, cool flames of dimethyl ether (DME) has been investigated at sub-atmospheric pressure of 7.3kPa using a laminar flat flame Hencken burner. This platform permits estimation of laminar propagation speed, as well as spatially-resolved temperature and species mole fractions along the burner axis. Stability mapping for a range of equivalence ratios (ϕ) was performed to determine the range of ozone concentrations for which a cool flame can be sustained. Based on the results of stability mapping, an ozone concentration of 6.1% in oxygen was chosen to investigate the characteristics of DME cool flames over a range of equivalence ratios from ϕ=0.4 to 1.4. Two distinct cool flame stabilization modes were observed in experiments: a burner-stabilized mode at low reactant flow rates, and a freely propagating mode at higher flow rates. From the transition between the two modes, cool flame propagation speeds from ϕ=0.4 to 1.4 were determined. Flame temperatures were measured at these equivalence ratios, with maximum temperatures decreasing from 885K at ϕ=0.4 to 779K at ϕ=1.4. A single equivalence ratio of ϕ=0.6 was chosen for detailed investigation of the flame structure, including spatial measurements of temperature and species as a function of height above the burner. Experimental results were compared to numerical simulations of a ϕ=0.6 cool flame. Experimental propagation speed was found to be within 25% of the numerical value, and significant agreement between experimental and numerical species profiles was observed.