Experimental study of the dynamics and structure of self-sustaining premixed cool flames using a counterflow burner

Self-sustaining premixed cool flames are successfully stabilized in a dimethyl ether/oxygen counterflow burner through ozone addition, creating a new platform for the quantitative measurement of cool flame extinction limits, ignition limits, and structure as well as the validation of low-temperature chemical kinetic models. First, results show that stable premixed cool flames can exist over a broad region of equivalence ratios and strain rates, which allows for the ignition and extinction limits of both cool flames and hot flames to be measured at a variety of conditions. It is seen that at low fuel concentrations the cool flame extinction limit surpasses the hot flame extinction limit, providing experimental validation to previous numerical predictions. Furthermore, the experiments demonstrate that the cool flame speed's dependence on equivalence ratio is far weaker than that of near-limit hot flames. It is also found that a hysteresis exists between cool flames and hot flames near the hot flame extinction limit at low fuel concentrations. The examination of cool flame structure through planar laser-induced fluorescence reveals that the CH2O profile of the premixed cool flame is much thicker than its hot flame counterpart at the same fuel concentration, confirming the importance of CH2O as an important cool flame product. Numerical calculations based on a detailed chemical kinetic model are able to capture these various trends and show good qualitative agreement with the experimental results. The present experiments provide a new method to study premixed cool flames in a laboratory setting and to advance the fundamental understanding of low-temperature chemistry and near-limit flame dynamics.