Chemistry and Dynamics of Counterflow Cool Flames

Abstract

Cool flame experiments offer an unexplored platform in addressing challenges relevant to the design of advanced engines. Advanced engine designs often focus on premixed or partially premixed strategies involving reduced flame temperatures and, consequently, near-limit combustion with heightened sensitivity to the fuel reactivity and ignition timing. Despite the emissions and efficiency advantages, however, these designs have not been widely implemented due to the limited knowledge of the combustion chemistry required to operate them. A complete understanding of the chemical reactivity of real transportation fuels has been particularly difficult to achieve when considering the complexity of low-temperature combustion phenomena. By investigating low-temperature cool flames in counterflow burners, this dissertation advances the fundamental understanding of the chemistry and dynamics of both nonpremixed and premixed cool flames.

In the first section of this dissertation, the counterflow cool flame platform is presented as an important tool in the quantitative validation of chemical kinetic models at low temperatures. Measurements of the nonpremixed cool flame extinction limits are shown to magnify relatively small differences in low-temperature chemistry, giving the platform a potential use for screening the reactivity of different fuels. It is found that kinetic model predictions of the cool flame extinction limits cannot reproduce experimental measurements accurately due to their inability to capture low-temperature heat release rates in cool flames. An updated kinetic model for dimethyl ether/methane mixtures is developed and validated by targeting reactions disproportionally important to cool flame extinction.

In the second part of the dissertation, the dynamics of both cool flames and hot flames are investigated. The structure and stability of lean premixed cool flames are measured over various conditions, and it is observed that cool flames can sometimes burn under conditions that hot flames cannot, resulting in an extension of the lower flammability limits. In some cases, hot flames can extinguish into cool flames via a transitional double flame structure, composed of a leading cool flame and a trailing hot flame. The interactions between double flames and vortices are also investigated, revealing new and interesting transient flame structures. These findings highlight the relevance of cool flames to near-limit combustion.