This paper presents recent kinetic and flame studies in plasma assisted low temperature combustion. First, the kinetic pathways of plasma chemistry to enhance low temperature fuel oxidation are discussed. The impacts of plasma chemistry on fuel oxidation pathways at low temperature conditions, substantially enhancing ignition and flame stabilization, are analyzed base on the ignition and extinction S-curve. Secondly, plasma assisted low temperature ignition, direct ignition to flame transition, diffusion cool flames, and premixed cool flames are demonstrated experimentally by using dimethyl ether and n-heptane as fuels. The results show that non-equilibrium plasma is an effective way to accelerate low temperature ignition and fuel oxidation, thus enabling the establishment of stable cool flames at atmospheric pressure. Finally, the experiments from both a nonequilibrium plasma reactor and a photolysis reactor are discussed, in which the direct measurements of intermediate species during the low temperature oxidations of methane/ methanol and ethylene are performed, allowing the investigation of modified kinetic pathways by plasma-combustion chemistry interactions. Finally, the validity of kinetic mechanisms for plasma assisted low temperature combustion is investigated. Technical challenges for future research in plasma assisted low temperature combustion are then summarized.

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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 CH 2 O profile of the premixed cool flame is much thicker than its hot flame counterpart at the same fuel concentration, confirming the importance of CH 2 O 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.

The formation and dynamics of premixed cool flames are numerically investigated by using a detailed kinetic mechanism of dimethyl ether mixtures in both freely-propagating and stretched counterflow flames with and without ozone sensitization. The present study focuses on the dynamics and transitions between cool flames and high temperature flames. The impacts of mixture temperature, inert gas temperature, and ozone concentration on low temperature ignition, cool flame formation, and flammable regions of different flame regimes are investigated. For the freely-propagating flames, three different flame structures (high temperature flames, double flames, and cool flames) are found. The present study shows that the flammability limit of dimethyl ether is significantly extended by the appearance of cool flames and that the conventional concept of the flammability limit of a high temperature flame ought to be reconsidered. Furthermore, the results demonstrate that the cool flame propagation speed can be significantly higher than that of near-limit high temperature flames and that ozone addition dramatically accelerates the formation of cool flames at low temperatures and extends the flammability limit. A schematic of a modified flammability limit diagram including both high temperature flames and cool flames is proposed. For stretched counterflow flames, the results also show that multiple flame regimes exist with and without ozone addition. It is demonstrated that at the same mixture enthalpy, ozone addition kinetically extends the cool flame extinction limit to a higher stretch rate. Moreover, with ozone addition, two different cool flame transition regimes: a low temperature ignition transition and a direct cool flame transition without an ignition limit at higher temperature, are predicted. The present results suggest that cool flames can be an important combustion process in affecting flammability limits and flame regimes as the mixture temperature, turbulent mixing, and radical production/recirculation are increased. The results also provide guidance in observing self-sustaining premixed cool flames in experiments.