Cool Flame Propagation Speeds

Lee

et al. 2017

The low-temperature oxidation of hydrocarbon fuels has received increasing attention as advanced engines seek to operate in less conventional combustion regimes. Large n-alkanes are a notable component of many real transportation fuels and possess strong reactivity in this important low-temperature range. These n-alkanes have been studied extensively in various canonical kinetic experiments but seldom in systems with strong coupling between lowtemperature chemistry, transport, and heat release. To address this issue, the present study investigates self-sustaining n-alkane cool diffusion flames in a counterflow burner. The extinction limits of both hot diffusion flames and cool diffusion flames are measured at atmospheric pressure for a range of n-alkanes from n-heptane to n-tetradecane. It is observed that while these fuels behave similarly for hot flames, the larger n-alkanes are substantially more reactive in the low-temperature cool flame regime. Moreover, ozone addition strongly enhances the low-temperature chemistry to the point where the differences in fuel reactivity are nearly suppressed.The experimental measurements are compared with numerical simulations employing both detailed and reduced chemical kinetic models of various sizes. Although the different kinetic models adequately predict the extinction limits of the hot flames, a large scatter is present in the model results for cool flames, and a general overprediction of the measured cool flame extinction limit is observed for all of the fuels studied. This implies that the cool flame heat release is not well agreed upon by the current chemical kinetic models, despite their capability to reproduce many homogeneous reactor experiments at low temperatures. Furthermore, it is observed that the cool flame heat release is spread over a substantial number of reactions involving large molecules, a trait that makes it particularly difficult to create reduced kinetic models that can accurately describe cool flame behavior. The results of this study suggest that the cool flame platform can provide crucial validation of the coupling between chemistry, transport, and heat release in flames at low temperatures. 2 3 4 5 6 7 8

Section: Introductionmentioning

confidence: 99%

Study of the low-temperature reactivity of large n-alkanes through cool diffusion flame extinction

Lee

et al. 2017

“…Of particular importance for internal combustion engines were experiments showing the connection between cool flames and engine knocking [8,9]. Recent investigations have even applied the cool flame to reduced gravity [10,11] and microgravity settings [12][13][14][15]. Yet despite these studies and others involving cool flames using heated surfaces [16,17], heated reactors [18,19], and stirred reactors [20,21], it has been notably difficult to establish self-sustaining cool flames in order to examine their detailed structure, extinction limits, flammability limits, and other dynamics in a laboratory setting without flame-wall thermal coupling.…”

Section: Introductionmentioning

confidence: 99%

“…However, there is no direct experimental data to support this finding due to the difficulty in experimentally establishing a self-sustaining cool flame. Although Foster and Pearlman [10,11] measured cool flame speeds in a spherical chamber, the initial mixture temperature was so high that the mixture was partially oxidized in front of the flame, and as a result the flame was not propagating in a well-defined non-reacting mixture.…”

Section: Introductionmentioning

confidence: 99%

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

2016

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.

“…Unfortunately, the previous studies on flame speeds, extinction limits, and flammability limits for stretched and unstretched flames were often limited to high temperature flames. It was not revealed how fast a cool flame could propagate compared to the high temperature flames, whether it burned leaner than a high temperature flame, or how mixture temperature and radical addition affected its extinction and flammability limits [37].…”

Section: Introductionmentioning

confidence: 99%

Numerical simulations of premixed cool flames of dimethyl ether/oxygen mixtures

2015

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.