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Oscillatory cool flame combustion behavior of submillimeter sized n-alkane droplet under near limit conditions

Published on Jan 1, 2019
· DOI :10.1016/J.PROCI.2018.05.151
Fahd E. Alam5
Estimated H-index: 5
(USC: University of South Carolina),
Ali Charchi Aghdam1
Estimated H-index: 1
(USC: University of South Carolina)
+ 1 AuthorsTanvir Farouk19
Estimated H-index: 19
(USC: University of South Carolina)
Abstract
Abstract This paper reports simulation results of oscillatory cool flame burning of an isolated, submillimeter sized n-heptane (n-C7H16) droplet in a selectively ozone (O3) seeded nitrogen-oxygen (N2-O2) environments at atmospheric pressure. An evolutionary one-dimensional droplet combustion code encompassing relevant physics and detailed chemistry was employed to explore the roles of low-temperature chemistry, O3 seeding, and dynamic flame structure on burning behaviors. For XO2= 21% and a range of selective ozone seeding, near-quasi-steady cool flame burning is achieved directly (without requiring hot flame initiation and radiative extinction). Under low oxygen index conditions, but with significant O3 seeding (XO3 = 5%), a nearly quasi-steady cool flame is initially established that then transitions to a dynamically oscillating cool flame burning mode which continues until the droplet is completely consumed. It is found that the oscillation occurs as result of a initial depletion of fuel vapor-oxidizer layer evolving near the droplet surface and its dynamic re-establishment through liquid vaporization and vapor/oxidizer transport. A kinetic analysis indicates that the dynamic competition between the reaction classes- (a) degenerate chain branching and (b) chain termination/propagation - along with continuous fuel and oxygen leakage through the flame location contributes to an oscillatory burning phenomena of ever-increasing amplitude. Analysis based on single full-cycle of oscillatory burning shows that the reaction progression matrices (evolution of heat and species) for QOOH➔chain propagation/termination reactions (here, Q = C7H14-) directly scales with the gas phase temperature field. On the contrary, the QOOH➔degenerate branching reactions undergoes three distinct stages within the same oscillatory cycle. The coupled flame dynamics and kinetics suggest that in the oscillatory burning mode, kinetic processes dynamically cross through conditions characterizing the negative temperature coefficient (NTC) turnover temperature, separating low temperature and NTC kinetic regimes. In addition, a parametric study is conducted to determine the role of O3 seeding level on the observed oscillation phenomena.
  • References (16)
  • Citations (3)
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References16
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#1Fahd E. Alam (USC: University of South Carolina)H-Index: 5
#2Sang Hee Won (USC: University of South Carolina)H-Index: 5
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Abstract Cool flame combustion of individual and isolated sub-millimeter sized n -heptane ( n -C 7 H 16 ) and n -decane ( n -C 10 H 22 ) droplets are computationally investigated for atmospheric and higher operating pressure (25 atm) conditions with varying levels of ozone (O 3 ) mole fractions in the surroundings. A sphero-symmetric, one-dimensional, transient, droplet combustion model is utilized, employing reduced versions of detailed chemical kinetic models for the fuel species and an append...
5 CitationsSource
#1Wenkai Liang (Princeton University)H-Index: 9
#2Chung King Law (Princeton University)H-Index: 83
Abstract A numerical and theoretical investigation has been conducted on the flammability limits of n- heptane/air mixtures by considering the role of the cool flame chemistry. It is found that the existence of the cool flame can substantially extend the lean and rich limits of the conventional high-temperature flame. Different levels of kinetic criteria based on the controlling branching and termination reactions at low temperatures are derived and shown to agree well with the numerical results...
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#1Christopher B. Reuter (Princeton University)H-Index: 9
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Abstract 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 low-temperature chemistry, transport, and he...
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Abstract 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 extincti...
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This paper summarizes the first results from isolated droplet combustion experiments performed on the International Space Station (ISS). The long durations of microgravity provided in the ISS enable the measurement of droplet and flame histories over an unprecedented range of conditions. The first experiments were with heptane and methanol as fuels, initial droplet droplet diameters between 1.5 and 5.0 m m, ambient oxygen mole fractions between 0.1 and 0.4, ambient pressures between 0.7 and 3.0 ...
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#1Tanvir Farouk (USC: University of South Carolina)H-Index: 19
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Abstract Recent experimentally observed two stage combustion of n -heptane droplets in microgravity is numerically studied. The simulations are conducted with detailed chemistry and transport in order to obtain insight into the features controlling the low temperature second stage burn. Predictions show that the second stage combustion occurs as a result of chemical kinetics associated with classical premixed “ Cool Flame ” phenomena. In contrast to the kinetic interactions responsible for premi...
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Abstract Flame extinction which belongs to one of the most important limit combustion phenomena plays a vital role in the combustion safety, efficiency, and stability, and additionally, it is also important for fire safety research. Hence studies on the flame extinction mechanism is of fundamental and practical significances in the combustion science. In this paper, the structure dynamics and extinction mechanism of dimethyl ether (DME) hot and cool spherical diffusion flames are studied by expe...
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