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Ozone assisted cool flame combustion of sub-millimeter sized n-alkane droplets at atmospheric and higher pressure

Published on Feb 1, 2018in Combustion and Flame4.12
· DOI :10.1016/j.combustflame.2018.01.015
Fahd E. Alam4
Estimated H-index: 4
(USC: University of South Carolina),
Sang Hee Won5
Estimated H-index: 5
(USC: University of South Carolina)
+ 1 AuthorsTanvir Farouk19
Estimated H-index: 19
(USC: University of South Carolina)
Abstract
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 appended ozone reaction subset. Comprehensive parametric computations show that the regime of the cool flame burning mode and the transition from cool to hot flames are sensitive to the changes of O 3 loading, pressure, diluent variation, the strength of initiation source, and the influence of fuel vapor pressure at the ambient condition. For both fuels and over a range of O 3 concentrations in the ambient, sustained cool flame burning can be directly produced, even for sub-millimeter sized droplets. Over some range of O 3 concentrations, operating pressure, and drop diameter, a self-sustaining, continuous cool flame burn can be produced without incurring a hot flame transition. For sufficiently high O 3 concentrations, combustion initiation is always followed by a hot flame transition. Fuel volatility is also shown to be important for initiation and transition to cool flame and hot flame initiation. For fuels having a flash point lower than the ambient temperature (e.g. n- heptane), atomic O radicals formed by O 3 decomposition react with the partially premixed, flammable gas phase near the droplet surface, leading to OH radicals, water production, and heat that auto-thermally accelerates the combustion initiation process. For fuels with flashpoints higher than the ambient temperature (e.g. n -decane), the reaction progress is limited by the local fuel vapor concentration and the necessity to heat the droplet surface to sufficiently high temperatures to produce locally flammable conditions. As a result, the initial transient for establishing either cool flame or hot flame transition is significantly longer for high flash point fuels. The transition of locally partially premixed reaction to diffusive burning conditions is more evident for high flash point conditions.
  • References (31)
  • Citations (3)
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References31
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#1Tanvir Farouk (USC: University of South Carolina)H-Index: 19
#2Daniel L. Dietrich (Glenn Research Center)H-Index: 14
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Abstract Observations of “ Cool Flame ” burning for large diameter isolated droplets on board the International Space Station have stimulated interest in combustion initiation/generation of non-premixed combustion modes. For a number of n -alkane fuels at large initial droplet diameters, the initiation process was observed to first establish a hot flame condition that radiatively extinguished, followed by a quasi-steady, “ Cool Flame ” droplet burning mode. However, recent large diameter n -deca...
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Abstract Due to their natural coupling of low-temperature chemistry and transport, cool flames are a valuable platform for drawing fundamental understandings of complicated phenomena relevant to real engines. In this study, self-sustaining partially premixed cool flames of dimethyl ether are investigated in detail through the use of an ozone-assisted counterflow burner. A double cool flame with distinct diffusion flame and premixed flames sides is visibly observed at increased fuel loading and e...
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#2Colin Banyon (National University of Ireland, Galway)H-Index: 7
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Abstract This work presents an updated experimental and kinetic modeling study of n- heptane oxidation. In the experiments, ignition delay times of stoichiometric n- heptane/air mixtures have been measured in two different high-pressure shock tubes in the temperature range of 726–1412 K and at elevated pressures (15, 20 and 38 bar). Meanwhile, concentration versus time profiles of species have been measured in a jet-stirred reactor at atmospheric pressure, in the temperature range of 500–1100 K ...
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Recently, large diameter, isolated n-heptane droplet experiments under microgravity conditions (aboard the International Space Station) exhibited “Cool Flame” burning behavior, resulting from a heat loss mechanism that extinguishes hot combustion and a transition into a sustained, low temperature second stage combustion. In atmospheric pressure air, a single combustion mode transition to “Cool Flame” burning is followed by diffusive extinction. But with increasing pressure, multiple cycles of ho...
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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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Experiments on the combustion of large n-heptane droplets, performed by the National Aeronautics and Space Administration in the International Space Station, revealed a second stage of continued quasi-steady burning, supported by low-temperature chemistry, that follows radiative extinction of the first stage of burning, which is supported by normal hot-flame chemistry. The second stage of combustion experienced diffusive extinction, after which a large vapour cloud was observed to form around th...
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