Can cool flames support quasi-steady alkane droplet burning?

Nayagam, Vedha; Dietrich, Daniel L.; Ferkul, Paul V.; Hicks, M.; Williams, Forman A.

doi:10.1016/j.combustflame.2012.07.012

Cited by 124 publications

(38 citation statements)

References 21 publications

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“…The numerical predictions show the peak gas temperature during the second stage to be approximately 700 K, consistent with the presence of low temperature chemistry. The numerical predictions (Farouk and Dryer 2013) identify the radiative heat loss as being responsible for initiating the 'cool flame', consistent with the experimental observation (Nayagam et al 2012). The model also predicts the presence of gaseous pure fuel vapor in addition to larger alkenes, ketohydroperoxides and cyclic ethers after 'cool flame' extinction that could condense to form the vapor cloud observed in the experiments.…”

Section: Heptanesupporting

confidence: 68%

“…2). The linear vaporization behavior in this 'cool flame' region occurs as a result of low-temperature chemical reactions that somehow are initiated by the visible hot-flame extinction (Nayagam et al 2012). This behavior is consistent with earlier results on combustion of large heptane/hexadecane droplets in 1-atm air that initially contained small amounts (as small as about 5.8% by mass) of hexadecane (Shaw et al 2001).…”

Section: Heptanementioning

confidence: 99%

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Droplet Combustion Experiments Aboard the International Space Station

Dietrich

Nayagam

Hicks

et al. 2014

Microgravity Sci. Technol.

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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 mm, ambient oxygen mole fractions between 0.1 and 0.4, ambient pressures between 0.7 and 3.0 atm and ambient environments containing oxygen and nitrogen diluted with both carbon dioxide and helium. The experiments show both radiative and diffusive extinction. For both fuels, the flames exhibited pre-extinction flame oscillations during radiative extinction with a frequency of approximately 1 H z. The results revealed that as the ambient oxygen mole fraction was reduced, the diffusiveextinction droplet diameter increased and the radiativeextinction droplet diameter decreased. In between these two limiting extinction conditions, quasi-steady combustion was observed. Another important measurement that is related to spacecraft fire safety is the limiting oxygen index (LOI), the oxygen concentration below which quasi-steady combustion cannot be supported. This is also the ambient oxygen mole fraction for which the radiative and diffusive extinction diameters become equal. For oxygen/nitrogen mixtures, the LOI is 0.12 and 0.15 for methanol and heptane, respectively. The LOI increases to approximately 0.14 (0.14 O 2 /0.56 N 2 /0.30 CO 2 ) and 0.17 (0.17 O 2 /0.63 N 2 /0.20 CO 2 ) for methanol and heptane, respectively, for ambient environments that simulated dispersing an inertgas suppressant (carbon dioxide) into a nominally air (1.0 atm) ambient environment. The LOI is approximately 0.14 and 0.15 for methanol and heptane, respectively, when helium is dispersed into air at 1 atm. The experiments also showed unique burning behavior for large heptane droplets. After the visible hot flame radiatively extinguished around a large heptane droplet, the droplet continued to burn with a cool flame. This phenomena was observed repeatably over a wide range of ambient conditions. These cool flames were invisible to the experiment imaging system but their behavior was inferred by the sustained quasisteady burning after visible flame extinction. Verification 66 Microgravity Sci. Technol. (2014) 26:65-76 of this new burning regime was established by both theoretical and numerical analysis of the experimental results. These innovative experiments have provided a wealth of new data for improving the understanding of droplet combustion and related aspects of fire safety, as well as offering important measurements that can be used to test sophisticated evolving computational models and theories of droplet combustion.

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Section: Heptanesupporting

confidence: 68%

Section: Heptanementioning

confidence: 99%

Droplet Combustion Experiments Aboard the International Space Station

Dietrich

Nayagam

Hicks

et al. 2014

Microgravity Sci. Technol.

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“…Effects such as radiative extinction [9,25,26,36] and LTC phenomena as first postulated by Nayagam et al . in 2015 [37] and subsequently analyzed with detailed numerical modeling [38][39][40][41] may also be important in this regime. An intermediate regime where both radiation and diffusive transport are important would bridge regimes I and III.…”

Section: Introductionmentioning

confidence: 99%

Comprehensive study of initial diameter effects and other observations on convection-free droplet combustion in the standard atmosphere for n-heptane, n-octane, and n-decane

Liu

Hicks

et al. 2016

Combustion and Flame

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“…As the extinction boundary moves to the front of the droplet, it enters a region of gradually enriched oxygen concentration and higher temperature due to the flow and the flame recedes to the back side as combustion is re-established, thereby completing one cycle of an oscillation. This sort of oscillatory phenomenon has often been associated with "cool flame" phenomena (Nayagam et al 2012;Dietrich et al 2014). Fig.…”

Section: Creating Spherically Symmetric Droplet Burning Conditionsmentioning

confidence: 99%

“…For large droplets, radiation can be important to promote extinction with the possibility of "cool flames" (Nayagam et al 2012;Dietrich et al 2014) that are characterized by a low-temperature combustion regime of burning. By varying D o from about 0.5 mm up to about 6 mm, the complete range of physical phenomenon a liquid fuel could experience in a combustion environment can be accessed.…”

Section: Creating Spherically Symmetric Droplet Burning Conditionsmentioning

confidence: 99%

Developing Surrogates for Liquid Transportation Fuels: The Role of Spherically Symmetric Droplet Combustion

Avedisian

2014

Novel Combustion Concepts for Sustainable Energy Development

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The finite supply of transportation fuels has generated renewed interest to improve their performance in power and propulsion devices. However, the complexity of real fuels prohibits developing their combustion chemistries and property databases needed for simulating performance in engines to identify operational regimes for improving fuel efficiency. Surrogates offer the means to address these concerns if they can be shown to replicate certain combustion targets of the real fuel, and to result in combustion properties similar to real fuels when burned in a suitable configuration that is amenable to detailed numerical modeling. This paper examines the role which droplet combustion can play in the development of surrogates for complex transportation fuels. Recognizing that spray combustion is far too difficult to model and that droplets represent the fine-grid structure of sprays, the combustion dynamics of fuel droplets are examined in an environment that seeks to remove external convective influences to simplify the transport field and produce spherical symmetry in the droplet burning process that can be modeled using a detailed numerical simulation approach. The one-dimensional flames and transport dynamics that result are shown to be well positioned to evaluate the efficacy of surrogate fuel performance. Recent efforts are summarized that have used the spherical droplet flame configuration to evaluate the performance of surrogate fuel blends. Experiments are discussed for promoting spherical symmetry, and results are presented to show the efficacy of some surrogate blends to replicate the performance of gasoline and jet fuels using the spherical droplet configuration. Some results are also included from detailed numerical modeling of biodiesel droplets that incorporate complex combustion chemistry, and unsteady transport, vaporization, and variable property effects that illustrate the potential for high fidelity predictions needed for developing surrogates using the spherically symmetric droplet flame configuration.

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Can cool flames support quasi-steady alkane droplet burning?

Cited by 124 publications

References 21 publications

Droplet Combustion Experiments Aboard the International Space Station

Droplet Combustion Experiments Aboard the International Space Station

Comprehensive study of initial diameter effects and other observations on convection-free droplet combustion in the standard atmosphere for n-heptane, n-octane, and n-decane

Developing Surrogates for Liquid Transportation Fuels: The Role of Spherically Symmetric Droplet Combustion

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