Combustion of droplets of liquid fuels: A review

Williams, Alan

doi:10.1016/0010-2180(73)90002-3

Cited by 281 publications

(55 citation statements)

References 71 publications

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“…[19][20][21] Although droplet regression can follow the classical d 2 -law under normal gravity, natural convection can distort the spherical flame shape and therefore negate the simplified spherical symmetry, 22 affecting burning rate behavior. Nevertheless, there exist certain conditions, where the length scales (and corresponding Grashof numbers) are considerably reduced-for example, a micronsized droplet burning in an environment of increased oxygen concentration (which keeps flame sizes small)-that favors the spherical flame shape.…”

Section: Introductionmentioning

confidence: 99%

Disruptive burning of precursor/solvent droplets in flame‐spray synthesis of nanoparticles

et al. 2013

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Flame spray pyrolysis is an established technique for synthesizing nanoparticles in the gas phase through aerosol combustion of precursor/solvent droplets. The combustion characteristics of isolated micron-sized precursor/solvent droplets are investigated experimentally. Pure solvent droplets burn uniformly and classically quasisteady, whereas precursor/ solvent droplets manifest disruptive combustion behavior. The fast onset of droplet disruption, which occurs only for solutions with dissolved metal precursors, is not due to solid-particle precipitation within the droplet. Instead, the mechanism of disruptive droplet burning is similar to that of slurry droplets, consisting of three main steps: (1) diffusioncontrolled burning of the high-volatile solvent, (2) viscous-shell formation due to decomposition of the low-volatile metal precursor, and (3) subsequent disruption due to heterogeneous nucleation. The time sequence of the three steps depends on the concentration and decomposition characteristics of the metal precursor, shortening with increased concentration and higher incremental decomposition temperature.

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Section: Introductionmentioning

confidence: 99%

Disruptive burning of precursor/solvent droplets in flame‐spray synthesis of nanoparticles

et al. 2013

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“…We pause to make several comme nts about the expressions obtained for ed (1], 7) and eY(1] , 7). Both contain a term proportional to the reduced time T which is a consequence of the second order pole at a = 0.…”

Section: H(p)mentioning

confidence: 99%

Evaporation of a liquid droplet

Kayser¹,

Bennett²

1977

J. RES. NATL. BUR. STAN. SECT. A.

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Two idealized models for the preheat s tage of liquid dro plets are ana lyzed theoretically . These models conta in the effects of transie nt heat conduction and e va porati on. It is assumed that the droplet surface a rea decreases linearl y with ti me. This assumption necessitates the soluti on of mov in g boundary pro ble ms . These models, howe ver, do not consider gas-phase mass transport . In the finit e-gradient model, the te mpe ratures of both the dro plet a nd s urrounding hot gases vary spatia lly a nd temporall y. In the zero-gradie nt mod el the gas tempe ratu re varies spatia lly and temporall y but the d roplet temperature vari es onl y tempora ll y, i. e., the drop let tempe rature is spati a lly uniform . Numeri cal e xamples, whi ch require exte ns ive ca lcu lations of connue nt hypergeometric fu nctions, are presented for typi cal va lues of the d ro ple t late nt heat and evaporation rate consta nt. The tem pe rature profi les give n by the fin ite-gradient and zero-grad ient models agree to within 20 percent of eac h othe r for a ll cases examined .Key words: Conflue nt hype rgeometric fun cti ons; d ro plet; mov ing boundary prob lem; pre heat; tra ns ie nt heat cond uction. . IntroductionThe ignition of a dropl et of conventional fu el consists essentially of two stages [l]l. During the first or preheat s tage heat fl ows fro m the hot surrounding gas to th e d ro plet, causing the droplet temperature to ri se and liq ui d fu e l to evaporate from the d ro pl et surface . In the second stage, ignition occurs in the gaseous mixture of fu el and oxidizer surrounding the droplet. The pre heat stage is dominated by transie nt processes a nd provides the motiva ti on for the subject of thi s pape r.Wi se and Ablow [2], Parks, et al. [3], and Waldman [4] ha ve previously analyzed the effects of transient d roplet heating by negl ecting the existe nce of internal c irculation and by neglecting selected terms in the heat conducti on equations. In addition they assume that the d roplet surface regresses linearly with time. In a previous publication, Bennett and Kayser [5] included the effects of internal c irculation but also neglected evaporation and regression of the droplet surface. We now include these effects in the present pape r.The neglect of internal circulation can in many instances lead to gross underestimates of the heat transfer rates within the droplet. Available experimental evidence indic ates that in most cases the heat transfer rate inside the drople t is muc h faster than is possible with heat conduction alone. El Wakil, et al. [6] observed vigorous circulation within the droplet and showed that the droplet temperature is uniform eve n during the initial trans ient period. This ind icates that the rate at which heat is transfe rred to the droplet is much slowe r tha n the rate of internal mixing. As pointed out by Law [7], assuming the d ropl et te mperature to be uniform in a sense circumvents th e difficult tas k of describing internal c irc ulation but still in cludes the eff...

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“…Numerous reviews of the subject are now available in the literature (Williams 1973;Faeth 1977;Law 1982;Avedisian 2000;Choi and Dryer 2001). An advantage of spherical symmetry is that only one spatial dimension enters the description of the combustion process, so that the one-dimensional, time-dependent conservation equations apply.…”

Section: Introductionmentioning

confidence: 99%

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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Combustion of droplets of liquid fuels: A review

Cited by 281 publications

References 71 publications

Disruptive burning of precursor/solvent droplets in flame‐spray synthesis of nanoparticles

Disruptive burning of precursor/solvent droplets in flame‐spray synthesis of nanoparticles

Evaporation of a liquid droplet

Droplet Combustion Experiments Aboard the International Space Station

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