Gas-phase quasi-steadiness and fuel vapor accumulation effects in droplet burning

Law, Chung K.; Chung, Suk Ho; Srinivasan, N.

doi:10.1016/0010-2180(80)90049-8

Cited by 107 publications

(29 citation statements)

References 11 publications

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“…As expected, larger radii correspond to larger mass flow rates for a given fuel mixture. The steady growth of the flames resembles earlier experimental results on the temporal variation of the flamefront standoff ratio for droplet combustion in micro-buoyancy [8,9], exhibiting the influence of fuel vapor accumulation.…”

Section: Resultssupporting

confidence: 80%

Microgravity burner-generated spherical diffusion flames - Experiment and computation

Tse

Zhu

Sung

et al. 1999

37th Aerospace Sciences Meeting and Exhibit

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Microgravity experiments were conducted in the 2.2-s drop-tower facility at the NASA Glenn Research Center to study the transient response of the burner-generated spherical diffusion flame caused by its initial displacement from the steady-state position. The experiment involved issuing H 2 /CH 4 /inert mixtures of constant fuel mass flow rates from a bronze, porous, 1.27-cm-diameter, spherical burner into atmospheric air. The experimental results on the flame trajectory were found to agree well with those obtained through fully transient computational simulation with detailed chemistry and transport, and appropriate initial conditions. Furthermore, although steady-state behavior should exist for such flames, the experimental and computational results indicated that it cannot be reached within the 2.2-s microgravity duration for the fuels and mass-flow rates tested.To assess the role of radiation on the flame dynamics and extinction, computations were performed without radiation, with radiation employing the optically thin approximation, and with radiation utilizing a detailed emission/absorption statistical narrow band (SNB) model. The computation showed that while the influence of radiative heat loss on the position of the flame is small, proper consideration of radiative effects is crucial in assessing the state of flame extinction. Specifically, while all simulations of the experimental cases studied incorporating radiative heat loss revealed that the flame extinguishes well before the attainment of steady state, simulations accounting for gaseous reabsorption of radiative emissions were required to adequately represent the experiments in terms of extinction time, with the optically thin simulations predicting premature extinction during the flame expansion process. Effects of heat loss to the porous burner were also examined, and the lack of correspondence between the visible flame luminosity and flame strength, as related to flame temperature and heat release rate, was noted.

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Section: Resultssupporting

confidence: 80%

Microgravity burner-generated spherical diffusion flames - Experiment and computation

Tse

Zhu

Sung

et al. 1999

37th Aerospace Sciences Meeting and Exhibit

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“…This observation is inconsistent with the quasi-steady theory, which states that the ratio of the flame-to-droplet diameter is constant [9]. Subsequent studies have clarified that the main factors causing this contradiction are gas-phase diffusive unsteadiness [10], droplet heating [11], and fuel vapor accumulation effect [12].…”

Section: Introductioncontrasting

confidence: 49%

“…When the initial fuel vapor concentration around the droplet cloud is low, immediately after ignition most of the fuel vaporized is stored around the droplet cloud rather than being consumed at the flame [12]. The group combustion theory shows that the distance from the droplet cloud to the envelope flame increases with decreasing droplet spacing.…”

Section: Introductionmentioning

confidence: 99%

Interactive combustion of two-dimensionally arranged quasi-droplet clusters under microgravity

Nagata

Kudo

Ito

et al. 2002

Combustion and Flame

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To investigate the mutual interactions between droplets in the spray combustion, combustion of 2-dimensionally arranged quasi-droplet clusters is studied under microgravity. Quasi-droplet samples, which are solid in room temperature and change into liquid just after the ignition, consist of alcohol (propanol, butanol, pentanol, or hexanol) and polyethylene glycol with a volumetric ratio of 2:1. Seven samples sustained by glass rods form a 2-dimensional quasi-droplet cluster. Electrically heated nichrome wires ignite all samples in the cluster simultaneously. Single envelope flames that surround the clusters appeared. The results show that the sample spacing has a strong effect on the shape and movement of the flame. Sample clusters with large sample spacings come to the external group combustion through the scavenging combustion mode, whereas the small spacing clusters start directly with the external group combustion. At large sample spacings, the distance from the edge of the sample cluster to the flame (flame distance) increases to a maximum value and then decreases with time. The period of flame growth is prolonged with decreasing sample spacing and finally, at a small enough sample spacing, the flame distance keeps increasing until the flame disappears. This flame movement is attributed to the fuel vapor accumulation effect, which becomes more dominant with decreasing sample spacing. The burning lifetime decreases monotonically and approaches the value of the single flame with increasing sample spacing. The flame distance decreases monotonically and approaches the single flame radius with increasing sample spacing also. These results render important confirmations of the external group combustion phenomena and prove the importance of the two kinds of unsteadiness, i.e., the scavenging combustion with large droplet interval and the fuel vapor accumulation effect with small droplet interval, in group combustion.-3-

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“…Applying smaller P has been attempted by Law [13] and it has been revealed that the spherical droplet(s) flame was successfully obtained. Lastly, "miniaturization (i.e., adopting smaller L)" can also reduce Gr most effectively even without any special facility likely drop-tower and vacuum chamber to manipulate G and P. Although it is quite obvious, no successful study of "tiny" droplet(s) combustion has been reported so far; this is simply because that it is hard to establish steadily and difficult to diagnose.…”

Section: Strategies To Reproduce a "Tiny-spherical" Flamementioning

confidence: 99%

Development of "Tiny" Droplet Flame Simulator: "Super-Stabilized" Micro-Jet Flame in a Hot Air

Fujiwara

Nakamura

Hirasawa

2011

JTST

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An attempt to form a "tiny" spherical flame over the small jet burner (so called microflame) as a model of a tiny droplet flame was made experimentally without any assistance brought by large facilities which could eliminate/minimize buoyancy effect. Using ceramic burner and high-temperature air effectively suppresses the quenching distance and such "super-stabilized" micro-jet flame would be fairly close to 1-D (droplet) flame. The temperature of air (up to 770 K) and the fuel (methane) flow rate were varied as experimental parameters. Fundamental characteristics of limiting and near-extinction behavior of methane-air microflame in high-temperature air are investigated. Results show that the flame shape under high-temperature air condition is hardly affected by the external disturbance and the quenching distance is minimized due to the increase of the temperature at burner tip. It is found that the theory developed by Kuwana et al., to predict the limiting behavior of small-scale flame would be applicable even for the one formed in high-temperature air. Minimum flame size achieved in high-temperature air is predicted as an order of hundreds micron; interestingly, this is identical to what is predicted with ideal adiabatic burner (i.e., no conductive and radiative heat loss to/from the burner) in our previous numerical work.

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Gas-phase quasi-steadiness and fuel vapor accumulation effects in droplet burning

Cited by 107 publications

References 11 publications

Microgravity burner-generated spherical diffusion flames - Experiment and computation

Microgravity burner-generated spherical diffusion flames - Experiment and computation

Interactive combustion of two-dimensionally arranged quasi-droplet clusters under microgravity

Development of "Tiny" Droplet Flame Simulator: "Super-Stabilized" Micro-Jet Flame in a Hot Air

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