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Experimental Study of Initial Diameter Effects on Convection-free Droplet Combustion in the Standard Atmosphere for n-Heptane, n-Octane, and n-Decane: International Space Station and Ground-based Experiments

Published on Jan 13, 2014
· DOI :10.2514/6.2014-1019
Yu Cheng Liu11
Estimated H-index: 11
,
Jeff K. Rah1
Estimated H-index: 1
+ 2 AuthorsC. Thomas Avedisian14
Estimated H-index: 14
Abstract
A comprehensive investigation is reported on varying the initial droplet diameter (Do) over a wide range on the burning characteristics of three normal alkane fuels that are representative of components found in practical fuel systems. The droplet burning characteristics of n-heptane, n-octane and n-decane, were studied experimentally in a low gravity ambience to minimize the influence of convection and promote spherical droplet flames as well as formation of a shell-like structure of soot aggregates that reside between the droplet and flame. Initial droplet diameters ranged from about 0.5 mm to 5.0 mm, and the experiments were carried out in the standard atmosphere (room temperature and normal atmospheric pressure) in a ground-based (drop tower) and a spaced-based (the International Space Station) facility. The range of Do investigated influences mechanisms related to radiative transport and sooting dynamics on the droplet burning process that determine the droplet burning rate, sooting dynamics and flame extinction mechanisms. The results show that the burning rate monotonically decreases with increasing Do. Varying Do over the range investigated promotes a transition from a soot-dominated process, with a minimal influence of luminous radiative affects, for small droplets to increased radiative losses for larger droplets that reduce heat transfer to the droplet surface. At a given time after ignition, the relative position of the flame to the droplet decreased with increasing Do. A rather abrupt increase in flame diameter was noted for Do ~ 1 mm followed by a monotonic decrease with further increases of Do for all of the fuels examined. The relative position of the soot shell to the droplet increased with time, while it also increased with Do for a given time after ignition. A three-staged burning process was found for Do > 3 mm suggesting several extinction modes. An early extinction mechanism is speculated to be the result of radiation losses from the flame rather than more diffusively controlled processes. Evaporation continues after the first extinction until reaching a second limit with a rather abrupt decrease in the droplet burning rate – which is speculated to be a “cool-flame” extinction. The morphology of the extinction process showed an oscillatory dynamic in which the flame would peel away from the droplet then re-appear before completely disappearing.
  • References (58)
  • Citations (3)
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References58
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#1Daniel L. Dietrich (Glenn Research Center)H-Index: 13
#2Vedha Nayagam (Case Western Reserve University)H-Index: 15
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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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#1Yu-Cheng LiuH-Index: 1
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#1Yu Cheng Liu (Cornell University)H-Index: 11
#2Tanvir Farouk (USC: University of South Carolina)H-Index: 18
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Abstract Combustion characteristics of isolated sub-millimeter sized methyl butanoate (MB) droplets are studied at low gravity (10 −4 m/s 2 ) in a 1.2 s drop tower. In the experiments, droplets were grown and deployed onto the intersection of two 14 μm silicon carbide fibers in a cross-string arrangement and exposed to symmetrically placed spark ignition sources. The initial droplet diameter was fixed at 0.54 ± 0.01 mm, and experiments were carried out in room temperature air at atmospheric pres...
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Diffusive extinction of methanol droplets with initial diameters between 1.25 mm and 1.72 mm, burning in a quiescent microgravity environment at one atmosphere pressure, was obtained experimentally for varying levels of ambient carbon-dioxide concentrations with a fixed oxygen concentration of 21% and a balance of nitrogen. These experiments serve as precursors to those which are beginning to be performed on the International Space Station and are motivated by the need to understand the effectiv...
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Emissions of nitrogen oxides (NOX) in sprays are a complex and crucial topic. The knowledge about NOX formation in sprays is based on research of droplet combustion. This paper presents the experiment setup for the combustion of an n-decane droplet array under microgravity conditions. Its focus is to investigate the array’s undisturbed burning characteristics and the related NOX formation mechanisms. Since the prevaporization time of the fuel droplets is the main experiment parameter, the extend...
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Abstract Recent droplet-combustion experiments onboard the International Space Station (ISS) have revealed that large n-alkane droplets, following radiative extinction of the visible flame, can continue to burn quasi-steadily in a low-temperature regime, characterized by negative-temperature-coefficient (NTC) chemistry. In this study we report experimental observations of n-heptane, n-octane, and n-decane droplets of varying initial size burning in oxygen/nitrogen, oxygen/nitrogen/carbon dioxide...
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