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Droplet Combustion Experiments Aboard the International Space Station
Abstract
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 a t m 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 diffusive-extinction droplet diameter increased and the radiative-extinction 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 C O 2) and 0.17 (0.17 O 2/0.63 N 2/0.20 C O 2) for methanol and heptane, respectively, for ambient environments that simulated dispersing an inert-gas suppressant (carbon dioxide) into a nominally air (1.0 a t m) 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 quasi-steady burning after visible flame extinction. Verification 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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  • References (27)
  • Cited By (27)
Vedha Nayagam11
Estimated H-index: 11
,
Daniel L. Dietrich13
Estimated H-index: 13
,
Michael C. Hicks7
Estimated H-index: 7
... more
The Flame Extinguishment (FLEX) experiment that is currently underway in the Combustion Integrated Rack facility onboard the International Space Station is aimed at understanding the effects of inert diluents on the flammability of condensed phase fuels. To this end, droplets of various fuels, including alkanes and alcohols, are burned in a quiescent microgravity environment with varying amounts of oxygen and inert diluents to determine the limiting oxygen index (LOI) for these fuels. In this st...
1 Citations Download PDF
G. M. Faeth60
Estimated H-index: 60
(Pennsylvania State University)
Abstract The present understanding of spray combustion in rocket engine, gas turbine, Diesel engine and industrial furnace applications is reviewed. In some cases, spray combustion can be modeled by ignoring the details of spray evaporation and treating the system as a gaseous diffusion flame; however, in many circumstances, this simplification is not adequate and turbulent two-phase flow must be considered. The behavior of individual droplets is a necessary component of two-phase models and rec...
447 Citations Source Cite
Benjamin D. Shaw12
Estimated H-index: 12
(University of California, Davis),
J. B. Wei4
Estimated H-index: 4
(University of California, Davis)
Reduced gravity and normal gravity combustion experiments were performed with individual fiber-supported n-heptane and 1-propanol droplets with initial diameters in the 1-mm size range. Experiments were performed with air-inert mixtures at 0.1 MPa or 0.3 MPa and about 298 K, where helium, carbon dioxide, or xenon were separately used as inerts. The amount of inert gas required to suppress combustion was generally higher in reduced gravity than normal gravity, and higher mole fractions of xenon w...
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2012 in Image Analysis & Stereology [IF: 1.42]
Christopher L. Dembia5
Estimated H-index: 5
(Cornell University),
Yu Cheng Liu10
Estimated H-index: 10
(Cornell University),
C. Thomas Avedisian15
Estimated H-index: 15
(Cornell University)
A simple automated image analysis algorithm has been developed that processes consecutive images from high speed, high resolution digital images of burning fuel droplets. The droplets burn under conditions that promote spherical symmetry. The algorithm performs the tasks of edge detection of the droplet’s boundary using a grayscale intensity threshold, and shape fitting either a circle or ellipse to the droplet’s boundary. The results are compared to manual measurements of droplet diameters done...
16 Citations Source Cite
2012 in Combustion and Flame [IF: 4.49]
Tanvir Farouk17
Estimated H-index: 17
(Princeton University),
Frederick L. Dryer63
Estimated H-index: 63
(Princeton University)
Abstract Quasi-steady burning and extinction of droplets are of interest from both fundamental and application viewpoints. The latter is related to combustor performance and fire safety issues in reduced gravity environments. Influences of diluent in the atmosphere on isolated droplet combustion characteristics including extinction provide insights to fire extinguishment phenomena and the effectiveness of various diluents as fire suppressants. Extinction of pure methanol and methanol–water dropl...
20 Citations Source Cite
2012 in Combustion and Flame [IF: 4.49]
Tanvir Farouk17
Estimated H-index: 17
(Princeton University),
Frederick L. Dryer63
Estimated H-index: 63
(Princeton University)
Abstract Tethered methanol droplet combustion in carbon dioxide enriched environment is simulated using a transient one-dimensional spherosymmetric droplet combustion model that includes the effects of tethering. A priori numerical predictions are compared against recent experimental data. The numerical predictions compare favorably with the experimental results and show significant effects of tethering on the experimental observations. The presence of a relatively large quartz fiber tether incr...
20 Citations Source Cite
2012 in Combustion and Flame [IF: 4.49]
Vedha Nayagam11
Estimated H-index: 11
,
Daniel L. Dietrich13
Estimated H-index: 13
(Glenn Research Center),
Paul V. Ferkul9
Estimated H-index: 9
... more
Abstract Experimental observations of anomalous combustion of n-heptane droplets burning in microgravity are reported. Following ignition, a relatively large n-heptane droplet first undergoes radiative extinction, that is, the visible flame ceases to exist because of radiant energy loss. But the droplet continues to experience vigorous vaporization for an extended period according to a quasi-steady droplet-burning law, ending in a secondary extinction at a finite droplet diameter, after which a ...
65 Citations Source Cite
1998 in AIAA Journal [IF: 1.56]
Vedha Nayagam11
Estimated H-index: 11
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J. B. Haggard1
Estimated H-index: 1
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R. O. Colantonio1
Estimated H-index: 1
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Results are presented from experiments on the combustion of freely floated n-heptane droplets in helium-oxygen environments conducted in Spacelab onboard the Space Shuttle Columbia during the first launch (STS-83) of the Microgravity Science Laboratory mission in April 1997. During this shortened flight, a total of eight droplets were burned successfully in nominally 300 K oxygen-helium atmospheres having oxygen mole fractions of 25, 30, and 35% at a total pressure of 1 atm. Initial droplet size...
67 Citations Source Cite
  • References (27)
  • Cited By (27)
C. Thomas Avedisian15
Estimated H-index: 15
(Cornell University)
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 ...
Source Cite
2015 in Combustion and Flame [IF: 4.49]
Vedha Nayagam4
Estimated H-index: 4
(Case Western Reserve University),
Daniel L. Dietrich13
Estimated H-index: 13
(Glenn Research Center),
Michael C. Hicks7
Estimated H-index: 7
(Glenn Research Center)
... more
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...
27 Citations Source Cite
Narugopal Ghata1
Estimated H-index: 1
,
Benjamin D. Shaw12
Estimated H-index: 12
A detailed numerical investigation of combustion of unsupported and fiber supported n-heptane droplets in reduced gravity is presented. The primary focus is on the effects of support fibers on the droplet burning rates and flame structure. A 21-step n-heptane reaction mechanism consisting of 20 species is employed to model the combustion chemistry. The volume-of-fluid (VOF) method is employed to capture the liquid-gas interface while allowing for time-dependent two-phase multidimensional flows. ...
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2013 in Experiments in Fluids [IF: 2.19]
Andrew Wollman2
Estimated H-index: 2
(Portland State University),
Mark M. Weislogel11
Estimated H-index: 11
(Portland State University),
Brentley M. Wiles1
Estimated H-index: 1
(Portland State University)
... more
Drop towers provide brief terrestrial access to microgravity environments. When exploited for capillary fluidics research, the drop tower allows for unique control over an experiment’s initial conditions which can enable, enhance, or otherwise improve methods to study capillary flows and phenomena at significantly larger length scales than can be achieved on the ground. In this work, a new, highly accessible, 2.1-s tower design is introduced for such research. Enabled in part by simple macro-fab...
25 Citations Source Cite
Benjamin D. Shaw12
Estimated H-index: 12
(University of California, Davis),
Chang L. Vang1
Estimated H-index: 1
(University of California, Davis)
ABSTRACTReduced gravity combustion experiments were performed aboard the International Space Station with individual methanol and n-heptane droplets that had initial diameters in the 1.2–5.0-mm-size range. Experiments were performed with air-inert mixtures at 0.1 and 0.07 MPa and about 298 K, where the monatomic gases helium and xenon were separately used as the added inert. These two gases have the same thermodynamic properties on a molar basis, but their transport properties are significantly ...
4 Citations Source Cite
Fahd E. Alam3
Estimated H-index: 3
(University of South Carolina),
Frederick L. Dryer63
Estimated H-index: 63
(Princeton University),
Tanvir Farouk17
Estimated H-index: 17
(University of South Carolina)
ABSTRACTThe ‘FLame EXtinguishment’ (FLEX) program conducted by NASA on board the International Space Station (ISS) has been assisting in developing fire-safety protocols for low gravity applications through microgravity droplet combustion experiments. A wide range of fuels, including alcohols and alkanes, have been studied in different ambient conditions that also encompass the use of various diluent species and concentrations. A prime focus of the work has been to observe the relative effective...
4 Citations Source Cite
2016 in AIAA Journal [IF: 1.56]
Vedha Nayagam11
Estimated H-index: 11
,
Daniel L. Dietrich13
Estimated H-index: 13
,
Forman A. Williams44
Estimated H-index: 44
A simplified model for droplet combustion in the partial-burning regime is applied to the cool-flame regime observed in droplet-burning experiments performed in the International Space Station with normal-alkanes fuels resulting in expressions for the quasi-steady droplet burning rate and for the flame standoff ratio. The simplified predictions are found to produce reasonable agreement with the experimentally measured values of burning-rate constants but not with their apparent dependencies on p...
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Yu Cheng Liu10
Estimated H-index: 10
,
Jeff K. Rah1
Estimated H-index: 1
,
Koffi N. Trenou1
Estimated H-index: 1
... more
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 aggre...
2 Citations Source Cite
2016 in Combustion and Flame [IF: 4.49]
Yu Cheng Liu10
Estimated H-index: 10
(University of Michigan),
Yuhao Xu4
Estimated H-index: 4
(Cornell University),
Michael C. Hicks7
Estimated H-index: 7
(Glenn Research Center)
... more
Abstract This paper reports the results of a comprehensive experimental study on the effect of initial droplet diameter ( D o ) over a very wide range (0.5 mm D o n -heptane, n -octane and n -decane – that are representative of components found in petroleum-based transportation fuels and their surrogates. Spherical symmetry in the burning process was promoted by carrying out the experiments in a reduced convection (stagnant ambience) and buoyancy (low gravity) environment using the facilities of...
14 Citations Source Cite
Tanvir Farouk17
Estimated H-index: 17
(University of South Carolina),
Daniel L. Dietrich13
Estimated H-index: 13
(Glenn Research Center),
Fahd E. Alam3
Estimated H-index: 3
(University of South Carolina)
... more
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...
8 Citations Source Cite

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