Flame Extinguishment Experiment (FLEX) - 04.25.18

Overview | Description | Applications | Operations | Results | Publications | Imagery

ISS Science for Everyone

Science Objectives for Everyone
FLame Extinguishment Experiment (FLEX) will assess the effectiveness of fire suppressants in microgravity and quantify the effect of different possible crew exploration atmospheres on fire suppression. The goal of this research is to provide definition and direction for large scale fire suppression tests and selection of the fire suppressant for next generation crew exploration vehicles.
Science Results for Everyone
Great balls of fire! Fire behaves differently in space, based on a complex interaction of fuel vapor and oxidizer transport; fuel vaporization; conductive, convective and radiative heat loss; and chemical kinetics. Effectively extinguishing fires in space depends on understanding these interactions. This investigation tested the effectiveness of various fire suppressants in microgravity, to provide direction for design of future spacecraft fire suppression testing and systems. The most surprising discovery is the apparent continued "burning" of heptane droplets after flame extinction under certain conditions, a unique observation with significant implications.

The following content was provided by Forman A. Williams, Ph.D., and is maintained in a database by the ISS Program Science Office.
Experiment Details


Principal Investigator(s)
Forman A. Williams, Ph.D., University of California, San Diego, La Jolla, CA, United States

Michael C. Hicks, Glenn Research Center, Cleveland, OH, United States
Frederick L. Dryer, Princeton University, Princeton, NJ, United States
Benjamin D. Shaw, University of California, Davis, Davis, CA, United States
Vedha Nayagam, National Center for Space Exploration Research, Cleveland, OH, United States

ZIN Technologies Incorporated, Cleveland, OH, United States

Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)

Sponsoring Organization
Human Exploration and Operations Mission Directorate (HEOMD)

Research Benefits
Information Pending

ISS Expedition Duration
October 2008 - October 2009; March 2010 - March 2011; September 2011 - May 2012

Expeditions Assigned

Previous Missions
The Fiber Supported Droplet Combustion Experiment, (FSDC), a predecessor to MDCA-FLEX was performed on STS-73 (October 20, 1995) and STS-83 (April 4, 1997). The Droplet Combustion Experiment (DCE), another predecessor was performed on STS-83 (April 4, 1997) and STS-94 (July 1, 1997). The fundamentals addressed by these experiments are essential to the fundamentals that impact fire sensing and suppression technologies applicable to space exploration environments.

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Experiment Description

Research Overview

  • The combustion and extinction characteristics of liquid fuel droplets in microgravity are determined by the complex interaction of fuel vapor and oxidizer transport, fuel vaporization, conductive, convective and radiative heat loss and chemical kinetics. All of these phenomena are critical in real fire scenarios and thus in understanding the complex issues involved in the design of an effective fire suppression system.

  • The simplicity of the droplet geometry lends itself to detailed theoretical and numerical studies. By comparing the results of the experiments to these detailed treatments it is possible to develop validated chemical kinetic and heat and mass transport mechanisms for liquid fuels. From these fundamental mechanisms it becomes possible to develop validated, predictive, reduced-scale mechanisms that are appropriate for modeling real fire suppression scenarios aboard future spacecraft.

  • The Multi-User Droplet Combustion Apparatus - Flame Extinguishment Experiment (MDCA-FLEX) investigation will yield both a quantitative measure of the effectiveness of various suppression agents and more importantly a set of predictive tools that will allow the effective, efficient design of future spacecraft fire suppression systems.

Flame Extinguishment Experiment (FLEX) will map the flammability boundaries for liquid fuel combustion in reduced gravity to quantify the suppressant efficacy of various gaseous suppressants over the range of candidate atmospheric pressures and oxygen concentrations. This investigation will develop predictive theoretical and numerical codes and chemical kinetic schemes to model flammability boundaries as a function effective gravitational acceleration on the unique ambient conditions encountered in space exploration applications.The development and validation of these models will require detailed spatially and temporally resolved measurements of droplet burning rate, flame extinction, flame radiation, soot concentration, soot temperature, etc. This investigation will also develop improved and validated reduced (simplified) theoretical and numerical sub-models of important physical processes (chemical kinetics, radiation, soot formation/destruction) that can be used in simulations of large scale, realistic fires.

The independent experiment variables analyzed by MDCA-FLEX are:

Oxygen Mole Fraction: The ambient oxygen mole fraction in a typical space environment can vary from high concentrations in EVA pre-breathing environments down to that typically found in air. At high oxygen concentrations, however, the chemical times are small enough (relative to the characteristic flow times) such that the droplets will burn to completion rather than exhibiting flame extinction. Therefore, it is also necessary to study low oxygen concentrations, down to the Limiting Oxygen Index (LOI). It is also important to determine the LOI in order to verify the chemical mechanisms. The oxygen mole fractions in the present study will vary from 0.10 to 0.50. The chamber must also be large enough such that there is no significant decrease in ambient oxygen mole fraction during an experiment (i.e. the droplet burns in an essentially constant ambient).

Diluent (non-suppressant): The ambient mixture will consist of oxygen mixed with a suppressant and the balance of an inert diluent gas. The diluent gas for these studies will be primarily nitrogen since that is the typical diluent on Earth and expected in space. There will be a small number of tests with a helium diluent gas. The reason is primarily for baseline comparisons with the Droplet Combustion Experiment, which used helium as the diluent gas and also to vary the physical properties of the diluent gas.

Suppressant Type: The tests will examine candidate gaseous suppressants that have widely varying physical, chemical and radiative properties. This will enable model and sub-model development and validation over a wide range of ambient conditions to improve the predictive capabilities of the models. The suppressants are carbon dioxide and helium. The expected concentrations range from 0.00 mole fraction to the limit where no flame can exist (0.70 expected for the least active suppressant).

Pressure: Ambient pressure does not significantly influence the droplet burning rate, but does influence chemistry at sufficiently low values. In addition, one strategy for extinguishing a fire is to isolate the habitat where it exists and vent the cabin to space. It is therefore beneficial to have verified suppressant data at low pressures. The pressure for the tests will range from 0.5 to 1.0 atm (standard atmosphere). As with the oxygen mole fraction, it is important that the test chamber be sufficiently large so that the pressure is essentially constant during an experiment (i.e. the droplet burns in a constant pressure ambient).

Fuel Type: The advantage of the droplet geometry is that the fuel is relatively simple and better characterized than typical fuels in fire safety studies (e.g. PMMA or paper). This is also a disadvantage since it does not represent a practical fuel. The study will use two typical hydrocarbon fuels, an alcohol, methanol (CH3OH ) and an alkane, heptane (C7H16 ). There is a relatively large experience base with these fuels. Methanol has a fuel-bound oxygen atom, and burns with a very dim blue flame (not much soot production) with a small standoff distance, so it has widely different radiative characteristics than heptane. Therefore, studying these two fuels gives a wide range of fire scenarios to verify model and sub-model performance over.

Droplet Diameter: The droplet is ignited in the flammable region and then burns in a constant oxygen mole fraction ambient until flame extinction. Under the assumption of quasisteady burning, the initial droplet size should not significantly influence the determination of the extinction droplet size. Transient influences, however, will be present, so some variation in initial droplet diameter is necessary to determine the deviation from quasi-steady behavior. The initial droplet size in the proposed study will vary between 2 and 5 mm.

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Space Applications
FLEX will help us develop more efficient energy production and propulsion systems on Earth and in space.

Earth Applications
FLEX will help in the understanding to deal better with combustion generated pollution, and address fire hazards associated with using liquid combustibles on Earth.

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Operational Requirements and Protocols
The FLEX investigation will be setup in the Combustion Integrated Rack (CIR). The CIR chamber will be filled with the appropriate atmosphere depending on the test point. A total of 196 test points are required for completion of FLEX. Fuel will be dispensed followed by ignition. All activities will be captured in near real-time download with the CIR color camera video.
Following setup and initialization of the FLEX investigation the chamber shall be filled with the appropriate atmosphere, which depending on the test point, will vary in pressure from 0.5 atm to 3.0 atm, will vary in oxygen concentration from 0.1 to 0.4 mole fraction, and will vary in suppressant concentration from 0 to 0.7 mole fraction. A settling time of approximately 2 minutes will elapse prior to initiating the test in order to ensure that the temperature and pressure of the chamber gases have stabilized. This settling time will be followed by the dispensing of a predetermined amount of fuel (based on the target droplet size) onto the support fiber. When sufficient fuel has been dispensed the dispensing needles will be retracted and a dwell period of at least 10 seconds will be allowed for the droplet internal fluid motion induced by deployment to subside.

This will then be followed by initiating power to the igniter for a selectable amount of time ranging from 1 second to 5 seconds after which the igniter will be retracted from the field of view. If the flow field is to be generated by translating the droplet then droplet motion would commence at the same time that the igniter is retracted. A near real-time download of the color camera video will be required in order to verify successful droplet deployment, ignition, and overall progress of the experiment. Pressure and temperature data of the chamber environment will also be required in near real time.

At least 2 minutes must be allowed after filling chamber to ensure that the chamber gas temperature and pressure has stabilized. For droplet dwell time allow at least 10 seconds to ensure all droplet motion imparted by droplet deployment and needle retraction has subsided. Chamber purity for fuel vapor mole fraction of less than 0.005 in the atmosphere for tests without carbon dioxide and less than 0.02 mole fraction (each species) of carbon monoxide, carbon dioxide and other products.

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Decadal Survey Recommendations

Applied Physical Science in Space AP6
Applied Physical Science in Space AP7
Applied Physical Science in Space AP8
Translation to Space Exploration Systems TSES8
Translation to Space Exploration Systems TSES9

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Results/More Information

From March to December 2009, the Flame Extinguishment Experiment (FLEX) completed the first round of experiments to investigate fire in space with more than 225 unique test points conducted using the Combustion Integrated Rack (CIR) of the Fluids and Combustion Facility (FCF) on board the ISS. To help understand how flames behave and burn in space, researchers ignite a small drop of fuel (heptane, methanol, etc.) inside the FLEX combustion chamber with a controlled atmosphere. As this little sphere of fuel burns for about 20 seconds, it is engulfed by a small (2.5 - 4 millimeter) spherical flame ball. The droplet shrinks until either the flame extinguishes or the fuel runs out.

In microgravity hot gases don't rise so an entirely different process, called molecular diffusion, drives flame behavior. So far, researchers have seen some unexpected phenomena from FLEX results. The most surprising discovery is the apparent continued "burning" of heptane droplets after flame extinction under certain conditions. First the droplet undergoes normal transient (short-lived) burning with a visible flame surrounding the droplet and extinguishes at a relatively large droplet size. This event is followed by continued rapid almost steady vaporization of the droplet without the visible flame, which ends abruptly, at a point called "cool-flame extinction", leaving behind a smaller droplet, which then either experiences normal evaporation in the hot surrounding environment or sometimes grows slightly through condensation of the vapor in the cloud that forms upon extinction or through migration of the vapor-cloud particles to the droplet surface. This behavior is not explained by conventional theories of droplet combustion, and is hypothesized as a low temperature chemical reaction (cool flame) that continues after the 'hot' flame extinction. Since cool flames typically occur in premixed systems (this is a diffusion flame), are transient (this is quasi-steady) and lead to a hot-flame ignition (this is a hot-flame leading to a cool flame that extinguishes), which is a very unique observation with very significant theoretical and practical implications.

Cool flames usually occur in premixed fuel systems, where they are known to spread and eventually lead to ignition that establishes hot-flame combustion, generating ignition pulses commonly associated with engine-knocks in automobiles. Also, flames in space burn with a lower temperature, at a slower rate, and with less oxygen than in normal gravity. This means that materials used to extinguish fires in microgravity must be present in higher concentrations. Through microgravity flame investigations scientists hope to gain a better knowledge of droplet burning, improved spacecraft fire safety and ideas for more efficient utilization of liquid fuels on Earth (Nayagam et al. 2012).

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Results Publications

    Liu YC, Xu Y, Avedisian CT, Hicks MC.  The effect of support fibers on micro-convection in droplet combustion experiments. Proceedings of the Combustion Institute. 2014 August; epub. DOI: 10.1016/j.proci.2014.07.022.

    Paczko G, Peters N, Seshadri K, Williams FA.  The role of cool-flame chemistry in quasi-steady combustion and extinction of n-heptane droplets. Combustion Theory and Modeling. 2014 July 24; epub. DOI: 10.1080/13647830.2014.934296.

    Dietrich DL, Nayagam V, Hicks MC, Ferkul PV, Dryer FL, Farouk TI, Shaw BD, Suh HK, Choi MY, Liu YC, Avedisian CT, Williams FA.  Droplet Combustion Experiments Aboard the International Space Station. Microgravity Science and Technology. 2014 October; 26(2): 65-76. DOI: 10.1007/s12217-014-9372-2.

    Farouk TI, Hicks MC, Dryer FL.  Multistage oscillatory “Cool Flame” behavior for isolated alkane droplet combustion in elevated pressure microgravity condition. Proceedings of the Combustion Institute. 2015; 35(2): 1701-1708. DOI: 10.1016/j.proci.2014.06.015.

    Nayagam V, Dietrich DL, Ferkul PV, Hicks MC, Williams FA.  Can Cool Flames Support Quasi-steady Alkane Droplet Burning?. Combustion and Flame. 2012 Dec; 159(12): 3583-3588. DOI: 10.1016/j.combustflame.2012.07.012.

    Shaw BD, Vang CL.  Oxygen Lewis number effects on reduced gravity combustion of methanol and n-heptane droplets. Combustion Science and Technology. 2015 July 16; epub: 150716065835002. DOI: 10.1080/00102202.2015.1072176.

    Shaw BD.  ISS droplet combustion experiments - Uncertainties in droplet sizes and burning rates. Microgravity Science and Technology. 2014 June 19; epub. DOI: 10.1007/s12217-014-9377-x.

    Farouk TI, Xu Y, Avedisian CT, Dryer FL.  Combustion characteristics of primary reference fuel (PRF) droplets: Single stage high temperature combustion to multistage “Cool Flame” behavior. Proceedings of the Combustion Institute. 2016 August 2; epub. DOI: 10.1016/j.proci.2016.07.066.

    Nayagam V, Dietrich DL, Hicks MC, Williams FA.  Methanol droplet combustion in oxygen-inert environments in microgravity. 8th U.S. National Combustion Meeting, Park City, Utah; 2013 May 19-22 11 pp.

    Vang CL, Shaw BD.  Estimates of liquid species diffusivities in n-propanol/glycerol mixture droplets burning in reduced gravity. Microgravity Science and Technology. 2015 July; 27(4): 281-295. DOI: 10.1007/s12217-015-9455-8.

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Ground Based Results Publications

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ISS Patents

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Related Publications

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Related Websites
ISS Research Project-MDCA-FLEX
How to Put Out a Fire In Space
Experiment Investigates How to Fight Fire in Space

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image Burning droplet from the Droplet Combustion Experiment, A predecessor experiment operated on the Space Shutlle image courtesy of Glenn Research Center.
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image NASA Image: C-2006-414: Flight Unit Multi-User droplet Combustion Apparatus (MDCA) Chamber Insert Assembly (CIA).
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image NASA Image: ISS018E010645 - Astronaut Michael Fincke, Expedition 18 commander, works on the Multi-User Droplet Combustion Apparatus (MDCA) Chamber Insert Assembly (CIA) in the Harmony node of the International Space Station.
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image Video Screen Shot of MDCA-FLEX Ignition 1 on March 5, 2009 (GMT 64/17:21). The methanol fuel droplet being formed on tip of needle (droplet size ~2.5 mm). Image courtesy of NASA.
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image Video Screen Shot of MDCA-FLEX Ignition 1 on March 5, 2009 (GMT 64/17:21). Beginning of combustion event in cabin air. Image courtesy of NASA.
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image Video Screen Shot of MDCA-FLEX Ignition 1 on March 5, 2009 (GMT 64/17:21). Igniters powered off & retracting, but still hot. Combustion event continues. Image courtesy of NASA.
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image Video Screen Shot of MDCA-FLEX Ignition 1 on March 5, 2009 (GMT 64/17:21). Igniters continue to cool. Combustion event continues. Image courtesy of NASA.
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image NASA Image: ISS026E025292 - NASA astronaut Scott Kelly, Expedition 26 commander, works on the Combustion Integrated Rack (CIR) Multi-user Drop Combustion Apparatus (MDCA) in the Destiny laboratory of the International Space Station.
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