Flame Extinguishment Experiment - 2 (FLEX-2) - 04.25.18

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Science Objectives for Everyone
The Flame Extinguishment - 2 (FLEX-2) experiment is the second experiment to fly on the ISS which uses small droplets of fuel to study the special spherical characteristics of burning fuel droplets in space. The FLEX-2 experiment studies how quickly fuel burns, the conditions required for soot to form, and how mixtures of fuels evaporate before burning. Understanding these processes could lead to the production of a safer spacecraft as well as increased fuel efficiency for engines using liquid fuel on Earth.
Science Results for Everyone
Information Pending

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

OpNom: FLEX-2

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

C. Thomas Avedisian, Ph.D., Cornell University, Ithaca, NY, 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
Mun Y. Choi, University of Connecticut, Storrs, CT, United States

NASA Glenn Research Center, Cleveland, OH, United States
ZIN Technologies Incorporated, Cleveland, OH, United States

Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)

Sponsoring Organization
NASA Research Office - Space Life and Physical Sciences (NASA Research-SLPS)

Research Benefits
Information Pending

ISS Expedition Duration
October 2009 - March 2016; March 2016 - April 2017; September 2017 - February 2018

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 forces, known as buoyant forces, which cause cold air to sink and hot air to rise in gravitational environments also cause fires to burn differently on Earth than they do in microgravity.

  • The FLEX-2 experiments are designed to explore the burning characteristics of droplets in the absence of buoyant forces.

  • Understanding how fires burn in microgravity will help to improve fire-safety in manned space vehicles and will also contribute to the development of liquid fuel burning engines by increasing their efficiencies.


The spherically symmetric combustion of a liquid fuel droplet in a quiescent oxidizing atmosphere is a classical problem in combustion research. The advantage of the spherical symmetry is that only one spatial dimension enters the description of the combustion process. This greatly simplifies the mathematics required for modeling combustion processes. Microgravity experiments provide the opportunity to achieve this spherical symmetry, a fact which NASA has used to its advantage in fundamental investigations for a number of years. The Fundamental Studies in Droplet Combustion and Flame Extinguishment in Microgravity (FLEX-2) investigation touches on a number of far-reaching combustion phenomena that can be uniquely addressed through the idealized geometry of a droplet burning in microgravity.

Studies of the liquid-phase mixing of the different components in fuels clarifies the way practical fuels turn to gas from liquid in the combustion process. Understanding soot formation during the combustion of liquid fuels is also studied. These studies are important due to soot’s negative effects on the environment and on human health. Knowledge of soot formation also has practical uses in certain industrial processes. The interactions of droplets with their gaseous environments and with other droplets are studied to reveal the fundamental dynamics of combustion.

In the absence of gravity, small droplets of fuel (i.e., from 2 mm to 4 mm in diameter) burn "one-dimensionally", which means the flame will be shaped a like ball about the size of a large olive that will be centered around the droplet. This one-dimensional nature of the droplet flame allows the science team to easily measure and understand important features of the burning fuel that would otherwise be impossible to obtain on the ground. This particular type of flame configuration allows measurement and observation of very complex interactions in a spherically one-dimensional system, providing insights into the behavior of combustion phenomena that would otherwise be difficult, if not impossible, to obtain in multi-dimensional systems that are typically found in most 1-g fires.

The Fundamental Studies in Droplet Combustion and Flame Extinguishment in Microgravity - 2 (FLEX-2) experiments involve dispensing, deploying and igniting either a single droplet or a pair of droplets, depending on the experiment configuration, in a known and controlled gaseous environment. The ignited droplets either burn to completion or the flame extinguishes at a finite droplet radius in a quiescent ambient. During the experiments involving binary droplet arrays, two droplets are placed along a support fiber at known locations and the droplets are ignited simultaneously and the subsequent burning is recorded. In the convective flow tests, a single droplet is subjected to a known and controlled sub-buoyant flow of the quiescent atmosphere around the droplet. The flow is realized when the droplet is tethered to a substrate. In this tethered mode the droplet is placed at the intersection of two fiber cross-hairs. The cross hairs are then moved through the quiescent atmosphere surrounding the droplet.

The experiments in FLEX-2 can easily be categorized into four separate groupings; these being, the pure fuel experiments, the fuel mixture experiments, the convective flow experiments, and the binary droplet array experiments. Each of these groupings are sponsored by one or more investigators comprising the investigation team and entail anywhere from 30 to 60 separate tests using different fuels, different ambient conditions, or different droplet configurations.

Pure Fuel Experiments: The objectives of the pure fuel studies are to examine steady and unsteady liquid and gas-phase phenomena, flame extinction, soot formation mechanisms and radiative heat transfer. Decane (C10H22) and ethanol (C2H5OH) fuels were chosen because of their combustion properties and to build on results from earlier flight experiments which also used an alkane and an alcohol. In these experiments droplet burning rates, flame dimensions, radiative emissions, and soot volume fraction are measured as a function of time and initial droplet size for different ambient conditions. When flame extinction occurs, the diameter of the unburned droplet at extinction is also determined. The objectives for these experiments are as follows:
  • Determine the diffusive and radiative flame extinction boundaries for ethanol and decane droplets in test atmospheres containing N2He at pressures ranging from 0.5 atm to 2.5 atm.

  • Measure droplet extinction diameters for ethanol and decane fuels for validation of detailed and reduced chemical kinetic models.

  • Measure soot volume fractions and soot temperatures for a range of initial conditions to determine influence on droplet burning and flame extinction.

  • Measure radiant emissions from both luminous and non-luminous flames in test atmospheres containing N2 or He and at pressures ranging from 0.5 atm to 2.5 atm.
Fuel Mixture Experiments: Bi-component fuel mixtures are chosen to examine the role of liquid phase transport effects as well as to study the burning characteristics of simulated 'real' fuels (i.e, surrogate fuels). Propanol/glycerol and heptane/hexadecane mixtures are used to examine the role of liquid phase diffusion, preferential evaporation, and mixing on the burning characteristics of fuel mixtures. Heptane/octane and decane/propylbenzene experiments focus on simulating the characteristics of 'real' fuels, like gasoline and jet fuels, which contain a variety of hydrocarbons with differing volatilities. The measurements made during these experiments are similar to the pure fuel studies; however, the initial fuel composition and the interpretation of the results are different. The primary objectives for this set of experiments are as follows:
  • Measure the droplet diameter (dc) , at the onset of flame contraction relative to the initial droplet diameter (d0). Correlate with theoretical predictions and provide estimates for effective liquid diffusivities.

  • Measure burning rate, burning time, extinction diameter, and flame luminosity for the surrogate fuel mixtures.
Convective Droplet Experiments: This set of experiments investigates the effects of slow convective flows on the burning characteristics of liquid-fuel droplets. The specific objectives of the present flight experiments are as follows:
  • Obtain radiative and diffusive flame extinction limits in the low Reynolds number flow regime over a range of test pressures and oxygen concentrations.

  • Measure droplet regression rates for sooting (decane) and non-sooting (ethanol) fuels as a function of imposed flow at low velocities over a range of ambient oxygen concentrations and pressures.

  • Measure flame length, flame shape, and temperature field measurements in low flow velocities under a range of test atmospheres for use in validation of detailed theoretical models (i.e., radiative and finite-rate chemical kinetic effects).
Droplet Arrays Experiments: The droplet array experiments investigate the droplet-droplet interactions on the diffusive and radiative extinction of the flame surrounding the droplets for various Length/Diameter ratios using pure fuels. The specific microgravity experimental objectives are as follows:
  • Determine the flammability map (diffusive and radiative extinction limits) for a binary droplet array as function of L/D0 ratio.

  • Measure variations in the burning rate constant (K) as a function of L/D0 for a range of ambient conditions to verify theoretical predictions.

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Space Applications
The FLEX-2 experiment measures soot buildup, flame heat and the burning rates of various types of fuels and fuel mixtures. Understanding how fuels burn in microgravity could improve the efficiency of fuel mixtures used for interplanetary missions by reducing cost and weight. It could also lead to improved safety measures for manned spacecraft.

Earth Applications
Watching fuel burn in a perfect sphere provides a unique view of fire that would be impossible to recreate on Earth. Better knowledge of fire’s dynamics could lead to improved fuels for vehicles and aircrafts, including efficient, environmentally friendly mixtures of chemicals that burn well together and produce less soot. Soot results from the incomplete burning of a hydrocarbon, and it is harmful to human and environmental health. The FLEX-2 experiment provides a unique view on soot formation that would be impossible under the influence of Earth’s gravity.

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Operational Requirements and Protocols

The FLEX-2 investigation, as with its precursor the MDCA-FLEX, will be setup in the Combustion Integrated Rack (CIR). The CIR chamber will be filled with the appropriate mixture of gases comprising different blends of O2/N2/He depending on the specifications of each test point. Since each test uses small volumes of fuel, the droplet test configuration can support a large number of test points and a total of 250 tests have been scheduled. If sufficient resources remain, after the first suite of tests is performed, there is the possibility of performing additional test points. Fuel is to be dispensed as either free-floating single droplets or as tethered droplets placed on an 80 micron SiC fiber in order to control position and spacing between droplet pairs. All activities will be captured with a “near real-time” download from the MDCA color camera.

Following setup and initialization of the FLEX-2 investigation, the chamber is filled with the appropriate atmosphere, which depending on the test point, varies in pressure from 0.5 atm to 3.0 atm, varies in oxygen concentration from 0.1 to 0.4 mole fraction, and varies in suppressant concentration from 0 to 0.7 mole fraction. A settling time of approximately 2 minutes elapses prior to initiating the test in order to ensure that the temperature and pressure of the chamber gases have stabilized. This settling time is 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 are retracted and a dwell period of at least 10 seconds is allowed for the droplet internal fluid motion induced by deployment to subside. This is 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 is required in order to verify successful droplet deployment, ignition, and overall progress of the experiment. Pressure and temperature data of the chamber environment is also 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 at least 10 seconds should be allowed 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 AP7
Applied Physical Science in Space AP8

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Results/More Information

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Results Publications

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Ground Based Results Publications

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ISS Patents

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Related Publications

    Nayagam V, Dietrich DL, Williams FA.  Partial-burning regime for quasi-steady droplet combustion supported by cool flames. AIAA Journal. 2016 January 11; epub: 5 pp. DOI: 10.2514/1.J054437.

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Related Websites
ISS Research Project - FLEX-2
How to Put Out a Fire In Space
Experiment Investigates How to Fight Fire in Space

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image Video Screen Shot of 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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NASA Image: ISS032E014576 - Expedition 32 Flight Engineer (FE) Joe Acaba working on the CIR (Combustion Integrated Rack) during IFM (Inflight Maintenance). MDCA (Multi-user Droplet Combustion Apparatus) hardware was assessed and replaced, to enable upcoming science operations with FLEX-2 Quiescent and Convective Flow test points. Three protective CIR alignment guides were also uninstalled and removed to allow ground-commanded micro-G operations.

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NASA image: ISS035E017712 - This is one of several photos documenting the Multi-user Droplet Combustion Apparatus (MDCA) Fuel Reservoir replacement in the U.S. lab Destiny. Here, Expedition 35 Flight Engineer Chris Cassidy removes and replaces one of the Fuel Reservoirs with the MDCA Chamber Insert Assembly (CIA) pulled partially out of the Combustion Chamber. The MDCA Fuel Reservoirs contain the liquid fuel used during droplet combustion experiments. This reservoir change-out was in support of the FLame EXtinguishment (FLEX)-2 experiment, scheduled to be executed by ground controllers.

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NASA Image: ISS037E012588 - Expedition 37 Flight Engineer (FE) Luca Parmitano working to replace the Combustion Integrated Rack (CIR) Multi-user Drop Combustion Apparatus / Chamber Insert Assembly (MDCA CIA) Igniter Tip on Base 2, to support operation of the Flame Extinguishment (FLEX) Experiment. FLEX uses small droplets of fuel to study the special burning characteristics of fire in space. The experiment studies the rate and manner in which fuel is burned, the conditions that are necessary for soot to form, and the way in which a mixture of fuels evaporate before burning. The results from these experiments provide insight into how fires behave in space and provide important information that can be useful in increasing the fuel efficiency of engines using liquid fuels.

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NASA Image: ISS043E052645 - Photographic documentation of the replacement of the Combustion Integration Rack (CIR) Manifold Bottle #2 in preparation for Flame Extinguishment Experiment (FLEX)-2 run. This experiment studies how quickly fuel burns, the conditions required for soot to form, and how mixtures of fuels evaporate before burning. Understanding these processes could lead to the production of safer spacecraft, as well as increased fuel efficiency for engines using liquid fuel on Earth.

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