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Flame Extinguishment Experiment (FLEX)

8th US National Combustion Meeting, May 19-22, 2013, Salt Lake City, Utah, USA


• “Isolated alkane droplet combustion in microgravity: ”Cool Flames” T. Farouk and F.L. Dryer (Paper#1H17)

• Effect of varying the initial diameter of n-octane and n-decane droplets over a wide range on the spherically symmetric combustion process: International Space Station Experiments,” Y.C. Liu, K.N. Trenou, J. Rah, M.C. Hicks, C.T. Avedisian (paper# 2G11)

• “Methanol droplet combustion in oxygen-inert environments in microgravity,” V. Nayagam, D.L. Dietrich, M.C. Hicks, F.A. Williams (paper#2G10)

• “Computational modeling of the effects of support fibers on evaporation of fiber-supported droplets in reduced gravity,“ N. Ghatta and B.D. Shaw (Paper#070HE-0020).



The FLEX experiment was designed to assess and quantify the effectiveness of inert-gas suppressants in microgravity and obtain the most conservative estimate of the limiting oxygen index for steady combustion.  FLEX is studying the behavior of near-limit diffusion flames examining in detail liquid- and gas-phase transport and chemical kinetics, and developed and is validating detailed and reduced-order transport and chemistry models that are the foundation for real engine simulations.


Below is an example of a combustion event using heptane liquid fuel. This video is imaged with a standard NTSC color camera and is essentially what you would see if you were looking through a window in the CIR chamber. The fuel droplet is suspended on thin Silicon Carbide (SiC) fiber.  The primary function of the fiber in this test is simply to pin the burning droplet to a fixed location.
The initial frame shows the fuel droplet suspended on the fiber and surrounded by two hot wire igniters and the fuel dispensing needles. This is taken with the CIR chamber light on. The light is off for the remainder of the video.  At about 1 second into the clip, the igniters are powered and the droplet ignites saturating the camera image.  The igniters can been as they retract from the ignition site. Afterward the combustion event can be observed as a thin bluish shell surrounding the liquid droplet.  The fiber is heated and glows at the two points where it intersections with the flame shell.  At approximately 8-9 seconds the flame extinguishes due to radiative heat loss and the fiber cools below the camera’s color range.   The flames shell collapses and the light from the flame diminishes, however the remaining liquid is still illuminated by the CIR laser backlight (partially visible at the lower portion of the image).   All of the above is representative of the primary FLEX science investigation.  
Then we have some unexplained phenomena. Over the next 12-15 seconds the fuel droplet is observed to exhibit a sustained rapid evaporation as if some reaction is still active. (This is more easily observed with the associated HiBMs camera images – not included here).  At the end of this phase we observe the remaining fuel droplet as it oscillates back and forth on the fiber. At about the 25 seconds the rapid evaporation ceases and the vapor atmosphere  begins to condense into a toroidal  cloud shape.  As the system cools the image appears to brighten to a deep red as the vapor continues to condense and increase in density.
The post combustion vaporization was not predicted by existing science and is of great interest to the community. Currently, there is  no clear understanding of the cause/process driving the post-extinction vaporization, nor is it understood whether the cloud formation consists of water, fuel or a mixture of both.  The phenomenon is repeatable. Should the cloud contain condensed fuel it represents a potential ignition hazard in micro-gravity environments even after nominal extinction occurs.


Communication clip from the intro about CIR (Mike Fossum on

FLEX in the news at ScienceDaily: external icon


February 2013

The FLEX testing that occurred on Feb 21, 2013, completes the FLEX science matrix dedicated to the CO2 Diluent Exchange Series of tests.    

Testing was performed with MeOH droplets in a chamber at 3 atm pressure in two different chamber compositions; the first was 21% O2 / 75% CO2 / balance N2 and the second test atmosphere, representing a return to a previous atmosphere that was unsuccessful, was a composition of 21% O2 / 50% CO2 / balance N2.

In both atmospheres, with the larger droplet diameters (i.e., Do = 6 mm),  some peculiar flame asymmetries were observed that are difficult to explain without further analysis of the recorded data.  It is not clear if these flame asymmetries are due to fiber effects, that  may be pronounced because of the elevated pressures and high diluent concentrations, or are artifacts of fuel quality.  These tests have to be carefully analyzed and compared with earlier MeOH tests.  

At a later date, additional Diluent Exchange Tests will be performed using Helium and Xenon as the diluents. These complementary tests will provide an excellent basis for comparing the effects of gas radiation (CO2) and thermal and mass diffusion rates; He (high) and Xenon (low).  Testing with these gases will also help shed light on the mechanisms driving the phenomena of spontaneous auto-ignition recently observed with n-heptane in elevated atmospheres.

December 2011 -  The FLEX Science and Engineering teams have accomplished the majority of the FLEX science operations.  As of December 13, 2011, FLEX has accomplished 225 unique test points; 82 of which have been taken since August 8, 2011.  All FLEX fuel reservoirs on ISS have been depleted.  Two additional fuel reservoirs (one heptane and one methanol) are planned to be launched on the HTV-3 flight, currently scheduled for July 31, 2012.  These fuels will allow for the completion of the Helium test point section of the FLEX science matrix and for the inclusion of droplet burns using Xenon diluent gas mixes.  Once these final two fuel reservoirs are depleted the FLEX investigation will come to an official end.  In the meantime, the CIR has been reconfigured to run the FLEX-2 investigation, which will start in January 2012.  A new radiometer package and FLEX-2 fuels were installed into the CIR on December 20, 2011. (See FLEX-2 for additional information.)

February 2011 -
On February 9, 2011, the ISS crew replaced a CIR manifold bottle, and installed new fuel deployment needles and fiber arm assemblies.  Also, the combustion chamber window in front of the LLL-UV camera was replaced by the spare window from storage.  It was noted that the replaced window was smudged.  The source of the contamination is unknown, but appears to be consistent with contaminated fuel droplets.  Ground testing will be performed to determine the nature of the window contamination and determine if a cleaning procedure can be developed.  The replaced window was put into ISS storage.

January 2011 - Two Flame Extinguishment Experiment (FLEX) test points to determine diffusion extinction limits in very low oxygen concentrations were completed on January 12, 2010.  The total number of test points achieved to date is 128 of 258 planned.  Newly designed, electrically conductive fuel deployment needle assemblies are still on track for launch on HTV-2.

December 2010 -Four Flame Extinguishment Experiment (FLEX) test points were taken on December 28, 2010.  The total number of tests achieved to date is 126 of 258 planned. 

April 2010 -
On-orbit operations continued for the Multi-use Droplet Combustion Apparatus’ Flame Extinguishment Experiment (MDCA/FLEX) this past week.  The depleted methanol fuel reservoir was replaced with another methanol reservoir on April 26, 2010.  The Molecular Sieve adsorber cartridge was replaced on April 27, 2010, with the Silica Gel adsorber cartridge.  The Molecular Sieve adsorber was installed for use with the nitrogen test points.

Multiple test points for the Multi-use Droplet Combustion Apparatus’ Flame Extinguishment Experiment (MDCA/FLEX) were run on April 19, 2010.  All were successful to at least some degree and good science data was collected.  This data will be transferred and downlinked to ground next week.  Three test points from the science matrix were accomplished.


Below is a selection of FLEX Test Point videos.

• Test #1 - Droplet diameter of 4 mm, with no support fiber. Droplet deployment was successful with a brief burn before radiative extinction. An afterglow from condensing vapor cloud and scattered backlight occurred approximately 30 sec after extinction. This afterglow phenomena typically occurs following radiative extinction.
• Test #2 - Droplet diameter of 4 mm, with support fiber and translation. Tethered deployment was successful with a brief burn before radiative extinction. Extinction occurred at “trailing surface ”after translation ceased.
• Test #3 - Droplet diameter of 4 mm, with no support fiber. Droplet deployment was successful with very little droplet drift. The burn was very brief before radiative extinction. The afterglow phenomena occurred again similar to Test #1.
• Test #4 - Droplet diameter of 4 mm, with support fiber and translation. Tethered deployment was successful with a brief burn before radiative extinction. A small amount of residual fuel from previous test was still on the fiber, which caused a brief secondary “ignition flash” that lasted less than 1 sec. Extinction occurred (similar to Test #2) at “trailing surface.”

Multi-user Droplet Combustion Apparatus (MDCA)
Burning droplet
Color image of a burning droplet
FLEX Chamber Insert Assembly Apparatus
FLEX Chamber Insert Assembly Apparatus
Flight Unit Avionics Package installed on Ground Unit Optics Bench Simulator and Flight Unit Chamber Insert Assembly.
Flight Unit Avionics Package installed on Ground Unit Optics Bench Simulator and Flight Unit Chamber Insert Assembly.
Flight Unit Chamber Insert Assembly
Flight Unit Chamber Insert Assembly
Flight Unit Avionics Package installed on Ground Unit Optics Bench Simulator
Flight Unit Avionics Package installed on Ground Unit Optics Bench Simulator
Flight Unit Avionics Package installed on Ground Unit Optics Bench Simulator
Flight Unit Avionics Package installed on Ground Unit Optics Bench Simulator

The Multi-user Droplet Combustion Apparatus (MDCA) is a multi-user facility designed to accommodate different droplet combustion science experiments.  The MDCA will conduct experiments using the Combustion Integrated Rack (CIR) of the NASA Glenn Research Center’s Fluids and Combustion Facility (FCF).  The payload is planned for the International Space Station.  The MDCA, in conjunction with the CIR, will allow for cost effective extended access to the microgravity environment, not possible on previous space flights.  It is currently in the Engineering Model build phase with a planned flight launch with CIR in 2007.   

The MDCA contains the hardware and software required to conduct unique droplet combustion experiments in space.  It consists of a Chamber Insert Assembly (CIA), an Avionics Package, and a multiple array of diagnostics.  Its modular approach permits on-orbit changes for accommodating different fuels, fuel flow rates, soot sampling mechanisms, and varying droplet support and translation mechanisms to accommodate multiple investigations.  Unique diagnostic measurement capabilities for each investigation are also provided.  Additional hardware provided by the CIR facility includes the structural support, a combustion chamber, utilities for the avionics and diagnostic packages, and the fuel mixing capability for PI specific combustion chamber environments.  Common diagnostics provided by the CIR will also be utilized by the MDCA.  Single combustible fuel droplets of varying sizes, freely deployed or supported by a tether are planned for study using the MDCA.  Such research supports how liquid-fuel-droplets ignite, spread, and extinguish under quiescent microgravity conditions.  This understanding will help us develop more efficient energy production and propulsion systems on Earth and in space, deal better with combustion generated pollution, and address fire hazards associated with using liquid combustibles on Earth and inspace.  

As a result of the concurrent design process of MDCA and CIR, the MDCA team continues to work closely with the CIR team, developing Integration Agreements and an Interface Control Document during preliminary integration activities.  Integrated testing of hardware and software systems will occur at the Engineering Model and Flight Model phases.  Because the engineering model is a high fidelity unit, it will be upgraded to a flight equivalent Ground Integration Unit (GIU) when the engineering model phase is completed.  The GIU will be available on the ground for troubleshooting of any on-orbit problems.  Integrated verification testing will be conducted with the MDCA flight unit and the CIR flight unit.  Upon successful testing, the MDCA will be shipped to the Kennedy Space Center for a post-shipment checkout and final turn-over to CIR for final processing and launch to the International Space Station.  

Once on-orbit, the MDCA is managed from the GRC Telescience Support Center (TSC).  The MDCA operations team resides at the TSC.  Data is transmitted to the PI’s at their home sites by means of TREK workstations, allowing direct interaction between the PI and operations staff to maximum science.  Upon completion of a PI’s experiment, the MDCA is reconfigured for the next of the three follow-on experiments or ultimately removed from the CIR, placed into stowage, and returned to Earth.

MDCA/CIR Testing and Integration 

Integrated testing between the MDCA hardware and CIR carrier will be performed on the engineering units of both pieces of hardware.  Both units are hi-fidelity, flight-like units.  Testing, planned for December 2002 will include a full array of sub-package testing, leading to a full end-to-end functional test.  Upon completion, the MDCA Engineering Model (EM) will undergo vibration & microgravity testing, EMI/EMC, and acoustical testing.  In parallel with EM environmental testing, the MDCA flight hardware will be procured and assembled.  Testing will be conducted on the flight unit in early summer 2005 in preparation for a turn-over of the hardware to CIR for flight integrated testing in August 2005.    

Launch of the MDCA Hardware

The MDCA hardware will be launch as stowed hardware on the same incremental flight launch as the CIR.  This hardware will include the MDCA common hardware and experiment unique hardware for the first droplet investigation, Flame Extinguishment Experiment (FLEX).  The Chamber Insert Assembly, MDCA Avionics Package, and experiment unique hardware will be separate stowed items.  Once on-orbit, the CIA and Avionics Package will be removed from stowage.  The avionics package will be installed on the CIR rack and the CIA will be inserted into the CIR combustion chamber.  Experiment unique diagnostics for the first experiment will be installed on the CIR optics bench. 



Contacts at NASA Glenn Research Center
Project Manager: Mark Hickman, NASA GRC

Project Scientist: Dr. Daniel Dietrich, NASA GRC

Principal Investigator Team: Prof. Forman Williams, UCSD (lead)
Prof. Frederick Dryer, Princeton
Prof. Mun Choi, Drexel University
Prof. Benjamin Shaw, UC-Davis
Dr. Vedha Nayagam, NCSER

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FLEX Related Documents
small acrobat icon   Methanol Droplet Extinction in Oxygen/Carbon-dioxide/Nitrogen Mixtures in Microgravity: Results from the International Space Station Experiments
small acrobat icon   MDCA/FLEX Overview Chart
small acrobat icon   FLEX-2 Overview Chart
small acrobat icon
MDCA-FLEX Short Overview Presentation
small acrobat icon   FLEX SRD (EIRD)
small acrobat icon   Publications & Presentations
small acrobat icon   FLEX and FLEX-2 Publications

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