header graphic
Flame Extinguishment Experiment-2 (FLEX-2)


A short mechanism for the low-temperature ignition of n-heptane at high pressures

Juan C. Prince (a,*), Forman A. Williams (b), Guillermo E. Ovando (a)

a Departamento de Metal-Mecánica, Instituto Tecnológico de Veracruz, Veracruz 91860, Mexico
b Department of Mechanical and Aerospace Engineering, University of California at San Diego, La Jolla, CA 92093-0411, USA


The low-temperature ignition of mixtures of n-heptane and air at elevated pressures (13–40 bar) is addressed. A base mechanism of 272 elementary reactions among 45 species, which models the ignition time of this fuel under a wide range of conditions for initial temperatures above about 1000 K is revised and augmented to produce the two-stage ignition and negative-temperature-coefficient (NTC) behavior seen experimentally for this fuel below 1000 K. By using available kinetic data, it is shown that by adding only 4 additional species to the original chemical mechanism, removing 8 of the original steps and replacing them by 14 new steps, the low-temperature ignition phenomena are modeled well. The numerical ignition-time predictions of this mechanism are compared with results of experimental shock-tube measurements in which NTC behavior and two-stage ignition are observed, showing reasonably good agreement. These results thus extend the range of applicability of the original mechanism to lower temperatures that are of interest in various applications, such as those for HCCI engines.

Amazing Flame Comes to Life in Space Station Microgravity Combustion Science (a YouTube video)external content

Science Overview


Content on this page requires a newer version of Adobe Flash Player.

Get Adobe Flash player


FLEX-2 Status

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).

      Bi-component fuel droplet  
    Image of Bi-component fuel droplet  
    Burning droplet subjected to convective flow  
    Image of burning droplet subjected to convective flow  
    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  
    acrobat icon
MDCA/FLEX Overview Chart
acrobat icon
FLEX-2 Overview Chart
acrobat icon
MDCA-FLEX Short Overview Presentation

January 2013

  Astronaut - CIR  
November 2012  


January 2011: FLEX-2 hardware is being launched to the International Space Station on various launch carriers.  The HTV-2 launch delivered fuel deployment needles and needle holders.  The upcoming STS-133/ULF-5 launch will deliver fiber arms, check gas bottles, argon/helium bottles, igniter tip holders, a Fuel/Oxidizer Management Assembly (FOMA) control unit, and a High Bit Depth/Multispectral (HiBMS) imaging package.  The FLEX-2 experiments will likely begin operation around July 2011.


The second in the FLEX series of experiments, the FLEX-2 investigation uses fuels and environmental conditions that mimic real combustor conditions.  The investigation will extend and advance the research into droplet combustion, studying the influence of sub-buoyant convective flows on combustion rates, determining the influence of a second burning droplet on a linear array, and beginning the study of practical fuels by burning bi-component and surrogate fuels.  As the research extends into increasingly complex fuels, FLEX-2 data can help verify models of real fuels used in transportation and industry.  Results of the FLEX-2 experimental data will help to develop verified detailed and reduced-order models of droplet combustion, particularly with flow-field and droplet-droplet interactions.


Extend the results of FLEX-1 to fuels and environmental conditions that mimic real combustor conditions

• Investigate bi-component fuels – real fuels are multi-componen

• Examine the influence of sub-buoyant convective flows – real combustors involve gas/droplet relative motion

• Study practical fuels and fuel surrogates

• Study binary droplet arrays – real combustors have droplet-droplet interactions

• Develop and validate detailed and reduced-order transport, chemistry and soot models that are the foundation for real engine simulations


The combustion of liquid fuels is the overwhelming energy source in the transportation sector

• Design future combustors to minimize carbon footprint (maximize fuel efficiency) and minimize pollutant emissions

• The development of surrogates (mixtures of pure fuels that simulate the behavior of real fuels) will allow quantitative evaluation of the performance of future fuels (e.g., oil shales, biofuels, etc.) in combustors

• Prior droplet results helped validate jet engine models by engine manufacturers

Development Approach

• Flight design leverages off previous flight design heritage (MDCA/FLEX)

• Multi-user, re-usable apparatus minimizing up-mass/volume, costs, and crew involvement



Project Management:

Contacts at NASA Glenn Research Center

Project Manager: John M. 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

Flame Extinguishment Experiment-2 (FLEX-2) News





View Archives