Ignition and Upward Flame Spread
Michael C. Johnston
Case Western Reserve University, NASA Glenn Space Academy 2011
Sandra Olson, Ph.D.
NASA Glenn Research Center
Case Western Reserve University
National Center for Space Exploration Research
An experimental apparatus for conducting large scale flammability tests was designed and is currently being constructed to investigate unanswered questions as to how NASA’s flammability flight qualification test STD-6001 Test 1 results on earth in 1 G actually relate to ignition, flame spread, and flammability limits in a microgravity environment. The apparatus is capable of sequentially burning five 0-5 cm wide x 2 m long thin fuel samples within an environmental chamber which can operate at any sub-atmospheric pressure and any oxygen concentration. A thin fuel combustion model from Case Western Reserve University has been modified to coincide with the experimental setup. An example calculation from the model is shown.
Fire safety and material flammability are vital to NASA’s space exploration goals. Currently, fire prevention aboard the Space Shuttle and International Space Station is primarily performed by ground based material selection and screening. New materials are flight qualified using NASA STD-6001 Test 1.
Test 1 is a ground based flame spread material screening test consisting of a 30 cm long solid sample suspended by its long edges. An igniter is mounted near the bottom edge of the sample and usually consists of a nitrocellulose chemical ignition source. If the flame spreads further than 15 cm, or if molten material ignites flammable paper at the bottom of the sample, then the material fails and is barred from space applications.
It was previously thought that the upward flame spread in 1-G can serve as a conservative estimate for material flammability in zero-G. Recent evidence , however, has suggested that this may not be the case. The lack of buoyant natural convection in a zero gravity environment, which allows hot gasses to stay in the vicinity of the combustion region, combined with the low speed forced convection from spacecraft ventilation systems, which helps feed oxygen to the flame zone, tend to extend the limits of material flammability in a way not seen in ground based tests.
There are other critical faults with test 1 as well. The test data exhibits a clear scatter in propagation distance. It is unclear whether this is an intrinsic property of the materials themselves, or the lack of a well defined ignition source creating a random ignition initial condition. The small scale nature of Test 1, also makes it difficult to separate the effects of ignition from the effects of extinction. In some cases, the ignition and extinction limits may not be the same.
Test 1 gives a rough applied approach to material selection; however, in addition to the problems mentioned above, it may not have a strong basis in combustion science. The only full proof method to determine material flammability in zero gravity experimentally may be to conduct prohibitively expensive flammability tests in microgravity. If instead, a computer numerical model could be validated to accurately simulate combustion processes in a spacecraft environment, it could be used to screen materials for microgravity by relating the microgravity combustion cases to the already large amount of data gathered in 1-G during STD-6001 Test 1.
An experimental apparatus has been proposed to conduct 1-G tests similar to test 1 in an atmospheric chamber at NASA Glenn Research Center. A few modifications from test 1 have been made.
- Radiative Igniter – a calibrated radiative lamp allows for a well known, easily characterized, ignition which can be accurately modeled in the computer simulation
- Large sample size – due to the difficulty and expense, large scale combustion tests have rarely been performed in the past. The 30cm sample size from test 1 will be extended to 200 cm to help separate the ignition event from the extinction limits.
Determining the exact heat release and transfer of nitrocellulose to various materials during test 1 can be difficult. By replacing this with a calibrated radiative ignition source, the only unknown variable is the materials radiative absorptivity. This radiative ignition source can be easily modeled in a combustion code which includes surface radiation.
Large scale combustion tests have rarely been performed in the past due to their expensive nature. This is especially true of tests performed in some of the proposed spacecraft environments (i.e. low pressure, increased oxygen). To help offset the costs of performing tests in a large environmental chamber, a sample indexing station has been proposed to allow up to five samples to be tested for each chamber fill. Since the chamber is so large, it is expected that the environmental conditions, namely the oxygen concentration, will not change much between sample burns.
The goals for summer 2011, and the ultimate goals of the experiment are summarized below. The computer code needs to be reconfigured for the exact conditions of the environmental chamber. The data will then be used to estimate the proper ignition energies. The experimental apparatus then needs to be completed, debugged, and tested. Once operational, this experimental apparatus can then be used to validate the model for accurately simulating 1-G combustion experiments.
- Modify the group’s high-fidelity 3D transient combustion model to match experimental boundary conditions (i.e. sample size, chamber size, pressure, oxygen percentage, igniter size, etc)
- Execute the model to estimate experimental parameters (i.e. necessary ignition energy)
- Complete experimental apparatus for use in environmental chamber
- Debug experimental apparatus
- Obtain some data points to show proof of concept and to compare with model
- Use a well characterized ignition source to determine if more reproducible results can be achieved as compared to NASA STD-6001
- Find the extinction limits of various fuels by utilizing minimal ignition energies
- Validate the computer numerical combustion model for 1-G tests
 S L Olson and F J Miller, “Experimental Comparison of Opposed and Concurrent Flame Spread in a Forced Convective Microgravity Environment,” Proceedings of the Combustion Institute, vol. 32, no. 2445-2452, 2009