Burning and Suppression of Solids (BASS) - 01.09.14
Science Objectives for Everyone
The Burning and Suppression of Solids (BASS) investigation examines the burning and extinction characteristics of a wide variety of fuel samples in microgravity. The BASS experiment will guide strategies for extinguishing accidental fires in microgravity. BASS results contribute to the combustion computational models used in the design of fire detection and suppression systems in microgravity and on Earth.
Science Results for Everyone
We didn’t start the fire – but we sure need to put it out. This investigation examined how to extinguish a variety of fuels burning in microgravity. With adequate ventilation, materials may burn as well in microgravity as in normal gravity, but putting out fires in space needs to take into account the geometry of the flame and characteristics of the extinguisher. Otherwise, some ground based type efforts may be ineffective or actually make the flame worse. These results will guide strategies for extinguishing accidental fires in microgravity and contribute to better fire detection and suppression systems on Earth.
ZIN Technologies Incorporated, Cleveland, OH, United States
Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)
Human Exploration and Operations Mission Directorate (HEOMD)
ISS Expedition Duration:
September 2011 - September 2013
Previous ISS Missions
ISS Expedition 23/24 is the first mission for the BASS experiment which utilizes the existing SPICE hardware on orbit aboard the ISS.
- Burning and Suppression of Solids (BASS) tests the hypothesis that materials in microgravity, with adequate ventilation, burn as well if not better than the same material in normal gravity with other conditions being identical (pressure, oxygen concentration, temperature, etc.).
- There are important differences in the suppression of fires in space compared to on Earth. On Earth it is understood that the best results are generally obtained when the extinguisher “attacks” the base of the flame, which is both the stabilization point and the point where fresh air first enters the flame.
- For a fire burning in microgravity, the best point of application of suppressant may not be immediately apparent, especially for a partially obstructed flame or a wake-stabilized flame. Depending on the geometry of the flame and the characteristics of the extinguisher (distance from flame, dispersion angle) it is possible that the suppressant stream will be ineffective or might actually make the flame worse through the entrainment of oxygen. Using nitrogen as a flame suppressant in microgravity provides a direct link to current and planned extinguishment techniques.
Burning and Suppression of Solids (BASS) utilizes slightly modified Smoke Point In Co-flow Experiment (SPICE) hardware within the Microgravity Science Glovebox (MSG) for observations of burning solid materials on board the ISS.
BASS consists of 41 fuel samples. There are three categories of samples: flat, solid spheres, and candles within tubes. Thin flat samples (12 cm long by 1 and 2 cm wide) yield concurrent-flow spread rate and limiting flame length. The cotton-fiberglass fabric blend Solid Inflammability Boundary at Low-Speeds (SIBAL) fuel is our principal thin material, and it was specially developed just for this purpose. Other thin materials are burned including Nomex® and Ultem®. Thick flat samples (5 cm long by 1 cm wide by 1 and 2 mm thick) of Polymethylmethacrylate (PMMA) and wax-saturated fiberglass fabric yield thickness effects on flame spread and extinguishment. Solid spheres of PMMA (1 and 2 cm in diameter) have the advantage of an axisymmetric geometry and permit multiple tests as the flame is extinguished and reignited. Ignition of either the front or back portion of the spheres is achieved. Finally, candles within a thin ceramic tube (6 mm in diameter by 25 mm long) are examined. Two types of wax are used, common paraffin and "Japan wax", which has a very low soot point. For many of these tests, the nitrogen suppressant system is engaged at a gradually increasing level until extinction is reached.
The important experimental observations from BASS with respect to the burning process include flame shape and appearance as a function of flow speed, flame spread rate (how fast the flame develops), and flame dynamics (pulsations, oscillations, etc.). With respect to extinction, the critical observations and data are the time to extinction as a function of fuel geometry, the nitrogen flow rate, and the flame distance from the nozzle. The dynamics of the flame before extinction are also important for comparison to the modeling work.
The modeling effort includes:
- Modeling flame spread over flat samples: For flat samples, the steady spread characteristics can be examined using the three-dimensional model currently available. Alterations are the new tunnel and sample geometry and the upstream boundary condition. For the flame growing phase, a transient model is currently being developed.
- Suppression by nitrogen injection: This can also be modeled readily using the current model.
- Modeling burning and extinction of PMMA spheres: Similar problem on modeling two-dimensional circular PMMA cylinder in cross flow has been performed. Some changes are needed for the sphere and the duct flow.
The current NASA spacecraft materials selection is based on a standard test method (NASA–STD–6001 Test 1) that segregates material based on 1-g behavior without consideration of low gravity effects. A critical element of this understanding is the radiative heat emission from the flame. These results are used in first order models and predictions of heat release in spacecraft fires and as a means to extend heat release data from tests like the NASA cone calorimeter test (NASA–STD–6001 Test 2) to a performance-based material selection process. Using nitrogen as a flame suppressant in microgravity provides a direct link to current and planned extinguishment techniques.
BASS results provide essential guidance to ground-based microgravity combustion research efforts. Detailed combustion models are validated using the simpler flow environment afforded by tests in microgravity. Once validated, they can be used to build more complex combustion models needed to capture the important details of flames burning in normal gravity. These models have wide applicability to the general understanding of many terrestrial combustion problems.
BASS is conducted inside the sealed MSG work volume. The crewmember is involved throughout the experiment to load fuel samples, initiate tests, ignite the fuel, adjust suppression, monitor and record data, exchange fuel samples, and replace the igniter. Forty-one test samples will be burned in a variety of flow conditions for a total of 89 test points.
Data is downlinked via video during or immediately after each flame test. Digital photos are downlinked after selected flame tests for ground confirmation before proceeding. BASS testing session must be conducted during periods when no major reboost or docking procedures are underway on the International Space Station (ISS).
The crewmember installs the BASS hardware in the MSG work volume. The BASS hardware consists of a small flow duct with an igniter and a small nozzle along with exchangeable fuel samples. During BASS operations a fan produces a co-flow of air through the duct. An anemometer is used to measure the actual flow rate. The crewmember adjusts the airflow from 5 to 50 cm/s. The flame is ignited and allowed to burn for about a minute. A nitrogen suppressant is then supplied via a mass flow controller, from 0 to 500 cc/min. A radiometer measures flame output. The crewmember conducts each test. They install the correct fuel assembly and set the air flow rate through the duct before igniting the flame. When the flame is ignited, the crewmember allows some time for the flame to stabilize then adjusts the flow of nitrogen suppressant through the nozzle until the flame goes out. After the test, the crewmember turns off the nitrogen flow and prepares for the next test. The science team on the ground monitors the video downlink to assist the crewmember in determining any peculiar flame behaviors and reviews the sensor data overlaid on the video image. Upon completion of the tests the crewmember stows the hardware and the stored images and data are returned to Earth for analysis.
Flat cotton-fiberglass fabric samples were burned in long-duration microgravity tests aboard the ISS. The samples were burned with air flow in the same direction and in the opposite direction of the flame. The custom-made fabric performed very well, with none of the complications caused by the burnout of ordinary cellulosic fuel samples like paper. The main influencing factor was air flow speed and it had a major effect on the flame as suggested in earlier studies. This is the first time that detailed transient flame growth data was obtained in purely forced flows in microgravity for a thin fuel material with uniform burnout characteristics. In addition, by decreasing same direction air flow speed to a very low value (around 1 cm/s), quenching extinction was observed providing a direct verification of the theoretically predicted U-shaped flammability boundary for a thin fuel. For the opposed flow configuration, the flame quickly reached steady spread for each flow speed, and the spread rate was fastest at an intermediate value of flow speed. These tests show the enhanced flammability in microgravity for this geometry, since, in normal gravity air, a flame self-extinguishes in the opposed-flow geometry (downward flame spread). For the concurrent-flow configuration, a limiting length and steady spread rate were obtained only in low flow speeds. However, flame base spread rate was constant and increased linearly with increasing flow for all tests. The valuable results from these long-duration experiments validate a number of theoretical predictions and also provide the data for a transient flame growth model under development (Ferkul 2013).
Ferkul PV, Olson SL, Johnston MC, T'ien JS. Flammability Aspects of Fabric in Opposed and Concurrent Air Flow in Microgravity. 8th U.S. National Combustion Meeting; 2013 May 19-22
Ground Based Results Publications
Zero-g facility test burning a 2 cm diameter PMMA sphere in 30 cm/s airflow. Left: 1 g; Middle: 0 g (1 s after drop); right: 0 g (4 s after application of nitrogen extinguishing agent). Image courtesy of Glenn Research Center.
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SPICE Hardware inside of the Microgravity Science Glovebox at Marshall Space Flight Center (MSFC). Image courtesy of Glenn Research Center.
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This image sequence from the BASS investigation shows a flame burning a 1-cm diameter sphere at very low air flow speed. The flame starts out blue and mostly spherical (left and center) but then as the fuel begins to heat up, after 2 minutes of burning, the vaporization from the fuel overpowers the air flow leading to highly irregular shapes.
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This image sequences a wake-stabilized flame burning a 2-centimeter-diameter acrylic sphere. Images are taken every second-and-a-half from left to right, then top to bottom. Maximum nitrogen flow nearly extinguishes the flame, but it is able to survive and even strengthen. When the nitrogen is turned off, the flame becomes significantly stronger, and only goes out when the air flow is shut off.
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