Burning and Suppression of Solids - II (BASS-II) - 07.29.14
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The Burning and Suppression of Solids –II (BASS-II) investigation examines the burning and extinction characteristics of a wide variety of fuel samples in microgravity. The BASS-II experiment will guide strategies for materials flammability screening for use in spacecraft as well as provide valuable data on solid fuel burning behavior in microgravity. BASS-II 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
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 2013 - September 2014
Previous ISS Missions
ISS Expedition 23/24 was the first mission for the BASS experiment which utilizes the existing Smoke Point In Co-flow Experiment (SPICE) hardware on orbit aboard the ISS.
Burning and Suppression of Solids - II (BASS-II) 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.).NASA tests materials for flammability using an upward burning test, which is considered to be the worst-case geometry for flammability of the material on Earth. One objective of the BASS-II tests is to identify what is the worst case for material flammability in spacecraft environments, and how does that compare to the terrestrial upward burning used to screen the materials for safe use aboard spacecraft.
The main variables to be tested are the effects of ambient oxygen concentration, ventilation flow velocity, and fuel type, thickness, and geometry. Many of the tests will focus on finding a minimum oxygen concentration or flow velocity where a material will burn in space, to compare with the Earth-based limits. Flame growth rates are also of interest, to determine how quickly a fire in space can grow and if the flames reach a finite size or continue to grow. This has implications for firefighting strategies in spacecraft.
Detailed combustion models can be validated by data obtained in the simpler flow environment 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.
Burning and Suppression of Solids - II (BASS-II) 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-II consists of 100 fuel samples and associated igniter wires. There are three categories of samples: flat samples, rod samples, and a section of a large solid sphere. Thin flat samples (10 cm long by 1 and 2 cm wide) yield concurrent or opposed-flow spread rates, limiting flame lengths, and extinction limits. The flat sample materials will include acrylic films and sheets of different thicknesses, and a cotton-fiberglass fabric blend Solid Inflammability Boundary at Low-Speeds (SIBAL) fuel which was previously tested in BASS. The rod samples are made of black or clear acrylic, and will provide solid fuel regression rates and extinction limits for both opposed and concurrent flow. The large solid spherical section, also made of acrylic, will be used to study ignition of thick materials and flame growth over the thick material.
The important experimental observations from BASS-II with respect to the burning process include flame shape and appearance as a function of ambient oxygen concentration, 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, flow, and ambient oxygen concentration. 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.
- 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.
A primary goal of BASS-II is improved spacecraft fire safety, improved understanding of combustion in space and how to avoid it. If you're on a mission far from Earth, a fire can be catastrophic. BASS-II helps engineers select the safest materials and improve firefighting methods on future space missions.
In outer space, fuels burn as oval balls rather than with an upward pointed cone flame as they do on Earth. This simpler combustion process enables researchers to evaluate computer models of fuel burning. These models can then be used to more accurately study flames on Earth, such as in wildfires, building fires, energy recapture from waste recycling, and other combustion problems.
BASS-II is conducted inside the sealed MSG work volume. The crewmember is involved throughout the experiment to load fuel samples, adjust nitrogen gas (GN2) concentration in the working volume vitiation of the working volume, initiate tests, ignite the fuel, 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-II 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-II hardware in the MSG work volume. The BASS-II hardware consists of a small flow duct with an igniter and a small nozzle along with exchangeable fuel samples. During BASS-II 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. Nitrogen vitiation of the working volume is then initiated using station GN2 at a flow rate of up to 500 cc/min for a prescribed time. The flame is ignited and allowed to burn for about a minute. 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.
SPICE flow duct hardware inside of the Microgravity Science Glovebox (MSG) on ISS. A still digital camera looks through the top window of the duct, and a tan video camera uses a mirror to look into the front door window. Image courtesy of Glenn Research Center.
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This image sequence from the BASS investigation shows a flame igniting (top left) and spreading upward across a 1-cm wide flat cotton-fiberglass sample with a 10 cm/s concurrent air flow (flow direction is also up in images). The images are just over 1 second apart. The flame reaches a steady size by 10 seconds, and burns the entire sample (bottom right). As the flame spreads, some smoldering of the fuel residue (glowing orange spots) continues.
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These two images from the BASS investigation are of ceramic encased candles burning in a very low velocity flow (flow is blowing upward in both images). On the left is a ‘normal’ candle where the flow is up and the candle is up. On the right, the candle is ‘inverted’. In normal gravity, the flames would be very different, but in microgravity with very low speed flow, the shapes are fairly similar and the primary difference is how the candle stabilizes around the candle. In the ‘normal configuration, the flame starts near the ceramic base and is primarily next to and above the wick, where the fuel starts. In the ‘inverted’ configuration, the flame starts below the wick where the air and fuel first meet, and the flame wraps downstream around the ceramic housing as the air and fuel continue to mix. Soot is formed in both geometries, with the maximum sooting (brightest yellow color) indicating the hottest part of the flame. The maximum sooting is on opposite ends of the candle in the two cases. The blue is chemiluminescence from the burning fuel. Rod samples to be tested in BASS-II will be functionally similar to these candle samples.
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(NASA Glenn Image).
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