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Why study combustion?
- In the United States, nearly 70% of our electrical energy is generated through the combustion of fossil fuels. For example, in 2012, electricity was generated in the U.S. by burning the following fuels, where the percentages indicate the fraction of the total U.S. electrical generation: coal (37%), natural gas (30%), biomass (1.4%), and petroleum (1%).
- Our transportation is heavily reliant on combustion, even for electric vehicles because most of our electricity is generated by combustion.
- With combustion, we heat our homes, water, food, etc. and also generate heat for industrial processes.
- Our reliance on imported fuel contributes to our national trade deficit and affects our national security.
- Combustion is a leading man-made source of greenhouse gases, where carbon dioxide is the most important example.
- Combustion is the primary man-made contributor to acid rain.
- Soot contributes to global warming and is a health problem.
- Fire safety!
- Given our pervasive use of combustion as an energy source, the U.S. consumes fossil fuels which cost on the order of a trillion dollars annually. Therefore, even small improvements in combustion efficiency would significantly reduce fuel needs and pollutant production.
Why study combustion in microgravity?
- Flames are strongly affected by gravity because the high-temperature combustion gases are much less dense than the cooler atmosphere which surrounds the flame. Gravity pulls more forcefully on the denser atmosphere and the hot gases are pushed upward as a result. This gravity-driven upward motion of material that is less dense that the surrounding fluid is referred to as buoyancy. This gravity-driven motion, referred to as buoyant convection, feeds the flame with fresh reactant – normally oxygen (in the air) – and removes the combustion products (e.g., carbon dioxide and water vapor) from the flame vicinity.
- Low-momentum flames are dramatically influenced by the effective elimination of buoyant convection, where the resulting effects are often advantageous for analysis.
- Spherically symmetric flames can be created enabling one-dimensional analysis. Two of the current ACME experiments take advantage of this feature.
- Flicker, which is a buoyancy-driven (i.e., gravity-driven) instability, is eliminated yielding quasi-steady flames.
- Length scales are increased in microgravity flames facilitating analysis of the flame structure.
- Momentum-dominated flames, which are important for most practical combustion, can be studied at low velocities to simplify analysis.
- Microgravity flames tend to have a much stronger sensitivity to their atmosphere and exhibit a much broader range of characteristics than normal-gravity flames because of the near absence of buoyant entrainment.
- The long residence times in microgravity flames can lead to strong soot production, but many microgravity flames are soot free.
- Microgravity flames are great for studies of limit and stability behavior where chemical kinetics are important. Soot, extinction, and stability limits are being studied in the ACME experiments.
- Microgravity is of course the appropriate environment for studies related to spacecraft fire safety.
Why study combustion on the ISS?
- Microgravity durations in drop facilities, such as the 2.2 Second Drop Tower and 5.18-second Zero Gravity Research Facility, are (1) too short for soot to achieve quasi-steady conditions, and (2) too short to establish a flame and then vary its flow rate, for example, to investigate stability or extinction limits.
- While research aircraft flying in parabolic maneuvers can provide reduced gravity durations of ~20 seconds, low-momentum flames are often dramatically disturbed by the aircraft vibrations. Although the jitter can be avoided if the experiment is floated within the aircraft, that reduces the low-gravity duration to mere seconds.