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FeuerTRUTZ-International-2-2016

Focus Fire in Space smoke do not evaporate very easily as accustomed due to the zero gravity. Fire and flames in zero gravity While we know that gravity on the earth determines the shape and behaviour of a flame, we are still at an early stage with the research of fire in zero gravity and in microgravity. In this context, Convection plays a significant role for fire fighting. The convection due to gravity has an effect that the density difference of warm air and soot climb (as they are lighter) whereas the flame sucks fresh air from underneath, that is colder and heavier. This process is called the chimney effect. In zero gravity there is neither an up or down and as a result, there is also no chimney effect as, due to the missing gravity, there are no different densities from warm and light air and therefore no convection currents can occur. This means that, e.g., a candle flame does not appear elongated, but as a sphere. As a whole, the flame is smaller and the fire burns slower which makes its discovery more difficult. It also develops a lower temperature as a comparable fire on the earth as the fire frequently smothers on its own emissions and the heat loss is higher than the head production. The oxygen required for burning can only reach the flame by diffusion or air flow. The shape and colour of the flame is also different: it has a spherical shape, free of soot and is blue, sometimes it is not even visible. High risk of fires The concentration of oxygen is slightly higher on the ISS and independent from that, there is a high risk of fire on the station 2 and 3. Heated motors e.g., represent an enormous risk as the heat is not discharged by natural convection. Certain substances are even highly combustible with low air flows in the microgravity 4. As on the earth, with fires on the ISS, the flames are not the greatest danger, it is the smoke. As one – as already mentioned – cannot step in front of the door in event of fire and take a deep breath, every astronaut has their own gas mask. Fortunately, there have not been any fires on the ISS yet  – except in experiments (especially with the Flame Extinguishment Experiment 5). Extinguishing with foam, carbon dioxide and water mist. On the ISS, water is generally the chosen extinguishing agent. On the ISS, a special water-foam fire extinguisher is only permitted in the Russian module. In the American section on the other hand, one was prepared to smother flames using carbon dioxide however, the carbon dioxide extinguishers were replaced by special water mist fire extinguishers 6. The first water mist fire extinguishers have been on-board the ISS since December 2015 7. The different supply voltages in the modules were the reason for the different extinguishing concepts until then. The American module operates with 124 V whereas 28 V is sufficient in the Russian module. Halon 1301 was used as extinguishing agent on the former American space shuttles. As however, this gas was incompatible with the Environmental Controls and Life Support System (ECLSS) of the ISS and toxic corrosive products also produced 8, an initial decision was made for carbon dioxide on the ISS as this, as with the carbon dioxide breathed out by humans, can easily be removed by the ECLSS. A candle flame on earth (left) and in zero gravity (right) Source: NASA/Wikimedia Commons However, a foam-water mixture as well as the carbon dioxide are problematic or even ineffective extinguishing agents under the zero gravity conditions on the ISS 9. The remains of the foam must be removed with great effort. On the other hand, carbon dioxide has benefits but it has to be applied in a high concentration (50% of the room volume) in order to displace the oxygen. Moreover, the carbon dioxide must not escape too quickly so that the carbon dioxide flow remains constant and its oxygen displacement does not come to a standstill too quickly 2. If the carbon dioxide loses its motion of flow, it also loses its extinguishing effect as the gas molecules behave differently due to the missing gravitation. Moreover, carbon dioxide cannot be eliminated by the respiratory protection filter masks used on the ISS so that there would be an immediate risk of suffocation for the astronauts 6. Water on the other hand has several benefits: it is not toxic, not corrosive, can be recycled, on the ISS or for long-term missions it is available for refilling, it is cost effective and the removal can be accomplished with the ventilation system via dehumidifiers. The extinguishing effect of the extreme fine water mist is based on the dispersion of the oxygen and the cooling effect. There is no danger for the electric systems as the water mist quickly evaporates on the hot electric components 10. A decompression of the module affected (extinguishing by vacuum) would also be possible (and is the only possibility in the Sojus space capsule) 11, but the fire may FeuerTRUTZ digital Subscribe to the free FeuerTRUTZ Magazine ePaper at www.feuertrutz.com/magazine FeuerTRUTZ International 2.2016 15


FeuerTRUTZ-International-2-2016
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