Phenomena
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Phenomena
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Cool Flames Cool flames are very weak flames that are more an unusually lively chemical reaction than a flame. This remarkable phenomenon is created by gentle oxidation rather than fully fledged combustion, when a fuel is heated in the presence of air. Sir Humphry Davy discovered these flames in 1817 and noticed the flames generated an incredibly small amount of heat, such that he did not burn his fingers when he inserted them into a cool flame of ether. Cool flames have been regarded a just a chemical "party trick" since then, but have recently become of great interest in combustion research. They have applications in heating systems and boilers, improving fuel efficiency, able to run on a variety of fuels and helping clean up their emissions, and now also in producing hydrogen for fuel cells and in vehicle engines. A cool flame is distinguished from a conventional flame by its chemistry. A conventional flame is self-sustaining and results in an increasing number of reactive radicals. However a cool flame is different. It can develop in most vapours of organic molecules, and forms spontaneously when the temperature mix of air and fuel vapour reaches around 2500C. The chemical processes in a cool flame release some heat, as in conventional combustion, and the reaction can even accelerate to create a normal flame in the process. More often, as the temperature rises, branching reactions gradually switch off. They are replaced by slower processes such as chain propagation, where only one reactive species is generated. The reaction rate stops increasing and so does the temperature and the cool flame stabilises at around 5000C. The picture below shows contours of temperature (filled contour), and contours of OH radical from CFD modeling of acetaldehyde Cool flame.
From: http://www.alexchirokov.narod.ru/cool_flame.htm Flames in microgravity Microgravity is a condition in which the effects of gravity are greatly reduced. In microgravity where the flows due to gravity are reduced, the flame is spherical, soot free and blue. Gravity complicates combustion by causing hot, less dense products of a flame to rise, and the cooler, denser surrounding air to fill in at the flame's base. This flow is called buoyancy-induced convection, and it makes other aspects of combustion difficult to see. By conducting studies in microgravity, where such convection is essentially eliminated, scientists are able to better observe the complete combustion process. So studies of combustion in microgravity aim to improve the understanding of combustion.
From: http://arrow.utias.utoronto.ca/~ogulder/FlamePage2a.jpg |
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