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#10
March 21st 04, 01:45 PM
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Jeff Liebermann wrote in message . ..
On 20 Mar 2004 23:28:38 -0800, (John Michael Williams) wrote: snipped lots of good stuff I think if I can see the spark, it can ignite gas vapor, provided the flame had a path out of the gap. I beg to differ. The ignition of a gasoline oxygen mixture requires a specific amount of energy to ignite. Anything less will not produce the requiste chemical reaction. Think spark plug heat ranges and glow plugs in model airplanes. I'll grind the numbers if you want, but it's now midnight, I'm tired of waiting for Windoze update, and I'm going home. The ignition of a gaseous oxygen-gasoline mixture, or a (potentially more sensitive) hydrogen-oxygen mixture does require a specific minimum amount of energy, which depends on the partial pressures of the oxygen and the fuel, and - IIRR - the partial pressures of any inert diluent gases around. Lesser amounts of energy can induce the requisite chemical reaction, but the reaction will fizzle out, rather than providing enough energy to ingnite the surrounding shell of a gas mixture and produce a self-propagating flame front. The controlling relationship is between the volume of the sphere in which the reaction is first initiated, and the surface area of that sphere - if the intial volume is too small, not enough energy is released to heat the surrounding shell of gas to the ignition temperature. Once you've got the basic idea,the thermodynamics is pretty straightforward. I had to work through the equations many years ago for an experiment intended to monitor the process in which one of the "Dewar benzenes" converted itself to normal - Kekule's - benzene, which is an enormously energetic process, involving about an order of magnitude more energy per molecule than you get out of TNT and PETN. I really didn't want to blast my experimental apparatus to smithereens. When I went through the calculations with my supervisor, he pulled a very long face - the motivation for the experiment had been some unexpected flashes of light seen when a dumb organic chemist had released small drops of liquid "Dewar benzene" into a hot cell, and my calculations made it clear that the flashes of light were just thermal radiation from a hot plasma, rather than fluorsecence from from an electronically excited state of Kekule benezene, which is what my supervisor had been hoping for ... For the difference between Dewar benzene and Kekule benzene see http://www.chemsoc.org/exemplarchem/...enzenering.htm ------- Bill Sloman, Nijmegen |
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