paranoid ?
in [ASM]
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From: Robert Redelmeier on 24 Jan 2008 18:50 Rod Pemberton <do_not_have(a)nohavenot.cmm> wrote in part: > True, but they are using multiple oil barrels packed with dynamite > to put out a relatively small fire by comparison to the explosion. The blowout area may be small, but the flow is very high velocity and the total flame (fireball) is not small compared to the explosives. > The relatively massive explosion consumes all combustible oxygen. No. High explosives like nitroglycerine and TNT contain their own oxidizer (the nitrate groups) and do not consume oxygen. The gasses generated however, are reducing and effectively smother the flame. It all happens extremely rapidly, disrupting the free-radical chain reactions of burning all at once. > By comparison, for airplane explosion, you have a small > explosion with a large volume of vapors. The problem is if the fuel is kerosene (jet), those vapors are too lean to burn until the fuel and the vapours are heated above the flashpoint (140'F). If the fuel is avgas (gasoline), those vapors are too rich to burn. > Gasoline is explosive at low pressure under certain > circumstances. Certainly. Between the LEL and UEL in concentration. Easily achieved by accident (see below) > At the moment, I can't seem to find the explosive limits of > gasoline vapor with water vapor... only with air. For good reason. Water vapor [steam] is not an oxidizer and is actually used when available to fight fires. > Gasoline: 1.4% LEL, 7.6% UEL (NIOSH) Jet A: 0 to 1% LEL, > and 6 to 7% UEL (Boeing) Yep. JP5 LEL is 0.7%v in air. The "problem" is that achieving those concentrations takes elevated temperatures (even beyond the flashpoint) >140'F. If the tmeperature is below this, not enough vapors are generated to support continued combustion. If the temperature was above this and cooled below, the vapors condense in a rather cool fog. > In fact, there was TV show a few months ago, where NTSB > officials claimed there are hundreds of airplanes accidents > resulting from fuel tank explosions, including many they > were reclassifying now that they understood the issue. Certainly in small gasoline powered aircraft. > As for gasoline, you only need watch video of firefighters > (unintentionally) blowing a few thousand sq-ft house to > pieces, as shown by a recent TV show. "He just took > a house to the face!" Morons! Gasoline is an extremely dangerous material when mixed with air in an enclosed space. > The reason is chemistry. Gasoline starts from oil which > has over 100,000 compounds. Gasoline is a complex and > random mix of hundreds of hydrocarbons (aromatics, alkanes, > alkenes, cycloalkanes) some easy to combust, some difficult. Yes, but they all very close to the same amount of chemical energy released through burning. About 19,500 BTU/lb > The chemical reactions that occur between oxygen and the > hydrocarbons change depending on the compression. No, they do not. Even without a catalystic converter, the engine exhaust is mostly N2 (inert from air), H2O (mostly from fuel combustion), CO2, CO, unused O2 and ppms of NOx. It doesn't change much at higher compression (a bit more NOx), not enough to affect the heat release. > Basically, a more efficient set of chemical reactions occur > when the compression is increased, because harder to burn > hydrocarbons (more stored energy) react more completely > (more released energy) with oxygen. No. A higher compression ratio creates a higher pressure after combustion pushing the piston harder and extracting more heat as work. Please study the thermodynamics of the Otto cycle. > You're referring to "water injection." I was referring to > "gasoline water emulsions." Chemically there is no difference. Emulsions might cause the gasoline to vaporize better, injection permits stratified charge. Both second-order effects. -- Robert
From: Rod Pemberton on 28 Jan 2008 07:11
"Robert Redelmeier" <redelm(a)ev1.net.invalid> wrote in message news:vv9mj.557$R84.17(a)newssvr25.news.prodigy.net... > Rod Pemberton <do_not_have(a)nohavenot.cmm> wrote in part: > > True, but they are using multiple oil barrels packed with dynamite > > to put out a relatively small fire by comparison to the explosion. > > The blowout area may be small, but the flow is very high velocity and > the total flame (fireball) is not small compared to the explosives. > .... > > The relatively massive explosion consumes all combustible oxygen. > > No. High explosives like nitroglycerine and TNT contain their own > oxidizer (the nitrate groups) .... > and do not consume oxygen. .... > The gasses > generated however, are reducing and effectively smother the flame. > It all happens extremely rapidly, disrupting the free-radical chain > reactions of burning all at once. Well, I knew for a _fact_ that I read that the oxygen was consumed. However, I also realized that you were correct about the explosive decomposition... It took me a few hours, but I found them. The first clearly says consumed the oxygen. The second says "deprives" which is probably more realistic, via shockwave or vacuum. It was pretty hard for me to imagine the shockwave creating a sufficiently large vacuum, but I guess it can be done with sufficient size and a vapor cloud... (The third is about daisy cutters). Due to the temperatures involved, the I believe the fire will reignite if the explosion fails to eliminate the oxygen... via some method. ;-) "They manoeuvre a large drum filled with dynamite or plastic explosives next to the fire and detonate it. The blast consumes all the oxygen in the vicinity, depriving the flames of vital air. If all goes well, when the smoke clears, the fire is out." - Wild Well Control http://www.newscientist.com/article/mg12917613.800.html "For the mechanism, slow-speed photography indicates that the explosion acts to temporarily drive fuel away from the point where the flame develops and deprive that immediate area of oxygen to support instant reignition." - Boots N Coots http://jwco.com/technical-litterature/p09.htm "The weapon works by first exploding and then spreading kerosene vapor into the air. A second explosion then ignites the fuel vapor creating a massive pressure wave, which sucks oxygen out from the surrounding area. This creates a powerful vacuum effect." http://www.globalresearch.ca/articles/BUC111A.html > > By comparison, for airplane explosion, you have a small > > explosion with a large volume of vapors. > > The problem is if the fuel is kerosene (jet), those vapors are too > lean to burn until the fuel and the vapours are heated above the > flashpoint (140'F). If the fuel is avgas (gasoline), those vapors > are too rich to burn. > Sealed tank, i.e, pressure buildup - many heat sources: sun, wind friction, engine, engine exhaust, hydraulic fluid lines, electrical system, air conditioner... ?... Without measurement or forensics of the event, any possible suggestion is a "shot in the dark..." > > At the moment, I can't seem to find the explosive limits of > > gasoline vapor with water vapor... only with air. > > For good reason. Water vapor [steam] is not an oxidizer and is > actually used when available to fight fires. > Not so... First water does contribute to an explosive effect with gasoline - even if I haven't been able to relocate the info recently. (Argh... sigh...) Second, water dissociates into H2, O2, O, H, OH above 1800K (2780F) - slightly above known thermite temperatures. So, using water to fight such fires, although rare at such a high temperature, would be stupid, in the least... Third, water is an oxidizer. It contains oxygen. It is therefore, by definition an oxidizer because that oxygen, if released, can be used to oxidize something. Most well known oxidizers contain "trapped" oxygen that is released via heat, chemical reaction, etc. Water is no different. You probably meant to say, "Water isn't a strong oxidizer, but an extremely weak one under typical circumstances and below temperatures of 1800K..." ;-) > > Gasoline: 1.4% LEL, 7.6% UEL (NIOSH) Jet A: 0 to 1% LEL, > > and 6 to 7% UEL (Boeing) > > Yep. JP5 LEL is 0.7%v in air. The "problem" is that achieving > those concentrations takes elevated temperatures (even beyond > the flashpoint) >140'F. If the tmeperature is below this, not > enough vapors are generated to support continued combustion. > If the temperature was above this and cooled below, the vapors > condense in a rather cool fog. > .... > If the temperature is below this, not enough vapors are generated to support continued combustion. Apparently, it's not that difficult. Flight 143 PAL (EI-BZG, Manila), and Flight 114 THA (HS-TDC, Bangkock), both blew up while sitting on the runway for a period. I've found another four the NTSB attributes to "fuel vapor" explosion via various reasons. According to you, this shouldn't have happened once, even with an electrical ignition source, because of the vapor and high temperature requirements necessary to sustain combustion... (I.e., what's wrong with your understanding?) > > In fact, there was TV show a few months ago, where NTSB > > officials claimed there are hundreds of airplanes accidents > > resulting from fuel tank explosions, including many they > > were reclassifying now that they understood the issue. > > Certainly in small gasoline powered aircraft. > I recall them being larger 737's, 747's, etc., but perhaps they were smaller... I mostly recall images of debris. > > As for gasoline, you only need watch video of firefighters > > (unintentionally) blowing a few thousand sq-ft house to > > pieces, as shown by a recent TV show. "He just took > > a house to the face!" > > Morons! Gasoline is an extremely dangerous material > when mixed with air in an enclosed space. > And, with moisture... :-) (Hey, I found the others...) > > The reason is chemistry. Gasoline starts from oil which > > has over 100,000 compounds. Gasoline is a complex and > > random mix of hundreds of hydrocarbons (aromatics, alkanes, > > alkenes, cycloalkanes) some easy to combust, some difficult. > > Yes, but they all very close to the same amount of chemical > energy released through burning. About 19,500 BTU/lb > Given that the energy releases are different due to the completeness of combustion under different pressures and that there is a decent mix of chemicals, I'd guess that you're looking at some averaged figure at a fixed compression, or perhaps just for isooctane. > > The chemical reactions that occur between oxygen and the > > hydrocarbons change depending on the compression. > > No, they do not. Even without a catalystic converter, > the engine exhaust is mostly N2 (inert from air), H2O > (mostly from fuel combustion), CO2, CO, unused O2 and > ppms of NOx. It doesn't change much at higher compression > (a bit more NOx), not enough to affect the heat release. > The book I skimmed some 16-17 years ago was by a number of PHD chemists who'd worked their entire careers for the automotive industry. Since I'm not a chemist and didn't understand the chemistry, I'm taking what I remember of their work as authoritative fact. I don't believe that the formulation for gasoline has changed that much since. The chart showing a steep exponential energy release upto 18:1 compression wouldn't make much sense either if that wasn't true. Their statements as to the fact they expected it to continue for substantially higher compression ratio's wouldn't make much sense either if that wasn't true. > unused O2 ....lean burning... extra NOx... the oxygen sensor will correct. > > Basically, a more efficient set of chemical reactions occur > > when the compression is increased, because harder to burn > > hydrocarbons (more stored energy) react more completely > > (more released energy) with oxygen. > > No. A higher compression ratio creates a higher pressure after > combustion I think that would be closer to a linear response, not an exponential. For the reasons stated, I believe that the higher pressure is the result of more energy being released due to more complete and differing combustion reactions, producing a hotter gas... Rod Pemberton |