Ice Czar Posted December 30, 2007 Share Posted December 30, 2007 "heat transfer is proportional to the temperature difference on the object. If the temperature differential doubles, the heat transferred doubles" Ive done a fair amount of twisting, and knew about this particular aspect of thermodynamics, but was quite surprised to see it so graphically illustrated when putting a little extra heat with a large oxy-acetylene rosebud. There you are with a cherry red bar in your twisting jig, drop the flame on a portion of it and watch as part of it turns yellow, but just down the way in both directions is a bit cooler than the rest of the bar, as the heat is robbed, flowing to the higher temperature section faster than it can be replaced by the remaining bar, the banding quickly equalizes once you take the torch away and the heat is transferring the other way from the hotspot to the rest of the bar. you may have noticed it, maybe not, but its pretty cool (pun intended ) Quote Link to comment Share on other sites More sharing options...
gerald Posted December 31, 2007 Share Posted December 31, 2007 I've always tjought it sort of amusing when I hear someone say "We'll quench one end of the piece to 'run the heat to the other end' of the piece." This goes against the Law of T'dynamics that says that "heat flows from a warmer to a colder area" (My appologies to Sir Isaac Newton). If you cool an area, the heat should want to run toward the cooler end rather than stay/run to the hotter end. Quote Link to comment Share on other sites More sharing options...
RainsFire Posted December 31, 2007 Share Posted December 31, 2007 I've seen this happen in my first propane forge design before the refractory gets up to heat, the jet will heat up the bar, with the acception of directly under neath it.. I always just assumed that this was because the fast speed of some of the gas was cooling it, but this makes more sense. weird stuff.. Quote Link to comment Share on other sites More sharing options...
Ice Czar Posted December 31, 2007 Author Share Posted December 31, 2007 we use propane at work and Ive seen that not exactly sure what is happening there it could be a combination of convective cooling from the yet to be fully ignited gas as you guessed and the effect described above where the flame front finally catches heats the steel around the dark spot and "robs" it of heat. one of the forges has three ports and its even more noticable at startup Quote Link to comment Share on other sites More sharing options...
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