venturi math?

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maybe venturi isn't the right word, but, similar concept.i have a 2-1/2" combustion exhaust tube, and i want to put a 4" stack over it that wont get dangerously hot, by drawing in fresh air from the void between the two pipes.my gut tells me that the velocity of the exhaust coming out of the 2-1/2" could accomplish this, if i knew an equation to figure out the overlap between the two pipes, if that makes sense. there has to be an equation that would say how far the 4" should slide over the 2-1/2" or if i need to change sizes to accomplish this.or maybe im missing something really important???bosses stuff:trailblazer 325maxstar 200my stuff:sa 200fronius transpocket 180100 amp Lincoln w/f97 f350 DITKevin
Reply:Are talking about like what is on Duramax/Chevy  exhaust tail pipe. Flowmaster Exhaust sells the Venturi reducer cones. Had thought about putting this on a Lincoln Short hood and a John Deere 450B dozer to remove heat from engine. My local Autozone ordered the cones, but this is far as have gotten on this.
Reply:yea! similar concept, but not automotive.this is a forced air heating tube in a sump. blower blows in flame at one end, heats water in sump, and exhaust comes out the other end. im just trying to make the exhaust "touchable" to the average passerby.bosses stuff:trailblazer 325maxstar 200my stuff:sa 200fronius transpocket 180100 amp Lincoln w/f97 f350 DITKevin
Reply:Not sure on math, but look at Duramax exhaust and see how far the tube goes in. Without the math, then maybe experiment and see where you get best flow.
Reply:Have seen the venturi design used in several applications. In farm tractors mounted on exhaust and plumbed to air filter to remove dirt from filter and out the exhaust. Also used as a sump pump, flowing compressed air to suck water out. Inside of chemical tanks, as pump circulates liquid the venturi ejector nozzle helps stir chemical. Last the Harbor freight air operated vacuum pump.
Reply:

Originally Posted by 92dlxman

maybe venturi isn't the right word, but, similar concept.i have a 2-1/2" combustion exhaust tube, and i want to put a 4" stack over it that wont get dangerously hot, by drawing in fresh air from the void between the two pipes.my gut tells me that the velocity of the exhaust coming out of the 2-1/2" could accomplish this, if i knew an equation to figure out the overlap between the two pipes, if that makes sense. there has to be an equation that would say how far the 4" should slide over the 2-1/2" or if i need to change sizes to accomplish this.or maybe im missing something really important???
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