No Weld Pulsejet

[Stephen H]

hey all... i was thinking the other day about bruces no weld pulsejet and how it works. Im guessing it works the same as any pulsejet but insted of having a cone going down to a smaller tail pipe from the CC it has a tube down the mid. so the area around the outside of the tube would be ther same as the inner diameter of the normal tail pipe ?.. is this right ?

If it is it wouldnt be to hard to do the dims. your self would it ?
and you could do it just using any pulsejet design there is!

Stephen

[Mark]

hey all... i was thinking the other day about bruces no weld pulsejet and how it works. Im guessing it works the same as any pulsejet but insted of having a cone going down to a smaller tail pipe from the CC it has a tube down the mid. so the area around the outside of the tube would be ther same as the inner diameter of the normal tail pipe ?.. is this right ?

If it is it wouldnt be to hard to do the dims. your self would it ?
and you could do it just using any pulsejet design there is!

Stephen

There might be a problem with cooling the interior tube, look at what happened to a tube Larry Cottrill put up inside his Dynajet tailpipe. It melted.
Mark

[Stephen H]

The inner tube is there to effectively reduce the cross-sectional area of the tailpipe so that it behaves as if it were a narrow pipe with a bigger diameter in the front (combustion chamber) part.

It's a whole lot simpler simply to stuff a tube inside a tube than it is to try and join a narrow tube to a wide one -- that generally requires some kind of pipe fitting that may not be easy to find, or some welding.

that is from the forum on brices site about the No weld Pjet... so it is like i decribed. I got bored so here is the Alpha Pjet (from this this website) with a inner tube!

Stephen

ps. forgot to say that it is drawn with a scale of 1:1 and the tail pipe dims. can be found on the original plans. The blue line is the original jet and the black is the inner tube virsion!!

[vhautaka]

How about using that inner tube to heat fuel?

Or even to boil water, the vapor to be injected into the exhaust?

Oh yeah, that would require welding again.... or would it?


- ville

[Stephen H]

i wasnt really thinking of this from a no welding point of view, just the idea of of the inner tube.. mabey have a cooling coil in the middle.

Stephen

[Mike Everman]

Stephen, that drawing is not right. The annular radius difference on both variants is the same. Check the geometry before you build anything from any drawing you find. This particular one should have the inner tube type have an annular difference (inner tube OD to outer tube ID) much less than the outer tube type radius for the areas to be the same. Just be cautious when looking at sketches, is all! ;=D
I don't know how many times a good idea was laid waste by drawing it to scale!

[Mark]

Nobody seems to be addressing the heat issue or the furnace you are going to subject the inner tube to. Maybe instead of putting it in the center, one could just put it to one side, and use the heat to weld it to the adjacent wall.
It might be fun to make the inside tube the fuel tank, if it got too hot well there would be the jacketing pipe to protect you and who really wouldn't like a big fireball effect from time to time? It would be tricky but fun to calibrate a fuel rod tube of fuel/fuel tank and nozzle diameter that sprayed the fuel toward the head of the jet. Maybe if you could actuate the fuel rod/tank in and out of the jet somewhat to get the heating right, trombone style.
I don't know how the flame front would behave traveling down a "in effect donut tube" either, resistance wise or otherwise. Might be interesting to try some donut tubing either fuel filled in the center or glazed with some sort of ablative substance, akin to some solid fuel ramjet designs.
In all seriousness though, haven't we already seen what the inside of pulsejet does to metal that isn't cooled? If you didn't have the outside air on a Dynajet for cooling, if not dissipated/radiated the calories would turn the Dynajet walls to butter as well. But you don't have to take my word for it.
Mark

[Stephen H]

Stephen, that drawing is not right. The annular radius difference on both variants is the same. Check the geometry before you build anything from any drawing you find. This particular one should have the inner tube type have an annular difference (inner tube OD to outer tube ID) much less than the outer tube type radius for the areas to be the same. Just be cautious when looking at sketches, is all! ;=D
I don't know how many times a good idea was laid waste by drawing it to scale!

sorry mike i dont get ya... can you explain it for the stupid ones here :)

Stephen

[marksteamnz]

Stephen, that drawing is not right. The annular radius difference on both variants is the same. Check the geometry before you build anything from any drawing you find. This particular one should have the inner tube type have an annular difference (inner tube OD to outer tube ID) much less than the outer tube type radius for the areas to be the same. Just be cautious when looking at sketches, is all! ;=D
I don't know how many times a good idea was laid waste by drawing it to scale!

sorry mike i dont get ya... can you explain it for the stupid ones here :)

Stephen

Stephen. You need to look at the crossectional areas and keep the ratios the same. Say you have a 4 cm valve body bit tapering to a 2 cm tail pipe. To replace the tail pipe with a 4 cm tube with a body up the guts of it the body will need to be about 3.5cm in diameter to have the same cross sectional area as a 2cm tube. Ie Cross sectional area of original tail pipe must equal the cross sectional area of the new (larger diameter) tail pipe LESS the crossectional area of the central core. Core diameter equals
Square root of ((the diameter of the large pipe squared) - (diameter of original tail pipe squared)). Note pi cancels.
Hope this makes sense from a very beer o'clock brain on a very warm day.
By the way I relise there are all sorts of skin, turbulence effects that will mean the exact geometric ratio may well require "fine" tuning but unless there is data avalible lets not wibble on about it people. I'd rather Stephen built some thing, then reports and measures what happens.
Set us old farts a good example Stephen.
If you need a valve plate email once you have progress and lets see if I can help out. Fill the cental body up with water and wet socks or something and have at it!

[Mike Everman]

Build something for sure! But calculate your geometry three times, measure twice, and cut once. ;-D
Oh and thanks mark for explaining! Every forum minute counts when you're stuck in the third world.

[Stephen H]

ohh i got ya... ill have alook at it again... mark i came accross the drawings of the didgeridoo pulsejet you were going to make for me awhile ago (back in the day when i had money)... ;)

Stephen

[Stephen H]

ok... this right.... excuse the long math in there its just how we have to do it in school!!.. i think thats right... if not.. im confused!!

Stephen

[Stephen H]

ok... this right.... excuse the long math in there its just how we have to do it in school!!.. i think thats right... if not.. im confused!!

Stephen

[Mark]

If the center tube is left open at the exhaust end, you might could push some sausages up inside and cook them if you are going to run it on propane.
Mark

[marksteamnz]

ok... this right.... excuse the long math in there its just how we have to do it in school!!.. i think thats right... if not.. im confused!!

Stephen

Close but no cigar. The formula piR^2 correct but you have used 3.14x22 that is piD where D is diameter and you have therefore calculated the circumference not the area.
Area is (3.14x22x22)/4 = 379.9mm2
the divided by 4 is to allow for diameter not radius being used in the equation. Now you have to subtract the areas and then back calculate the diameter of the central body. Have another look at the simplified formula I wrote to go directly to the diameter of the central body.