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I posted this in another forum, at the site pulsejets.com, hoping that someone would help me with figuring out how to accurately calculate what size nozzle diameter to make for a given-sized chamber...
Oh well, as no one cared to offer any mouse-fart of a hint as where to look for information. - Everyone seems to be caught up in a recent post about this new "Thunderchine", which does, btw, look awesome!
I drifted out of building "spudguns", venturing into playing with blackpowder cannons. They were fun, but no rocket-science calculations to make one, so I quickly got bored with it.
This new-found hobby is my current obsession! I (as anticipated ) just HAD to build somethin' a little bigger, seeing as how I scored a FREE piece of 2-1/2 thinwalled chrome steel pipe (probably off of a Fast and the Furious wannabe's ricer) and also a scrap piece of 5" diameter mild steel exhaust tubing.
Here's the recent post:
Based on the dimensions of something that works (Eric Beck's 3 pound thrust Thermojet plans), I thought that I could combine my extensive knowledge of "potato cannon" design & fabrication with my extremely limited knowledge of pulse jet design.
Eric's "Combustion Chamber" volume of the 3 lb. Thermojet is estimated at 28.27 Cubic Inches. This, of course, is not to include the estimated 5.62 Cubic Inch volume of the truncated cone. - I decided this truncated cone volume was negligible, simply for ease of calculating the new design. The "Barrel" volume of the 3lb. Thermojet is estimated to be 17.79 Cubic Inches ... C:B (Chamber : Barrel) Ratio is: 1.58:1
The estimated volume of the FREE piece of 5" diameter x 8-7/8" Length Exhaust Tubing is 165.65 Cubic Inches. This volume is 5.859 times the volume of the Combustion Chamber on 3 lb. Thermojet. The Barrel volume is estimated at 117.80 Cubic Inches. This gives me a C:B (Chamber : Barrel) Ratio of 1:40:1 --- Smaller than the 3 lb.'s , but it would appear that C:B ratios are hardly a concern, because I've seen plans on the internet with C:B Ratios as little as 0.35:1 .
My #2 main concern, in "upscaling" Eric's design, was the combined volume of Intake Tube(s), which are : 1/2" I.D. x 3" Length (x2) and equates to : 1.17809625 Cubic Inches. So, seeing as how my new one was 5.859 times larger (volume-wise), I made sure that the intake tube length was upscaled as well, to match or slightly exceed this volume.
1.178 x 5.859 = 6.903 Cubic Inches
The estimated volume of the 1" Diameter conduit is 0.7853975 / Inch (length). Dividing the required 6.903 cubic inch volume of the intake tubes by 0.7853975, gave me a length of 8.789 Inches. Divided by 2, gave me a required length of 4.394", for each intake tube... Eh, I rounded it off to 5" each.
The 10" combined length, an "odd multiple" of the 24" length of the Exhaust (Tailpipe) seemed to be OK with what I learned by reading Bruno Ogorelec's Valveless Pulsejets 1.5 - Quoted from the section:
KADENACY OSCILLATION, THERMAL BREATHING AND ACOUSTIC RESONANCE / Elements of Resonance: "... So, the tailpipe length must be an odd multiple of intake pipe lengths for the engine to work properly".
My #1 concern in getting this thing to work, was the size of the injector orifice. As much research as I've done, I'll be damned if I can find even a hint towards how to accurately calculate the required nozzle opening! Although I wanted (so badly) to purchase Eric Beck's "4lb Thrust Thermojet Valveless", my wife (as a surprise) bought me a $120 Thermojet, off of ebay... It came with a lot of information (regardless of how vague, at some points), the biggest of which was the fact that the nozzle diameter was revealed in the included instructions... The diameter of the nozzles were a #71 drill size, which I found to be .025" - .026" in diameter. (Conflicting dimensions on different websites)
Well, the area of a .025" diameter hole is 0.0004908734375". Since my new thermojet's chamber volume is 5.859 times the volume of this little 3 lb.'er, I multiplied 0.0004908734375 by 5.859. - This gave me 0.0028760274703125" . (Mind you, I was doing all this math on my cell phone's calculator) Messin' around with different numbers, first .040, then .050, led me to find that a 0.060" hole is (well, I'll be damned!) ; 0.002827431"
Knowing that a 1/16" drill bit is .0625" , I made the injector(s) out of 3/16" Steel Brake Line, which I picked up at Checker Auto Parts. Cutting the flared tip off of one end, I used a tiny ball-peen hammer to shape the end of the brake line to a rounded point. This rounded point was tapped on end, until it completely closed the hole. Using a small automotive steel pick, I worked the pick until it poked a hole in the end of the line. The hole was worked until (by eyeball- Ha!) was slightly smaller than the 1/16" Drill Bit.
The chamber ignited on the very first try, idling at less than 5psig line pressure. As it warmed up over about a one-minute period, I slowly increased the line pressure to 20psig. - Any higher than that, it started to sputter slightly. Between 20-25psig, a short/slight blast of compressed air allows it to immediately run at a high oscillation. <-- Correct term?
2.7 Liter Valveless Thermojet Video
I absolutely love this new-found hobby! The recent posting of the "Thunderchine", pushed me slightly farther over the edge, to build something a little bigger than my spot-welded party-popper. I've been lucky thus far, after making an altered-version of Beck's 3lb. Thermojet, and another out of an oil filter casing, which I got from work.
Someone... please - Steer me in the right direction towards the proper way to calculate nozzle diameters? It would make this SO much easier.
... I just thought that you guys would get "just as much a kick" out of something that is so easy to build and so obnoxiously loud.
Damn, that looks good.
Can't help with the math on diameters, volumes, etc. Not sure if anyone really knows how to calculate that kind of stuff anyway.
I wonder if the best approach might be to build the engine so that one or more of it's characteristics can be changed easily.
Can't think of any easy way to change chamber diameter, chamber volume, exhaust diameter or inlet diameter. Those all seem to be set when the engine is built.
Two things that might be easily changeable are the exhaust and inlet lengths. Can you add "telescopic" type extensions to those two pipes? Does the engines efficiency, power etc. change as the two lengths are varied? Is there and optimal length for each, or an optimal length ratio for the pair?
Perhaps make the chamber a big-ass piston in cylinder so the length (hence the volume) of the chamber can be easily changed? Sealing the piston will be a challenge. Though perhaps it doesn't need to be sealed all that well
I do not believe that there is just a regular formula for nozzle diameters.
You will have to make/find an advanced computer simulation in order to calculate the optimal size.
As you probably aren't going to do that, find it experimentally.
Till the day I'm dieing, I'll keep them spuddies flying, 'cause I can!
Spudfiles steam group, join!
look sgreat but what will you use it for, after all this is more than just shits and giggles, this has potentual!
"Did you ever stop to think that out of the seven deadly sins envy is the only one which doesn't give the sinner even momentary pleasure"-George Will
Well in my time reading various forums people are more inclined to help those who
1.use good grammar
2.Don't ask to be spoon fed <--that's this forum
3.Keep there post short and sweet so you don't end up looking through a novel to find the questions
I would have just posted this;
Well, of course! - It compliments my aluminun foil helmet and 3D night vision goggles.
The pipes would have to mate where they can still slide but have enough of a seal as to not suck air... Thermal expansion would lock them up after a very short period of run time.
Yes. - And basically, a pulse jet is (in a sense) a sound generator. If the intake pipe(s) is/are an even multiple of the length of the tailpipe, the engine will not run. - Something about quarter and half wavelengths- Would have to reread the article that I got that info from.
Yes. There is a point where you achieve the highest frequency. -Trial and error.
jimmy101, engineers calculate how much fuel/air is required to operate an engine at a certain rpm... Injector size, flow-rate-- There's gotta be a way to calculate all this, but I guess there's not all that much R&D, in the realm of pulsejets.
Yep. - And I've realized this, as I've implied earlie in my other replies. I really appreciate some of the positive comments.
This is ALL just for shits and giggles! I like them because they're obnoxiously loud and glow red-hot. - There's always that fear of it rupturing and spraying you with molten metal. - Not that it really would, but you still think there's a large chance that it might! -Ha!
1.) Grammar is not an issue with my post, nor has it ever been.
2.) I've done more spoon-feeding in the last five years, than almost anyone here on this forum. Detailed explanations, plans, video - you name it.
3.)In another RECENT topic on the forum that I asked for help, a veteran member clearly stated that the information exists (IN the forums) on how to correctly calculate how much fuel a given sized engine would need. - I simply asked to be steered in the right direction. 60 views and not one reply.
Leads me to think they are too stuck-up to even bother to reply- Even if only to tell me that my "so-called thermojet" looks like a P.O.S. ?
My long-winded post was simply to let them know that it made perfect sense to me, that if an engine is 5.89 times the size of one that they KNOW needs a certain (optimal-sized) nozzle diameter, then why not make the nozzle diameter itself... 5.89 times the area of the smaller one?
Doesn't take a rocket-scientist to figure out how to upscale a design that has been extensively tested. Just looking for some feedback, in the wrong neck of the woods.
No need to be a dick about things, you clearly don't know who DR is, he is one of the pioneers of spudgunning and a RESPECTED member of the community.
Current project: Afghanistan deployment
Nice work DR, have you compared it to others such as the focused wave and all of that? Actually, come to think of it it's not much different. How about a lockwood hiller?
By the way Jonny:
1. He doesn't need good grammar, it's alright as long as the post is easily understandable.
2. He wasn't asking to be spoonfed, he was asking a question which the odds are someone knows. It's one simple part, not the entire thing and it could save him a lot of time and money.
3. He doesn't have to keep it short. Learn to skim read (read the important parts..)
I know my grammar isn't perfect..but as you insist:
*Waits for somebody to scrutinize my post*.
All I'm saying is; if you can read the post without having to squint and think hard then leave it.
Sorry DR, I can't give you a proper formula for that.
You could try this site, but it focuses more on reed valved pulsejets and a lockwood hiller. You may still be able to find what you need, or ask for advice there.
frankrede, I appreciate your reply...
innonickname, I have read many parts of that site as well, although I do than you for going to the trouble of including the link.
I have not compared it to anything else, as I've only built thermojets of this particular design. - It stated that they are easy... (easiER) to start, and the design is simple to replicate without having to hand-form cones.
Well, I'm sure Ragnarok could weigh in on the subject. He's a little further along in math than most of us are (I believe at least) and this problem is certain to involve some type of fluid flow through a fixed orifice.
Another thing you could try is go to McMaster-Carr and look at their flow-control orifices. By using their table of flow values at a constant pressure you could do some sort of regression with a program or calculator (I would go with a logarithmic regression, given the nature of what you're modeling but again I'm not sure) and see what you can find out.
By the way, jonnyboy, this is far from spoon-feeding. The material DR is working on obviously requires knowledge most people don't learn until their upperclassman years of college.
Good to see you're still tinkering around, DR. And what a thing to tinker on! Me and my friend played around with the idea of a pulsejet a while ago but eventually gave it up, as our small model required a constant supply of shop-compressed air to keep it running on our lowest fuel flow. Probably more to do with construction materials than anything.
Ever think about building a twin chamber? Wonder how that would sound running???
If I didn't know your style so well, I'd take offence to that!
When life gives you lemons...throw them back they suck!
Actually, it would OK if they suck a bit of air, as long as they suck/blow significantly less gases than does the bore of the pipe.
Heating will lock'm up, cooling should unlock'm (as long as they don't fuse toghter).
The tricky part would be finding nested pipe sizes.
It looks to me like this thing is a set of three coupled oscilators; the exhaust pipe, the inlet pipe and the chamber. The inlet and exhaust are open at both ends and will resonate at;
f = nv/2L
where f is the frequency of resonation, n is an interger (1,2,3,... the harmonic number?), v is the speed of sound (which is very dependent on the temperature of the gases) and L is the length of the tube. There is a small correction to f for the diameter of the tube, often the correction is small enough to be ignored.
The chamber is closed at one end so it resonates at half the frequence;
f = nv/4L
where the variables are the same as before except n is an odd integer (1,3,5,...).
The inlet and chamber are strongly coupled oscillators. The outlet and chamber are also strongly coupled. The inlet and outlet are perhaps only weakly coupled.
Now, if my math and physics weren't so rusty, I should be able to derive the equations for the interaction of these three coupled oscilators. The tricky part is going to be that the resonant frequency changes with the air temperature. For example, during the high pressure part of the cycle the barrel has 2000C (?) combustion gases in it and the speed of sound is something like 3000 FPS. During the low pressure part of the cycle (fresh air being sucked back into the chamber) the air temperature is much lower and the speed of sound is back around 1100 FPS. So the resonant frequency of the barrel changes by about a factor of three as the engine cycles. The gases in the inlet and chamber are also changing temperature, hence their resonant frequencies are changing as well.
Furthermore, a pipe has "impedance". The same way an AC electrical circuit does. The friction between the pipe and the moving gases changes depending on the relationship between the frequency of the gases' movements and the length (and diameter?) of the tube. Depending on how the gases are moving relative to the pipe you can have a "high pass filter" or a "low pass filter" (equivalent to high and low pass RC filters in electronics).
I wonder if the impedance affects are really what makes this type of engine work. The inlet and exhaust have different impedances. During the combustion phase the exhaust pipe has low impedance and the inlet has high impedance. The result is most of the gases are forced out of the exhaust. During the "suck back phase" the barrel has high impedance and the inlet has low impedance and more of the gas flow is through the inlet.
One more complicating factor. For air resonance in a tube there is no net movment of the air in the tube, the waves are "standing waves". In these jets there is massive net movement of the gases. Not exactly sure how that changes things.
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