Hello, I’m John and this is my first posting on this forum.
I’ve been interested in pulsed combustion devices for some while, making Reynst combustors out of ... well ... almost anything that came to hand, really. The plastic ones melted, the glass ones shattered, the metal ones (sometimes) ran for a while ... :) ... but I’ve never owned or run a pulse jet.
A pleasure yet to come.
As it happens I have a one of the stainless steel, tig-welded mini-thermojets on the way - but being a strange individual my intention is not to RUN it (Shock! Horror! It’ll go all brown and dull) but to polish it to a mirror finish and keep it as a house ornament that’ll make people ask “what the hell is that?â€
New Member's Musings - Long and probably wrong! ;)
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Graham C. Williams
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Graham C. Williams
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larry cottrill
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Tools
Click on the Forum Index (above left, just above the blue bar). From there, choose the 'Tools and Construction Forum'.toakreon wrote:Sorry, I'm being stupid here ... where is the "tools section of this forum"
L Cottrill
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larry cottrill
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Theory
John -
If you want a brief (for me) dissertation of valveless pulsejet theory, go to Wikipedia.org and enter 'valveless pulsejet' in the search window. This article has apparently not changed significantly since I originally wrote it. It will not explain how to design, but MIGHT illuminate somewhat why things behave the way they do, and why the engines are shaped as they are.
Actually, I might as well give you the link that goes right to it:
http://en.wikipedia.org/wiki/Valveless_Pulsejet
L Cottrill
If you want a brief (for me) dissertation of valveless pulsejet theory, go to Wikipedia.org and enter 'valveless pulsejet' in the search window. This article has apparently not changed significantly since I originally wrote it. It will not explain how to design, but MIGHT illuminate somewhat why things behave the way they do, and why the engines are shaped as they are.
Actually, I might as well give you the link that goes right to it:
http://en.wikipedia.org/wiki/Valveless_Pulsejet
L Cottrill
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toakreon
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Thanks Larry ... :)
As it happens I'm just discovering the delights of UFLOW1D. I've not yet really "got to grips with it" but it's fascinating to put the dimensions of my "design" (for want of a better word - or a better design) for a very simple valveless into it and set it with an initial high pressure in the combustion chamber.
You can watch the pressure pulses flow along the inlet and exhaust and watch as the flow then reverses in the inlet (and to a lesser extent the exhaust) hopefully drawing in fresh air for the next cycle with the combustion chamber pressure peaking again about 0.003 seconds later - 333Hz operation, anyone?
I don't know how accurate it is but it looks astonishingly "believable" - and also hopeful for this basic engine design ... now named the "Sneezing Gerbil" in honour of my expectations for its total thrust ... :).
John
As it happens I'm just discovering the delights of UFLOW1D. I've not yet really "got to grips with it" but it's fascinating to put the dimensions of my "design" (for want of a better word - or a better design) for a very simple valveless into it and set it with an initial high pressure in the combustion chamber.
You can watch the pressure pulses flow along the inlet and exhaust and watch as the flow then reverses in the inlet (and to a lesser extent the exhaust) hopefully drawing in fresh air for the next cycle with the combustion chamber pressure peaking again about 0.003 seconds later - 333Hz operation, anyone?
I don't know how accurate it is but it looks astonishingly "believable" - and also hopeful for this basic engine design ... now named the "Sneezing Gerbil" in honour of my expectations for its total thrust ... :).
John
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Rossco
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Good observations
Welcome John.
Some very keen observations there derived from obviously a hell of a lot of reading! Great start!
Now just get in and build something simple and based on your own understanding as you have suggested.
This is definitely the best way to learn, and no, you are not "strange" for it, many of us have started that way since hey, "these things are too simple to be comlicated!" ha... we all have to learn about that one eventualy too!
Some suggestions on your initial understandings.
I am not dissagreing with you here, or anyone else for that matter. Its just that you cant think about all that is going on in there to start with. You need to prove or disprove ONE concept at a time!
I like to first think in terms of resonance... this is a biggy, you need to get an understanding of this one first. All well and good having computerized fueling, timing and tourque control on your first piston engine, but if it doesnt have pistons or a crank your theorys might not come off as you expected.
(i dont come up with as good analagy's as you... i need something in here about a mouse blowing an elephant... or maybe not ;))
So, think in terms of intake and part CC : tail and other part of CC.
This you seem to be looking at well.
Ignore volumes and such for now.
Think of the Pressure waves, how fast they are going in the local temperature gas, where they go, come back to, and cross.
The intake is cold (relatively), the waves travel slow. Accousticaly this is a long path.
Visa virsa on the tail.
In something that does not run the intake section in some harmonic, you can start to see that these accoustic lengths are looking about the same!
Now your talking.
Then think on the pressure antinode. The consturctive interferance of the waves, and how this all forms a standing wave.
Just keep it logical, and work one step at a time.
I very much like your findings to err on a longer rather than shorter tail pipe. You hit the biggest obstical on the head already here!
But remember, the intake is the balance on the other side. 1cm (a long way) can thow it out compleatly. These are your timing. Longer intake needs a longer tail (loosely speaking)
Get the intake looking about right, and adjust with the over long tail.
Your diameters, or restrictions do effect this greatly, although if you work it within the range of standard engines, you will have a running engine in no time from thinking purely accoustics.
Getting good power, let alone efficiency is another game altogether!
Google hyperphisics, and read up on resonance in air columns, standing waves and, well, everything else on there. Very handy place.
Getting something running is really down to adjusting it to resonate, then getting the fueling right.
Nearly anything will run in the end, as it is quite difficult to stop a column of air resonating with input energy.
Just some pointers straight off the top of my head.
Hope i havnt rambled too much, i just wanted to put something up here usefull, as i was impressed with your first post.
OOOO, (EDIT) Please build it bigger than you have suggested!
They are not dangerous, they will not blow up and although they do make a little more noise (sometimes) it is far less penetrating.
They are 100 fold easier to handle! Small engines are a pain in the but!
60mm or more CC i would say for starter.
A 20mm tail (what i remember you suggesting? now i dont know) is quite a small engine...toooo many headaches! Just my 2 cents.
and EDIT again. To short, to short! dont start of trying to break records here. A short engine is even more of a pain than a small volume one!
(no more edits)
Just one more EDIT, please revise your thoughts on specific fuel consumption!!! ;)
Rossco
Some very keen observations there derived from obviously a hell of a lot of reading! Great start!
Now just get in and build something simple and based on your own understanding as you have suggested.
This is definitely the best way to learn, and no, you are not "strange" for it, many of us have started that way since hey, "these things are too simple to be comlicated!" ha... we all have to learn about that one eventualy too!
Some suggestions on your initial understandings.
I am not dissagreing with you here, or anyone else for that matter. Its just that you cant think about all that is going on in there to start with. You need to prove or disprove ONE concept at a time!
I like to first think in terms of resonance... this is a biggy, you need to get an understanding of this one first. All well and good having computerized fueling, timing and tourque control on your first piston engine, but if it doesnt have pistons or a crank your theorys might not come off as you expected.
(i dont come up with as good analagy's as you... i need something in here about a mouse blowing an elephant... or maybe not ;))
So, think in terms of intake and part CC : tail and other part of CC.
This you seem to be looking at well.
Ignore volumes and such for now.
Think of the Pressure waves, how fast they are going in the local temperature gas, where they go, come back to, and cross.
The intake is cold (relatively), the waves travel slow. Accousticaly this is a long path.
Visa virsa on the tail.
In something that does not run the intake section in some harmonic, you can start to see that these accoustic lengths are looking about the same!
Now your talking.
Then think on the pressure antinode. The consturctive interferance of the waves, and how this all forms a standing wave.
Just keep it logical, and work one step at a time.
I very much like your findings to err on a longer rather than shorter tail pipe. You hit the biggest obstical on the head already here!
But remember, the intake is the balance on the other side. 1cm (a long way) can thow it out compleatly. These are your timing. Longer intake needs a longer tail (loosely speaking)
Get the intake looking about right, and adjust with the over long tail.
Your diameters, or restrictions do effect this greatly, although if you work it within the range of standard engines, you will have a running engine in no time from thinking purely accoustics.
Getting good power, let alone efficiency is another game altogether!
Google hyperphisics, and read up on resonance in air columns, standing waves and, well, everything else on there. Very handy place.
Getting something running is really down to adjusting it to resonate, then getting the fueling right.
Nearly anything will run in the end, as it is quite difficult to stop a column of air resonating with input energy.
Just some pointers straight off the top of my head.
Hope i havnt rambled too much, i just wanted to put something up here usefull, as i was impressed with your first post.
OOOO, (EDIT) Please build it bigger than you have suggested!
They are not dangerous, they will not blow up and although they do make a little more noise (sometimes) it is far less penetrating.
They are 100 fold easier to handle! Small engines are a pain in the but!
60mm or more CC i would say for starter.
A 20mm tail (what i remember you suggesting? now i dont know) is quite a small engine...toooo many headaches! Just my 2 cents.
and EDIT again. To short, to short! dont start of trying to break records here. A short engine is even more of a pain than a small volume one!
(no more edits)
Just one more EDIT, please revise your thoughts on specific fuel consumption!!! ;)
Rossco
Big, fast, broke, fix it, bigger, better, faster...
[url=callto://aussierossco]
[/url]
[url=callto://aussierossco]
[/url]-
larry cottrill
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Pressure vs Mass Flow, Velocity, etc.
John -
If you're interested in seeing changes in pressure, mass flow, Mach no. and density, this was shown and discussed at some length here:
http://www.pulse-jets.com/phpbb2/viewtopic.php?t=2736
This is not an animation, but it shows progressions at a reasonable granularity and I tried to break down the cycle into recognizable 'phases' (which almost no one considered useful ;-).
A similar thing was done comparing various tailpipe designs (grabbed out of thin air) here:
http://www.pulse-jets.com/phpbb2/viewtopic.php?t=2757
While the designs shown are purely fictional, it does illustrate the powerful influence of tailpipe design on the internal behaviour.
Enjoy!
L Cottrill
If you're interested in seeing changes in pressure, mass flow, Mach no. and density, this was shown and discussed at some length here:
http://www.pulse-jets.com/phpbb2/viewtopic.php?t=2736
This is not an animation, but it shows progressions at a reasonable granularity and I tried to break down the cycle into recognizable 'phases' (which almost no one considered useful ;-).
A similar thing was done comparing various tailpipe designs (grabbed out of thin air) here:
http://www.pulse-jets.com/phpbb2/viewtopic.php?t=2757
While the designs shown are purely fictional, it does illustrate the powerful influence of tailpipe design on the internal behaviour.
Enjoy!
L Cottrill
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toakreon
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OK ... :) ... First a couple of BIG thank-yous.
To GRAHAM
Thanks for pointing me in the direction of UFLOW1D. This I have downloaded and started playing with.
Great stuff - I love CFD programs! I used one (a different one, obviously) to design my "Elipso-Parabolic" air-foil sections for radio-controlled model aircraft several years ago. I'm rather proud of my EP sections - they appear to perform competitively (or even marginally better in terms of drag) than the equivalent NACA "4-digit" air-foils.
It's nice to find something I can "play with" similarly for pulse jets.
I tried downloading NUDiS - but I understand you need Excell for it to work and I only have Corel Quattro Pro ... :(
Still - UFLOW1D is keeping me well occupied for the moment ... :)
To LARRY
I had a good read through your Wikipedia article on valveless pjs. Very interesting indeed.
I also had a VERY careful nose through your UFLOW1D-produced graphs showing pressures, mass flows, mach numbers and densities.
SUPERB! Exactly what I was looking for - particularly as I'm able to "see what gives" with my own design efforts in UFLOW1D - but to be honest I was not that sure (perhaps you should read that as "totally unsure") about what SHOULD give!
To Rossco
Thanks for your encouragement and pointing in the direction of resonances. Thanks also for your suggestion that I should try for something somewhat bigger than my initial idea for a first attempt.
So let me introduce (please imagine a fanfare here) the ... SQUIRREL
At the moment this is sitting in UFLOW1D and I'm "hitting it" with nice, explosive detonations in the combustion chamber and comparing the results to Larry's graphs while "playing" with inlet and exhaust lengths and temperatures.
The Squirrel is at a very, VERY early stage at the moment - but I knocked it up as a starting point, the intention being that it should be about "AFW engine" sized (about 500cc - in combustion chamber) at least. Here are my initial figures - all subject to great change!
COMBUSTION CHAMBER
- 8cm (3.2") diameter
- 12cm (4.7") long
- Centre 8cm parallel sided with 2cm (0.8") long "cones" reducing to the size of the inlet at the front and the exhaust at the rear
This gives a combustion chamber volume of 534cc
EXHAUST/TAIL PIPE
- 3.33cm (1.3") diameter
- 62cm (24.4") long
- Currently parallel, but I shall certainly be experimenting with non-parallel tailpipes in UFLOW1D - and with varying the length
INLET
- 2.36cm (0.93") diameter
- 16cm (6.3") long
- This will be varied in length in UFLOW1D "experimentation"
LAYOUT
Very, very simple. A small "barrel" with a long pipe sticking out of one end and a short one out of the other!
OVERALL LENGTH - 90cm (35.4")
I hope this sounds a bit "more like it" - and if anyone sees anything that's glaringly and obviously wrong - please yell at me ... :)
John
To GRAHAM
Thanks for pointing me in the direction of UFLOW1D. This I have downloaded and started playing with.
Great stuff - I love CFD programs! I used one (a different one, obviously) to design my "Elipso-Parabolic" air-foil sections for radio-controlled model aircraft several years ago. I'm rather proud of my EP sections - they appear to perform competitively (or even marginally better in terms of drag) than the equivalent NACA "4-digit" air-foils.
It's nice to find something I can "play with" similarly for pulse jets.
I tried downloading NUDiS - but I understand you need Excell for it to work and I only have Corel Quattro Pro ... :(
Still - UFLOW1D is keeping me well occupied for the moment ... :)
To LARRY
I had a good read through your Wikipedia article on valveless pjs. Very interesting indeed.
I also had a VERY careful nose through your UFLOW1D-produced graphs showing pressures, mass flows, mach numbers and densities.
SUPERB! Exactly what I was looking for - particularly as I'm able to "see what gives" with my own design efforts in UFLOW1D - but to be honest I was not that sure (perhaps you should read that as "totally unsure") about what SHOULD give!
To Rossco
Thanks for your encouragement and pointing in the direction of resonances. Thanks also for your suggestion that I should try for something somewhat bigger than my initial idea for a first attempt.
So let me introduce (please imagine a fanfare here) the ... SQUIRREL
At the moment this is sitting in UFLOW1D and I'm "hitting it" with nice, explosive detonations in the combustion chamber and comparing the results to Larry's graphs while "playing" with inlet and exhaust lengths and temperatures.
The Squirrel is at a very, VERY early stage at the moment - but I knocked it up as a starting point, the intention being that it should be about "AFW engine" sized (about 500cc - in combustion chamber) at least. Here are my initial figures - all subject to great change!
COMBUSTION CHAMBER
- 8cm (3.2") diameter
- 12cm (4.7") long
- Centre 8cm parallel sided with 2cm (0.8") long "cones" reducing to the size of the inlet at the front and the exhaust at the rear
This gives a combustion chamber volume of 534cc
EXHAUST/TAIL PIPE
- 3.33cm (1.3") diameter
- 62cm (24.4") long
- Currently parallel, but I shall certainly be experimenting with non-parallel tailpipes in UFLOW1D - and with varying the length
INLET
- 2.36cm (0.93") diameter
- 16cm (6.3") long
- This will be varied in length in UFLOW1D "experimentation"
LAYOUT
Very, very simple. A small "barrel" with a long pipe sticking out of one end and a short one out of the other!
OVERALL LENGTH - 90cm (35.4")
I hope this sounds a bit "more like it" - and if anyone sees anything that's glaringly and obviously wrong - please yell at me ... :)
John
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Rossco
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Guys, we have a problem!
Someone that actualy listens to us, and looks for the relevant information!
John, you never know where you could end up doing that!
Sounds good mate, much better size now.
Im drawing it up to get a hands on feel for it, will post.
Do you have any 3d handling software?
Ive got a plugin here somewhere that is very simple, with just a few buttons for spin, zoom, add and remove parts.
I should get Mikes thoughts on something in which we can "hand over" a concept for discusion, so that the viewer can turn it around and have a good look at it.
Rossco
Someone that actualy listens to us, and looks for the relevant information!
John, you never know where you could end up doing that!
Sounds good mate, much better size now.
Im drawing it up to get a hands on feel for it, will post.
Do you have any 3d handling software?
Ive got a plugin here somewhere that is very simple, with just a few buttons for spin, zoom, add and remove parts.
I should get Mikes thoughts on something in which we can "hand over" a concept for discusion, so that the viewer can turn it around and have a good look at it.
Rossco
Big, fast, broke, fix it, bigger, better, faster...
[url=callto://aussierossco][/url]
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Vermin
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OK this is the way I was mentored on pressure and reactions (generaly) ...take a marshmellow (solid) and try to push it through a 2" tube...no problem right although slow it will go through...so now push it through a 0.5" tube so some of the marshmellow is going through and some will be going through later only so much mass and so much velocity can be acomadated at one time and in one place ....so you have to undersand that you cannot change the physical aspects of matter just attempt to contol it.... now if you want to apply gasious thermal fluid dynamics into the equasion everything changes because the available, potential energy (available thermal potential) / (energy mass transfer) changes with the configuration of the duct and how it converts thermal energy into mass transfer energy ...so there you have all the fun stuff that we are working with in a nutshell.
So lets all try to convert heat into dynamic energy other than ICE.
I hope this is as abreviated as possibele....it fits in my head.
So lets all try to convert heat into dynamic energy other than ICE.
I hope this is as abreviated as possibele....it fits in my head.
Vern
A desire to destroy as many man made hydrocarbon compounds as possible in one lifetime.
A desire to destroy as many man made hydrocarbon compounds as possible in one lifetime.