deLaval Nozzles for Small Ramjets?

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Bruno Ogorelec
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Re: re: deLaval Nozzles for Small Ramjets?

Post by Bruno Ogorelec » Sun Aug 21, 2005 9:56 am

Eric B. wrote:From what I recall, a supersonic nozzle is only required at supersonic speed. What would be the purpose of installing one on a subsonic ramjet?
Not so. To put it very simply, in terms of thrust generation, the important parameter is the momentum imparted to the ejected gas relative to the vehicle, not relative to the ambient air. You can reach supersonic speed with subsonic exhaust flow and you can fly at subsonic speeds with supersonic exhaust flow. (It is more complex than that in reality but this is it in rough terms.)

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Re: re: deLaval Nozzles for Small Ramjets?

Post by Bruno Ogorelec » Sun Aug 21, 2005 10:32 am

pezman wrote:Bruno's comment on reflection got me thinking -- what if you had a "quasi-valveless" that had a short intake w/ a bluff on a spring. When it inhales, the bluff is drawn into the chamber a bit. When it exhales, the bluff is forced up into the throat a bit, making it into a deLaval nozzle -- the high-presure side of the nozzle ought to rectify the pulse a bit. Whatever goes out the "intake" can be mechanically added to the thrust in the normal way (external rectifier, rear-facing intake etc.).
Sorry to disappoint you, but it would not work too well. At small speeds that are the rule within the chamber, the nozzle would behave too much like a nozzle and too little like a reflector.

That's why I used it in my first published pulsejet concept, the BCVP. I needed it to work as a nozzle in one part of the cycle and as a reflector in another part.

The idea was to have two combustion zones, one at each end of a tube. The tube ends would have de Laval nozzles. So, a 'bang' at one end of the tube would send some hot gas out of the nozzle (generating thrust) while the rest would expand down the tube as a hot pressure front.

This hot blast would hit the mixture at the other end of the tube at very high speed. The nozzle would not pass much of the charge out because the high-speed pressure front would be reflected off its bluff end, so that most of the mixture in this zone would be compressed by the blast (and ignited by the hot gas).

The new blast would travel in the opposite direction, back to the first combustion zone, which has meanwhile refilled with fresh charge. It would compress and ignite the charge there etc.

Hot gas fronts would shuttle at high frequency in the combustor, which would be exerting thrust alternatively from one end and the other. If bent into a U-shape, both ends would push in the same direction.

So, I used the de Laval nozzle as a nozzle for hot gases generated right at its mouth (which had low speed), but as a reflector for the pressure front coming from the opposite end of the long combustor tube (which was accelerated to near sonic speeds).

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re: deLaval Nozzles for Small Ramjets?

Post by Viv » Wed Aug 24, 2005 10:15 pm

So how small do you wan't your nozzle then Larry? is this small enough for you:-)

Just doing some spinning experiments in the shop and came up with this:-)

1/2" 316L stainless steel tube, neck is down to 1/8"

Viv

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re: deLaval Nozzles for Small Ramjets?

Post by Mike Everman » Thu Aug 25, 2005 6:46 am

Thanks for this effort, Larry. Very thought provoking. I trust your motors arrived safely?
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Re: re: deLaval Nozzles for Small Ramjets?

Post by larry cottrill » Thu Aug 25, 2005 1:20 pm

Mike Everman wrote:Thanks for this effort, Larry. Very thought provoking. I trust your motors arrived safely?
Yes, got them in fine shape - I was going to email you right away to thank you, but quickly forgot about it. Thanks for getting them back to me.

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Re: re: deLaval Nozzles for Small Ramjets?

Post by larry cottrill » Thu Aug 25, 2005 1:23 pm

Viv wrote:So how small do you wan't your nozzle then Larry? is this small enough for you:-)
Looks like it would be great for rocketry!

Viv, is there any way you can get me the ID accurately measured at several stations of the nozzle (at least 10 or 12 spots)? We could then assume some reasonable temps and get the chamber pressure that would make it work using UFLOW1D, and I would post it here or in the Rocket Forum (probably more appropriate, since I think the critical pressure's going to be pretty high).

L Cottrill

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Re: re: deLaval Nozzles for Small Ramjets?

Post by Viv » Thu Aug 25, 2005 2:53 pm

[quote="Ben"]How much do you want for one, Viv, that size? Email me if you don't want to post a price.[/quote]

Ben, the material and my time comes to about $15 CDN shipping a package to the US would be about another $15 CDN from experiance.

So about $30 CDN or $25.09 USD

I have to say these are done by eye! I had the shape of the orbit motor on the last stage of a Satern V in mind while I was doing this:-)

If you want a differant profile just let me know.

I am limited on time so if you buy it can you do Larry a favour and map it for his analysis:-)

Viv

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Re: re: deLaval Nozzles for Small Ramjets?

Post by Viv » Thu Aug 25, 2005 2:58 pm

[quote="Larry Cottrill"][quote="Viv"]So how small do you wan't your nozzle then Larry? is this small enough for you:-)[/quote]
Looks like it would be great for rocketry!

Viv, is there any way you can get me the ID accurately measured at several stations of the nozzle (at least 10 or 12 spots)? We could then assume some reasonable temps and get the chamber pressure that would make it work using UFLOW1D, and I would post it here or in the Rocket Forum (probably more appropriate, since I think the critical pressure's going to be pretty high).

L Cottrill[/quote]

Time is m problem Larry, we work on average 14 hours a day and 6 days a week on our projects so I just don't have much spare time for any others even small ones:-)

Ben wanted a price so if he likes it maybe he could do the mapping for you?

We are just a bit busy as you can imagine with our new third generation series pressure jets.

Viv

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Re: re: deLaval Nozzles for Small Ramjets?

Post by Viv » Fri Aug 26, 2005 12:49 am

[quote="Ben"]Stupid weak USD. It's a bit more than I was hoping, but shipping from your boondocks to my boondocks isn't cheap, and time is money. You take Paypal?

Larry, I don't have a good micrometer any more, so dimensions would be entirely guessed.[/quote]

Sorry about the USD Ben but wait five minutes and I am sure it will change:-)

Yes we take PayPal, luncheon vouchers or other trade goods, some times even cash:-)

I will get Luc to send you a Paypal invoice to make things easy and often they do a better exchange rate.

Thanks

Viv

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Re: re: deLaval Nozzles for Small Ramjets?

Post by marksteamnz » Fri Aug 26, 2005 10:43 am

I'd pay US$30 for a DVD of how you spun the shape. That's a very impressive piece of work especially as it's in stainless.




Yes we take PayPal, luncheon vouchers or other trade goods, some times even cash:-)

I will get Luc to send you a Paypal invoice to make things easy and often they do a better exchange rate.

Thanks

Viv[/quote]
Cheers
Mark Stacey
www.cncprototyping.co.nz

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Re: re: deLaval Nozzles for Small Ramjets?

Post by Viv » Fri Aug 26, 2005 2:47 pm

[quote="marksteamnz"]I'd pay US$30 for a DVD of how you spun the shape. That's a very impressive piece of work especially as it's in stainless.




Yes we take PayPal, luncheon vouchers or other trade goods, some times even cash:-)

I will get Luc to send you a Paypal invoice to make things easy and often they do a better exchange rate.

Thanks

Viv[/quote][/quote]

:-)

Terry Tynan does a much better DVD for about $40 usd, plus its a two DVD set:-)

Check out his metal spinning workshop web site (google it) he is a nice guy and very experianced in all aspects of spinning.

The nozzle is spun in 316L most spinners consider this the upper limit for realatively easy spinning, we spin 321 grade for nearly all our parts and if you ask a spinner he will tell you its almost impossible to spin 321 for the nose cones we need.

We were told after we had worked out how to spin 321 cones for the pressure jets by two profesional spinners that it could not be done, we did not know so went ahead and did it any way:-)

Spinning 316L or lower grades is just for play or practice shapes for us now.

I have worked out how to do repeatable and exact profiles for these little rocket nozzles now so if any one wants more I will make a tool, just give me a profile that you all want for the tool.

Viv

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re: deLaval Nozzles for Small Ramjets?

Post by Greg O'Bryant » Mon Feb 20, 2006 9:51 pm

This may sound like a dumb Question and doesn't really relate to anything.
Can you get air to move faster than the speed of sound, using nozles etc, while the nozle is going slower than the speed of sound? For example say you had a convergent nozle that was travling about .8 the speed of sound as the air travles into the constriction it should speed up, but to what point, does it start compressing once it reaches the speed of sound or does it go faster?

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Re: re: deLaval Nozzles for Small Ramjets?

Post by larry cottrill » Tue Feb 21, 2006 3:33 am

Greg O'Bryant wrote:This may sound like a dumb Question and doesn't really relate to anything.
Can you get air to move faster than the speed of sound, using nozles etc, while the nozle is going slower than the speed of sound? For example say you had a convergent nozle that was travling about .8 the speed of sound as the air travles into the constriction it should speed up, but to what point, does it start compressing once it reaches the speed of sound or does it go faster?
Greg -

This is very easy if attempted at a very small scale. You can set up a little compressor with the usual hose and push-button "blowgun" tool (used for dust-off, etc.). At only 35 PSI the air from the tiny nozzle will be supersonic, while the air in the hose running up to it will be flowing at only a few ft/sec.

The problem is when you try to size it up. Theoretically, you could make a 4-inch intake feeding a 1-inch (or even larger) throat and get it - but I'll bet neither you nor I could afford the air mover that would drive it to a high enough pressure (e.g. something like a big leaf blower wouldn't even come close).

I think making a finely-crafted lathe-turned tiny supersonic ramjet driven by an ordinary air compressor would be a fine project. Probably NOT for beginning machinists. Might take acetylene or MAPP to run it. Imagine posting a photo of a tiny pencil of blue flame with Mach diamonds laid out like a little string of pearls!

L Cottrill

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re: deLaval Nozzles for Small Ramjets?

Post by Greg O'Bryant » Wed Feb 22, 2006 12:31 am

Larry;
I saw on a web page once a guy who made a jet engine with an industrial sized air compressor. The ones that you would power a jack hammer with. It had very defined mach diamonds he said he was going to use it for snow and ice removal. Any way about my question I quess I could sum it by asking at what piont does a supersonic ram inlet start compressing air? At below supersonic a convergent nozle should increase the speed of the flow until it reaches the speed of sound and then start compressing even though the jet is going at a speed lower than sound?

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Re: re: deLaval Nozzles for Small Ramjets?

Post by larry cottrill » Wed Feb 22, 2006 1:56 pm

Greg O'Bryant wrote:Larry;
I saw on a web page once a guy who made a jet engine with an industrial sized air compressor. The ones that you would power a jack hammer with. It had very defined mach diamonds he said he was going to use it for snow and ice removal.
Yes, I remember that one. His "engine" was a combustion chamber scavenged from a big turbojet, and man, that DID put out the stuff! The compressor it took to drive it was about half the size of my Voyager minivan, though.
Any way about my question I quess I could sum it by asking at what piont does a supersonic ram inlet start compressing air? At below supersonic a convergent nozle should increase the speed of the flow until it reaches the speed of sound and then start compressing even though the jet is going at a speed lower than sound?
Here's how the "textbook example" works: The diffuser section consists of two basic elements, a small nozzle-like inlet (called the "supersonic diffuser") which guarantees that you get a sonic shock developed in the throat, followed by a LONG conical diffuser (the "subsonic diffuser") that leads to a much larger diameter chamber. Remember, since the air is expanding from supersonic flow, the inlet throat can be MUCH smaller than the chamber, in terms of cross-sectional area.

Here's how I would build a demo model: Come out of your air hose fitting with a fairly long straight section of parallel tubes forming a "flow straightener". Then an open straight section of the same net area, leading to your inlet nozzle (supersonic diffuser) which should be lathe turned as a very nice smooth radius "flare". The outer diameter would be small, and the throat ID should be workable at 60 or 70 percent of that, just guessing - say 8 or 10 mm rim diameter for the inlet and a 5 or 6 mm throat (unless you have a heck of a compressor). From the throat, you'd expand outward with the subsonic diffuser cone to maybe as much as 25 mm for the chamber ID, at which point you'd want a miniature flameholder. I'd make the chamber fairly long, say a few inches, to make sure you really get combustion fully developed. The fuel can come in at about the midpoint of the subsonic diffuser. CRUCIAL: The subsonic diffuser cone should have an included angle of NO MORE THAN 5 OR 6 DEGREES and the entire inlet and diffuser section should be mirror smooth inside! As for the exhaust nozzle, the throat could probably be about half the ID of the chamber. I would make several interchangeable exhaust nozzles that would thread on, so you can experiment with different designs - remember that, as I tried to show above, a deLaval nozzle will only work correctly if you can achieve a pretty high chamber pressure (significantly better than 2 atmospheres absolute) - below that, a simple convergent nozzle will be best. Note also that you will have to control BOTH air pressure and fuel flow to keep a reasonable mixture as you increase the air input. Don't expect this to amount to much until the magic moment when you actually get the shock developed in the throat of the diffuser.

That's how I would approach it. Turning the diffuser section would be a bear, with that small throat diameter and the need for perfect smoothness, but an experienced machinist could figure out the right way to do it, I'm sure. You could try it without the "flow straightener" front end, but I think providing that would greatly increase your chance of success - that part should have the same cross-sectional area as your inlet rim (I don't think I made that clear before).

L Cottrill

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