Variable frequency valveless pulsejet

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emile
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Variable frequency valveless pulsejet

Post by emile » Tue Mar 29, 2016 6:04 pm

I'm assessing the feasibility of the following:

A "valveless" pulse jet that can run at several discrete pulse-per-minute rates, in a range from 110hz to 220hz, or 220hz to 440hz, understanding that the operational frequency changes with the engine temperature. This is a boondoggle, by the way. I am building it to let people play music on a jet engine. Some people would call it "art".

It doesn't have to fly, so weight, aerodynamics, and thrust are irrelevant. I've seen some jets control frequency via fuel throttle and some via harmonic length ratio adjustments. One video in particular demonstrated the pulse frequency changing as a propane fuel line was inserted deeper into the intake pipe.

While changing the exhaust geometry hot seems challenging, I'd thought I could get some variance by changing the intake geometry with solenoid valves.

Am I barking up the wrong tree? I came to pulse jets as a solution because operating at the frequency range I want required active airflow into the combustion chamber, and this seemed like a good fit. I'd studied some "machine gun simulator" designs but they rely on solenoid valve timing and don't appear to approach 220hz.

I've seen videos of pulse detonation engines being dialed into specific, high-frequency ranges but from what I've read, a PDE is beyond my metal shop skills.

Thanks for supporting my inanity.

Mark
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Re: Variable frequency valveless pulsejet

Post by Mark » Wed Mar 30, 2016 1:27 am

He're a rudimentary attempt at something vaguely similar to what you want to do perhaps if you haven't viewed this before. It's not as dulcet as I would have liked.

Satans Calliope Burning Man 2012
https://www.youtube.com/watch?v=-zU4u7SSDKw

"The calliope is centered around a cluster of 17 acoustic pulse jets, tuned chromatically from G1 through B2. Fourteen aluminum organ pipes tuned from C3 through C#4 fan out on the sides and are blown with propane."
http://www.leonardo.info/gallery/burnin ... sking.html
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Joris
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Re: Variable frequency valveless pulsejet

Post by Joris » Wed Mar 30, 2016 7:14 am

I don't know the frequency range you would get from regulating gas throughput, in my own experience gas sure does make a difference but it would at max have an amplitude of 20 hertz, i can fiddle with mine to get a more specific range of frequencies.
Changing the exhaust/intake length might have something to it, if you made a slide mechanic however a slide may also be difficult due to the expansion of metals causing the outer tube to get stuck.
However the intake oftentimes does not expand by much, if at all, when the jet's running so you could try using a design with a non tapering intake (perhaps pyrojoe's recipe) and slip a second tube around the intake, important to note have it stik out just enough for you to have a taper which moves along whilst sliding out the tube. I'll include a rough sketch, the sliding can be done by making 4 pins onto the outer tube one on the front and back of opposite sides and attaching these to a chain to move it along.

i like to ramble so i hope you can figure out what i mean, i'm not 100% sure if it'll affect the frequency and not just flare out.
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Mark
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Re: Variable frequency valveless pulsejet

Post by Mark » Wed Mar 30, 2016 3:08 pm

Here's a couple of tidbits/crumbs that come to mind.
Of course sleeves have been used in studies to vary the length of the exhaust tube. It's not that critical to have a tight fitting sleeve I don't think. As an aside, Tharratt found that by varying the length of the exhaust tube in a single direction the thrust would increase, then decrease, and then rise back up again but not quite to the ideal thrust in one of his tests. There was another paper citing the use of a trombone exhaust but by a university student I think. Anyway here's someone toying with the effect.
https://www.youtube.com/watch?v=FOI0vAK4_4c

Here's one that burbles along and then is modulated to a somewhat higher pitch by varying the fuel flow, with a crazy long exhaust tube.
https://www.youtube.com/watch?v=iqknDswOUWA
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Joris
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Re: Variable frequency valveless pulsejet

Post by Joris » Wed Mar 30, 2016 8:38 pm

The trombone one is new to me, the exhaust tube may however not be adjustable if it does run for a longer period of time due to heat expansion.
I thought the fit would have to be somewhat tight, i'm not looking to build one myself right now, i'm mixed up in a different project but i do like the idea so i thought i'd help brainstorm. If i get time i might mess around with making a small jet with a variable intake and exhaust.

metiz
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Re: Variable frequency valveless pulsejet

Post by metiz » Wed Mar 30, 2016 9:36 pm

Your best bet is to go with a thermojet I think, because you can realy abuse those and they'll keep running.
You can change frequency by one, or a combination of, the following (some are already mentioned above):

Make the exhaust tube variable in length;
Make the intake(s) variable in length;
Make the fuel injector(s) variable in depth;
Adjust throttle;
Add nitrous oxide to your fuel injection

Any one of those will change the operating frequency of the engine to varying degrees. Getting it to change by a factor 2 is going to be a real challenge and will probably require several of the options above at the same time, if it can be done at all.
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Joris
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Re: Variable frequency valveless pulsejet

Post by Joris » Thu Mar 31, 2016 7:52 am

You could maybe attach the fuel injectors to the sleeve intakes so the injector depth and intake length vary at the same time. If you rig it right you could also have a sleeve on the exhaust that moves along with them but i don't know whether shortening the intakes and exhaust at the same time would counteract each other in some way. I can make a sketch later today to make it more clear.

Joris
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Re: Variable frequency valveless pulsejet

Post by Joris » Thu Mar 31, 2016 9:00 am

It's a sketch so it's not to scale, the chains would have to be longer and should not be turned so far that the chain attachments would reach it. The injectors are attached to the intake sleave and the intake to the outtake via the chain. the opposite gears should be linked and the axles should be mounted to the same frame as the jet via ballbearings.
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Schets meerdere bewegende delen.jpg

emile
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Re: Variable frequency valveless pulsejet

Post by emile » Fri Apr 01, 2016 7:08 pm

metiz wrote:Your best bet is to go with a thermojet I think, because you can realy abuse those and they'll keep running.
You can change frequency by one, or a combination of, the following (some are already mentioned above):

Make the exhaust tube variable in length;
Make the intake(s) variable in length;
Make the fuel injector(s) variable in depth;
Adjust throttle;
Add nitrous oxide to your fuel injection

Any one of those will change the operating frequency of the engine to varying degrees. Getting it to change by a factor 2 is going to be a real challenge and will probably require several of the options above at the same time, if it can be done at all.
I am assuming the NO injection is to raise combustion temperatures? Would lowering it with an inert gas be effective? And for throttling / injector depth, I am thinking something like a manifold with a needle valve and solenoid on each port, with a feed line hitting a different depth in the intake. That way I could adjust the injection site and fuel volume for each note

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