Acoustic length

[dwayne247]

Usually in pulse jets, the chamber isn't of equal diameter through out. Its broader at the CC and narrow at the tail pipe. Does this effect the acoustic length of the engine? or would the acoustic length be the same if the diameter was the same throughout. (in other words do changes in the volume of the engine effect its acoustic length)

[paul fellows]

not as far as i am know
i posted this viewtopic.php?f=3&t=5747#p69585 a while ago and on one as corected it
how ever you realy need the advice of a practiced builder
good luck

[larry cottrill]

That's a neat question. I'll make a guess, since no one else has spoken to it. (This will probably be wrong ;-)

If we had the super simple case of a pipe that was full of stationary air of constant temperature and density (like an organ pipe at ordinary temperature), I think we would have no reason to think that acoustic length would depend on oddities of shape. However, no pulsejet is that simple in real life.

In a running pulsejet, the heated air is always moving, at least a little, in different regions of the pipe. The temperature, pressure and density of a moving gas are all affected by passage through larger and smaller diameters. The pulsejet is more complex than a ramjet in this aspect, since even massflow varies from one section to another (in a ramjet the massflow is constant throughout, even though the speed varies greatly). So, in the case of the pulsejet, I would say that section area and other geometric variables will have a definite effect.

I'm no acoustic engineer, but I think the accurate calculation of acoustic length for a pulsejet must be a pretty complex business, because of the wildly changing conditions in the pipe during even a single cycle.

L Cottrill

[dwayne247]

thank you Paul and Larry

the reason this topic came to my mind is because of a doubt i have about scaling PJ plans.

Larry, taking your FWE PJ, when its running at certain frequency , it is resonating at a certain wavelength. The engine now is running efficiently, delivering good thrust.
Now if i were to scale up the FWE plans, I understand from what you said about the 'section area and other geometric variables will have a definite effect', that the engine will not run efficiently or may not start at all. The acoustics will also go off (atleast thats what i read will happen if i scale up or scale down engine plans).
But shouldn't the scaled up version of the engine have its own natural resonating wavelength that it will run at when started up? Why will the acoustics go off? The engine should just resonate at a new frequency and be as efficient as the original.

This doubt has been bugging my mind and it would be really nice if it could be cleared.

Thank you.

[hinote]

I'm no acoustic engineer, but I think the accurate calculation of acoustic length for a pulsejet must be a pretty complex business, because of the wildly changing conditions in the pipe during even a single cycle.

Larry:

You've nailed it pretty good there.

Dwayne:

If you're capable of asking questions like you are, you should also be capable of finding some of the answers.

Take a look at this: http://hyperphysics.phy-astr.gsu.edu/hb ... pe.html#c1

if you plug in some temp values you'll find a surprising range of velocities will be the result. Now, if you look at a good-running pulsejet you'll see a huge range of colors (indicating heat). The overall resonance of the engine is only the final result, with sound changing velocities all over the place. I've (again!) included a photo of one of my better running projects to illustrate the wide range of heat which can be observed. (BTW modern digital cams generally exaggerate the color, due to excess IR sensitivity)

BTW if you google around you'll find a site which will allow you to estimate the temp of the metal based on its observed color.

One of our more active members from the past proposed a value which he termed the acoustic temp; instead of trying to estimate the temp, the formula in the hyperphysics site above is turned around to solve for T instead of V. The assumption is that if 2 engines have the same length, the one with the higher operating frequency is achieving an overall higher operating temp. This sidesteps the issue of individual components with their attendant temps but is a useful tool for comparison purposes.

Bottom line is, there's a lot of calculation and iterative processes which make the modern computer invaluable for PJ design.

BTW I still think the Belfast paper is one of the best for a good description and analysis of the operation of a valveless PJ.

Bill

[larry cottrill]

But shouldn't the scaled up version of the engine have its own natural resonating wavelength that it will run at when started up? Why will the acoustics go off? The engine should just resonate at a new frequency and be as efficient as the original.

It is a kind of pulsejet truism that a conventional valveless engine (especially a straight tailpipe engine like the early FWE engines) can be scaled UP and get you a running engine, IF every dimension (both lengths AND IDs) are scaled carefully. However, this is largely because big engines are much more forgiving of small construction errors than small ones. It is not a matter of "efficiency" (a term which has no single standard definition in jet work) but of whether the engine will run optimally (i.e. in well-tuned fashion). Scaling a good-running big engine DOWN is almost hopeless, because the tiniest variance from scale proportions will throw it just enough out of tune that it won't sync up and run.

Yes, the big engine will have a new (lower) frequency than its smaller counterpart. Contrary to popular belief, the frequency is NOT simply determined by length. It is determined by chamber volume, tailpipe and intake pipe impedances, and the internal operating temperatures achieved (which are usually higher in large engines than small ones) -- and to a much lesser degree other factors (such as internal surface smoothness).

Proper tuning is a balance between the impedance effects of the tailpipe and intake. Their respective areas determine how much of the chamber contents will be carried by each, and the temperatures at which they operate will NOT proportion out the same in two versions at vastly different scales. Thus, a perfectly balanced design half a meter long will probably not be perfectly balanced even if perfectly scaled up to one meter long.

One of the problems for beginners is that the valveless engine is mechanically so simple, they naturally expect the internal operation to be simple and the design straightforward. But you are dealing with the chaos of chemical explosions and all the variables that this entails. Appearances are deceiving, and heated air does not always behave the way we would like it to.

L Cottrill

[nickwilson]

I am down sizeing a PJ i made , I am almost done we will see if it will run, If not it will look cool. I will post pics tomorrow if i can

[Viv]

That's two very good posts from Bill and Larry on this subject.

Viv

[larry cottrill]

Thanks, Viv! Good to hear from you.

I am down sizeing a PJ i made , I am almost done we will see if it will run, If not it will look cool. I will post pics tomorrow if i can

Nick -

Good luck! The best that can happen is that you will be VERY fortunate and have a running motor. If not, I'm sure you'll find that some slight adjustment will bring it in. I am not saying that a scale-down will always be grossly wrong. Usually, it will just be a little bit off from perfect proportions. The problem is that with a small engine, a little bit means a lot -- usually enough to keep the engine from starting. Often, just a little tailpipe length adjustment is all that's required. It depends a lot on what design you're scaling, though.

L Cottrill

[dwayne247]

Thank you so much Larry and Hinote for actually taking the time to give me an answer. From what i see is that the acoustics aren't everything and they are highly dependable on working conditions which are highly dependable on the shape. Resulting in greatly varying performance as the engine is scaled up and down.

I put my current pulse jet construction on hold for these doubts to be cleared up. I didn't want to build something that i was confused about. I want to approach the whole pulse jet scene with a pretty firm understanding about the topic. I know many of you started out this hobby with little or no understanding of the topic but have grown to understand so much of these engines now. i really admire that.. I'm also going to do my research in this field and hopefully contribute something one day. >_<

Dwayne

[dwayne247]

One of the problems for beginners is that the valveless engine is mechanically so simple, they naturally expect the internal operation to be simple and the design straightforward. But you are dealing with the chaos of chemical explosions and all the variables that this entails. Appearances are deceiving, and heated air does not always behave the way we would like it to.

L Cottrill

How true!!

[nickwilson]

403 mm long

[metiz]

ha that one looks a lot like the M40 mark 1. If you based your design of of that I realy hope you didn't simply devided all dims by X, it won't work like that, probably

[nickwilson]

Here are all the PJ i have made, The big one is the metiz's M40, Also larry's Short Lady,The other ones are mine.They are made of several types of metal,Just started making them out of stainless, Next i am going to make Metiz's M40a out of stainless.Just waiting on money for stainless wire. They might not look like much but i like these things.