Not so simple

1fastneonrt · Post by **1fastneonrt** » Wed Feb 13, 2008 8:52 am

Hello every one on the forum. Well I am over here beating myself up and just maybe someone can shed some light. After my recent fwe build, like my usual self, I began to wonder. Now I know how a pulsejet works in it's general ways, but I always dig deeper into the subject. I am very curious on how you can get an "acoustically correct" engine with no previous plans. Now I know that there are no definite calculations and or documentation but there’s got to be some kind of path to follow. I am kinda figuring that the volumes of certain parts of an engine play a role. Now I just can't stop there guys.
Even though a pulsejet looks very simple and requires not much time to construct, it's basics on how and why it runs is very difficult. These basics are what I am interested in. I have look at some areas of interest but am not totally clear on the subject. Some of the stuff I have looked into is:
1)sound waves
2)acoustics (in general)
3)tube resonance
4)fluid dynamics
I would like to know how these engines really work and in the future not have to follow any pre-built plans. I would like to help some of you guys design a more efficient and powerful engine in the future if all possible.
So with that said, is there any recommended reading material that you guys know of.
I know, I know search the forum, which I have and found some useful info. Mr. Larry Cottrill you have got some nice information out there....
Sorry not trying to single out any one else.
I have got a special interest in pulsejets that just makes me....ah happy
I just have the need to know more.
Thanks

Al Belli · Post by **Al Belli** » Wed Feb 13, 2008 3:08 pm

Hi 1fastneonrt,
SEE MY 02/17 POST FOR ADDRESSES----
Here is My collection of theoretical papers on ( mostly ) valved pulsejets for Your information.

I will send this to You when You send Me Your E-mail address.

About 100 Mb.

Al Belli

larry cottrill · Post by **larry cottrill** » Wed Feb 13, 2008 6:31 pm

1fastneonrt wrote:These basics are what I am interested in. I have look at some areas of interest but am not totally clear on the subject. Some of the stuff I have looked into is:
1)sound waves
2)acoustics (in general)
3)tube resonance
4)fluid dynamics
I would like to know how these engines really work and in the future not have to follow any pre-built plans.

fast one -

If it was really simple, everyone would be doing it ;-)

The problem with discussions of 1), 2) and 3) is that they are usually illustrated with examples where the fluid temperature never varies. This results in oversimplified "rules of thumb" such as: "Frequency is determined by length." With pulsejets, the inner workings are highly dynamic because the internal temperatures vary wildly over very brief periods of time. I might even claim that the temperature variations are what make a pulsejet a pulsejet (just as you might claim that the temperature RISE is what makes a ramjet a ramjet - ha).

In particular, the valveless pulsejet has to do two basic functions to set up each cycle:
1. Load an inert mass to use as a "bullet" to fire - this mass is usually called the "tailpipe piston mass" or just "tailpipe piston" or "tail piston".
2. Load a fresh mixture of fuel and air to form up the firing charge to propel the piston.
Once these are accomplished, it's just a matter of setting off the charge. But, of course, these two actions must be tightly coordinated in time - on a small engine, we only have a couple of thousandths of a second to get this done.

There are quasi-engines that simplify things further, of course. If we eliminate the tail piston altogether, we have a "jam jar" combustor. If we try to accomplish both the breathing and the tail piston formation in a single duct, we have a "snorkeler" (Mark's original term, I think). It is probable that neither of these is efficient enough for full-scale propulsion, though.

In the full-blown valveless pulsejet, the two functions are separated into two sections of the engine - however, these two sections merge into each other so there is only a functional separation, not a geometrical one. The station where these two sections merge is called the "pressure antinode" or "velocity node" (implying that it is the locus where the air stops moving!). I have arbitrarily named the larger section (which holds the tail piston) the "mortar" and the smaller section (which breathes in the explosive charge) the "flask". And yes, the volumes are highly important in understanding the difference.

The mortar is characterized by the fact that its volume and total mass is so large that it is not fully cleared by the explosive blast. This does NOT mean that Kadenacy action (breathing in after initial mass ejection) is absent, merely that it is not prolonged enough to fully purge the duct or to subsequently recharge the entire duct, including the explosion zone. The flask is characterized by complete (or very nearly complete) clearing of the blast mass by the explosion energy. Basic engine resonance is achieved when the fundamental frequency of the flask equals or very closely matches the fundamental frequency of the flask, with the actual internal temperature swings normally attained in the cycle. That last bit, in bold, is important.

Both the flask and mortar undergo temperature swings during the cycle, and both achieve reasonable gas speeds at the "far end" (the end opposite the explosion zone). The speeds achieved by the flask are MUCH higher than those achieved in the mortar, however. This is because the masses moved are much smaller in the flask.

The flask is like one of Mark's snorkelers: A large belly that maintains a hot range of temperatures with a neck that is generally quite cool because of the total displacement of explosion gas and replacement by outside air (with fuel mixed in, of course). This means that the pressure wave from the blast traverses the neck (i.e. the intake stack of our engine) quite slowly, even though the mass velocity swings are quite large. This slow movement of the pressure wave is why the intake pipe is so much shorter than the tailpipe of our engine to be perfectly in tune with it.

The mortar, on the other hand, is always fairly hot throughout, though NOT uniformly so. It is cooled only moderately by the outdoor air it takes in, because the blast mass it handles is not anywhere near completely purged. So, when in-breathing occurs, it is a mixing action, not a total mass replacement. Because of this, the wave velocity is quite rapid (on the order of three times as fast as in the flask neck). However, the mass velocity is relatively slow. It can be shown that in a typical pulsejet it is IMPOSSIBLE for a molecule to travel from the explosion zone out through the end of the tailpipe in a single cycle - the velocity is far too low!

So, to summarize in engine terms, we have an intake that runs relatively cool with very fast gas mass velocities and very slow wave velocities, and a tailpipe which runs relatively hot with relatively slow gas velocities and very fast wave velocities, connected by a chamber that stays very hot and has fairly high pressure swings and low internal gas velocities. Viewed in the light of my two-function simplification, we have a mortar consisting of a tailpipe duct and part of the chamber, and a flask consisting of the intake duct and a (usually) smaller part of the chamber.

Under the conditions just described, the axiom that "length determines frequency" is only crudely true. What actually determines operating frequency of either the mortar or the flask is the set of conditions that create Kadenacy action. These are the internal gas masses in different parts of the structure, and the impedance of the channel by which these masses get in and out. Going back to the jam jar for a moment, these conditions are fairly simple: we have a certain volume (meaning a certain mass for a given temperature) in the jar, and a port impedance determined entirely by the diameter of the hole (we are assuming that the thickness of the lid is insignificant). Under these conditions, it will be found that the larger the hole (i.e. the lower the impedance), the higher the frequency - without changing the length of ANYTHING at all! We would also find that a BIGGER jar with the SAME SIZE PORT will have a lower frequency. So, we can say in general, that:
The natural frequency of a Kadenacy driven system is roughly INVERSELY proportional to the contained mass, AND is roughly INVERSELY proportional to the port impedance.
This is simple for the jam jar because impedance is practically determined by ONLY the port diameter.

In the mortar and flask, things are not so simple, because impedance itself becomes more complex. The impedance increases with the length of the neck (crudely speaking, due to "drag", though this is too easy an explanation) AND with the the gas mass contained within the neck (the mass has "inertia" and it takes some energy to accelerate it). So, we see that, yes, ALL the volumes have something to do with the natural frequency that will be attained by Kadenacy action, either in the flask (where the neck volume is fully replaced) or in the mortar (where only partial replacement occurs). In terms of engine tuning, changes in diameter are FAR more powerful than length changes of the same amount (though it has to be admitted that changing length is usually much easier to do ;-). Because of the temperature difference, changes in the intake length are about three times as effective as the same change taken at the tail end.

If we design and construct the flask section and the mortar section correctly, we will get the tight coordination of their independent frequencies and achieve the overall resonance needed for the engine to cycle, assuming the required energy input (flammable fuel). I have sometimes argued that the action is crudely similar to a radio oscillator followed by a resonant amplifier (or perhaps better, an oscillator connected to a properly tuned antenna).

These are "the basics" as I perceive them. My approach is entirely non-mathematical, of course -- I coerce things like UFLOW1D to do all the math for me, and would be lost (in terms of actual design) without such a tool. Graham's re-work of NUDiS is even better.

L Cottrill

1fastneonrt · Post by **1fastneonrt** » Fri Feb 15, 2008 8:28 am

Alrigthy, Mr. Al Belli I have sent you a pm with my e-mail attached, thank you.
Larry I have briefly read through your post and understood some of it. I will have to take some time and read it fully so I can get a better understanding. On the short note though I am an amateur radio operator with a full understanding on impedance matching, so some of my thought on how the "flask" (intake) and "mortar" (exhaust if I understand it right) work are in close speculations. It kind of coincides with your tuned antenna theory

My thinking was somewhat right on that the large mass of air in the exhaust pipe is too much for it to completely clear and that the mass in the intake is cleared almost completely. So being that the intake is shorter and the masses of gases move through at a high velocity is why the intake does not get red hot. Also being that the exhaust has slow moving gases (per say) with a high wave velocity that is in tune with the intake is what makes the engine run. I do kind of understand the temperature differential. These engines are more or less made under-sized to say. It's only when they are warmed up that they tend to be more "in tuned"(objects expand when there heated). Which is why trying to get one to run in the cold is a major pain. Please please correct me if I'm wrong.
I am highly fascinated with pulsejets and you can never have enough info.
I really appreciate your help. Thank you

larry cottrill · Post by **larry cottrill** » Fri Feb 15, 2008 1:21 pm

1fastneonrt wrote: My thinking was somewhat right on that the large mass of air in the exhaust pipe is too much for it to completely clear and that the mass in the intake is cleared almost completely. So being that the intake is shorter and the masses of gases move through at a high velocity is why the intake does not get red hot.

I believe that to be exactly the case.

Also being that the exhaust has slow moving gases (per say) with a high wave velocity that is in tune with the intake is what makes the engine run.

I believe that to be the most basic explanation of valveless pulsejet operation. Establishing the temperature difference is essential for resonance to be achieved.

I do kind of understand the temperature differential. These engines are more or less made under-sized to say. It's only when they are warmed up that they tend to be more "in tuned"(objects expand when there heated). Which is why trying to get one to run in the cold is a major pain. Please please correct me if I'm wrong.

Sure thing -- you are wrong. However, this is not meant as a severe criticism, because this part happens to be somewhat counterintuitive.

It is generally observed that the pulsejet increases significantly in operating frequency during the first few seconds of operation. This has practically nothing to do with the thermal expansion of the metal structure. If it did, the frequency would drift lower (because the pipe is lengthening), not higher. In actual fact, a red-hot pulsejet of ordinary size is only a few mm longer than the same engine dead cold - that would be a TINY fraction of the wavelength. No, the situation is more complicated:

The engine metal acts as a "black body absorber" or "black body radiator", which is to say that the color and intensity of all radiation coming from it is determined entirely by its temperature. Unlike a solid metal bar, however, the pulsejet shell radiates internally as well as externally. As it absorbs heat from the burning gases, its temperature rises until an equilibrium value is reached where it is hot enough to radiate away all the heat it is absorbing. A little of this is visible light, but most of it is infrared radiation (radiant heat). Again, part of this radiation is internal, and that part represents the engine "giving back" absorbed energy to the gas stream from which all the heat was absorbed.

So, what happens in the first half minute or so of your engine running is that at first there is a large absorption of heat from the combustion gases with very ineffective radiation. Another way of saying this is that the metal is fairly effectively cooling the moving gas while it is itself collecting heat. In response to this, the steel temperature rises with a consequent increase in black body radiation, which begins to cause heat to be given back to the contained gases -- in effect, the absorption of heat from the gases to the steel becomes less efficient, and the average gas temperature rises. The rate of energy loss by radiation is highly nonlinear with temperature, so as the steel temperature rises, the re-radiation into the traveling gases becomes more and more effective until the point of equilibrium is reached (at this point, the steel is still MUCH cooler than the gas temps!). Because this represents less and less effective cooling of the combustion gases, the average gas temperature in the tailpipe increases over this time frame, and so the wave speed increases, causing the increase in running frequency for the same quarter wavelength mortar.

This effect CAN be so significant that an engine won't run when it heats up. This only means that the mortar is a little short for the flask. An opposite effect is possible: an engine that only sustains after the shell has been warmed up significantly by forced air running (and/or, by artificially heating the shell from the outside). That means that the mortar is a little long for the flask. In essence, heating of the engine "shortens" the tailpipe acoustically (by which I simply mean that it moves toward higher frequency operation).

A practical engine will be almost perfectly tuned when it is fully hot, and "close enough" when it's cold, for startup. An engine that starts right up cold and keeps running is a perfect compromise between hot and cold tuning. It is possible that optimum tuning for high performance might carry with it the price of difficult starting or (more likely) the need for a few seconds of warmup under forced air. But, this is also true of many automotive engines tuned for racing!

Strangely, outside air temperature has little theoretical bearing on this, even though it does to some degree affect how well the engine shell manages heat absorption and re-radiation. The reason it has little effect is that the whole range of outdoor air temperature variation is such a small fraction of the combustion gas absolute temperature. An engine that would not start "out in the cold" due to resonance effects would be a very highly tuned beast, indeed! The problems with cold starting are MUCH more due to things like the difficulty of getting adequate delivery pressure in fuel gases such as propane, or the difficulty of getting good vaporization of liquid fuels such as gasoline (petrol). In other words, the same problems you have with cold starting any internal combustion engine.

Eric Beck, with his careful methods and vast experience, has been able to start his stainless engines in sub-freezing temperatures using propane, and you should be able to hunt out some of his posts from winter a couple of years ago to see pics and videos of this.

L Cottrill

Al Belli · Post by **Al Belli** » Sun Feb 17, 2008 6:50 pm

Post by **Mike Everman** » Tue Feb 19, 2008 4:22 am

thanks so much Al,
I highly recommend the SAE doc and the tm1131

1fastneonrt · Post by **1fastneonrt** » Tue Feb 19, 2008 7:54 am

The engine metal acts as a "black body absorber" or "black body radiator", which is to say that the color and intensity of all radiation coming from it is determined entirely by its temperature. Unlike a solid metal bar, however, the pulsejet shell radiates internally as well as externally. As it absorbs heat from the burning gases, its temperature rises until an equilibrium value is reached where it is hot enough to radiate away all the heat it is absorbing. A little of this is visible light, but most of it is infrared radiation (radiant heat). Again, part of this radiation is internal, and that part represents the engine "giving back" absorbed energy to the gas stream from which all the heat was absorbed.

Oh O.K. your right I see now. It's kind of like a "give-take" situation, were there’s a temperature exchange between the combusted gases and the body of the pulsejet. This is kind of why the body doesn't go into meltdown mode. Can this also be in conjunction with the barrier layer as well?

I have notice something that I think follows the "black body radiator" effect. I have been experimenting with liquid fueling on my pulsejet. I first start it with propane and let the engine warm up a bit (red hot). I have noticed that when I start feeding the liquid fuel I get random cold spots on the body for a few seconds. Now when I say cold spots I mean that the redness of the body fades away (darkens). I can guess at first the air/fuel ratio is a little too rich, cooling the combustion, thus taking some heat away from the body. Second is that the fuel is cooler than the air inside the combustion chamber, thus cooling the combustion and so on.
After every thing is heated back up, the heat from the combustion heats the body back up.

A practical engine will be almost perfectly tuned when it is fully hot, and "close enough" when it's cold, for startup. An engine that starts right up cold and keeps running is a perfect compromise between hot and cold tuning. It is possible that optimum tuning for high performance might carry with it the price of difficult starting or (more likely) the need for a few seconds of warmup under forced air. But, this is also true of many automotive engines tuned for racing!

So the difficulty in cold starting (certain engines) is basically the fact that the heat first generated by the combustion is absorbed into the body and away from the gases. Of course after ruling out fuel related problems with the cold. Thus changing the air and wave velocities in the exhaust tube (just a guess).

Strangely, outside air temperature has little theoretical bearing on this, even though it does to some degree affect how well the engine shell manages heat absorption and re-radiation. The reason it has little effect is that the whole range of outdoor air temperature variation is such a small fraction of the combustion gas absolute temperature

Right, because the center of the combustion is greatly higher in temperature.

O.K. things are making sense

Also one quick thought; could you compare a pulsejet to a Helmholtz Resonator.

Ohhh and before I forget - Thank you Al for the reading material

larry cottrill · Post by **larry cottrill** » Tue Feb 19, 2008 1:09 pm

1fastneonrt wrote:Oh O.K. your right I see now. It's kind of like a "give-take" situation, were there’s a temperature exchange between the combusted gases and the body of the pulsejet. This is kind of why the body doesn't go into meltdown mode. Can this also be in conjunction with the barrier layer as well?

The boundary layer must have some effect; I'm not sure exactly what. Probably a minor issue, though, since its mass is such a small part of the whole.

I have notice something that I think follows the "black body radiator" effect. I have been experimenting with liquid fueling on my pulsejet. I first start it with propane and let the engine warm up a bit (red hot). I have noticed that when I start feeding the liquid fuel I get random cold spots on the body for a few seconds. Now when I say cold spots I mean that the redness of the body fades away (darkens). I can guess at first the air/fuel ratio is a little too rich, cooling the combustion, thus taking some heat away from the body. Second is that the fuel is cooler than the air inside the combustion chamber, thus cooling the combustion and so on.
After every thing is heated back up, the heat from the combustion heats the body back up.

Seems like a reasonable hypothesis. Mark and others have noted the pronounced cooling effects of liquid fuels, but mostly pertaining to the front ends of valved engines (where the fuel undergoes carburetion). I for one would be very interested in the details of your liquid fueling.

So the difficulty in cold starting (certain engines) is basically the fact that the heat first generated by the combustion is absorbed into the body and away from the gases. Of course after ruling out fuel related problems with the cold. Thus changing the air and wave velocities in the exhaust tube (just a guess).

Exactly. The wave velocity is very sensitive to temperature and density. Fortunately, it usually just takes a few good bangs to get the average temps inside the pipe quite a bit higher than the air we started out with. The wave speed will always increase with an increase in temperature and a reduction in density. We must always keep in mind that in a running engine, the temperatures are never the same throughout the whole pipe, and in fact are rapidly changing throughout the cycle at every point (with the possible exception of the velocity node itself).

Strangely, outside air temperature has little theoretical bearing on this, even though it does to some degree affect how well the engine shell manages heat absorption and re-radiation. The reason it has little effect is that the whole range of outdoor air temperature variation is such a small fraction of the combustion gas absolute temperature
Right, because the center of the combustion is greatly higher in temperature.

For most of us, our whole lives are spent within a total air temperature range of about 120 degF (or about 70 degC) -- that range of variation pales in comparison to the hundreds of degrees represented by red hot metal or the thousands represented by blue-hot combustion gas. On the other hand, things like liquid flow, evaporation and droplet formation are profoundly affected by quite ordinary changes in temperature.

O.K. things are making sense :D

Ah ... now you're really in trouble ;-)

Also one quick thought; could you compare a pulsejet to a Helmholtz Resonator.

Purists will quibble (it's what they do best, after all) but yes, I view the mortar and the flask as rather awkward Helmholtz resonators with full acoustic coupling. Ha -- take that, purists! Also, in our case, we apply a fairly severe perversion by adding heat at a certain point in every cycle. So, it is a kind of "forced" Helmholtz action, or we might call it a "boosted" Kadenacy action. Of course, you could argue that the Helmholtz chambers in an engine muffler are "forced" in a different way (by the pulsations of exhaust gas pressure), so the concept is not completely pulsejet-specific.

In the pulsejet, there is a kind of "symbiosis" between the natural oscillation and the application of combustion energy. At first glance, we might think of it as combustion keeping the oscillation going -- but it is also the natural oscillation of the system keeping combustion going. Another "balanced asymmetry" like the relationship between mortar and flask.

1fastneonrt · Post by **1fastneonrt** » Wed Feb 20, 2008 7:18 am

The boundary layer must have some effect; I'm not sure exactly what. Probably a minor issue, though, since its mass is such a small part of the whole.

I agree but it might also effect starting a bit. At first the boundary layer is not prevalent, thus effecting air and wave velocities as well. Also, to thick and or to thin of a layer during running might have the same effect. Although the effect might be so minuscule that it wouldn't bother the engine at all (depending on the size of the engine).

Seems like a reasonable hypothesis. Mark and others have noted the pronounced cooling effects of liquid fuels, but mostly pertaining to the front ends of valved engines (where the fuel undergoes carburetion). I for one would be very interested in the details of your liquid fueling.

I will have to post up some pictures of my fuel set-up. If you’re interested as well, I will try to get a video of what my engine does during the change over.

Ah ... now you're really in trouble

Yeah I know! LOL

In the pulsejet, there is a kind of "symbiosis" between the natural oscillation and the application of combustion energy. At first glance, we might think of it as combustion keeping the oscillation going -- but it is also the natural oscillation of the system keeping combustion going. Another "balanced asymmetry" like the relationship between mortar and flask.

O.k. I see and I seem to view blowing over the top of a pop bottle as a kind of forced Helmholtz action as well. I guess you can slightly compare that to a jam jar if you wanted too.
You had mentioned chambers in a muffler, which the thought of a muffler lead me to my next idea. I know that a pulsejet is somewhat of an acoustical device and much more. I would like to experiment a little with a sort of tuned baffling system either in the C.C. or the exhaust tube. I guess it would be to see if you could change wave velocities and manage them a little better. Kind of like an antenna tuner on the back of a radio. You might be able to achieve more thrust, a quieter operation or a non-operational engine.

Just an idea...

larry cottrill · Post by **larry cottrill** » Wed Feb 20, 2008 2:07 pm

1fastneonrt wrote:You had mentioned chambers in a muffler, which the thought of a muffler lead me to my next idea. I know that a pulsejet is somewhat of an acoustical device and much more.

Strike "somewhat" -- it is FUNDAMENTALLY an acoustical device. This can be easy to miss, simply because of what we want to do with it; i.e. its raison d'etre is usually (at least for us) mechanical propulsion.

The pulsejet (and especially its valveless form) is what it is because of the acoustic wave properties of air. Beginners often mistakenly try to divorce the pressure wave action from the movement of air mass, but this is an impossible disconnect. There is NO motion of air in the pipe that is not caused and directed by the pressure change that travels through the pipe as a "wave". The valveless pulsejet is only different from an organ pipe in two ways: The driving energy is much higher, and the temperature (and hence, density) differences are much greater. That is all.

Wave motion is the natural mode of pressure change in air. It is that way in a pulsejet, an organ pipe, a blown soda bottle, and even in a fully closed steel tank that's being gradually filled by a compressor.

I would like to experiment a little with a sort of tuned baffling system either in the C.C. or the exhaust tube. I guess it would be to see if you could change wave velocities and manage them a little better. Kind of like an antenna tuner on the back of a radio.

Many of us have thought of this as a very interesting study. But, none of us have gotten around to doing anything about it, if I am remembering rightly. I say, go for it! I know that Bruno has posted some information from a research paper on this very topic, but it's buried in a thread somewhere and I haven't been able to ferret it out.

You might be able to achieve more thrust, a quieter operation or a non-operational engine. :-)

Probably the latter. An awful lot depends on exactly where you place it, i.e. at what point you tap into the main engine duct. That is just one of the reasons the stuff Bruno posted would be useful. For noise reduction to be effective, it might take multiple chambers of different sizes, tuned to at least the two or three strongest harmonic frequencies. The "sharpness" of the explosive sound is largely from the "ringing" of the pipe with these higher frequencies, I think (speculating here).

L Cottrill

larry cottrill · Post by **larry cottrill** » Fri Feb 22, 2008 12:40 pm

larry cottrill wrote:
I would like to experiment a little with a sort of tuned baffling system either in the C.C. or the exhaust tube. I guess it would be to see if you could change wave velocities and manage them a little better. Kind of like an antenna tuner on the back of a radio.

Many of us have thought of this as a very interesting study. But, none of us have gotten around to doing anything about it, if I am remembering rightly. I say, go for it! I know that Bruno has posted some information from a research paper on this very topic, but it's buried in a thread somewhere and I haven't been able to ferret it out.

You might be able to achieve more thrust, a quieter operation or a non-operational engine. :-)
Probably the latter. An awful lot depends on exactly where you place it, i.e. at what point you tap into the main engine duct. That is just one of the reasons the stuff Bruno posted would be useful. For noise reduction to be effective, it might take multiple chambers of different sizes, tuned to at least the two or three strongest harmonic frequencies. The "sharpness" of the explosive sound is largely from the "ringing" of the pipe with these higher frequencies, I think (speculating here).

Well, I mis-spoke here. When I mentioned the stuff Buno had posted, that was strictly a study of noise suppression, not the knd of "tuning" you appear to be thinking of. Sorry, I just got side-tracked by your "muffler" reference.

Just to throw out an opinion, it's hard to imagine chambers and baffles that would have a generally positive effect. This is just based on the general observation that in pulsejets, simplicity is usually good and complications are usually bad. Of course, performance is "in the details", so why not try it? The most difficult part (I think) is that whatever you do will have to be "just right" for both directions of gas motion and for the wave and flow timing at the chosen location. Keep in mind, too, that internal structures (in the absence of some kind of forced cooling) will tend to be destroyed fairly quickly by the combustion gases, which DO contain significant unused oxygen.

Unfortunately, things like chambers and oddly located/oriented baffles are three-dimensional in nature, and one-dimensional analysis tools like UFLOW and NUDiS won't let you model them in any accurate way (except for the special case of simple "washer-like" baffles in the duct - maybe that's all you're really talking about). Particularly, these tools will fail to show real-life effects such as "dead zones" and turbulence.

L Cottrill

1fastneonrt · Post by **1fastneonrt** » Wed Feb 27, 2008 9:58 am

Ahhh finally have a chance to get back on here.

O.K. Larry I do know now that a pulsejet is an acoustical device. Some things threw me off at first but I am on track now. I always seem to think it was an acoustical device but my mind began to wonder.

Just to throw out an opinion, it's hard to imagine chambers and baffles that would have a generally positive effect. This is just based on the general observation that in pulsejets, simplicity is usually good and complications are usually bad.

I see your point of view and the more complicated things get the more difficult the problems are to solve. I seem to think though that if you can some how carefully and accurately reshape the rarefaction waves or possibly the compression (pressure) waves you might be able to obtain more inward flow of air or more thrust. I think that if you could reshape a rarefaction wave and make it longer or shorter while keeping all of the other waves the same length you might be able to gain from it. Now my terminology of baffles and chambers might be a little off. You might be able to use a sloping type "baffle" (half heart shape) either in the combustion chamber or tailpipe. Another thing you can do is incorporate a type of resonate chamber within' the combustion chamber utilizing a kind of baffling system.
Now it might come down to reshaping the design of the engine. Utilizing correctively placed inward bumps on the C.C. or tailpipe. We can even try a wavy tailpipe or a slight spiral. Now correct volumes and areas will have to be maintained but shouldn't be too much of a problem.
These are all the things that have gone through my head and I know I am not an expert but purely an amateur learning the ropes still.

I know that most of what I have written above is probably not correct and is far fetched but at least I am trying.

I am sorry for rambling on and I thank you for putting up with my on going learning and questions.

www.pulse-jets.com

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