heat insulation

[haaken]

I am wondering if it is possible to coat the inside of a pulsejet with some kind of heat reflektive material. On large pulsejets this can possebly increase power output .

It will altso make it a lot safer to operate , because the themperature of the surface will be cooler.

[Andrew Parker]

I believe this was discussed at length in the old forum (Kenneth, what happened to the old forum archives?). IIRC, it was determind that the environment inside the tube is very hostile to most, if not all, currently existing heat insulating materials. The discussion also covered the possibilities of forming the tube out of industrial ceramic. Also, iirc, it was stated that painting the outside of a mild steel tube would extend its life by cutting down on oxidation. That is all I can remember. Hopefully, an expert will chime in.


Andrew Parker

[Greg O'Bryant]

This is an old post, but I was having some ideas along similar lines. This is all just an idea but I want to know what your inputs are on this subject. I have in mind a pulse jet that is cast out of the material that they make the tiles out of on the space shuttle. Imagine a big block of ceramic that has a cavity cast in it's center that is a perfectly shaped lockwood or other respectable pulse jet shape. Also imagine that it doesn't crack or the core doesn't distort or break down, what kind of efect do you think it would have on the engine? Even the smallest pulse jet looks like it throughs off at least a couple thousand watts of power in the from of heat. I can't help but think about how much power is lost as the gases slow down heating up the tube. When you watch a pulse jet start up it will heat up in the CC first and then the hot zone will progress down the tail pipe until it reaches a happy medium of radient heat and power loss. So I was thinking that in an engine, like the one I'm describing, the tail pipe and CC would quickly heat up, on the inside, until there was no where for the heat to go but out the tail pipe to add thrust. It should work good with a thrust augmenter and the tile material is actually pretty light. Once you got the cast made it should be pretty easy to mass produce them. That is if it works good.

[Bruno Ogorelec]

Even the smallest pulse jet looks like it throughs off at least a couple thousand watts of power in the from of heat. I can't help but think about how much power is lost as the gases slow down heating up the tube. When you watch a pulse jet start up it will heat up in the CC first and then the hot zone will progress down the tail pipe until it reaches a happy medium of radient heat and power loss. So I was thinking that in an engine, like the one I'm describing, the tail pipe and CC would quickly heat up, on the inside, until there was no where for the heat to go but out the tail pipe to add thrust.

Greg, I've often thought about a well-insulated pulsejet (like a thermos, for instance) and it might be a very interesting experiment. However, it need not necessarily work at all, much less work better than the usual pulsejet. The pesky tube is a very complex thing despite looking so simple. It could well be that it needs to shed heat in order to maintain the working cycle.

Reynst called it ‘thermal breathing’ – heating the gas causes it to expand (and the engine to ‘exhale’) while the cooling of the gas due to convection of heat to the cooler chamber walls leads to contraction, and the engine ‘inhales’. This process helps maintain the cycle. If you stop it by preventing heat convection, you may stop the cycle.

I think it should be tried, however. The easiest way to do it would be to purchase some 'exhaust wrap' (silica-impregnated tape) used by engine tuning shops and wrap up a proven working pulsejet to see what happens.

What just might happen is meltdown, so inconel or ceramics or something might be necessary for the thing to work (if it does work without heat convection).

My personal conviction is that we should encourage heat convection by means of cooling fins, and transfer the heat to passing air in a profiled ramjet duct. (See the engine cooling setup in Lockheed P-38 Lightning for a good example.) It complicates things in terms of weight, bulk and cost but might be worth it in terms of performance and fuel economy.

[larry cottrill]

Even the smallest pulse jet looks like it throughs off at least a couple thousand watts of power in the from of heat. I can't help but think about how much power is lost as the gases slow down heating up the tube.

Greg -

Back on the old forums, when I was complaining one time about the wimpiness of my newly acquired leaf blower, someone calculated for me that the total power input to my Dynajet was something like 87,500 watts! Almost all of this is lost as radiated heat, of course - very little of it goes into motive power.

If anyone thinks such a value must be an exaggeration, compare this to the red-hot ribbon inside an ordinary kitchen toaster - probably far less than an ounce of solid material running red hot, while converting 1000 to 1500 watts of power to radiant heat plus a tiny amount of visible light.

I have also thought of Bruno's suggestion of an external shroud providing 'thermal ramjet' thrust. It happens that a Dynajet shape inside a simple long cone frustrum is almost perfect for this. The cone would be sized for a very close fit around the rear of the chamber [say a gap of 3 to 5 mm] and a fairly close fit at the rear of the exhaust pipe. This would form a convergent intake at the front [around the head and chamber], nozzling into an expansion space right around the nozzle zone of the engine, exactly where it runs hottest. If the tail end of the cone extends just slightly past the tail end flare of the engine, the exhaust flow and Kadenacy effect at the tail of the engine will help pull the air through, so it will work even in static running! The tricky part will be getting the spacing and overhang just right at the tail end, for optimum efficiency.

I have always been astounded that nobody [that I know of] has bothered to try this on a flight engine! [Of course, I have owned and run a Dynajet for 40 years and I've never tried it, either.] It seems like an absolute shoo-in for improved performance. Bruno has pointed out one potential serious [or fatal] flaw: The outer shroud will itself form a resonant pipe, which may not work well with the resonance that keeps the engine going! However, it seems to me that tuning the exact length of the shroud at the front end could take care of that difficulty. Another practical problem is the inherent difficulty of mounting the engine once you've got it bottled up inside another flimsy tube, while maintaining streamlined flow throughout the space.

I DO know of one application where a shroud has been used effectively around a pulsejet: in some of the Dynafog jet insect foggers. I would speculate that this serves not only as a cooling shroud protecting the surrounding structure from radiated heat but also effectively augments the total machine air flow as well, for better dispersion of the insecticide. Only Dynajetjerry knows for sure ...

Ah ... I just thought of another potential problem. The shroud will introduce some very hot air into the tailpipe during the Kadenacy breathing of the engine. This could impair performance considerably due to reducing the tailpipe piston mass taken up in the low-pressure part of the cycle. As usual, some experimentation required.

L Cottrill

[Greg O'Bryant]

Wow I didn't know that the heat loss was so high. I was thinking maybe 6000 watts or so. I have thought about making a hybrid ram-pulse jet too. Except mine is a little more complicated then a shaped shroud around the engine. I don't want to say to much becuase it is one of my ideas that I truely beleive nobody else has had (yet). Thanks for the input guys. Imagine if a well insulated pulse jet did run all of that extra energy could be going to added thrust. Even if the exhaust gases weren't going that much faster it would still be hotter and should work better with an augmentor.:)

[Greg O'Bryant]

My next thought is do larger pulse jets perform better then smaller ones? What I'm trying to get at is, a larger pulse jet has a smaller surface area to volume ratio and in effect shoud radiate less heat for volume than a smaller one. I know other factors come into play like less surface area=less friction and a larger pipe also changes the ressonance as well, but maybe less radiant heat loss should also be acounted for (if larger ones work better), and if they don't it may also shed some light on the question about insulation.

[leo]

snecma has some patents on ram pulsejets.
http://164.195.100.11/netacgi/nph-Parse ... PN/2834183
and
http://164.195.100.11/netacgi/nph-Parse ... PN/2834181
you can find more in my post on patents.
http://www.pulse-jets.com/phpbb2/viewtopic.php?t=1887

[Bruno Ogorelec]

Ah ... I just thought of another potential problem. The shroud will introduce some very hot air into the tailpipe during the Kadenacy breathing of the engine. This could impair performance considerably due to reducing the tailpipe piston mass taken up in the low-pressure part of the cycle.

Larry, don't make the tail ends of the two ducts coaxial.

[larry cottrill]

Ah ... I just thought of another potential problem. The shroud will introduce some very hot air into the tailpipe during the Kadenacy breathing of the engine. This could impair performance considerably due to reducing the tailpipe piston mass taken up in the low-pressure part of the cycle.

Larry, don't make the tail ends of the two ducts coaxial.

Hmmm ... now, let's see ... I have a cylindrical chamber and a cylindrical tailpipe inside a closely fitting long conical shroud. Now, to keep them from ending up being coaxial, I would just ... uh ... just ...

L Cottrill

[Bruno Ogorelec]

snecma has some patents on ram pulsejets.

The Bertin one reminds me strongly of Steve's pulse-turbojet. Jut add compressor at the front and a turbine at the back...

[Bruno Ogorelec]

Now, to keep them from ending up being coaxial, I would just ... uh ... just ...

One simple way is to have the tail end of the outer shroud _much_ wider than necessary and then pinch (crimp) it in at several places so that the end is not a circular duct but a series of smaller ducts. It would look like a flower with ramjet ducts for petals and the pulsejet duct for the center. So, your hot accelerated air streams through the petals and your fresh air streams between them. The layout also has a structural use, holding the central duct in place. No need for radial brackets. Plenty of refinements are possible, of course.