origonal ???

[paul fellows]

i am quite new to pulse jets,
however being the XXXXXXXX that i am i thought i would invent my own, with a little feed back from you's

the targest are useable levels fo thrust.
quiet operation.
and high "super sonic" exaust velosity.

if we can manage ether of the last two without losing the firstit will be a worth wile efort.

[metiz]

No tuned engine is quiet and no normal pulse-jet has gas speeds reaching or exeeding mach 1
600-650mph is about the maximum for a straight pipe pulse-jet exhaust velocity

[paul fellows]

i thought sonic choking was one of the problems to be dealt with?
the fact that the gas coming down the exaust gets to the speed of sound then can not go any faster, dispite having the enregy avialabel to do so if the phisics alowed!

[paul fellows]

the first sujsted chainge i am puting forword for people to concider is to make the combustion chamber; circular, spherical, or toroidal.
and to have the intake and the exaust enter it tangetialy, such that they make air in the chamber spin around. this should alow the air to continue to enter the chamber even after the fuel is burning, alowing higher chambr pressures to develop.

[Kool]

the combustion chamber; circular, spherical, or toroidal.

This dependents a lot on which design you go. For an spherical pulsejet, you should look for an engine made by Leo, (on this forum, with about 8 exhausts). Besides this designs like ThermoJet, Chinese, and Marconette use al different shapes of combustion chambers

[Jutte]

The problem is going to be the nature of unsteady combustion.
However - in saying that...interested to see your ideas to combat this problem.

[paul fellows]

Sorry for the crudeness of the drawing
this PJ has one witch serves as both exhaust and inlet, this is not as radical as some might think, Bruno Ogorelec on the bottom of his web site Valveless Pulsejets 1_5 By Bruno Ogorelec.mht. He Talks about the Reynst, witch is a modified version of the 'jam jar' that can be seen on this site.
The change that I am offering for consideration is having the second pipe. But having it stopped at about half the length of the working throat, to give a quarter wave oscillation at the same frequency as the working throat.. And who's positive reflected wave meets the negative 'rearification' wave coming back up the working throat. As these two waves are out of phase they will cancel each other out.
So each pulse produces its own acoustic waves that do what ever magical and mystical thing acoustic waves are supposed to do and then cancel them selves out, rather then being driven to ear splitting resonance.
Mean wile a PJ works on Kadenacy the movement of gasses! Not the movement of sound through gasses.
It is an unfortunate fact that the geometries that produce good Kadenacy usually also produce strong acoustic resonances but dose that mean that those resonances are necessary or even desirable???
that is a genuine question! I believe the answer is NO, but I could be wrong?
As an aside
back in the days of the incandescent filament light bulb, bulbs produced lots more heat than they did light. This was conciderd normal and even necessary to there operation! Now we know better!

[paul fellows]

there should have been a picture with this

[paul fellows]

[paul fellows]

a pj is just a combustion chamber

[Jutte]

OK - take this with a grain of salt or whatever( your comments greatly received etc).
Had a chat with my High Speed Aerodynamics tutor regarding one of my pulse jets and what I observed.( Yes -I'm stuying that now).
What I wanted was his take (and input) on my theory on what was happening.
My theory was that this pulse jet flow went - or had at least a supersonic - component to it.
This was seen in -what I termed - a semi-standing shock wave + later on seeing
shock diamonds forming ( yes I did see that -wish I took a picture but it was so damned LOUD that a VERY
upset neighbour came over and had a 'chat').
He ( the tutor) more or less confirmed my theory -(remember it's only a theory to explain what I saw).
Speed of sound in high temp is higher than cold air - so when flow exited out of tail pipe
a shock wave formed on the rounded edge of the tail pipe (flow went sonic re cold air -lower speed of sound)- hence the observed shock wave and more than usual hideous noise.So when the flow went laminar and exited the shock diamond started up.( was real nice too - hearing wise it was way into the pain threshold...)
Improbable ? - most likely: Possible ? - could be, could be,might be - I don't know...
This is what I observed - doesn't mean squat much with out pictures etc.
Also that particular pulse jet has been changed since then(duh!-didn't know what I do now) and I haven't got the means to run stuff now-(poor student etc etc).
As a note - that was the only time a neighbour came over and spat the dummy out big time (heh heh heh).
Also - this pulse jet was the only one that did this.

[Bruce Simpson]

Yes, given the correct degree of confinement, the tailpipe flow can approach the speed of sound at the temperatures involved (which would be supersonic at normal (external) ambient temperatures)

However, I've never seen (nor heard of) the flow being sufficiently fast or sustained to create a shock diamond.

The problem is that pulsejets are intermittent-flow engines and therefore the dynamics of steady-flow don't necessarily apply.

When the pressure wave exiting the tailpipe hits the colder external air, the result is a great deal of turbulence and in many cases a degree of toroidal circulation around/behind the tailpipe. This seems to kill any chance of a mach-diamond forming at this point.

Also, mach-diamonds seem to form more readily when the flow passes through a convergent/divergent nozzle -- which facilitates the formation of the shockwaves that create them. A pulsejet tailpipe is generally either just a straight tube or a divergent cone -- which further reduces the chances of a mach-diamond forming.

Finally, the pressures we're dealing with in a pulsejet are comparatively low, further reducing the chances of forming any kind of shockwave in the exhaust.

[Jutte]

Hi Bruce - hows things up North ?,
I agree exactly -shouldn't be diamonds in unsteady flow - but there it was.
A nice contradiction in all that is 'proper' - I'm not worried though...things to think about and all that.

[paul fellows]

The double ended Lockwood, and the double ended Lockwood variant

make a Lockwood pulse jet but with the exhaust pipe ending level with the exhaust pipe inlet. Then build a second matching combustion chamber and inlet pipe on the end of it. This will give you a symmetrical design with two 'intakes' / working throats at one end. Followed by two combustion chambers. Joined together by a long 'U' bend, the length of the exhaust of a normal Lockwood.
It might improve the performance of this if the ends of the combustion chambers where the 'U' bend joins it is flat, and of equal cross section along its length.

If I have got the theory right, and it is always possible that I haven't. Then once started the two engines will run exactly out of phase with each other. As one throat is sucking fresh air the other is blowing hot gas.


The variant is just the same thing with acoustically cancelling circular combustion chambers.

[paul fellows]

[quote="Jutte"]OK - take this with a grain of salt or whatever( your comments greatly received etc).
Had a chat with my High Speed Aerodynamics tutor regarding one of my pulse jets and what I observed.( Yes -I'm stuying that now).
What I wanted was his take (and input) on my theory on what was happening.
My theory was that this pulse jet flow went - or had at least a supersonic - component to it.
This was seen in -what I termed - a semi-standing shock wave + later on seeing
shock diamonds forming ( yes I did see that -wish I took a picture but it was so damned LOUD that a VERY
upset neighbour came over and had a 'chat').


evey one seams to think that it is imposible? so here is what i think if the end of that paticular PJ was ragged then it might have set up micro voticies witch could have kept the flow contained for a short distanct away from the PJ, if the first part of the cooled and slowed the back end of the pulse would pile in to it,
this could only happen during the 'pulses'.
however if the frequancy of the PJ was more then 12 Hz pecistance of vission would make it look like it was always there, instead of being breafley formed with each pulse.
there is 3 ifs a might and a could have in there
this mightbe tested by taking a lenth of old hacksaw blade and positioning it around the inside of a PJ, so that only the top of the teeth are showing, to temparerily create a raggy ended PJ.