questions about pulse jets

[dmjc66]

formerly entitled: 3 legged PJ

Hi.

Heres my plan (not scaled, not proportioned):

http://img230.imageshack.us/img230/7997/3tppj.jpg

I gather it should 'work' (thanks Metiz).

But how well? (could work better with a valve, not shown).
and
Will it be 'harmonic' (remember the BeeGees)?

Dave

[dmjc66]

I think this idea is as old as the hills:
To drive a turbine from the eflux from a pulsejet.

But since it hasnt worked reliably to date, what exactly are the problems involved?

Is it just a case of exhaustive testing and finding soulutions that work?

And does anyone know how a 'settling chamber' might work in better detail, a chamber/buffer that presumably acquires pressure, from the tailpipe (through a 2 way valve?) for turbine airflow smoothing, but still allows unrestricted breathing from the tailpipe which to me seems contradictory?
Even if further energy (heat/pressure) were added at this stage to drive the turbine.

Dave (UK)

[dmjc66]

I have seen recently how tuned pipe exhausts are routinely fitted with small silencers (on scooters).

Since a pulse jet operates on the same principles, as a tuned pipe exhaust, is it not entirely feesable to add a silencer to a well designed pulsejet, given that thered still be enough residual thrust to do what we wanted with it?

Dave

[Eric]

The thrust left over after the sound reduction, would be equivalent to the same sound produced by simply running the engine at a lower throttle level with no silencer to begin with.

[paul fellows]

I have seen recently how tuned pipe exhausts are routinely fitted with small silencers (on scooters).

Since a pulse jet operates on the same principles, as a tuned pipe exhaust, is it not entirely feesable to add a silencer to a well designed pulsejet, given that thered still be enough residual thrust to do what we wanted with it?

Dave

here are 2 possibles for silencers for bench testing pulse jets!

1) take 2 circles of scrap metal (you could even use wood if it is kept wet ) with a radius of the same length as the exhaust pipe, make an hole in the centre of both large enough for the jet to come through unimpeded, place them together level with the end of the exhaust pipe, start the jet, move the circle that is furthest back backwards until it is at the same frequency as the jet. this will not interfere with the jet for bench testing . but it would be murder on the aerodynamics and power to weight ratio if you try to fly it
the way that this will lesson the sound of the pulses ( but not the noise of the jet ) is that once tuned the sound of the pulses will be coming from the circumference of the circle, so the sound from the point closet to you will be exactly out of phase with the sound coming from the point furthest from you, and these should cancel each other out. there will be two point ,'top' and 'bottom' , from which the sound dose not cancel at all. and between these two extremes there will be various degrees of cancelling.
of cause if you place your head at the centre of the circle you will briefly hear the sound from all point at the same time

2)this aplyes to any type of jet. take several circles of thin workable metal, 5 or 6 times the diameter of the exhaust pipe, and cut a hole about 2 pipe diameters in the middle of them, split them in to two equal piles. Bend the disks in one pile over a solid sphere such that they form part of a spherical surface. Do the same with the other pile but use a sphere of a different size. Stack them 'dome' side up one at a time from alternate piles. Fasten them together and pack the out side gaps with wet rags or other non-combustible sound absorbing material (rock wool ).

One for Eric to think about “those who are quickest to say that something is impossible, are also often quickest to say that it is obvious when some one dose it!”

[dmjc66]

Very interesting Eric.
Any chance of a drawing?
Just to show proportions etc.
Dave

[paul fellows]

hi
can i ask why a turbine?

if you are thinking about genorating electricity, then you might want to look up MHD (magnetohydrodynamics (PROBABLY NOT SPELT LIKE THAT)). it is a system that uses the hot gas of a jet engin to genorate direct curent electricity.

if you are thinking of runing a shaft to a compressor to make some sort of turbo jet arrangement the be worned it will have the worst of both devices! complexity, high speed moving parts and vibration. not a healthy combination.

if you have an other ground based use of high speed rotaion in mind, you might think about this.
a pulse jet as its name sujests provides its power in pulses and a turbine needs a steady flow.
So the problem needs to get a fairly steady one directional flow, out of a pulse jet system?
This well require an elastic store of energy.
My suggestion would be to use wet steam that is being sucked into a venturi tube by the action of a pulse jet .
So with the engine at full power it will be sucking steam from a duct into a venturi and trancfares to energy in the forms of motion and heat between pulses this stored enregy will help to fill in keeping a steader out put.
i can not give any asuerances about this but thinking about this pormted me to post desalination in the off topic forum

or you could just use a fast electric moter

[dmjc66]

Hi Paul.

Your spelling belies your sound thinking.

As Bruce suggests a pressurised fuel tank bleed off the Combustor works OK, using a valve....
then presumably a PJ would do well to have a compressed air tank for 'intermittent' Compressed Air-Motor Propellor drive? Or as you say using potential energy stored in the compressed air tank to start in the first instance, supplemented by bleed pressure 'in-flight'

Just a thought. A Carbon Fibre Compressed Air Tank, to extend range, alternating jet thrust with Prop thrust cyclically (would also reduce noise if succesive Pulsejet restarts can be achieved at intervals of several seconds or less?)


Dave

[dmjc66]

Hi Im trying to determine if this straightpipe Pulsejet worked/works with any degree of success.

http://www.aardvark.co.nz/pjet/noweld.shtml

Sorry Bruce, but I did level this query privately to no avail in the first instance.

Dave

[paul fellows]

a second quick thought

have you ever heard of a thing called the Wells turbine?

it turns the same way witch ever air is blown through it!
it was developed for collecting wave energy in a thing called an OWC.
it might possibly work with the out put from a pulse jet.

[dmjc66]

Moving forth after raissing the query as to whether Combustor bleed air can suitably pressurise fuel tank for feed pressure I came up with this:

http://yfrog.com/j7bleedingpulsejetj


Basically the on board compressed air tank is refurnished at intervals with bleed air, such that the Jet thrust and Prop thrust are alternatable.

Any thoughts/
Dave

[dmjc66]

Hi Paul.

Ive amended the lat drawing:
http://yfrog.com/5jpropjetj

The Moderator rudely dumped over half my posts into a new thread, without any warning (I hope he has no problems with me).
I like the idea you proposed to use a kind of vibrating steam valve to eject steam in the eflux of a PJ during the 'rarefactions'.
Maybe using a valve on the combustor, linked to a valve at the steam nozzle?
Designing such a nozzle with careful thought as to how it might also attenuate noise (mixing?) & quite how its achieved without preventing resonance might require some thought.

Secondly the Wells turbine is very close to an autogyro, that used zero incidence on the blades, but still keeps turning when dragged forward through the air at an angle (like a parachute: lots of drag here too). On this note an Autogyro needs pre-rotation, but once its going it wont stop til the revs drop below a pre-determined minimum value.

Dave

[paul fellows]

he as done the same elsewere
mixed 2 or3 short readable threads to make a longer hard to read thread.

going back over the tread it seems i have gotten hung up rotery macheins the are not that good around pulse jets,
when what you are looking for is a way to compress the air in to a pulse jet?

if that is the case have you thought abuot resonance, it is what they use to make valveless pulsejets work!

also 'flight'
pulsejet valves have a tendency to go from working well, to not working at all, in less than a second. not good if its your life thats at risk if it fails at the wrong time!
PJ valves also have very short lifes

most of the well documented flights where done with valveless macheins. see bruno's stuff in viewtopic.php?f=3&t=2256

finaly i think what the moderator is up to is to keep all the newbie's stuff in one place

[dmjc66]

Its surprising to me if as you say valveless pulsejets are better flight propulsion units.
Since Argus opted for reed valves and nigh all models sport 'Dynajets' there must be advantages with reeds in flight mode.
One website points at efficiency.
But it strikes me conflicting information is banded around, which suggests a 'black Art' which really shouldn't be the case now that we have computational modelling of PJ operation.
The valve length also determines its own natural resonance, which in my mind explains why they are kept a certain length (the smallest valved jet has a single piece of 6 thou. feeler guage, like a 'cat flap') to match the wavelength of the jet or a divisible (such as in the case of harmonics).
Also I note that on the whole the Combustor runs cooler on Dynajets to similar sized Valveless types, but again that might be just my impression.
Arguably less fuel is inducted since the valve stops suction at the venturi all the while its closed.
Even so theyre all still humongously thirsty and noisy, both aspects I'm adressing through various schemes somewhat in the dark however, it seems?
:S
Might be wrong but thats I think why my queries have been thrown in a heap.
As a designer I like to view everything thats gone before, but I also need to know the likely results based on previous similar experiments, or more precisely what physical factors were present previously.

Dave

[paul fellows]

i did no say that say that valveless PJs are better for flying.
the 2 points i was raising where:
1 that you might get the compression that you are looking for by using resonance like they do with valveless PJs.
2 PJ valves have a short life, and unless that problem can be sorted, anyone flying a machien powerd by them may incounter the same problem.