Towards a wood fueled pulsejet

[ace_fedde]

Vturbine,

(Viv, P-Joe, Grim, please confirm, debate or correct what I’m writing here since I’m not an experienced PJ-designer)
I think that mass flow is inherent to PJ performance, whether the chosen performance is thrust, or heat production, or extreme sound production.
The simple fact that combustion needs it’s fresh air supply and has to got rid of exhaust gasses explains that.

The amount of thrust relies on several factor’s, for example: The pressure in the CC, the mass flow, the exhaust gas exit speed at the end of the tailpipe, and the surface of the end of the tailpipe.
I don’t know the exact formula for how they relate to each other.

As far as I know or understand, the exit speed is highest for PJ’s with a straight tailpipe. Fitting a tapered tailpipe causes a lower exit speed. In turbine and rocket science you would then expect a lower thrust.
In PJ operation however, the (well designed) tapered tailpipe also causes a larger mass flow because more fresh air can be sucked in before it is blowed out.

So: same CC pressure, lower exit speed, higher mass flow (not through CC), higher thrust (at low device speed).
There is a point however (for finding that point we need the formulas) where slowing down the exhaust stream by increasing the mass flow, will not increase the thrust anymore.

If it would be possible to design a tailpipe that is so much tapered that the exit speed becomes almost zero, but the acoustics are not tempered, you’ll have your ideal amplitude producer.
You could try to do something in Nudis, unfortunately I can’t get it running.

Fedde

[vturbine]

I''ll take a stab at explaining the kinetics,
With less desire for mass flow and more desire for Helmholtz action, what would the layout be? Maybe a stupidly big CC with a smallish tailpipe to clear exhaust?
Joe

Exactly. The snorkel.

I've continued to think about them ever since I heard that fabulous sound. We've got to get them working on propane.

I'm thinking the injector needs to be positioned exactly as Grim did it. Through the upper corner, at an angle.

I also think there should be a platinum glow plug in the bottom.

And a shuttable draft pipe for starting clearing.

[vturbine]

Vturbine,

(Viv, P-Joe, Grim, please confirm, debate or correct what I’m writing here since I’m not an experienced PJ-designer)
I think that mass flow is inherent to PJ performance, whether the chosen performance is thrust, or heat production, or extreme sound production.
The simple fact that combustion needs it’s fresh air supply and has to got rid of exhaust gasses explains that.

The amount of thrust relies on several factor’s, for example: The pressure in the CC, the mass flow, the exhaust gas exit speed at the end of the tailpipe, and the surface of the end of the tailpipe.
I don’t know the exact formula for how they relate to each other.

As far as I know or understand, the exit speed is highest for PJ’s with a straight tailpipe. Fitting a tapered tailpipe causes a lower exit speed. In turbine and rocket science you would then expect a lower thrust.
In PJ operation however, the (well designed) tapered tailpipe also causes a larger mass flow because more fresh air can be sucked in before it is blowed out.

So: same CC pressure, lower exit speed, higher mass flow (not through CC), higher thrust (at low device speed).
There is a point however (for finding that point we need the formulas) where slowing down the exhaust stream by increasing the mass flow, will not increase the thrust anymore.

If it would be possible to design a tailpipe that is so much tapered that the exit speed becomes almost zero, but the acoustics are not tempered, you’ll have your ideal amplitude producer.
You could try to do something in Nudis, unfortunately I can’t get it running.

Fedde

No, I didn't say you could get rid of mass flow, I said reduce it. If you don't need thrust, you definitely don't need as much fuel and air as the typical pulsejet uses.

The tailpipe I had in my own mind was from my understanding of 2 cycle exhausts. Basically looking like a funnel stuck into the tailpipe with a closed end at the top (the diaphragm) and a side pipe coming in radially and extending into the funnel chamber to near the centerline. Possibly the centroid of the volume. This is the exhaust pipe.

The total height to the diaphragm will be a tuned length that will work with the CC rather than damp it.

Actually a form similar to a classic tuned 2 cycle resonator with dual cones and a side outlet as above and diaphragm at the top is what I have in mind for a snorkler. The side outlet must function as inlet and outlet alternately and the tuned dual cones should ram the fresh air charge back into the CC, just as it does in a 2 cycle engine.

[vturbine]

Actually a form similar to a classic tuned 2 cycle resonator with dual cones and a side outlet as above and diaphragm at the top is what I have in mind for a snorkler. The side outlet must function as inlet and outlet alternately and the tuned dual cones should ram the fresh air charge back into the CC, just as it does in a 2 cycle engine.

Actually, maybe I can dispense with the top cone and go back to the simple funnel shape with a cylindrical extension to the diaphragm to suit timing. The second cone on a 2 cycle engine is there because we want an RPM range. But in this application, we probably just need to tune for a small band, so the second cone can just be a wall -- the diaphragm.

[PyroJoe]

Don't get me wrong I love snorklers, but the snorkle pipes on average are typically 1 CC length,... to reach a point of good throttle range typically takes a tailpipe of 5 CC lengths. Not sure how to get around that parameter.
Joe

[vturbine]

Don't get me wrong I love snorklers, but the snorkle pipes on average are typically 1 CC length,... to reach a point of good throttle range typically takes a tailpipe of 5 CC lengths. Not sure how to get around that parameter.
Joe

The resonator/diaphragm will probably look like a variable length pipe. As the load gets greater the diaphragm will resist deflection returning a stronger pulse/charge to the CC and timing will also be altered.

I don't know Joe, guess we'll have to find out what happens with something different than a straight pipe.

EDIT: If we drop that center pipe down it starts looking like a Reynst with extractor cone, except closed at the end with a diaphragm, and with a separate pipe inlet/outlet

[vturbine]

The Reynst cone is greater than 1 CC in length.

I wonder if a simple snorkler could have a longer pipe IF the pipe diameter is increased enough not to shotgun.

[PyroJoe]

May be Mark has found a extra long snorkle length/setup that may help.
Joe

[GRIM]

Did anyone catch this , posted by Viv , viewtopic.php?f=10&t=5450

I thought immediately of a diaphragm, full auto reload , etc , of course nothing to do with this thread but amazing just the same

[ace_fedde]

Haha, was thinking the same when I saw it!

[Viv]

Exhaust power takeoff.png

EDIT: If we drop that center pipe down it starts looking like a Reynst with extractor cone, except closed at the end with a diaphragm, and with a separate pipe inlet/outlet

now you see why I put in the Reynst stuff, your cone will tend to lengthen the duration of the pulse but lower its amplitude, ie thats what the long cone in a 2 cycle resonator does to improve the rpm range.

Viv

[vturbine]

I just uploaded a video of a startup to youTube because even after compressing 350 megabytes it was still 20 megs long. I don't imagine it's going to excite anybody there. But if you're into pulsejets you can hear the long gradual transition in this engine. I have to throttle it up very slowly (as far as I know -- maybe practice will change that).

http://www.youtube.com/watch?v=PD2r_vMDC_A

[vturbine]

Exhaust power takeoff.png

EDIT: If we drop that center pipe down it starts looking like a Reynst with extractor cone, except closed at the end with a diaphragm, and with a separate pipe inlet/outlet

now you see why I put in the Reynst stuff, your cone will tend to lengthen the duration of the pulse but lower its amplitude, ie thats what the long cone in a 2 cycle resonator does to improve the rpm range.

Viv

I guess the degree of modification varies -- it depends on the cone angle and length. The book I read years ago said that the primary function was to invert the pressure and do an evacuation of the cc. I wrote an excel spreadsheet calculation program from that book back in 1996 or so. I was flying two cycle ultralights at the time and got interested in exhaust tuning.

[GRIM]

Thanks for the video ,
Did you throttle it up more than that ? It should go a lot higher , if it wont then you have got a restriction in the fuel line,

I never actually got mine to flame out , just got huge flames from the exhaust ,
I think the injector position although good for starting is very poor for efficiency and a lot of the fuel goes up the exhaust , also the combustor is surely very inefficient as it has no cone , and I am sure the shape is less than ideal ,

just the same I am glad that it served to give you some idea about pulsating combustion , I learned a lot from mine ,

[Viv]

Hi V

We use some of the same maths as its the same type of effects, cone length and angle being some of the varibles, controlling the reflected pulse sign, amplitude and duration is the same for these types of engine.

Viv