Another pulsejet kick from Sweden

[erik]

I am just going to write a short reply because its 04:30 here and i need some sleep...

I have some good news, i got the material for the reeds!
250 pieces 0.25mm stainless spring steel cut in the right dimensions,
i was planing to let a friend make the holes with EDM but because of the short time left i had to improvise.
The solution was to make a simple fixture to hold the valves and let me drill the holes in the right place.

Another modification is to place 3 of the 6 fuel nozzles inside of the CC, mainly to make the starting process safer.
When i got the engine running on the internal nozzles i just switch on the second fuel pump and inject the rest of the fuel in the intake, at least that's the plan.


'

[erik]

Success!!!
I made a few test runs and it really roars!
First start:
http://www.youtube.com/watch?v=rBZcqLm9ynI
http://www.youtube.com/watch?v=lW8PZee7UHQ

http://www.youtube.com/user/LaRdArms?feature=mhw4

After the testing i checked the valves and they are like new!

[WebPilot]

That's grrreat news, Erik - congratulations!

A pulse-jet log mover -- who would have thought? Is this some sort of Swedish thrust measuring rig?

I would be interested in knowing how many running minutes these valves last for my records.

Earlier I estimated your jet's

• fundamental (driving) resonant freq at 83.72 Hz ( ±5%), and
• your then valve (0.3mm thick) mode 1 vibration at 200.3 Hz
• resulting in an estimated dfr of 83.72/200.3 = 0.418

For Valve Glide ™  to occur, I suggested we need to get this as close as possible to 0.431.

Since then I "measured" your jet's fundamental from your very first video and it was 76 Hz.



Using my spreadsheet of the "Barton formulation" for a valve thickness of 0.25 mm, I obtain a Mode 1 frequency of 167 Hz.

The new dfr computation using these revised parameters, is dfr = 76/167 = 0.455 which differs by +5.59% from the requisite value of 0.431. These valves should still last a while.

There isn't much difference between 0.3 mm (0.01181 in) and 0.25 mm (0.00984 in), only 0.002" by my count, but what a difference it makes in your case.

Again, I'm very happy things worked out for you.

Now go "kick some butt" at the races with your Argus-ette!

Later

[PyroJoe]

Good job!
Looks like it runs strong, is the fuel injected in the CC in those runs?
Good luck in the races.
Joe

[WebPilot]

 Are there 3 different videos on youTube or just two?

[WebPilot]

 I've seen these two.


            pulsejet_first-start
            


            Pulse-jet kick testing

[erik]

Thanks.
WebPilot: There is only two videos, when i get time i can edit and upload rest of the material .

PyroJoe: Yes, using a 7, 10 and 13.5gal/h nozzles , fuel pressure is 8 bar at the moment so around 35gal/h.
I am not sure if i what to just inject all fuel in the CC or use both the nozzles in the CC and intake?
Its easy and saves time to just the CC injectors but injecting fuel before he valves may give them a longer life.
If i am going to inject all fuel into the CC maybe i change to a slightly bigger nozzle to allow a lower fuel pressure with the same flow.

[Eric]

Good to see you got it running, though I'd say its still only running at medium throttle. Whats the length of the intake from the front of the valves to the intake opening, and whats the tail length?

If there is any blowback coming from the intake you might need to lengthen it a bit, the intake actually resonates on its own if its a proper length, and at that length, timed with the valves opening, can stuff a lot more fuel and air into the engine.

[WebPilot]

Of the three links above, two show the same video.

I told you I thought your valves were close.

I think the big change you did was injecting directly some of the fuel into the cc. Obviously, it's cold where you live and I get the the impression you're pre-heating the tube in this fashion, before actual start-up.

What is your starting procedure? Are you turning on only the cc injectors first, blowing in air, run it this way for a short period, and then turning on additionally the forward injectors?

Are you NOT using valve stops?

Why does the "flame tail" want to point towards the ground in your videos? Are you using so much extra fuel that raw fuel is blowing out the tail pipe and 'falling' on the ground? The jet flame is then following the fuel trail?

How fast can you drag one of those timber logs? I think you just created an event for your celebration; pulse-jet log pulls, similar to tractor pulls here in the states.

If nothing else, dragging a log(s) will be a great max-thrust tuning aid.

[Eric]

Web,

The 'falling flame' is one interesting type of ground effect, the flow sticks to the ground and re-ingestion zone is biased towards the top of the engine, creating a higher average static pressure because the average velocity is closer to zero and its easier to pull air out of that region, then from the ground, so the flow goes out straight but then snakes down and follows the path of least resistance. If you have multiple obstacles you can make the exhaust do all kinds of funny things.

Whats happening in the video is basically a 1D version of what happens in an augmenter, the gas slug continues to pull air through the augmenter until it exists the back, at which point the augmenter is full of another cool air charge to get fired out by the next blast of exhaust.

When stuff like that happens you can typically improve thrust considerably by raising the engine up off the ground, it needs a doughnut shaped ring around the exhaust clear of any objects to breath properly.


Erik,
Is there any way for you to add some pure methanol to the fuel mix? If you put a bit of gas in a tin tray and light it, it burns, but yellow and with lots of soot. If you put methanol in the tray it burns blue, and fairly slowly. If you mix gasoline and methanol, you get a blue flame with yellow tips that burns very clean, very high, and much faster. Gives you a lot more thrust too. It will also cool your valves considerably, and make the fuel air mixture more dense further increasing power.


Eric

[WebPilot]

 Thanks for the reply, Eric.

 So, if I understand you, we can explain this phenomenon via the "Bernoulli effect", well,
sort'a kind'a.



 Namely, if we draw a control volume of unit depth surrounding a portion of the exhaust gas jet,
at the top of which is where the gas velocity is zero and likewise at the bottom or ground level.

 Since the air is accelerated (or has a velocity) through the control volume, the pressure there,
P int, is less than atmospheric, P atm. This produces a net force per unit length on the upper surface,
or (P atm - P int) x 1.

 Since the control volume is NOT a rigid shell, the net result of all of this is to "push" or to accelerate the control volume (or flame) downwards.


            'Falling Flame' Phenomenon

 That's pretty neat.

[Eric]

Yep that pretty well describes it.

Also the point that it dips at, would be the same point in free space, that would create a virtual venturi, with the 'nozzle' being made up of a toroidal region of static / recirculating flow, and happens to be where an augmenter should be positioned.

I just posted in the valveless section on augmenter design, with some exhaust plume pics that pretty clearly illustrate the flow in toroid form.

In theory, if Erick put down a plate on rollers on a flat smooth surface at this point, the plate would move forward, against the direction of flow. It is also therefore possible in theory, for pulsejet exhaust flowing over a properly shaped section of wing, to create a 'flat' augmenter.

[erik]

Webpilot: in the video i am only injecting into the CC. Because of the crude fuel system used the engine died when i tried to open the ballvalve to the intake injectors.
And i am not using any valve stops.

Eric: i will try to get some methanol in time for the race, do you think the hydralic lines will react to methanol?

Could the big exhaust flame be caused by bad mixing betwen the fuel and air, and maybe i could get more power in injecting more of the fuel into the intake?

And if i get time. Maybe adding a short augementor on the exhaust?

[PyroJoe]

8 bar should be plenty of pressure to atomize the fuel, the difficult part is flowing the fuel charge from the intake injector to the back of the CC.

By direct injecting into the CC it has lost its mixing path/length. Methanol will help, but correct mixing along with adding methanol should help reduce the "rich" condition.

It may be useful to look at the system and ask yourself, what is the most restrictive components in flowing the fuel from the intake injector to the back of the combustion chamber?

Another thing that may have helped in the design is injecting into the narrow diameter of the intake venturi. The injection point looks to be some distance after the narrow of the venturi.

All in all your doing fine as is, really would like to hear the engine in person, it must shake the ground.

A neighbor from about a quarter mile away stop by the other day, they mentioned how my Kraken engine was vibrating the window blinds in their house. I failed to tell them the engine was running well below 50% and not pointed in their direction.

[WebPilot]

And i am not using any valve stops.

He, he, he ...