Best Runner, yet

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HPSCL
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Best Runner, yet

Post by HPSCL » Sat Mar 27, 2010 9:04 am

Based on a small amount of recently acquired spoon-fed info, I was finally able to build another pulse jet engine. It took very little "tweaking" to get the injector nozzle the correct size, as well as setting the depth at which it sits into the Intake.

I finally went with a "rosscojettor" (sp?) type of injector nozzle and am very pleased at how well the gas flows out of it. I can be impatient, when it comes to finally getting to the point where you're almost ready to fire a new engine up for the first time. Because of this, I've never taken the time to make the end of an injector "pretty"... for lack of a better word, at 3 a.m.

The end of the injector was squared off with a file, with most of the attention given to getting the opening as close to a perfect rectangle shape, as possible. Final trimming was done with (of all things) a safety razor. The end result was well worth the small effort.

The "augmentor" is just for looks, as I had that scrap piece of tubing, leftover from an unfinished project. This is (by far) the most obnoxious of all, in terms of "annoying decibel level". - Wonder if the augmenter is acting much like a "loudspeaker"? - Ha!

Anyway, it was a fun build and starts pretty easy... ( Now that I finally figured out the technique for starting this particular one ).




VIDEO #1

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

VIDEO #2 (Just another phase of the previous test run)

http://www.youtube.com/watch?v=_LG6iRjqhLk
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metiz
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Re: Best Runner, yet

Post by metiz » Sat Mar 27, 2010 12:47 pm

sweet little runner!

Any thrust estimates?
Quantify the world.

Kool
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Re: Best Runner, yet

Post by Kool » Sat Mar 27, 2010 2:18 pm

nice pulsejet, I like the way you don't need any conical sections
...It's better to generate heat efficiently, than recover it efficiently...

HPSCL
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Re: Best Runner, yet

Post by HPSCL » Sat Mar 27, 2010 5:34 pm

metiz wrote:sweet little runner!

Any thrust estimates?
Thanks! - As far as the thrust estimate, I have no idea? I thought of putting the stand on some pipes and using my fish scale, but wonder how I'd keep the stand from vibrating sideways, off of the pipes? Going to search these forums, to look for ideas on how to set-up a nice looking one.

The stand is tied down to one of my little workbenches with rope, on the front & rear of the stand. I saw (on the first video) that when the engine went from a low idle to immediate high-throttle, the engine pulled pretty hard on the front rope. I've got those ropes sinched down pretty tight, because I didn't want the engine vibrating all over the table. - I know the rope is prone to stretch more, but know it more than just a little force, to move that stand.

I'd laugh though, if I went to all the trouble to actually build a thrust test-stand, only to find out that the engine put out less than 5 lbs. of thrust.

HPSCL
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Re: Best Runner, yet

Post by HPSCL » Sat Mar 27, 2010 5:40 pm

poech prut wrote:nice pulsejet, I like the way you don't need any conical sections
Thanks!
I would like to have used a cone section, but it's SO much easier to build them, without. I just add extra volume to the CC, to compensate for lack of the cone. I would venture to guess that the cone makes a MUCH better transition for the gases to flow at a high cyclic rate, so I also try to compensate by making sure that the transition from the front CC plate into the INTAKE / TAILPIPE is blended well.

Eric
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Re: Best Runner, yet

Post by Eric » Sat Mar 27, 2010 6:17 pm

The problem with flat plates is that eventually they will cause cracks. the thermal expansion can only go out perpendicular to the engine, where a cone can have perpendicular and lengthwise expansion.

Mild steel wont be too bad because its a lot more conductive, but I personally dome everything at least a little bit, since it reduces expansion forces by an enormous amount, though having offset intakes and tail tubes, high expansion in some areas, low expansion around the intake, etc, can cause a lot of stress in a lot of directions. Not to mention inward and outward flexing from pressure variations.

You dont have to go too crazy with making the injector perfect, smooth flow is not really desireable, because it takes a lot longer to mix than a really turbulent flow.

For example a straight injector might need to be at the very front of the intake, where an injector optimized for turbulent mixing can actually extend down into the combustion chamber well past the intake.
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Talking like a pirate does not qualify as experience, this should be common sense, as pirates have little real life experience in anything other than smelling bad, and contracting venereal diseases

HPSCL
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Re: Best Runner, yet

Post by HPSCL » Sun Mar 28, 2010 6:29 am

Eric wrote:The problem with flat plates is that eventually they will cause cracks. the thermal expansion can only go out perpendicular to the engine, where a cone can have perpendicular and lengthwise expansion.

Mild steel wont be too bad because its a lot more conductive, but I personally dome everything at least a little bit, since it reduces expansion forces by an enormous amount, though having offset intakes and tail tubes, high expansion in some areas, low expansion around the intake, etc, can cause a lot of stress in a lot of directions. Not to mention inward and outward flexing from pressure variations.
I've actually thought of the stresses caused by running the engine at such high pressure, even though the metal tubing is pretty darn thick. The end plates are 1/8" , but I guess it wouldn't matter if they were even a quarter of an inch thick, because having the metal red hot several times will eventually weaken it, no matter how thick they are.

Using 1/8" thick steel for the end plate may seem redundant, but it does make for a much nicer end result in the welds, when the steel is close to the thickness of the exhaust tubing and EMT.

Running the engine at a lower fuel pressure is (of course) a lot less stressful on the entire assembly, but it's difficult to resist the urge to "crank-it-up" as high as you can, if/when you have a lot more available pressure.
Eric wrote: You dont have to go too crazy with making the injector perfect, smooth flow is not really desireable, because it takes a lot longer to mix than a really turbulent flow.

For example a straight injector might need to be at the very front of the intake, where an injector optimized for turbulent mixing can actually extend down into the combustion chamber well past the intake.
Would that theory be true, if one was planning on running liquid fuel, through the same injector? I ask, because I do notice that the smoother the "laminar flow" is, the easier that any engine is to start. If one were running liquid fuel through the injector, then it would make sense that a more turbulent flow would be desirable, as it would help a great deal with atomizing the fuel.

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Re: Best Runner, yet

Post by Eric » Mon Mar 29, 2010 7:31 am

Smooth flow might help the engine bring in air for that first pulse, but once it is actually running, and is a good running design, turbulent mixing will result in much more power, and higher fuel efficiency.

The farther the injector is in the intake the less blowback there is. But at the same time you want to balance the flow characteristics for the engine you have.
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Talking like a pirate does not qualify as experience, this should be common sense, as pirates have little real life experience in anything other than smelling bad, and contracting venereal diseases

HPSCL
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Re: Best Runner, yet

Post by HPSCL » Tue Mar 30, 2010 3:45 am

Eric,

I took your advice and made another injector today. It's 1/4" Steel Brake line, with the end piched together. I filed a slot, with a hacksaw, until it barely made an opening in the tubing. This one sits halfway down the length of the Intake. I adjusted the depth until the engine seemed to run at it's highest.

Starts easier now and will almost start cycling, with no air. Got the engine to do that twice, but it's intermittent. No big deal, as it just barely needs a little spurt of compressed air to get it running now.

As an aside, I would love to build one of those static ramjets, like the one you have on your ThermalPulse 180. I've searched the internet, but can find no plans on the internals... just a rough 2D schematic, which does not include the flameholder assembly.

If this is another trade secret, then I apologize for even asking. Thanks, for all of your previous input, as it was greatly appreciated.

EDIT: DUH! - Found plans for a subsonid ramjet, on the home page of this site.

Dave.
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HPSCL
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Added another faux augmenter

Post by HPSCL » Fri Apr 02, 2010 5:16 am

...Had a scrap piece of 2-1/2" Exhaust Tubing lying around and thought it would make a decent-looking Intake Tube Augmenter. I uploaded another cheesy video on youtube, showing the engine running, with the augmenter. Line pressure is approx. 80psig vapor.

Pulsejet with Intake Augmenter

As I've already implied, the augmenters on this engine were not designed properly. - There more for show, than anything else.

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Re: Best Runner, yet

Post by Eric » Fri Apr 02, 2010 6:00 am

The internals are proprietary, a basic ramjet will work somewhat with an average pulsejet but for really good performance you need to do more.

The big flame in the augmenter is actually a sign that there is quite a bit of fuel blowback from the intake, the augmenter slows the average velocity down and confines it enough to allow the excess fuel to burn.

try using a flared tube with a V shape plug ground from a small piece of round stock, which will form a conical shape spray nozzle. A 90 degree overall < will give good turbulent mixing, shallower, will give better no-air starting. Higher turbulence will let you put it farther into the intake, meaning even higher power, and less wasted fuel.

Valveless pulsejets can be quite efficient if you actually do everything you need to in order for them to run properly.

Unfortunately this is something that 99% of people pass over and just sum up all valveless engines as being low power / low efficiency, when actually almost all decent designs can have better fuel efficiency than valved engines, and the most advanced ones approaching turbofan efficiency for certain applications.
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Talking like a pirate does not qualify as experience, this should be common sense, as pirates have little real life experience in anything other than smelling bad, and contracting venereal diseases

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