Big Test Lady FWE arrives in Aero/Combustion Laboratory

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larry cottrill
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Re: Big Test Lady FWE arrives in Aero/Combustion Laboratory

Post by larry cottrill » Tue Apr 28, 2009 11:57 am

I should have also said that I think moving the spouting point of the nozzle forward will probably also help, by getting the mixing action farther into the chamber.

I think this motor would be a great place to try the wonderful 3-point nozzle that was proposed a year or so ago. Greg, if you missed it, it was done by squeezing the end of the tube in a 3-jaw drill chuck. I didn't invent this, I just wish I had! In our case, I think we want something pretty high velocity (and you have all the pressure you need to drive it) so I think you could squeeze it down to well under 1 mm clear space in each of the three lobes. That would surely give you far more flow than the defective nozzle I supplied. Then, carefully center the spouting point at or near the front end of the intake tube (right under where the transition cowl breaks downward toward the chamber wall).

The only problem I can see with such a location is that you won't be able to get there with a bent tube fed through your 1/2-inch fitting. The bent tube I provided is about as much of a forward leg as you can get in there, because of the location of the fitting and the limitations of the intake ID. See drawing attached.

L Cottrill
Attachments
Big_test_lady_fuel_pipe_full_size.png
Fitting the fuel pipe into the fitting provided. This is full size when printed at 96 DPI.
Drawing Copyright 2009 Larry Cottrill
Big_test_lady_fuel_pipe_full_size.png (10.18 KiB) Viewed 3297 times

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Re: Big Test Lady FWE arrives in Aero/Combustion Laboratory

Post by larry cottrill » Tue Apr 28, 2009 12:39 pm

Greg has sent along the attached photo, with this comment:
Something else that came to mind: I am shooting into one of the cell's exhaust ducts, could that be screwing with me wave-wise?
I responded thus:

Hmmm ... I don't know. On the one hand, it seems so big that as long as
it's fully open to the outside, it shouldn't mean much. But, looking at it
another way, Rossco pointed out that closely coupled ducts act
acoustically as continuous piping, as though the physical break doesn't
exist. The real reason for this is that the circular wall reflects the
pressure wave back into the center, so the wave still propagates along the
length of the larger tube.

Is it possible to move the mounting rig forward a foot or two, so there's
a "de-coupling" between the end of the engine and the big duct? It would
be very interesting to see if that makes a difference.

In other words, I have no way to be sure by just seeing the photo, but
yes, it's possible you're shooting yourself in the foot with that setup. I
think you MUST resolve that experimentally. If it's easy to move the rig
forward, even just a foot, you should be able to determine if it's really
an issue. I'm REALLY glad you asked about this, and provided the photo.

And again:

I just thought of another thing you can do that may help, since you're
working inside a hard-walled "box", the test cell:

Rotate the mounting rig 25 or 30 degrees to the right or left, as well as
space it out from the duct. What I mean is, you can still have the
tailpipe pointed at the mouth of the duct, but shooting at it from an
off-center position at an odd angle. Operating inside a hard box (no
matter how spacious it may seem), the things you should not allow are:
- Having the tailpipe aimed "straight at" one wall (direct reflection)
- Having the engine at the center point between two parallel walls,
whether end-to-end or side-to-side (standing wave generation)
- Having the tailpipe aimed directly into a corner (corner reflection)
- Having a secondary duct closely coupled to the tailpipe or intake
(propagation and negative reflection)

The ideal setting would be a room with no parallel flat walls -- like a
correctly designed concert hall!

Comments, gentlemen?

L Cottrill
Attachments
image003.jpg
The test setup as it now stands. I think the large duct is tightly coupled to the
engine, even though it is quite large. Photo Copyright 2009 Greg Meyers

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The double-null

Post by milisavljevic » Tue Apr 28, 2009 3:10 pm

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Edit: As this post has served its purpose, not to mention Larry quoting it at length, it is now made *poof*!
Last edited by milisavljevic on Tue Apr 28, 2009 6:18 pm, edited 1 time in total.

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Re: The double-null

Post by larry cottrill » Tue Apr 28, 2009 4:09 pm

Thank you, sir, for weighing in. My friend does not have much time to beat a dead horse, but he does have time to make meaningful changes. So, my response is as follows:
milisavljevic wrote:I see that you have gassed on at length about the fuel injector (yes, it does have issues) and even the exhaust duct adjacent to your pulsejet (no, this setup is not an issue; neither is the enclosed test cell).
He has already proven a couple of newly minted injectors to be far better; He has verified already that changes in the engine placement and orientation do NOT appear to make any sensible difference.
There are two (2) fatal errors in the duct design: (1) the intake is ridiculously long and (2) the tailpipe
is ridiculously short. FWE intakes typically run to the short side, but here you went with a ca. 5.5:1 L/D
intake duct that aspirates more than twice the oxygen needed to support combustion. Being as long as
it is, the intake couples to the main duct at a very low lock up ratio (I know: heresy), which the duct is
incapable of supporting at a realistic acoustic temperature; the intake needs to be shortened > 90 mm.
That should be doable; he has all the workshop facilities needed. There should be enough material clear of fittings to take out the 90 right in front of the flare so it can be re-used.
If we ignore the intake for a moment, we find that the Helmholtz modes for the duct are some 100 Hz
below the open and closed pipe fundamental: I believe the saying is, "this dog won't hunt". Impossible.
To correct this deficiency, the tailpipe would need to be extended by ca. 1400 mm. Now, if the intake were shorted and the tailpipe lengthened in a coordinated action, then your required tailpipe extension decreases to ca. 600 mm. If your associate has time, tools and materials required to correct the above,

The tailpipe is designed to be easily replaceable; the mod is reasonable as presented. I believe you are saying that IF the 90mm were taken out of the intake length, the tailpipe would only be increased by 600 -- a mere two feet, not at all outlandish for the material used.
... you would still be left with a deficient injector positioned at the wrong place in the intake (in x and z).
At least that issue is easily rectified.
That's right; he can handle that easily.
I do not understand why, when given this opportunity, you did not use a proven design (eg., Eric Beck's
advanced FWE), a motor for which reasonable amounts of test data are available. Data that could then
be correlated with any new measurements. I am sure you have reasons, but these exist beyond reason.
You are quite right, of course; My only real constraints were the tubing I could get for the intake and the pipe that was reasonably available to him for the tailpipe. I should have stuck closer to proven design.

I will pass these recommendations on to Greg, with appropriate apologies. And many thanks to you sir, for contributing at this crucial point.

L Cottrill

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Re: The double-null

Post by milisavljevic » Tue Apr 28, 2009 5:03 pm

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larry cottrill wrote:Thank you, sir, for weighing in.
Larry, it seems that you have a profound misunderstanding about what "being a dick" is about. :wink:

That said, high-fidelity corrections for the intake and tailpipe are now available; will post soon.

M.
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Re: The double-null

Post by milisavljevic » Tue Apr 28, 2009 5:30 pm

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Edit: As this post was rendered obsolete by the disclosure of critical information, it is now made *poof*!
Last edited by milisavljevic on Wed Apr 29, 2009 12:19 pm, edited 2 times in total.

larry cottrill
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Re: Big Test Lady FWE arrives in Aero/Combustion Laboratory

Post by larry cottrill » Tue Apr 28, 2009 5:32 pm

Again, sir, many thanks! Much obliged, etc.

I will pass these along immediately via email to Greg, and we'll see what we get.

L Cottrill

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Re: Big Test Lady FWE arrives in Aero/Combustion Laboratory

Post by PyroJoe » Tue Apr 28, 2009 5:59 pm

Hi Larry,
She may start airless at those proportions. Just for kicks, may want to try the 1285mm tail length before cutting the intake. At that tail length, I often find there is some forgiveness at the intake.
Joe

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Re: Big Test Lady FWE arrives in Aero/Combustion Laboratory

Post by Dang911 » Tue Apr 28, 2009 9:30 pm

M, Thanks for your advice and effort, I will get on that tomorrow morning. Since it takes no effort at all to do a quick run, I will take Joe's advice and give it a quick go before going on to the intake. Overall I think by days end I can have those changed made and a running engine. As for exhaust I would have done Joe's length first as it was no sweat except I already had the pipe cut. With that being said I don't have a new long piece to put on and chances of getting one by Friday are slim. I am going to have to do my best and do a full annular nichrome tack weld with the remaining 12" of pipe. Then I have rolled a sheet of aluminum to act as a floating tuner which I can get another 12" out of. Those together should get me close to your numbers and give me a hell of a lot better chance to it self sustaining.
Louder is ALWAYS Better!!!

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Re: The double-null

Post by milisavljevic » Wed Apr 29, 2009 9:40 am

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Edit: As this post was rendered obsolete by the disclosure of critical information, it is now made *poof*!
Last edited by milisavljevic on Wed Apr 29, 2009 12:20 pm, edited 1 time in total.

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Re: Big Test Lady FWE arrives in Aero/Combustion Laboratory

Post by larry cottrill » Wed Apr 29, 2009 11:54 am

M, thank you -- but once again I have to apologize. We DO have a few "as built" dimensions that should have been revealed already:

Both 245mm dimensions shown on the original plans really came out a bit short; the location of the flare is actually 485mm aft of the front chamber cone edge rather than 490. My most important correction is that the ID of the intake was incorrectly shown on the plan (I showed the OD instead of the ID) -- taking the .035-inch wall thickness into account, the actual ID is 42.4mm. I meant to reveal that much earlier on, but somehow it slipped past me. And that, of course, affects the intake length correction. As for the chamber and dome dimensions, these are hard to verify after construction; all I can say is that all pieces were hand-filed to VERY accurately fit the computer-generated cone patterns.

Greg - Can you provide us with the as-built overall length, from front chamber edge to tailpipe rim? Not easy to measure, but I think that would be very helpful.

The rather large flare at the tail end shouldn't be too hard to do, since that's your aluminum section. Make this flare BEFORE you weld that 12" section on, so you can use the pipe section as an internal support for your aluminum wraparound! Seize it onto the pipe with a couple of big hose clamps so you have maybe 2 inches of aluminum out beyond the pipe edge. It might be better to use a short length of smaller pipe as your "hammer" for making the flare, so you don't get any sharp dings in the aluminum.

L Cottrill

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Re: The double-null

Post by milisavljevic » Wed Apr 29, 2009 12:24 pm

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larry cottrill wrote:We DO have a few "as built" dimensions that should have been revealed already:
Argh! ...And that's not pirate talk! :(

Remodeling in progress. If you have anything new to add...

M.
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for ye merchants who do the prop'r t'ing only if
ye be haul'd-up on charges b'fore ye ship-mates
an' threat'nd wit' forfeiture of all ye precious loot
hear this - so-called stand-up guys YE BE NOT

avast!
Cap'n M.

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Re: Big Test Lady FWE arrives in Aero/Combustion Laboratory

Post by larry cottrill » Wed Apr 29, 2009 1:06 pm

If I have anything new to add, I will submit to walking the plank instead. Aaargh ...

L Cottrill

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Full length pipe now available!

Post by larry cottrill » Wed Apr 29, 2009 2:49 pm

Time to walk the plank. But, basically good news -- Greg says he WAS able to obtain a much longer tailpipe section, over a metre in length.

Basically, this means we should be able to handle whatever M wants us to do to optimize. This may even eliminate or minimize the tail flare previously described. We'll see.

For right now, NO MORE CHANGES until dictated by M (if he's still willing to talk to us).

L Cottrill

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Reported Running, Briefly

Post by larry cottrill » Wed Apr 29, 2009 3:09 pm

Greg reports that with the new tailpipe attached, she will sustain for about 10 seconds. Usually, I would offer that this means we are just a hair short. However, in this case, I still have to bow to M's criticism of the excessive intake length -- there is simply too much fresh air volume (and mass) there for the design to be reasonable, even if it would happen to sustain indefinitely. It does, of course, at least show that the basic front end configuation is capable of working, which is good to know.

M has pointed out that any data derived will be MUCH more meaningful to everyone if the motor design is better optimized. I have to agree with that -- if we're going to get data to share, we owe it to everyone to make sure its the best we can do with what we have available.

L Cottrill

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