Uflow and Concentric PJ

[hagent]

Hello Everyone!

Happy Holidays!

I was wondering if someone could please answer a not so simple question.

When using UFLOW I have been modeling the intake and CC as one model and the exhaust as a separate model for my concentric PJ. I wanted to confirm how I should build the exhaust model in UFLOW. Should it be closed at one end or open at both ends?

The exhaust tube fits down the middle of the intake tube, and the intake tube is welded to the CC.

Thanks so much for your assistance.

Hagen

[larry cottrill]

Hagen -

In my opinion, an 'exhaust only' model should be a front-end-closed model that includes the chamber, just as if the intake did not even exist. The main flaw this has is that the rarefaction wave will appear more powerful and longer-lasting than it really is, because there's no intake to let the engine breathe to relieve the low pressure. As long as you understand that you're not really modeling the whole engine, such a model can be very useful.

The original FWE (aka 'Short Lady') was modeled in exactly that way, with the intake added on later to satisfy the 'Hinote Criteria'. You'll recall that the independently built prototype (Steve Bukowsky) fired and sustained the first time it was tried.

The other thing you can do is mathematically "swing" the intake around to the front, using an intake pipe diameter that gives you the clear area you actually have between your intake and tailpipe, then adjust the intake length as you look at what UFLOW shows you on the graphs. But, this "linear engine" model will probably be hard to get right, and hard to interpret in the case of your concentric engine, which most definitely does NOT resemble a one-dimensional situation.

L Cottrill

[hagent]

Thanks Larry,

If I model the intake on the front and exhaust on the rear won't that drasticly change the accoustics?

Should I not model the intake with the CC but with the correct used area and length?

Also is the goal to have the intake resonate at twice the frequency of the exhaust? Or is there a different magic number?

Or should I just model them both open ended without the CC and get the magic ratio between the frequency, and let the CC do what it wants?

Thanks

[larry cottrill]

If I model the intake on the front and exhaust on the rear won't that drasticly change the accoustics?

Actually, no, if you do it right. Remember, the front plate of your engine is an acoustic "mirror", so the intake "sees" the tailpipe out in front, and the tailpipe "sees" the intake out in front, acoustically. The problem in modeling this machine is not basically acoustic - it is more mass flow and velocities.

Should I not model the intake with the CC but with the correct used area and length?

No - the intake does not handle the air mass as if the tailpipe weren't there! That pipe takes up a lot of the area. The acoustic length of the intake is not much different from its actual length - it should not need huge end corrections because of the presence of the tailpipe. But again, acoustics aren't the big problem - the presence of the tailpipe GREATLY reduces the effective diameter of the intake, so you should model an ID based on that "net" area.

The biggest problem is that the embedding of the tailpipe in the intake means that the chamber cones will be way off in the model, both on the intake and exhaust ends of the chamber, once you adjust that intake diameter. So, what can we do?

What I would do is "reflect" the intake end around the end plate to the front - even including reflecting the chamber itself! The front plate disappears, of course. Then, reduce the chamber volume by half, by multiplying its ID by 71 percent. This will give you the proper chamber volume, the proper tailpipe volume, and the correct intake volume (because you've reduced the ID to get the proper area). This also reduces the area of the end cones, bringing them closer to reality. This should give you a linear model that will roughly act like your highly multidimensional real engine.

Also is the goal to have the intake resonate at twice the frequency of the exhaust? Or is there a different magic number?

That's one way to approach it, but the real thing you want is to adjust intake and/or tailpipe until the engine as a whole resonates nicely - that's the goal!

Or should I just model them both open ended without the CC and get the magic ratio between the frequency, and let the CC do what it wants?

In my opinion, no. You need to model the chamber as an integral part, even if the model doesn't seem to resemble the real chamber that much (in what I've described above, twice as long and half the cross-sectional area as real life). The main reason for this is the acoustic path length, which has to include the chamber, as well as the expansion that has to take place there.

There is going to be no such thing as a perfect 1D model of such an engine design, because of the blending of the pipes. But, you might at least have something useful if done as described.

L Cottrill

[Mike Everman]

I like the two closed end models with proportionately smaller chamber area approach, Larry. Very creative. I've usually rotated the intake around. I'll try that some time.

[Eric]

Hagent could you post a sketch of your engine layout?

Eric

[hagent]

Hagent could you post a sketch of your engine layout?

Eric

Hi Eric,

I haven't finalized it yet. I'm trying to keep it as versatile as I can by allowing myself the ability to adjust the intake and exhaust. I'll come up with a few different versions and post them. Let me know what you think.

Thanks,

[hagent]

Hi Eric,

Here is the first version of my new concentric PJ. The exhaust and intake are even in the CC.

I have the ability to move the intake flare, exhaust tube, and intake tube with attached fuel ring.

I hope that this setup will give me enough lea way to get this running. I'm still unsure about how UFLOW is predicting things.

Spark plug is not shown.

Any ideas?

Thank you,

[hagent]

A couple of more pictures.

[larry cottrill]

Beautiful, Hagen. This is the kind of creative thinking that I always enjoy seeing. Keep it simple, but make it flexible and adaptable.

Very Nice. I really like the basic idea of the concentric design, and this should be a lovely way to experiment with it.

L Cottrill

[Mike Everman]

Yeah, nice model.

[hagent]

Thanks Larry and Mike for the kind words.

I have so far finished the fuel ring and intake tube as shown. I just finished one of the two cones on the CC.

I've never made cones before. I just worked it by hand until it looked right. I know it's not perfect but I don't have a slip roll. Luky for me I didn't buy a thicker gauge. I'm currenlty using 24 gauge mild steel for the cones. The intake tube was from a piece of exhuast pipe. The fuel ring was from a decorative 1/2 inch square stock made round 4" OD and then I just welded caps on them and then did a leak test. I decided to use Gorilla glue to seal it against the intake tube becuase of the different thickness in metal, it would be hard to weld without making a mess. Also since it is the intake tube I'm hoping the glue will survive the test, and the Propane gas should help cool it too.

Tomorrow I will try and finish the CC.

[hagent]

Yeah, nice model.

This will give you a better idea of the flexibility. The exhaust tube can also move.

Hagen:)

[thecheat]

interesting, so basicly this is a "telescoping" pulsejet made so you can fine tune it. I'd love to see this run!


PS: don't get me wrong, but, is it me or does that look like a baseball bat stuck inside a vase :roll: :lol: not sure if it was meant that way, but, that's what pops into my mind first! looks like a promising project though!

[hagent]

PS: don't get me wrong, but, is it me or does that look like a baseball bat stuck inside a vase :roll: :lol: not sure if it was meant that way, but, that's what pops into my mind first! looks like a promising project though!

Now that you mentioned it, I have been eying a few vases around the house :) I wish they were made out of steel though.

Now the real cool thing would be if the adjustments could be made with actuating mechanisms while the engine was running!