Square Thrust Rectifier

mk · Post by mk » Thu May 27, 2004 8:07 pm

Hello!

Has anybody thought of building a thrust rectifier (Escopette type) with a square cross-sectional area?!

Would be relatively easy to build (at least two stripes for inside and outside bend, perhaps internal stripes as well; two U-shaped plates for the sides)...

Drawing coming soon...

mk · Post by mk » Fri May 28, 2004 10:13 am

Here is an image of what I'm writing about.

It could be easily turned into a augmenter by chosing the cross-sectional area bigger than that of the ejector pipe and adding an end-cone after the "U" or by flaring the "U" itself.

Bruno Ogorelec · Post by **Bruno Ogorelec** » Fri May 28, 2004 12:32 pm

Marten, it is an excellent idea. It works well, too. SNECMA used square section for U-bends on a number of Ecrevisse models.

Frankly, I fail to see why square section couldn't be used for the entire engine. It would probably bother a constant pressure engine (like a turbojet or ramjet) but flows in a pulsejet are so jerky that I'm certain the section can only have negligible influence on performance.

Anyone remember the triangular-section engine by Ben Brockert? The most ridiculous idea I have heard of. It worked fine.

larry cottrill · Post by **larry cottrill** » Fri May 28, 2004 12:52 pm

brunoogorelec wrote:Marten, it is an excellent idea. It works well, too. SNECMA used square section for U-bends on a number of Ecrevisse models.

Frankly, I fail to see why square section couldn't be used for the entire engine. It would probably bother a constant pressure engine (like a turbojet or ramjet) but flows in a pulsejet are so jerky that I'm certain the section can only have negligible influence on performance.

Anyone remember the triangular-section engine by Ben Brockert? The most ridiculous idea I have heard of. It worked fine.

Just a couple of comments:

The main reason a circular cross-section is generally used is that it minimizes material -- the circle is the smallest shape that will bound a given area.

A cylindrical duct is stronger in some respects than any flat-sided shape. It is much easier to collapse a flat sided duct inward due to negative pressure. It is much easier to bulge a flat sided duct outward due to positive pressure. And of course, these tendencies are enhanced by getting the duct material really hot.

The flat sided duct will absorb and radiate more heat from the flow than a round duct of the same flow area, because there is a much larger surface area in contact with the flow boundary. For the same reason, the drag of a flat sided duct is significantly higher than that of a round duct.

For the same thickness of material, a rectangular duct can be made harder to bend than a round duct of the same internal flow area, especially if braced at intervals along its length. This is partly because more material is required for a given flow area, but mostly because the concentration of material toward the outside edges is greater.

The flow will be much less uniform in a flat sided duct. The corners of a rectangular duct tend to have much lower flow velocities than the center flow, and significantly lower velocities than the near-boundary area of a round section. This is especially true where a flow is made to turn a corner to go in a different direction [in air conditioning work, corners often include carefully design turning vanes to try to smooth out the flow].

L Cottrill

Bruno Ogorelec · Post by **Bruno Ogorelec** » Fri May 28, 2004 2:41 pm

Larry, agreed on all points. You explain it nicely and clearly. However, a number of those points need not be a problem for a pulsejet builder.

I have already said that I doubt that uneven flow due to corners will affect a pulsejet much. Air-conditioning units etc. are probably bothered much more. A fast series of explosive blasts is not the same as continuous flow.

Structurally, a short curved duct will probably be more than strong enough to take the loads. It worked for SNECMA. This is especially true for the recuperator/thrust augmenter in which the stresses are comparatively negligible.

I wonder what happened to that wonderful triangular curved recuperator that looked like evening out the gas speeds through the bend? Who was it that proposed it? Was it Tundra Man? I loved that idea! Kirney, was it you?

Post by **Mike Everman** » Fri May 28, 2004 3:28 pm

Bruno, 'twas I:
http://www.pulse-jets.com/phpbb2/viewto ... 9&start=15

And then Chris (resosys) Brick made a triangular cross-section engine and ran it:
http://www.pulse-jets.com/phpbb2/viewto ... sc&start=0

Bruno Ogorelec · Post by **Bruno Ogorelec** » Fri May 28, 2004 3:43 pm

Mike, I'm getting too old for this. Can't even get my triangles straight.

Bruno Ogorelec · Post by **Bruno Ogorelec** » Fri May 28, 2004 3:46 pm

I still LUUUUV that tringular U-bend. I'm a sucker for simple and elegant solutions.

Post by **Mike Everman** » Fri May 28, 2004 4:07 pm

I aim to please, buddy. Viv was going to weld one up, but got sidetracked.

Pieter van Boven · Post by **Pieter van Boven** » Fri May 28, 2004 6:42 pm

Hello,

this is a very interesting trhead. At the moment I am searching for the best method to make the Ecrevisse trust rectifier or U-bend for the Ecrevisse-project. In stead of the mentioned square shape, I was thinking of an octogan trust rectifier. After welding the, 8 pre-shaped parts together, the complete rectifier could be welded on two pieces of round material (inlet-, outletdiameter) so that a fitting could be attached for hydro- or hydraulick forming. The rectifier-shape will keep up with the in- and outlet-diameter.

Pieter.

Pieter van Boven · Post by **Pieter van Boven** » Fri May 28, 2004 7:19 pm

Here is a drawing of what I mean.

Pieter.

Post by **Mike Everman** » Fri May 28, 2004 7:48 pm

That's a lot of welding, Pieter. Hydroforming the square section to round will give good results, I think. Remember you can beat it into shape when it's pressurized... That's how they get the wrinkles out of it.

Viv · Post by **Viv** » Fri May 28, 2004 8:25 pm

Mike Everman wrote:I aim to please, buddy. Viv was going to weld one up, but got sidetracked.

Hey buddy i am still waiting for the welding teacher to come up with the goods, if he does I am game on!

Viv

Grant · Post by **Grant** » Sun May 30, 2004 11:51 am

Old two stroke tuners would dimension there expansion pipes for the rpm where powerband was desired. They would then make a straight pipe with the required cones and reverse cones, and then cut as shown in the picture below to shape ithe pipe around the bike. the great thing with this method is that acoustically the pipe is not altered, but naturally there is a bit more fluid drag. This cutting technique can be used on section of pipe where opposing walls are not parrallel (such as a cone). Any angle is achiveable. The masterpieces that can be created when a straight expansion chamber is cut and rewelded 20 or thirty times are unbelievable...they end up looking quite smooth and organic!

This book covers off on expansion chamber design and manufacture and lots of other two-stroke tuning voo-doo
http://www.amazon.com/exec/obidos/tg/de ... 4?v=glance

Grant · Post by **Grant** » Sun May 30, 2004 11:59 am

Here is another link describing whats going in a two stroke expansion chamber...i just thought it might be useful.

http://www.vf750fd.com/blurbs/stroke.html

Square Thrust Rectifier

Square Thrust Rectifier

Heres another link