Exhaust Augmenter Design Question

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HomeBuilt101

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Can someone please direct me to some information and/or provide some advice on the topic of exhaust augmenter design?

I am very familiar with "injector pumps" and such because of my aircraft systems background however I have never designed one...until now.

The project is a bit off topic for experimental aviation because it is an RV project however the RV does take me to airshows and it lives at an airport so hopefully that is close enough.

I have built a sound enclosure for a portable generator and I want to use the exhaust blast to ventilate the box.

So specific to the exhaust augmenter tube, I need to figure out what is the best location for the genny exhaust pipe tip (how far into the augmenter tube should it be .

The exhaust pipe on the genny is 1 inch in diameter and the "cherry bomb" muffler exhaust pipe is 3 inch ID so I need to figure out how far into the 3 inch pipe the tip of the genny exhaust pipe should be. I think just 1 inch insertion should work because as the genny exhaust pulse blasts into the tube that burst will open up and expand like a mushroom as it transits the 3 inch pipe and that mushroom should drag a bunch of air with it through the "muffler". The cherry bomb has almost no obstruction just some perforated holes around the inside of the pipe so it should not provide much back pressure.

I know there has to be some great deal of design parameters of a real air injector system with the size difference between the inside pipe and the outside pipe and the contours of the shape of the tip etc so hopefully I can get the design close enough to produce some ventilation for the box.

THANKS!!!
 

Riggerrob

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I found this some years ago, hope it helps.
Good illustration of the venturi tube inside an exhaust augmentor.

Also look at the augmenters installed on DHC-3 single-engine Otter if it still has the original Pratt & Whitney radial engine. I suspect that Otter augmenters were primarily intended to increase cooling air flow at low airspeeds.
 

KeithO

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1657139796577.png

If you take a look at the stock exhaust tips on GM duramax diesel trucks you will see a more practical example of the kind ot tip you are looking for. The goal of these tips was to mix ambient air into the hot exhaust stream to cool it down to lessen the risk of fire when the engine was doing a regen.
 

HomeBuilt101

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I found this page on the web. It is most likely a simplified drawing however it sure looks fairly simple...like I was wanting to do. Just a small exhaust pipe blasting through the center of a larger diameter pipe.


1657139402352.png

Here is a picture of the Otter exhaust. Still looks pretty simple.

1657139641500.png


Maybe I should buy this airplane augmenter tube for 300 bucks. The picture looking down the tube is kinda what the cherry bomb "muffler" looks like


Here is a Caribou exhaust system. The website says that it adds a little extra thrust. Maybe this will propel my generator on my RV

1657140216085.png



That is about all I found on the subject...I guess I broke The Google
 

Victor Bravo

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There seems to be some question (at least on an airplane augmentor) whether you are trying to use the power of the exhaust to help drive increased engine cooling airflow, or whether you're trying to use the engine cooling airflow to help lower the pressure in the exhaust pipe and reduce the pumping losses of the engine (engine "breathing" easier).

I doubt that any system could accomplish both at the same time, otherwise you'd somehow be creating more energy than you put in.
 

aeromomentum

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While total "cooling drag" can be reduced and cooling can be improved, any "jet pump" including an exhaust augmenter will be less than 100% efficient so it will reduce the possible thrust from the exhaust. Yes, exhaust can produce thrust. The most efficient way to get thrust from the exhaust is straight from the pipe with the pipe pointed aft.

To use an augmenter to reduce the engine pumping loss, the velocity in the entry of the augmenter must be higher (lower dynamic pressure vector...) than the velocity outside the aircraft. This is very unlikely but possible.
 

Toobuilder

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I'm trying to use the exhaust pulse to increase airflow through the cooling system at low speed and lower dynamic pressure. It has been very successful. My total exit area is miniscule by the conventional "rules of thumb", and I'm STILL overcooling on all but the hottest days.
 

HomeBuilt101

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>The most efficient way to get thrust from the exhaust is straight from the pipe with the pipe pointed aft.

Of course all I want to do is to ventilate my new generator enclosure...however...

Lets say you are wanting to try recapture some of the exhaust pulses to try to make some kind of thrust instead of just wasting that pulse energy so you want to compare the following two exhaust systems:

-Exhaust pipe hanging out under the fuselage in the free air stream and pointed straight aft

-Exhaust pipe inserted concentrically into an exhaust augmenter pipe that is pointed straight aft.

With the first option, the air streaming past the tip of the exhaust pipe that is hanging out into the breeze is already traveling at 100 MPH so the puff from the exhaust pipe does not have any slower air to push against so I would think that puff of exhaust would be completely wasted.

With the second option, the air streaming through the exhaust augmenter pipe has previously been smashed through the cowl opening, stagnated and gained pressure inside the top section of the cowling, then forced through the cylinder fins and has finally found itself expanded into the lower cowling and is somewhat stagnant and now drifts inside the augmenter tube and now the puff from the center exhaust pipe is pushing back against that slow moving air mass.

The advantage in addition to the "recovery" of that energy pushing aft against the air inside the augmenter tube is that the air in question needs to come from somewhere so it creates a lower pressure inside the lower cowling and that helps draw the cooling air down through the cylinders.

The trick is to design a augmenter pipe that can withstand the heat, point aft in the appropriate direction, and be light enough so that the weight and resultant drag penalty does not counteract the benefit
 

BJC

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With the first option, the air streaming past the tip of the exhaust pipe that is hanging out into the breeze is already traveling at 100 MPH so the puff from the exhaust pipe does not have any slower air to push against so I would think that puff of exhaust would be completely wasted.
Any thrust comes from the exhaust gasses having had their momentum changed, not from pushing against something. Rockets work in a vacuum such as space.

Exhaust gasses that exit and then need to have their momentum changed disrupt the near air flow and, therefore, create drag.


BJC
 

Old Koreelah

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NACA did quite a bit of experimenting with exhaust augmentors in the 1940s and the documentation is easily accessed. I couldn’t understand why this great idea wasn’t widely adopted, so I installed augmentors on each side of my fuselage. They worked well, helping cooling and adding about 4% to power.

Unfortunately their exit was just below the cockpit canopy, emitting an intolerable howl on full power. My hearing was at risk, so they had to go.

Some GA aircraft seem to use the augmenting effect in their cooling design. Have a look under many Cessna, Piper, etc. aircraft: many have quite small outlet cowls with the exhaust pipe right in the middle.
 

Bill-Higdon

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NACA did quite a bit of experimenting with exhaust augmentors in the 1940s and the documentation is easily accessed. I couldn’t understand why this great idea wasn’t widely adopted, so I installed augmentors on each side of my fuselage. They worked well, helping cooling and adding about 4% to power.

Unfortunately their exit was just below the cockpit canopy, emitting an intolerable howl on full power. My hearing was at risk, so they had to go.

Some GA aircraft seem to use the augmenting effect in their cooling design. Have a look under many Cessna, Piper, etc. aircraft: many have quite small outlet cowls with the exhaust pipe right in the middle.
Early 310's had them
 

Marc Bourget

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Homebuilt said:

I have built a sound enclosure for a portable generator and I want to use the exhaust blast to ventilate the box.

I have some experience with genset installations in Recreational Vehicles. I assume the generator is a single cylinder?

Portable gensets usually have a cooling fan installed and the task will be to separate the supply side from the discharge side so you can utilize the fan. You can also install an ejector pump to reduce downstream temps.

John Thorp, in a December '63 Sport Aviation article gave guidelines and insight to designing an effective exhaust ejector pump. (attached).
 

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Bill-Higdon

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I seem to remember an article in a Sport Aviation later than the one mentioned above that discussed exhaust augmenter design which I believe David Thurston was mentioned in it or maybe wrote it.
 
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Can someone please direct me to some information and/or provide some advice on the topic of exhaust augmenter design?

I am very familiar with "injector pumps" and such because of my aircraft systems background however I have never designed one...until now.

The project is a bit off topic for experimental aviation because it is an RV project however the RV does take me to airshows and it lives at an airport so hopefully that is close enough.

I have built a sound enclosure for a portable generator and I want to use the exhaust blast to ventilate the box.

So specific to the exhaust augmenter tube, I need to figure out what is the best location for the genny exhaust pipe tip (how far into the augmenter tube should it be .

The exhaust pipe on the genny is 1 inch in diameter and the "cherry bomb" muffler exhaust pipe is 3 inch ID so I need to figure out how far into the 3 inch pipe the tip of the genny exhaust pipe should be. I think just 1 inch insertion should work because as the genny exhaust pulse blasts into the tube that burst will open up and expand like a mushroom as it transits the 3 inch pipe and that mushroom should drag a bunch of air with it through the "muffler". The cherry bomb has almost no obstruction just some perforated holes around the inside of the pipe so it should not provide much back pressure.

I know there has to be some great deal of design parameters of a real air injector system with the size difference between the inside pipe and the outside pipe and the contours of the shape of the tip etc so hopefully I can get the design close enough to produce some ventilation for the box.

THANKS!!!
 
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