Energy efficient air amplifiers 2

[dspDad]

Does anybody know a non-mechanical way to convert high pressure (or high velocity) air flow to a larger mass of slower flowing air in an efficient manner.

I am looking for a no-moving parts method of performing what a high-bypass fan does in a jet engine.

So far I've tried a venturi type system, and discovered that the momentum is conserved, so thrust is unchanged.
Coanda air amplifiers do a little better, but the best I have found is only about 20% efficient at creating 350 fps air flows.

Exair sells a coanda type 'super air amplifier' whose published specs indicate a 70% efficieny (which would be great), but my own measurements indicate a 20% energy efficiency.

Are there any methods I've missed that are applicable?

Thanks,
Dspdad

 

[Sparweb]

A nozzle arrangement of some sort is the only system that I can think of that would work, without moving parts. Try different types of ejector pump. Remember that the speed of sound is a limitation. If V_out is 350 fps, at what speed does the source flow?

If you're trying to generate thrust from something, remember that energy has to get into the working fluid from some external source, before being expelled.


Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[GraviMan]

"So far I've tried a venturi type system, and discovered that the momentum is conserved, so thrust is unchanged."

Hmmm, I thought this was the point? Thust stays the same, but you accelerate more air to a lesser extent to achieve it. This is why bypass engines are more efficient. Are you sure you haven't just got a divergent nozzle? If you allow more air to be sucked in along the path of the HP air (ie slots along the expansion), then you will shift a lot of air.

I tried this on a friends exhaust many moons ago. The idea was to generate a vacuum for a mild boost supercharger at t'other end. Using the vacuum to generate anything more than a mild pressure boost proved to be the problem, but it was fun...

Mart

 

[dspDad]

Graviman,
The high bypass uses jet power to turn a ducted fan, and achieves what I would call impedance matching. As a result, the thrust is higher and the speed is lower, so energy is conserved. This only works for 'low' speeds, but mach .8 is low compared to jet exhaust, so all commercial jets use this technology.

This set of experiments was triggered by a post you had a while back about tip-jet helicopters. If there was a cheap, reliable way to get some impedance matching, then driving a helicopter rotor with compressed air or jet exhaust would be a lot more promising. So far, in the absence of a turbine and fan arrangement, I'm not sure how to do it. The turbine/fan has a significant problem with precessional forces if mounted on a rotating blade, which is why I asked about non-mechanical solutions.

Any other ideas?

Thanks,
DspDad

 

[GraviMan]

"This set of experiments was triggered by a post you had a while back about tip-jet helicopters."

Hehehe - if there is one thing I really enjoy it is stimulating new ideas! I like your line of thought...

One idea i did have was to pump compressed air into the rear section of the chopper blade. You then have a series of nozzles on the upper trailing edge. The idea is to inject fluid energy into the most draggy region, since this is where boundary layer (from laminar to turbulent) becomes thickest. Would also stop stall in fixed wings, but I digress.

The main point would be spread the nozzle area, and induce flow in the surrounding air. You'll have to play with the nozzle position, but my guess is start with the trailing edge then work forwards. Trailing edge might even induce flow in both top and bottom sides, so who knows.

Whatever you try, have fun doing it. That's the who;e point of science and technology! ;-)

Mart

 

[JohnFortier]

I'll probablty get shot doen in flames for suggesting this, but I remember using a Jetex "engine" in model aircraft with what was called an augmentation tube.

The Jetex was really a simple slow burning rocket engine, with a small hole at the rear instead of a venturi, {had to be affordable to teen and sub teen boys) and it produced enough thrust to propell a simple model quite well, albeit for a pretty short time. The augmentation tube fitted around the Jetex. It was basically a half venturi, with a wide bell mouth and a long tube, itself larger in diameter than the Jetex. the Jetex was mounted at the inlet to the tube.

The idea was that the Jetex produced a high speed stream of gas which sucked air through the tube and mixed with it. The tube was long enough to ensure that deceleration of the rocket exhaust and accellaration of the surrounding air was pretty complete by the time the end of the tube was reached. At the speeds that the models I built operated, the lower speed exhaust from the augmentation tube was more efficient than the high speed exhaust from the Jetex, so performance was improved.

Strikes me that, if your high speed air flow was simply directed into a similar installation, you would achieve most of what you need. The downsdide, of course, is the length of the augmentation tube. Also, you'd probably have to undertake quite a bit of experimentation to determine the best match of bell mouth diameter, tube diameter and tube length. The other downside, unfortunately, is that the flow from the tube is very turbulant, which robs some of the efficiency of the system.

Just a suggestion, but Ihope it helps.

John