DIY Group Project - Eductors

[Geoff]

Originally Posted by tdwyatt

so what is the plan?

we will discuss what they are for a bit.

then give out the parts list (apparantly most have already found the parts list, and there have been a run at some of the local HD's. )

then we get to see what results everyone has had.

G~

[tdwyatt]

will this be based on the original eductor diy you had last year?

[Weatherman]

I found a paper, about a year ago, which provided an excellent summary of how eductors work and some design considerations to make them more efficient. I can’t find the paper, but I did find my notes:


Note… Eductor, ejector and thrust augmentor are all names for the same type of device.

Some pertinent quotes

How it works:

“An exhaust ejector is a device, which converts a high velocity fluid flow of given mass flow rate into a fluid flow of lower velocity. This conversion is achieved by the transfer of momentum and energy through viscous interaction of the high velocity (primary) fluid flow with a lower velocity (secondary) fluid flow within a mixing tube…”


Design problem:

“The conventional mixing of the primary and secondary flow in a mixing tube occurs very slowly, which is performed mainly by a small scale viscous mixing in a shear layer. Thus, a conventional ejector requires a long mixing tube to entrain the secondary flow, and a long mixing tube results in large wall friction loss, extra weight and higher cost.”

In other words, the longer the eductor, the more mixing will occur between primary and entrained flows, resulting in an increase in the efficiency of the eductor, up to a point. The downside is that a longer mixing tube will also result in more loss in energy through friction with the tube walls. This suggests that the short, penductors may not be as efficient as the longer eductors.


But, here is a little tidbit for do-it-yourselfers…

“…a lobed exhaust ejector/mixer system (Fig. 2), in which a lobed nozzle was used as a primary nozzle, was proposed in the past several years. It was found that a lobed nozzle can cause large scale streamwise vortices to be shed at the trailing edge of lobe structures, so the downstream of the flowfield is embedded with arrays of large scale streamwise vortices, and a rapid exchange of momentum and energy was achieved by means of intense mixing within a short downstream with very little loss.”

In other words, if you want to turbocharge your eductor, add little lobes to the side of the nozzle section. There is, however, a little caveat:

“It has been found that the geometrical parameters of the lobed structure, such as lobe height, the lobe penetration angle, the form of the lobed structure, the area ratio of the primary nozzle, mixing tube etc, can affect the aerodynamic performance of a lobed ejector/mixer system very much.”

It ain't easy to build your own.


Flow separation effecting efficiency (longer diffuser code is better):
http://www.processinnovation.com/venturi.htm

[Krux]

i am interested in building one, even though i dont have a high flow pump, as i think they look nifty!

[Viv]

Ok, I told you I was game! Don't laugh-can't draw a straight line much less drill one-but here it is...

[Viv]

You'll notice in the second pic that the barb piece can fit all the way into the outer covering...does it matter whether it does or not? It would extend the nozzle a little if pulled slightly apart.

[Krux]

it has been some time since i saw one of these, but isnt the barb supposed to fave the other way somehow?

[Geoff]

OK, you all. you are getting ahead of the game.

Vince, you are missing a piece.

here is an exploded view of mine.



G~

[Geoff]

Originally Posted by Weatherman

It ain't easy to build your own.

way to burst my little bubble there. Weatherman.

G~

[tims]

i take it each piece can be bought at the Home depot or like store?

[Geoff]

Originally Posted by tims

i take it each piece can be bought at the Home depot or like store?

i will post a parts list pretty soon. i was hoping to discuss which nozzle sizes do what and why you would want one instead of the other, but if everyone just wants to start building than we can do that.

vote?

G~

[Krux]

my returns are 3/4" lockine, so whatever can be hooked onto that is what i vote for!

[robsmith32]

noticed someone asked about measuring flow on these, and said that about out of the water, you won't measure the benefit you're getting that way, as the benefit is that the force of the inner water pulls water in the system surrounding it through similar to how a venturi works but pulling water instead of air.

[Geoff]

they work exactly the same way as a venturi does, but with water.

they are difficult to measure because the way the water is drawn into it. plus any measureing device may add back pressure to the eductor dropping efficiency of and messing up the measurements.

G~

[Wiskey]

I want detailes on nossle sizes, because I think only with that info can we build the best one for our needs.

That is my vote anyway.

Thanks,
Whiskey