an impeller blade of another kind

[Shakey Jake]

I've been thinking about this for some time and would like some input from some of the thinkers lurking here abouts.
Assume you have a mandrill with impeller blades in a sine wave configuration (any number of blades) mounted on it. This assembly rotates inside a close fitting shroud, as in a ducted fan unit. The blades are mounted at an angle in relation to the mandrill, so that the blade's angle of attack is increased at the leading edge and the center section of the blade has the ability to move air back instead of just pushing it sideways.
The front half of the blade is very similar to a standard impeller blade. I'm curious as to what effect the rear half of the blade will have on the air stream.
Will it give an additional push to the stream as it leaves the blade, therefore increasing thrust? What would be happening on the back side of the blade? Would the reverse curvature at the rear of the blade create additional/less drag….additional/less turbulence?
It just seams to me that there might be some additional thrust lurking somewhere in this set up. One could play with the length (frequency) of the sign wave, angle of attack and number of blades endlessly. Or is this a dumb idea that has been tried & trashed? I'm thinking about building one with three blades just to see what it will do. Any insights on this, anyone?
The drawing isn't too hot but maybe you can get the gist of it anyway.

[Matt M]

Basically, what you have drawn is one blade of an axial flow compressor. Similar to a ducted fan with smaller blades.

It would produce thrust with circumferential blades as you have drawn, but I don't think that it would be very efficient.

Maybe someone who has done something similar can shed some more light on this.

Cheers,
Matt

[gkamysz]

I've never seen an axial flow anything that looked quite like this.

Get a copy of this book if you would like to learn how to desgin fans. I ordered through B&N or Borders for about 100USD.

http://www.cranfield.ac.uk/public/me/me063a/books.htm


Bascially you can achieve 90% efficiency with conventional blade shapes, which have been in use for 50+ years. In our size fans you won't get 90% but all the information still aplies. The only way to get more thrust is to improve efficiency. Hard to do.

Greg

[Shakey Jake]

Matt,
I have seen industrial compressors that have vanes similar to this, but they were in a spiral configuration like a screw. Don't know how efficient they are but they are generally used where high volume and high pressure are required. I guess they must not be too inefficient, or they would use the old piston type compressors instead.
Greg,
I have a book on ducted fan design, but all it discusses are the usual air foil type blades. I'm not really interested in trying to re-hash their designs. I'm sure I couldn't do any better than they have with those. I'm just one of those idiots who thinks there has to be a better way out there somewhere. Another infernal tinkerer I guess.
My main reason for putting this up on the Zone was to get some other thoughts on how air will flow around a blade of this type. Guess I didn't make that too clear. Mumble mumble…
The way I see it is:
1. The forward third of the blade takes in air as it rotates, and compresses it (increases velocity)
2. The center section angle would possibly continue the increase in velocity…???
3. The back third of the blade---Will it give additional thrust, or will it just unload due to the reverse curvature in that section??
4. On the back side forward third a low pressure area will develop (increased velocity) aiding the next blade in its effort to take in additional air to be compressed
5. On the back side rear third of the blade another possible low pressure area could develop creating possible vortex = drag, or would the high pressure created by the following blade fill this area with high velocity air and come off the blade with a smooth non-turbulent flow = less drag.
Well, go ahead, tear me apart or….add a thought or two.

Thanks Guys

Shakey
:p

[gkamysz]

Our fans don't need high pressure to operate. The only way to improve on the designs we have is to improve efficiency. If you think this will do it, then you'll have to build one and try. I don't see how this has any benefit over a standard design.

As far as industrial compressors it could offer no efficiency advatnage over a piston compressor, but noise and maintainance benfits would alone be worthwhile. Do you have any pics of these kinds of compressors? I couldn't find anything online that resembled your picture or description.

Greg

[BMatthews]

First off we need to remember that turbine blades operating on air are not much more than a ducted propeller. And propellers are no more than little rotating wings. Now what you've drawn is basicall a highly reflexed airfoil. Reflexed airfoils don't do well at very high lift coefficient's as they give up a bit of the section's lift for stabilizing forces. And if there's anything that needs to operate at a high CL it's got to be ducted fans and other axial compressors.

So I think what you have here would not work as well as using the conventional fan airfoils at producing VELOCITY in the airflow. And for flying ducted fan models it's the velocity coupled with decent mass volume that we want.

But perhaps the reflex at the rear of the blades would act to reduce the flow velocity and thus produce higher pressure more effectively if the downstream system was restrictive. Only testing or someone brighter than me could tell....:D

[Shakey Jake]

Greg,
Sorry no pictures. The only picture I've ever seen of one of these setups was a cut away view of one in an old WW Grainger catalogue some years ago. I have seen some on job sites running jack hammers and such.
Mat,
You may well be right. I had given some thought to tapering the blades and shroud from front to back by maybe 3% to 5%. Two reasons for this. The reduction would increase both pressure and velocity, I think, which would possibly help keep the rear section of the blade from unloading.
Well in any case you are probably right about this not being an improvement over conventional blades. More of my convoluted thinking I suppose. I have a twisted mind that forces me to try goofy stuff like this. Think I'll give it a go anyway. These little projects make life interesting.
Thanks for the comments.

[bobbdd]

Going in that direction with an impeller blade,is leaning to more pressure,at the cost of more power needed to spin it.As in injction molding,...moveing a blob of viscous matrial insted of something like air.For a DF we want thin,minimal crossection airfoils.;)

[Arbo]

If you really are dealing with nothing more than a little wing... what sort of airfoil are you looking for?

One might think a high lift foil, with a minimal AOA. Thinking high lift will mean pushing air out the back more than a low lift.. (would that be efflux or thrust?)

Or do you want a more semetrically foil, and just put it at a higher AOA (aka pitch)?

I'm trying to understand how you get higher efflux vs higher static thrust, etc.

Paul

[Kevin Cox]

Originally posted by gkamysz
......As far as industrial compressors it could offer no efficiency advatnage over a piston compressor, but noise and maintainance benfits would alone be worthwhile. Do you have any pics of these kinds of compressors? I couldn't find anything online that resembled your picture or description.

Greg

Greg,
We have these type of compressors at work and they are called "Screw type". The part that is missing from this thread that will help you understand how they work is that the screw is used to pump hydrualic fluid and the high speed oil is sent into some type of ventrui and air is sucked in due to this action. I may not have all the steps correct (I only work on the electrical part) but this is the idea behind it. If you are interested I can get pic and info for work.

[gkamysz]

I still haven't been able to find anything like it. The screw type compressors I found did not operate as you said. All were similar to a type of supercharger found in cars.

I'd really like to find al ink to somebody that makes these. They would have enough info to see what it's about.

Greg

[Kevin Cox]

Now that you have said that supercharger part, I think you are right. I will see what I can dig up at work.