Can I make a blower fan operate effectively in a linear fashion with a directed duct? 2

[salukikev]

(As opposed to the typical perpendicular flow)

In other words, is this going to work? I have a small diameter motor & concentric fan blade confined in a cylinder and I need to move some air. Of course I could use a conventional propeller, BUT I want air to move in the same direction regardless of the direction of the motor. Also any pressure/volume increase would be a nice benefit. I've never tried something like this before, so thanks for any tips!

I think if this were to work, it's effectiveness would only be improved by the use of a "blocking" plate under the blades to improve directional airflow. Also reduced the support cage to 3 struts instead of 4 and simplified things for the view. Here is an updated illustration.

 

[lukaiENG]

isn't this basically a mixed flow fan?

 

[salukikev]

Is it? I thought the straight blades and arrangement of the motor may differentiate it. Will a mixed flow fan always blow the same regardless of rotation?

 

[hydtools]

Yes, it will work. The fan will intake air at its center and discharge it at its periphery. The air cannot go back to the intake, go it must go forward regardless of rotation direction.

Ted

 

[lukaiENG]

Mixed flow fans are typically designed for one direction of rotation. The fan impeller blades are curved to minimize dynamic losses to improve the efficiency of the fan.
By trying to design a fan which can rotate in both directions you are most likely going to have to compromise on the fan impeller design and therefore efficiency.
This may lead to energy code restrictions depending on your jurisdiction - in the USA fan performance needs to meet certain performance ratings to be allowed.

can you share the background to this project so we can view it in context?

 

[salukikev]

Sure- I should have done that from the start as none of the codes apply. This is actually part of a toy mechanism and so I'm trying to get a lot of function from a single motor.
Now that we've established that it works in principle, I'd welcome any tips on optimizing the performance/design. Specifically I'm wondering (and forgive my improvised terminology) what diameter the opening vs. the "blocking disc" should be, and what height the fan blades should be. Also wondering if there's any benefit to having the blade tips curved to match the deflector- maybe I just just chop them off straight?
The fan itself is about 5/8" in diameter.
Thanks!

 

[lukaiENG]

You're getting outside of my area of expertise - I'll hand off to other members more versed in fan design!
If you wanted to model it, there are free CFD software packages available (OPENfoam for example). This would allow you to model the performance for different designs for fine tuning.
If you search for OPENfoam within these forums you will find good discussions about these free packages. I've not personally used them but have heard good things.

 

[Compositepro]

The main reason this design will be inefficient compared to "optimized" designs is that the air leaving the impeller will have a lot of swirl, which is energy that will be lost as heat after the fan. Most radial vane blowers have volutes that convert the swirling velocity to linear velocity down a duct. But volutes are directional. You could design baffles so that at the outlet of your fan, only a 90 degree sector is open for air flow, and at the edges of this opening are curved vanes to smoothly redirect the swirling air to move axially down the duct. That is, if the improvement in efficiency is worth it.

Note that your design is almost identical to the cooling fan on electric motors.

 

[salukikev]

That's a good point on the motors, although I think that the motor fans are usually located in the middle (between the main armature and the brushes) of DC motors that I'm imagining, so the air flow situation I envision is pulling air from both the front and back of the motor and sending outward immediately at the fan tips rather than redirecting it.

Regarding the volutes, I wonder about how much efficiency I can pick up, but I have some other mechanisms that are generally linear so hopefully they'll act as deswirlers. Thanks for the info! I'm curious about the aspect ratios regarding the opening and backing plate (note the differences between the two linked images).

 

[salukikev]

Hi again,
Ok, I'm still working at this and trying to get the most of my one motor in a limited space (this is because this is a toy- so we're not high precision- just focusing on tiny, and especially CHEAP!) We have to drive our motor in both directions, but in either case want the same flow direction from the fan. In this iteration, I was thinking we could use a flexible blade axial fan instead of trying to make a centrifugal (mixed flow) fan happen. In a much earlier and different prototype we did get really good results using a standard axial microfan, but of course that was with optimized rigid blades moving only one direction. Definitely not cost effective but made for an convincing proof of concept.

Anyway, I'd appreciate any feedback on this attempt(vs. the earlier link here) using a flexible axial fan blade (likely silicone or maybe polyethylene).
Thanks!