Is there a simplified method for calculating fan performance?

[Donny]

Hello!

Is there? I have a propeller fan and I want to know what is the pressure, CFM, speed, etc. based on it's blade length, the number of blades, etc.

I already have a wind turbine generator and I seem can't find one for fans (propeller fan). Or is there a way to find those parameters?

Any help will be appreciated. :)

Thanks.

 

[DyslexicHobo]

We're actually doing an 8-week long lab designing a wind turbine, and I happen to have the exact equation you're looking for. :) We're learning more about it tomorrow (the equation isn't complete, because as of now the equation states the as you tend towards an infinite number of blades the power also tends toward infinity (which is of course not the case).

But here's some equations that might be relevant:
(Sorry I don't know how to use LaTeX)
Total rotational force = (1/2) * (density of air) * (sqrt(Windspeed^2 + omega^2 * radius of blade^2)) * num of blades * (chord length * dr) * (Lift coefficient * sin(phi) - Drag coefficient * cos(phi))

Phi = arcsin(Initial windspeed / (sqrt(Windspeed^2 + omega^2 * radius of blade^2)))

Again, this may or may not help ya, and it's very poorly written. But ah, I thought it might.

 

[Donny]

Thank you for your reply.

I don't really understand the concept. Could you please explain it to me a little bit more?

Is it possible to calculate the pressure produced by the fans? the speed in m/s or ft/min?

 

[DyslexicHobo]

There is a derivation that goes along with the equation, but I didn't have much time to post and explain. It also wouldn't be that useful because the equation isn't complete. It's ideal and unrealistic.

I have my engineering lab tomorrow so when we get a little more information I'll be sure to post back and help. Right now we can go as far as calculating and estimate for the total amount of energy harvested from the potential energy of the wind (which I'm guessing could be reversed... the amount of energy put into the air by the rotating blades).

 

[Donny]

Alright then, I guess we have to wait for it.

Thanks for your help :)

 

[DyslexicHobo]

Okay, if you thought the equation that I gave you the other day was complex and hard to understand... well... now it just got about twice as confusing.

I'm very sorry, but I'm not going to be of much help to you. All I can say is that you may want to look up Blade Element Theory and Blade Momentum Theory. Both of these were discussed in my lecture and led to an equation that could calculate the amount of kinetic energy from the wind was transferred to the turbine.

Using this equation, though, I assume you could measure how much energy the fan was using and then plug that into the equation to find the amount of kinetic energy is put back into the air (CFMs).

Unfortunately, unless you have an EXTREMELY specific spec sheet for the fans you're using, you will not be able to get a very accurate number. The equation must have exact values for things such as blade widths at every specific point, angle of attack at every point on the blade, etc. I believe you would also need to know the RPM of the fan.

I think you'd have more luck just trying to measure the wind speed and area of the fan and estimate the energy transfer to the air using some simple fluid mechanics equations (or perhaps you could even use basic conservation of energy equations).

Again, sorry I can't be of more help. Good luck!

 

[Topher925]

One of my internships while I was an undergrad was working on engine cooling systems for an automotive supplier. We used our own software which was based on correlated data from testing. However this was mostly because the fan geometry was rather complex and therefor its models were highly non-linear.

For simple fans though, you might be able to use the Eulerian description for axial flow turbo machinery. You will probably be off about 25% but it will at least give you a ball park estimation.