Expert: Paul Soderman - 11/26/2010

Question

We are designing a Wind Tunnel as part of our senior design project. Here are the dimensions which we have for our tunnel.

Length of Settling Chamber (contains honeycomb and screens) ----- 1.625 ft
  - We are using straws and an 18 mesh, 0.01 inch wire diameter screen
Length of Contraction ----- 3.25 ft
  - Its a 3D contraction. Contraction ratio is 10.5:1. We derived a fifth order equation using Bell & Mehta as a      reference.
Test Section Size ------12 inch x 12 inch x 36 inch
  - We are adding chamfers along the corners of the test section (and the whole duct system) to reduce losses.
  - Max. Velocity in Test Section ------ 45m/s
Diffuser ------ Conical diffuser with a divergence angle of 6.34 degrees. Its length is 9 ft. End of the diffuser is 36 inches(diameter). We added 2 extra feet after the diffuser for fan housing.

Losses ------ Preliminary calculations gave us a pressure drop of 1.5 inches of water.

Fan Size ------ We are looking for a fan that can generate 9,000 CFM (ballpark) and has 30-36 inch diameter.

Total Length of our tunnel is 20 ft.

Where we are stuck:
  * Sizing the fan motor. Is it as simple as choosing a fan that can provide 9,000 CFM at 1.5 inches of water?
        - We calculated 2 hp as our power requirement (theoretically) but we are wondering if it is the             actual power requirement or if we need to calculate (or find) the fan efficiency, the shaft power,             and the motor efficiency in order to get the actual power requirement?
  
  * What do you think of our design in general? Do you feel that the numbers which we got sound OK?
        - We initially thought about contacting you at the beginning of the semster but then we realized             that we will understand you advice better if we know about the design of wind tunnels.
  
  * How should we deal with the model as an obstruction to flow?
  
  * Are there any specified guidelines for positioning the model in the tunnel?
        - (Example: distance from the side walls and distance along the length of the test section (this is             the only thing on which we are confused))   
  
If you can please provide us an email address, we would like to send you our Design Report. We would really appreciate if you can give us your expert comments on our design.

Thank you for your time & Happy Thanksgiving.

- Senior Design Team (2010)
 Mechanical Engineering Department
 Wichita State University  

Answer
Tapan
Off hand, your design sounds good, but I am on the road for a week and a half and cannot check my reference material.  For example, I would check Rae & Pope:  Low- Speed Wind Tunnel Testing to answer your questions.  Perhaps you can get a copy while I am on the road.  My email is psoderman@gmail.com.  You should try to estimate the efficiencies of the fan system.  I imagine you can get some ballpark numbers on line.  80% would be a decent starting point.
Paul