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# Fan inquiry for open circuit wind tunnel

## Fan inquiry for open circuit wind tunnel

(OP)
Hi all,

I am wondering if anyone can help me with fan selection for a open circuit wind tunnel design. I understand that the fan static pressure is required in order to select a fan, but I am not sure what exactly is the pressure value for this wind tunnel design; I am attaching a sketch for the wind tunnel below.

The pressure at each end is atmospheric pressure since its open circuit. The theoretical fan is located at the front of the tunnel, which would theoretically produce a flow of 15.56m/s flow speed and -.02 psi static pressure. Right below exiting the outlet the static pressure of the tunnel drops to -1.16 psi. In this case which value would be the static pressure needed to size the fan? Would it be -.02 psi or -1.16 psi or is it something else?

Thank you

### RE: Fan inquiry for open circuit wind tunnel

Pick one system of units and use it.

PSI is a non-typical unit of pressure for "fans".

Fans increase pressure. Your sketch shows a fan that somehow decreases it.

Air flows from high pressure to low pressure. How does the air leave your tunnel on the right?

### RE: Fan inquiry for open circuit wind tunnel

(OP)
@MintJulep

I am not really familiar with fan nor wind tunnel lol. So say if the fan chosen has static pressure of 4 WC (which I assume is the typical unit of pressure for fan?), then the pressure right after the fan would be 4 WC? Then the design criterion becomes to choose a fan which would produce a high enough static pressure for the outlet pressure to be greater or equal to atmospheric pressure?

### RE: Fan inquiry for open circuit wind tunnel

#### Quote (sen_yang)

I am not really familiar with fan nor wind tunnel
And yet you are going to try to design one?

Fundamentally a wind tunnel is just a duct.

You have a velocity requirement.

I hope that you have a diameter or other dimensions if it is not a circle in cross-section.

Velocity and area get you flow.

What is the temperature? Density matters.

The duct has friction, and will resist the flow of air through it.

You need to pick a fan that matches the pressure drop of the duct at the desired flow.

### RE: Fan inquiry for open circuit wind tunnel

First, fans create both velocity pressure and static pressure. Your diagram shows decreasing ststic pressure which, given the evasee, meets the laws of physics. Total pressure = static pressure + velocity pressure

The best book on fan/air pressures is "Industrial Ventilation" published by American Conference of Governmental Industrial Hygienists.

### RE: Fan inquiry for open circuit wind tunnel

(OP)
@MintJulep

Yeah I have the dimensions for the tunnel and curves for the contraction cone, and the pressure, velocity, density and temperature at each section of the tunnel are all calculated under compressible air condition. I am just not familiar with fans at all, so I wasn't quite sure how to approach that.

Sorry for keep bothering you, but is the fan selection the concept as pump selection? You map out the system curve and choose a fan curve and the intersection point is the operation point?

And also does it matter if the fan I choose is an axial fan or centrifugal fan, or if it's located upstream or downstream, the selection process should be the same (map out two curves and the intersection is operating point), right?

### RE: Fan inquiry for open circuit wind tunnel

(OP)
@willard3

Thank you, I will look at it.

### RE: Fan inquiry for open circuit wind tunnel

All fan types have their own characteristics and FC (forward curved) fans have very different operating parameters than BI (backward inclined) fans, have very different operating characteristics than axial fans for instance.

Buffalo Forge publishes a "Fan Engineering" handbook that will be helpful

You are going to have to do some in-depth search on your own to answer your questions; fans are not at all simple.

### RE: Fan inquiry for open circuit wind tunnel

#### Quote (sen_yang)

Sorry for keep bothering you, but is the fan selection the concept as pump selection? You map out the system curve and choose a fan curve and the intersection point is the operation point?
Fundamentally, yes. But with fans there are other factors to consider as Willard said.
Is 15.56 m/s the absolute maximum velocity the wind tunnel is intended to test at? It is useful for wind tunnels to be able to adjust their velocity in the test section, so you will want a fan that can handle the absolute maximum air flow/velocity and associated pressure drop through the tunnel.
Have you factored in pressure drops for different objects in the test section?

#### Quote (sen_yang)

And also does it matter if the fan I choose is an axial fan or centrifugal fan
Yes, the shapes of the fan curves vary. As does the velocity profile off the fan.

#### Quote (yen_sang)

if it's located upstream or downstream
Not really, but very possibly. This choice has many variables that need to be considered, not all of which are a matter of velocities and pressure drops.
Things to consider are construction of the wind tunnel and how well it handles negative vs. positive pressures.
Where are you pulling air from, where is it exiting?
Available space upstream of the test section.

### RE: Fan inquiry for open circuit wind tunnel

Google Design, Construction & Testing of an Open Atmospheric Boundary Layer Wind Tunnel by Harold Sherwood Boudreau III

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