The wind tunnel uses two furnace blowers in an about 3 ft cube box. The 3 ft cube is connected to the test section by a diffuser that comes into one side of the cube. The blowers are setup to exhaust air from the cube which sucks air from the diffuser and the test section. The cardboard contraction section and diffuser sections are just very quick rough cuts to try to get some idea what the flow velocity at the test section will be – they will be replaced by real parts.

The test section is to be 14 by 14 inches (1.36 sqft).

Each furnace blower is rated for about 2500 cfm, so it I got the full 5000 cfm, it would give 61 MPH in the 14 inch by 14 inch test section. I’m only getting about 33 MPH, so, something is cutting the flow down almost by half.

From what I’ve read about furnace blowers, the output does not drop dramatically with pressure drops up to nearly half and inch of water. But, something about my setup is causing quite a bit of drop in cfm – not sure what?

The furnace blowers are nice from the point of view that they move quite a bit of air, and they are multi speed. And, they are usually free from HVAC contractors that are replacing old furnaces. On the down side, they are bulky and heavy.

Left to right – mockup Contraction section, mockup diffuser section, fan box. The test section would go in between the contraction section and diffuser (where the clamp is). The Contraction section and diffuser are just quick cardboard mockups just to get some idea what the airflow will be.

With both blowers on, the air velocities are:

Entering the contraction 16 MPH

At test section 33 MPH (this where I hoped to get 50 to 60 MPH)

Diffuser output to fan box 23 MPH

At blower outlets 39 MPH