My first tests of the wind tunnel were not encouraging. This morning I turned the fan around so that it draws air through the wind tunnel instead of blowing (I left the egg-crate between the test chamber and fan cowling) and the results were excellent!
At the open end of the test chamber the flow rate was uniformly 3.6 to 3.7 m/s. Taking measurements at the walls at increasing distance into the chamber, the flow rate gradually dropped as would be expected for the developing boundary layer. Rotation in the test chamber was nearly unmeasurable.
Because the fan switch is now inside the cowling, I set the switch to its highest setting and plugged the fan into a variable autotransformer. Adjusting it so that the pitch of the fan dropped about one octave (1/2 frequency), the flow rate dropped to 2.1 m/s, slightly more than half. So the flow rate is a roughly linear function of fan speed. I intend to put an opto-interrupter sensor on the fan to monitor its speed precisely.
I also ran a test without the egg-crate. Uniformity was not as good, and there seemed to be some rotation in the test chamber.
Because I can't achieve 10 m/s in the tunnel, I must make my convection measurements at lower speeds. This means that I must get more accurate measurements and estimates for the other modes of heat transfer than if the speed were higher (in order to achieve the same experimental accuracy). Lower speeds also mean less heat flow; so the block of aluminum I bought for the plate is thicker and will have longer settling times than is needed.
Here is the tunnel in operation with the anemometer showing 3.7 m/s:
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