We went out to dinner and music last night at Bud and Mary's over at the river. They had a fire in the woodstove. (We haven't had this year. I know not why). Sitting on stove was one those little fans that is activated by the radiant heat from the stove. They are all the rage these days, it seems, and come in a variety of sizes and shapes. They're pretty cool to watch. As the stove get cooler the fan slows down, indicating you need wood. If it ain't moving you need to build a fire.
The thing doesn't seem to move air, as advertised. Bud and Mary have had "discussions" about whether it does move air or not. I didn't feel any air movements when I put my hand in front of it. Bud says if you strike a match the smoke will go toward the moving fan. I don't know 'cause I haven't tried it. My notion is that it is moving air, but because it isn't "contained" by a shroud, like an electric fan. I don't know. I'm sure engineer Bob will weigh in on this one.
Then came the fun part. Mary says if you turn a LED flashlight on, and shine it on the fan when it turns, you can see the individual blades in very slow motion and slowly, they will begin to go backwards, like the wheels of a wagon in the old cowboy movies. The LED's in the flashlight are oscillating, that particular flashlight had two setting, and the low setting seemed to work best, according to Bud. It causing the blades to appear to stop, and reverse.
Mary made a comment that all is not what it seems, depending on how you look at it. She went on to make a comment about how samples can be skewed by not looking at the data in the right light...
What seems to me is as long as that fan is standing still and an LED oscillating light is shone on it...nothing will happen. Before LED's this whole conversation would have been moot.
I'm gonna' get me one of them fans to play with.
Pat: The shroud you mentioned improves efficiency, but is not absolutely essential. We are dealing with multiple issues with these thermo-electric devices. Small amounts of power are generated and it must overcome friction and mechanical inertia of the blades. In order to move air, the blades also must have an angle of attack. Imagine completely flat blades that would fit on a dish. Rotation would produce no forward or backward air movement, just turbulence. When you add the blade angle to move air, you require more energy. Another factor is that the hot stove surface produces a convection current that sends the air up (i.e. vertical) In the particular case you observed, the air flow off the fan might be 'swallowed' up by the vertical convection current so that you sense no flow when you put your hand in front or behind it. If that is the case, you are just looking at a toy, like an indoor whirli-gig. <bb
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