Every once in a while, something comes up which challenges your perception of how things work. When we were down at the in-laws over the Christmas holiday period a new addition was sitting on top of the wood burner, a stove top fan. Now, I've be suggesting one of these to the father in-law for a while as I had heard good things about then when I had been looking at wood burners for our house, however, as we went for a recessed burner, we cant have one.
The fan in question was much like the one above and when we first saw it, it was running nicely on top of the burner. The wife asked how it worked and I preceded to tell her that it worked due to the thermal currents created by the burner and the heat sink arrangement at the back of the fan. In simple terms, the air around the heat sink heated up, rising, drawing in more air over the blades of the fan and as the burner got hotter and so the there was more air current, the fan would start to turn, helping the air to circulate. Sound like a solid theory to me and seems to fit with all of the physics that I know. But then on closer inspection I spotted this;
Photo is a bit dodgy, as I was taking the photo while the burner was on and very hot and the fan was spinning at a fair rate of knots and I didn't fancy loosing a finger or something! Anyway, wires? Why would a fan that worked on air pressure and thermal currents need wires? The father in-law wasn't sure either, he was of the same opinion as me and hadn't taken much notice when he had placed the fan on top of the stove. We discovered this fact at about 23:45, just as we were heading up to bed, so doing what any sane person would do, we left it until the morning to investigate. Er, no, actually we launched in to a whole series of theories and investigations, finally culminating in discovering that stove top fans generally work in one of two ways.
1) Electric or more specifically thermoelectric. In these, there are two plates separated by a small gap, that, when heated to different temperatures create an electric current that powers a motor connected to the fan. This is the most common type of fan and is the reason for the heat sink at the top, which is there to ensure a heat difference between the top and bottom plates in the "battery". https://simple.wikipedia.org/wiki/Stove_fan is a good place to get a better explanation.
2) Mechanical or Stirling type. These use a small engine, which runs thought the expansion and contraction of air. Its kinda hard to explain, mainly as I don't really understand it myself, but the basics as far as I can tell are that the air heats up in the hot cylinder, pushing the piston up and turning the fan blade at the same time it expanse in to the cold cylinder, pushing that up to. The air is then forces in to the cold cylinder where it contracts and is drawn back in to the hot cylinder by the momentum of the fly wheel. Once in the hot cylinder in heat up again and the cycle repeats. Its a lot easier to see what i'm talking about here: https://en.wikipedia.org/wiki/Stirling_engine
So, all in all, I learnt something, mostly that I didn't have a clue what I was talking about. I have since discovered how stove top fans really work and also learnt a bit about Stirling Engines, which seem to be pretty cool.
Its at time like these that I wonder what else I have misunderstood over the years. I suppose time will tell.
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