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Dearest Lovelies,Fans are great...they keep you cool and make a pleasant whirring sound.Like most fans..the blade portion is what 90% blade ? and 10% gap ?...so how come I can see right through it when the blade is spinning ?With such a small gap one would think that the solidity would still be there but just slightly faded in colour....but no !..it's very transparent indeed...and the faster it goes the less blade you can see, the more transparent it becomes...why's that then ?
Quote from: neilep on 19/04/2007 14:44:43Dearest Lovelies,Fans are great...they keep you cool and make a pleasant whirring sound.Like most fans..the blade portion is what 90% blade ? and 10% gap ?...so how come I can see right through it when the blade is spinning ?With such a small gap one would think that the solidity would still be there but just slightly faded in colour....but no !..it's very transparent indeed...and the faster it goes the less blade you can see, the more transparent it becomes...why's that then ?not an answer, but you do realsie that fans do not actually cool your room. don't you?
The fact that you do not see the rotor spinning is an other case of what is illustrated by a test everyone must have seen in physics class : a circular disk is divided in seven or 14 sectors, each showing an other colour of the rainbow. When you make the disk spin, you do not see the different colours but just gray. So in this case you do not see the different wings but you do see what shows up repeatedly behind themYou can see the fan spinning if you use a stroboscope. If the frequency of the flashes is synchronized with the speed of your fan, it will look as if the fan stands still, if the frequency is marginally lower, you'll see the fan turning forwards, if the frequency is higher it will turn backwards. A single TL-tube will have the same effect, but since you can not change its frequency, you would have to change the speed of the fan. (Twin tube ornaments will have the tubes arranged in such a way that one is on while the other is off and vice versa, so no stroboscopic effect.)And concerning that other point : you feel colder because the blast of air helps your transpiration evaporate, which requires energy.
Remember that you are hotter than your surroundings and are continually losing heat through the surface of your skin - not just through evaporation of sweat but also through thermal radiation and natural convection. If you were the same temperature as your surroundings it would usually imply that you'd died....You don't actually feel temperature - your body responds to the rate at which you are losing heat, which is a good surrogate measurement except when something in your environment artificially boosts or reduces the rate of heat loss. So, for example, if you stand outdoors on a cold day you will feel much colder if there is a wind blowing as this gives 'forced convection' a more efficient process; continually moving cold air across your skin and removing heat. This is the idea of 'wind chill' you sometimes hear mentioned on weather forecasts. Though no matter how strong the wind, it won't cool anything down to a lower temperature than that of the 'still' air.A fan is just a mini-wind. By moving cold air across your skin it is causing 'forced convection' and taking away heat faster than 'natural convection'.
imagine yourself as the kid from the old readybrek advert, you have that red glow around you. that would be your body heat.if you place the fan facing you, it will blow this heat away and cool you down by cooling your sweat and/or creating a cooler current flow over you.also, lots of people leave fans running in a room "to cool the room", this can actually heat the room depending on what setting you have the fan on. the heat generated by the motor can put more heat into the room than the fan removes by circulating the air.i have this problem at work sometimes when the aircon. goes down in the server room, they want to put fans in there to cool it down when all it is actually doing is moving the warm air around not cooling it down. as i demonstrated with a good thermometer.
I just thought of another example....I once spent some time inside a shutdown nuclear reactor. Although shutdown it was still sitting at about 65 degrees Celsius (150 F). This was done in a fully enclosed air-cooled suit - a bit like a space-suit. The cooling effect came from blowing cold air around inside (and then out of) the suit. If the cooling had only come from enhanced evaporation of sweat I think I might have suffered from severe dehydrated...
Was it a drill or a genuine emergency ?
THANK YOU PAUL.......this is amazing that the heat of the fan (dependent on the setting) can in fact serve to counteract it's very purpose !!......does this really apply to household fans though ?...I've never really noticed them getting too hot !
Hi Neil, yes household fans! you may not have noticed them getting warm but the heat generated from the fan put more heat in to the room. Try it, put a good thermometer in a room, turn the fan on and close the door. i guarantee (no money back) the temperature will not go down, but may well go up.