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Hot air in a car.

  1. Jan 12, 2014 #1

    sophiecentaur

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    I was driving home today in my car, which had been parked in an exposed spot with a cold wind on it. It was very cold when I got in. I turned the heater (water valve control) on full but only had the fan on '1'.
    After a couple of miles, the engine thermostat cut in and the water temperature blipped up to 60 or 70 C but I was not getting any obvious heat into the cab. I went fast round a roundabout (traffic circle) and suddenly a load of warm air engulfed my head. The warm air from the heater must have found itself up near the middle of the roof above my head and was suddenly displaced by the cooler air lower in the cab due to the centripetal acceleration of the car. As I turned sharply, the force was clearly enough to 'slosh' the air about inside. I managed to repeat the affect a few times but the air all warmed up eventually and it stopped.
    The effect was the same that you can easily get with a Helium balloon (which leans into the curve) but was 'invisible', in this case.

    This is a really cool effect and I have never experienced it before, in years and years of winter driving. I shall try to repeat the trick again. I think it would not work well with the fan turned on high because that will just stir up the warm and cold air layers too much.

    Has anyone else noticed this? It's an uncanny experience and worth trying for. The faster you turn, the better - so be careful of icy surfaces.
     
  2. jcsd
  3. Jan 12, 2014 #2

    russ_watters

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    We've discussed it before. I searched, but can't seem to find any threads though. Now that you noticed it once, you're guaranteed to notice it almost every time you drive. Next time you stop hard at a stop light, you'll notice the same effect.
     
  4. Jan 12, 2014 #3

    Bobbywhy

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    Since cooler air is more dense than warmer air it ought to react to the angular acceleration force more than the warmer air. If this is true, then it seems likely that your "sloshing" theory is plausible.

    No, I've never experienced it, but now, owing to your observations, I'll attempt to duplicate it here. I assume the laws governing our experiments are the same, regardless of the ocean that separates us.
     
  5. Jan 13, 2014 #4

    Borek

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    Cool :smile:

    At home we have a fireplace used for heating. It happens that the house is built in a rather strange manner, with kind of ribs sticking from the ceiling and dividing rooms into smaller chunks. When I start the fire (and before the forced circulation fan kicks in, moving the hot air around the house), I have a pool of hot air under the ceiling in the part of the room where the fireplace is. You can literally stick your finger into hot air just by raising your hand.
     
  6. Jan 13, 2014 #5

    sophiecentaur

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    The warm air will go the the back of the car, if you do that, unfortunately. What's really needed is to floor the accelerator in a high performance car, to cause all the warm air to be displaced forward. My Ford Focus engine will hardly knock the skin off a rice pudding, I'm afraid so I am stuck with the g forces I can get with good tyres on a dry corner.

    @Borek The kitchen ceiling is a great example of hot air collecting in a dramatic way when you've got two ovens and three gas rings going full bore. It's almost hot air paint stripper conditions up there. If I could take the house fast round a sharp bend, life could be really interesting.
     
  7. Jan 13, 2014 #6

    russ_watters

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    I guess it depends on how your heating works. In my car, I run the top and bottom vents, which does a good job of heating the front half of the car, but the back half gets nothing. Coming to a fast stop makes the cold air in the back half of the car slosh forward.
     
  8. Jan 13, 2014 #7

    sophiecentaur

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    I have ducts that go to the rear, under the seats. The only time this effect would happen in mine would be, I think, with the demise / top vents with the fan on low.
    What really annoyed me was that I've been driving cars since 1962 and never noticed the effect. Still, better late than never; I will be able to tell (bore) Gabriel about it when I get to the Pearly Gates.
     
  9. Jan 13, 2014 #8

    Pythagorean

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    Is there also a chance that you had an air bubble in heat exchange (where the coolant transfers heat to the heating system) and you cleared it with the turn? I guess it would have been obvious if it was coming from the heater rather than ambient air already heated.
     
  10. Jan 13, 2014 #9

    sophiecentaur

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    Except it did it more than once and the hot air was sloshing around my ears and not blown in my face. I'm pretty sure it was an actual layer of hot air. (Very much suited to some threads on PF I think.)
     
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