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re: Sitting in an open thermos, which reaches room temperature (20°C) faster?
Posted on 6/1/21 at 8:21 pm to IT_Dawg
Posted on 6/1/21 at 8:21 pm to IT_Dawg
quote:
And the humidity
I'm assuming no heat exchange with the thermos and the surroundings. So it's just out the top.
I would think the boundary layer would be thinner for the warm water because of natural convection mixing the air. Additionally while evaporation probably isn't the most important part, you'd get more evaporation off the warm water. So the mass of the warm one would be a little less over time.
So the hot one seems like the right answer, but it depends on the assumptions you make about the system.
1
Posted on 6/1/21 at 8:42 pm to Duke
Assuming the fluid is filled to the top of the thermos, and the lid is on, so no air, and no evaporation / condensation, and both thermoses are shaped the same.
I believe both will require the same amount of energy transfer to reach the desired temperature.
Also assume no or at least the same air motion around both thermoses to allow equal opportunity for heat transfer, and it comes down to the natural convection within the thermos maximizing the temperature gradient between the air – thermos – water.
So which will have the greater convection? I would guess the hot 1, due to slightly less dense?
The hot one would have the fluid around the walls falling and an internal / Central updraft, while the cold would see the fluid Rising at the walls and falling centrally. Assuming identical geometry, I don't think that would create an intrinsic difference, but it's interesting to visualize
I believe both will require the same amount of energy transfer to reach the desired temperature.
Also assume no or at least the same air motion around both thermoses to allow equal opportunity for heat transfer, and it comes down to the natural convection within the thermos maximizing the temperature gradient between the air – thermos – water.
So which will have the greater convection? I would guess the hot 1, due to slightly less dense?
The hot one would have the fluid around the walls falling and an internal / Central updraft, while the cold would see the fluid Rising at the walls and falling centrally. Assuming identical geometry, I don't think that would create an intrinsic difference, but it's interesting to visualize
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