Convection and Gravity

Thread Starter

Ratch

Joined Mar 20, 2007
1,070
jpanhalt,

Wrong. That is not convection.

The requirement of gravity (or acceleration by any means) for convection was studied long ago. A researcher named Lunar demonstrated practical electrophoretic separations about 40 years ago in which he largely negated the adverse effects of convection by having the supporting fluid rotate in a sheet to simulate zero gravity over time. The process was later demonstrated in real microgravity on the shuttle.
From: Thermodynamics, an Engineering Approach, Yunus A. Cengel and Michael A. Boles, Second Edition, p. 96

"Convection is the mode of energy transfer between a solid surface and the adjacent liquid or gas which is in motion, and it involves the combined effects of conduction and fluid motion. ... In the absence of fluid motion, heat transfer between a solid surface and the adjacent fluid is by pure conduction."

From: Random House Webster's Unabridged Dictionary

con·vec·tion n.
1. Physics. the transfer of heat by the circulation or movement of the heated parts of a liquid or gas.

e·lec·tro·pho·re·sis n.
1. Also called cataphoresis. Physical Chem. the motion of colloidal particles suspended in a fluid medium, due to the influence of an electric field on the medium.

Now, would you be so good as to explain why a definition from a good thermodynamics textbook and a dictionary is not correct. And if you please, also explain what electrophoresis has to do with the definition of convection.

Ratch
 
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jpanhalt

Joined Jan 18, 2008
11,087
I also said that induced air flow can support convection. And no air flow can still support conduction. A hard vacuum stops both convection and conduction.

Ratch

Wrong. That is not convection.

The requirement of gravity (or acceleration by any means) for convection was studied long ago. A researcher named Lunar demonstrated practical electrophoretic separations about 40 years ago in which he largely negated the adverse effects of convection by having the supporting fluid rotate in a sheet to simulate zero gravity over time. The process was later demonstrated in real microgravity on the shuttle.

John
 
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Thread Starter

Ratch

Joined Mar 20, 2007
1,070
jpanhalt,

I agree on the conduction part, but you said convection. Partitioning based on density requires gravity.
I also said that induced air flow can support convection. And no air flow can still support conduction. A hard vacuum stops both convection and conduction.

Ratch
 
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Thread Starter

Ratch

Joined Mar 20, 2007
1,070
studiot,

OK so Newton's law of cooling says that the rate of cooling is proportional to the temperature difference and Fourier's Law says that the rate of heat transfer is proportional to the temperature gradient.

Descibe a situation where these laws don't apply.
Newton's Law of Cooling if for convection, and Fourier's Law of Cooling is for conduction. So neither of those methods work in a isolated vacuum, such as outer space or a vacuum chamber. Gotta go with radiation if you want to dissipate heat energy in those places.

Ratch
 
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thingmaker3

Joined May 16, 2005
5,083
To minimize distraction from the original topic, I have moved the debate into it's own arena. Please enjoy your debate - I'm certainly enjoying it.

Please also accept my apology for any inconvenience resulting from the loss and subsequent restoration of these posts.
 

jpanhalt

Joined Jan 18, 2008
11,087
Now, would you be so good as to explain why a definition from a good thermodynamics textbook and a dictionary is not correct. And if you please, also explain what electrophoresis has to do with the definition of convection.

Ratch
Glad you agree on using a dictionary to define words. Here's what the OED has to say about it:

convection

• noun transference of mass or heat within a fluid caused by the tendency of warmer and less dense material to rise.
As for the example in which I referred to the effects of convection in electrophoresis and a demonstration that in effectively zero gravity it does not occur, I think the context is clear.

John
 

Thread Starter

Ratch

Joined Mar 20, 2007
1,070
jpanhalt,

Glad you agree on using a dictionary to define words. Here's what the OED has to say about it:
Not just a dictionary, but a good thermodynamics text book also. Text books are usually better and more complete than dictionaries, especially technical with terms.

Quote:
convection

• noun transference of mass or heat within a fluid caused by the tendency of warmer and less dense material to rise.

As for the example in which I referred
Rather incomplete, don't you think? Doesn't say anything about forced air convection. True, one does not get natural convection without weight, but that is not the only type of convection.


As for the example in which I referred to the effects of convection in electrophoresis and a demonstration that in effectively zero gravity it does not occur, I think the context is clear.
Since electrophoresis is an application of an electric field, it works just fine in weightless conditions, I think you need to explain better how convection ties into that method.

Ratch
 
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