Physics Help Forum Heat Transfer

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 Feb 26th 2018, 10:11 AM #1 Junior Member   Join Date: Feb 2018 Posts: 4 Heat Transfer Hello, all. I'm a newbie in physics and am looking to pursue mechanical engineering as a degree. Thermodynamics is very interesting, but I'm stumped on a question without any way to test it out. This isn't for school, but rather for my own studies. Does the presence of heat in a conductor affect convection negatively? From what I understand, the heat transfer between two objects will always flow to the one with the lower temperature in relativity to the other. Similarly in electricity, major changes (80 degrees Fahrenheit for example) causes very small changes in resistance. Does anyone have any experimentation to prove or disprove this theory? Thanks, -R
Feb 26th 2018, 10:46 AM   #2
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 Does the presence of heat in a conductor affect convection negatively?
Could you try again to explain what you mean here please.

By conductor do you mean heat conductor?

And what do you mean by the presence of heat?

 Feb 26th 2018, 10:55 AM #3 Junior Member   Join Date: Feb 2018 Posts: 4 RE Is the forced transfer of heat hindered by the receiving object's increased temperature?
Feb 26th 2018, 10:59 AM   #4
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 Originally Posted by Ryudamasu Is the forced transfer of heat hindered by the receiving object's increased temperature?
Is your receiving object the conductor?

Feb 26th 2018, 06:29 PM   #5
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The question has been asked in two ways now ...I think I'm getting what has been asked ...sort of ...

 Originally Posted by Ryudamasu Does the presence of heat in a conductor affect convection negatively? Is the forced transfer of heat hindered by the receiving object's increased temperature?
First terminology ....it's not quiet correct to talk about "heat in a conductor "...some things are just hotter than others (the atoms/molecules vibrate at a faster rate)

You use the term convection this is just one heat transfer mechanism , that involving moving fluid currents (liquid or gaseous) quite often air.

Conduction involves the passing of heat through matter without noticeable movement of the mater itself ...

If two objects of different temperatures are close there will be heat transfer between them (not 'forced') ...the speed of this transfer is greater when the temp difference is large ... as this transfer continues their temperatures will come closer together and this will slow the heat transfer rate.

So yes .. heat transfer is " hindered by the receiving object's increased temperature "

Feb 27th 2018, 02:25 AM   #6
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 Originally Posted by Ryudamasu Does the presence of heat in a conductor affect convection negatively?
Heat-driven convection occurs when there is a temperature difference across a cavity, so it depends on what the conductor is, where it is in the cavity and its temperature.

Can you provide a more specific example of the kind of scenario you're investigating?

 Feb 27th 2018, 02:36 AM #7 Senior Member   Join Date: Apr 2015 Location: Somerset, England Posts: 820 Perhaps you are thinking of a transfer cooling system called a heat pipe, used in electronics, particularly computers?
Feb 27th 2018, 11:24 AM   #8
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RE

 Originally Posted by studiot Is your receiving object the conductor?
Yes

Feb 27th 2018, 11:32 AM   #9
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 Originally Posted by benit13 Heat-driven convection occurs when there is a temperature difference across a cavity, so it depends on what the conductor is, where it is in the cavity and its temperature. Can you provide a more specific example of the kind of scenario you're investigating?
For example, if there are two heated coils, one that is room temperature and one that is 100 degrees Fahrenheit. In the same instance, there are two heat guns aimed at the heater coils at the same settings. Would the 100 degree heat coil be slower to climb in temperature, or gain at the same rate as the room temperature coil?

Feb 28th 2018, 03:18 AM   #10
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 Originally Posted by Ryudamasu For example, if there are two heated coils, one that is room temperature and one that is 100 degrees Fahrenheit. In the same instance, there are two heat guns aimed at the heater coils at the same settings. Would the 100 degree heat coil be slower to climb in temperature, or gain at the same rate as the room temperature coil?
In practise, heating coil temperatures are dominated by the actual heat rate of the technology, so they're not affected much by the surroundings. Therefore, for convenience, heat problems tend to neglect the temperature of the actual heating coil and just assume that the heat is provided by an constant injection. However, if its desirable to model the temperature of the coil itself, what happens depends on the temperature of the air inside the cavity.

If the temperature inside the room is intermediate between the two coil temperatures, then one heat rate will increase and the other will decrease. The difference in temperature growth rates then depends on how much convection is happening (high surface area and high heat transfer coefficient).

If the temperature in the room is not intermediate between the two coils, then the growth rates can both be boosted or damped by the effect of convection, since heat will go into or out of the room air.

Let's say we have two parallel heating coils separated by an air gap (with adiabatic side walls) with temperature $\displaystyle T_{room}$. If each coil has a constant heat rate $\displaystyle \dot{q_{heat}}$, and variable temperatures $\displaystyle T_{1}$ and $\displaystyle T_{2}$, then

$\displaystyle \dot{q_1} = \dot{q_{heat}} + hA(T_{room} - T_{1})$
$\displaystyle \dot{q_2} = \dot{q_{heat}} + hA(T_{room} - T_{2})$

since in our problem set-up we have $\displaystyle T_{1} > T_{room}$ and $\displaystyle T_{2} < T_{room}$, we expect $\displaystyle \dot{q_2} < \dot{q_1}$.

A more standard problem is to consider two plane walls separated by an air gap and then embed the heating coils in the plane walls. Then, the heating coils are just modelled as injections of heat into the walls. Then, you can include the effect of conduction across the wall in addition to the convection from the surface.