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rate of heat transfer

Posted 11 févr. 2015, 04:13 UTC−5 Heat Transfer & Phase Change 5 Replies

P2p
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I am new in Comsol and might have a fairly simple question. How can I simulate this situation:

There is a block of a material 10mm*10mm*1mm. If a part of this material with dimension of 0.1mm*.01mm*0.01mm (in the center) has a temperature of 2000 degree. The surrounding ambient is always 25 degree.
now the question is: how long does it take that the whole block goes back to 25 degree?
5 Replies Last Post 11 févr. 2015, 09:33 UTC−5
Lasse Murtomäki
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Posted: 10 years ago 11 févr. 2015, 05:14 UTC−5
1. What is the material, i.e. its thermal conductivity or thermal diffusivity?
2. What is the heat transfer coefficient between the block and its surroundings? It is not guaranteed that the outer edges of the block remain at 25 °C all the time, unless you force it somehow.

Knowing these parameters the simulation is most straightforward.

br
Lasse
1. What is the material, i.e. its thermal conductivity or thermal diffusivity? 2. What is the heat transfer coefficient between the block and its surroundings? It is not guaranteed that the outer edges of the block remain at 25 °C all the time, unless you force it somehow. Knowing these parameters the simulation is most straightforward. br Lasse
P2p
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Posted: 10 years ago 11 févr. 2015, 07:57 UTC−5
Thank you for the prompt reply.

For the parameter I have:
k = 42 w/m.k
C = 750 j/kg.k

about the environment around the block, how can i introduce or force it to interact with sample at a certain temperature?
there are two surroundings
1- on top of the surface (where the heat release on a) there is water which moving really fast and doesn't heat up itself and just take the heat away from the sample

2- at the bottom of sample there is air which can be heated up
Thank you for the prompt reply. For the parameter I have: k = 42 w/m.k C = 750 j/kg.k about the environment around the block, how can i introduce or force it to interact with sample at a certain temperature? there are two surroundings 1- on top of the surface (where the heat release on a) there is water which moving really fast and doesn't heat up itself and just take the heat away from the sample 2- at the bottom of sample there is air which can be heated up
Lasse Murtomäki
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Posted: 10 years ago 11 févr. 2015, 08:25 UTC−5
Heat transfer from the solid to the surrounding fluid is taken into account through Newton's cooling law:

q = h·(Text - T)

where Text is the fluid temperature and T is the solid temperature; h is the heat transfer coefficient which depends on materials. For steel-air boundary it is 5-10 W/(m²·K) and for steel-water boundary 300-400 W/(m²·K). Exact values are impossible to give as they depend on the rate of convection, fouling, etc. But I do not think this value plays an important part, because your solid body is very close to 25 °C at all times, hence hardly no heat transfer takes place.

In Comsol this BC is under Heat Flux --> Convective Heat Flux.

Your plate reaches constant temperature of 25 °C in less than 0.5 ms (see attached image).

br
Lasse
Heat transfer from the solid to the surrounding fluid is taken into account through Newton's cooling law: q = h·(Text - T) where Text is the fluid temperature and T is the solid temperature; h is the heat transfer coefficient which depends on materials. For steel-air boundary it is 5-10 W/(m²·K) and for steel-water boundary 300-400 W/(m²·K). Exact values are impossible to give as they depend on the rate of convection, fouling, etc. But I do not think this value plays an important part, because your solid body is very close to 25 °C at all times, hence hardly no heat transfer takes place. In Comsol this BC is under Heat Flux --> Convective Heat Flux. Your plate reaches constant temperature of 25 °C in less than 0.5 ms (see attached image). br Lasse

Attachments:

P2p
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Posted: 10 years ago 11 févr. 2015, 09:08 UTC−5
Perfect, thanks for calculation.

is it possible to share the simulation file. I'm interested to see how you define the environment's temperature to be constant at 25 degree.
Perfect, thanks for calculation. is it possible to share the simulation file. I'm interested to see how you define the environment's temperature to be constant at 25 degree.
Lasse Murtomäki
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Posted: 10 years ago 11 févr. 2015, 09:33 UTC−5
Hi

I removed everything else but geometry and physics in order to keep file size small. Meshing would bring about a much bigger file.

br
Lasse
Hi I removed everything else but geometry and physics in order to keep file size small. Meshing would bring about a much bigger file. br Lasse

Attachments:

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