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heat loss in infinite medium (air)

Posted Nov 9, 2010, 7:23 p.m. EST Heat Transfer & Phase Change, Modeling Tools & Definitions, Parameters, Variables, & Functions Version 4.3 4 Replies

Minh Vuong
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I need to model an object that loses heat into air, which I consider an infinite medium. I can put the object into an enclosed box that contains air. That works OK except that I need to make the box very large compared to the object, which makes meshing difficult.

Is there a way to do this without the enclosing box? I have tried "Convective cooling" and that works OK but it's really conduction that I am interested in. I have tried making the box into an infinite element but that causes COMSOL to crash.

This seems like such an elementary problem. There must be an easy way to do this.

Please help. Thanks.
4 Replies Last Post Nov 11, 2013, 3:23 p.m. EST
COMSOL Moderator

Hello Minh Vuong

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Michel LeBaron
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Posted: 1 decade ago Jan 27, 2011, 3:46 a.m. EST
Hi,

you might look at this example describing natural convection cooling of a thermos under laminar flow :

www.comsol.com/showroom/gallery/1448/

Another solution might be to use the Convective Cooling feature of the Heat Transfer package, and then to specifiy an user-defined heat transfer coefficient. In the model "power_transistor" of v4.0a, COMSOL used h=5 W/m^2.K. I thought it was more like 50, but who knows...

Note that if you have a simple object (convex shape, such as a cube) with one of its faces is at a fixed temperature, and a moderate heat source, the boundary conditions at the other faces is of little importance. For instance, using h=5-50 W/m^2.K or even the Outflow option doesn't change much the maximum temperature nor the gradient.
Hi, you might look at this example describing natural convection cooling of a thermos under laminar flow : http://www.comsol.com/showroom/gallery/1448/ Another solution might be to use the Convective Cooling feature of the Heat Transfer package, and then to specifiy an user-defined heat transfer coefficient. In the model "power_transistor" of v4.0a, COMSOL used h=5 W/m^2.K. I thought it was more like 50, but who knows... Note that if you have a simple object (convex shape, such as a cube) with one of its faces is at a fixed temperature, and a moderate heat source, the boundary conditions at the other faces is of little importance. For instance, using h=5-50 W/m^2.K or even the Outflow option doesn't change much the maximum temperature nor the gradient.
Edgar Kaiser
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Posted: 1 decade ago Jan 27, 2011, 3:54 a.m. EST
Hi,

I think the predominant heat transfer mechanisms at room temperature are radiation and convection. Air is quite a good thermal insulator, so conduction losses are low.

Best regards
Edgar
Hi, I think the predominant heat transfer mechanisms at room temperature are radiation and convection. Air is quite a good thermal insulator, so conduction losses are low. Best regards Edgar
Akmal
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Posted: 1 decade ago Dec 23, 2011, 3:41 a.m. EST
Hi,

create the air cube a little bit bigger that object. Define cube domain as infinite element (general or something that similar). Define convective cooling in every outer cube boundary, at free convection h about 5-25 W/m²K (h depens on deltaT (air and outer temperature of object), charakterisitc length and air properties)

regards
akmal
Hi, create the air cube a little bit bigger that object. Define cube domain as infinite element (general or something that similar). Define convective cooling in every outer cube boundary, at free convection h about 5-25 W/m²K (h depens on deltaT (air and outer temperature of object), charakterisitc length and air properties) regards akmal
Scott Rauscher
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Posted: 1 decade ago Nov 11, 2013, 3:23 p.m. EST
Hi there,

I am trying to do the same thing, but my air cube is around imported geometry from AutoCAD (3D). The problem I am having is that once I create the air cube around my imported geometry, the imported geometry seems to disappear (it becomes unselectable and invisible) and I can no longer assign it a material property. I definitely don't have it hidden, and I've used the same imported geometry to run a heat loss analysis before and that worked fine.

Any ideas?

Thank you
Hi there, I am trying to do the same thing, but my air cube is around imported geometry from AutoCAD (3D). The problem I am having is that once I create the air cube around my imported geometry, the imported geometry seems to disappear (it becomes unselectable and invisible) and I can no longer assign it a material property. I definitely don't have it hidden, and I've used the same imported geometry to run a heat loss analysis before and that worked fine. Any ideas? Thank you

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