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heat transfer coefficients
Posted 23.05.2013, 05:07 GMT-4 Heat Transfer & Phase Change, Materials 2 Replies
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I'd like to know more about the heat transfer coefficients present in the material library.
I see that it depends from the surface temperature, from the temperature at infinite distane and from a geometric parameter (height for a vertical surface, area/parameter ratio for an horizontal surface, isnt't it?).
But i suppose that in reality also the surrounding conditions influence this parameter.
For example the superior face of a box suspended in the middle of a room full of air is cooled differently respect to a box placed on the floor of the same room.
Or the same face of that box can be cooled in a differant way if that box is alone, or is laterally surrounded by other identical boxes.
Since there is no way of considering those situations in the setting of the parameter h from the library, I suppose that those effects are considered secondary effects, neglected, and whose effects have to be considered in a different way if the results of the simulations are not compatible with experimental data for this reason.
But I'd like to know in which situation the library coefficient is the one exactly correct to use, and if there is a 'rule of thumb' for considering it incorrect when experimental data, or multipysics simulations including the ai flow inside the volume of air are not available.
In addition I'd like to know how I should behave when I have to consider not a single surface, but when a surface is composed by different boundaries: can i still use the library heat transfer coefficient? which geometric parameter should I consider, the one relative to the whole surface, or, for every boundary, I should use the one relative to that surface?
Thanks for the attention, I hope of being clear, if I wasn't , please ask.
Francesco
I see that it depends from the surface temperature, from the temperature at infinite distane and from a geometric parameter (height for a vertical surface, area/parameter ratio for an horizontal surface, isnt't it?).
But i suppose that in reality also the surrounding conditions influence this parameter.
For example the superior face of a box suspended in the middle of a room full of air is cooled differently respect to a box placed on the floor of the same room.
Or the same face of that box can be cooled in a differant way if that box is alone, or is laterally surrounded by other identical boxes.
Since there is no way of considering those situations in the setting of the parameter h from the library, I suppose that those effects are considered secondary effects, neglected, and whose effects have to be considered in a different way if the results of the simulations are not compatible with experimental data for this reason.
But I'd like to know in which situation the library coefficient is the one exactly correct to use, and if there is a 'rule of thumb' for considering it incorrect when experimental data, or multipysics simulations including the ai flow inside the volume of air are not available.
In addition I'd like to know how I should behave when I have to consider not a single surface, but when a surface is composed by different boundaries: can i still use the library heat transfer coefficient? which geometric parameter should I consider, the one relative to the whole surface, or, for every boundary, I should use the one relative to that surface?
Thanks for the attention, I hope of being clear, if I wasn't , please ask.
Francesco
2 Replies Last Post 29.05.2013, 18:41 GMT-4
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Posted:
1 decade ago
nobody could help?
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Posted:
1 decade ago
Dear Francesco,
The heat transfer coefficients built-in COMSOL use standard formulae available (and explained) in heat transfer textbooks. They cover common cases like horizontal/vertical surfaces under natural/forced convection, etc. I suggest consulting these handbooks for the constraints on the application of these expressions as well as the meaning of the input values. The COMSOL documentation has some descriptions as well, but it is not as elaborate as a heat transfer reference.
If the geometry of the convection surface is not close enough to one of the conditions for which formulae are available you should ideally explicitly model the heat transfer and fluid flow in the surrounding fluid. That complicates the modeling significantly however. In many problems you don’t need that level of accuracy and it is sufficient to find reasonable upper and lower bounds for heat transfer.
Check out Gallery example 1448 if you are interested in explicitly modeling the surrounding thermal flow problem. It compares the predictions of the standard convection formulae with the surrounding thermal flow results for a relatively simple geometry.
Nagi Elabbasi
www.veryst.com
The heat transfer coefficients built-in COMSOL use standard formulae available (and explained) in heat transfer textbooks. They cover common cases like horizontal/vertical surfaces under natural/forced convection, etc. I suggest consulting these handbooks for the constraints on the application of these expressions as well as the meaning of the input values. The COMSOL documentation has some descriptions as well, but it is not as elaborate as a heat transfer reference.
If the geometry of the convection surface is not close enough to one of the conditions for which formulae are available you should ideally explicitly model the heat transfer and fluid flow in the surrounding fluid. That complicates the modeling significantly however. In many problems you don’t need that level of accuracy and it is sufficient to find reasonable upper and lower bounds for heat transfer.
Check out Gallery example 1448 if you are interested in explicitly modeling the surrounding thermal flow problem. It compares the predictions of the standard convection formulae with the surrounding thermal flow results for a relatively simple geometry.
Nagi Elabbasi
www.veryst.com