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Heat transfert in a bounded medium
Posted 31 mars 2014, 05:06 UTC−4 Heat Transfer & Phase Change Version 4.2a, Version 4.3 1 Reply
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Hi all,
I work with the Heat Transfert module (comsol 4.2). My work consists to find the temperature of a small sphere of radius R = 10 cm and made of plastic embedded in air or water. The convection and radiation are take into account. The initial temperature of the sphere is 80 °C and the temperature of the external environment is 20 °C (when air) or 50 °C (when water)
These are my settings : Heat Transfert (ht) / Heat Transfert in Solids
The only geometry is a sphere of radius R = 10 cm.
- Solid enbedded in air
Materials (properties of the solid) :
k = 0.1 W/(m.K) (Thermical conductivity)
rho = 100 kg/m^3 (Density)
Cp = 1000 J/(Kg.K) (Heat capacity at pressure constant)
Initial values : 80 °C (initial temperature of the sphere)
Convective cooling : h = 2 W/(m^2.K) (heat transfert coefficient defined at the boundary of the sphere)
External temperature : 20 °C (temperature of the external environment when air)
I do not understand...Never, the properties of the external environment (air in this case) are defined. Therefore, I cannot to do it when the solid is embedded in water (or an other liquid). Also, when the external environment is bounded (for example when the sphere is embedded in 1 m^3 of water or 1 m^3 of air...), I cannot defined the heat transfert coefficient at the surface of the sphere since COMSOL "forces" me to define it only at the surface of the external environment.
Have you an idea to take into account the properties of the external environment and/or the finite size of this environment ?
Thank you !!
I work with the Heat Transfert module (comsol 4.2). My work consists to find the temperature of a small sphere of radius R = 10 cm and made of plastic embedded in air or water. The convection and radiation are take into account. The initial temperature of the sphere is 80 °C and the temperature of the external environment is 20 °C (when air) or 50 °C (when water)
These are my settings : Heat Transfert (ht) / Heat Transfert in Solids
The only geometry is a sphere of radius R = 10 cm.
- Solid enbedded in air
Materials (properties of the solid) :
k = 0.1 W/(m.K) (Thermical conductivity)
rho = 100 kg/m^3 (Density)
Cp = 1000 J/(Kg.K) (Heat capacity at pressure constant)
Initial values : 80 °C (initial temperature of the sphere)
Convective cooling : h = 2 W/(m^2.K) (heat transfert coefficient defined at the boundary of the sphere)
External temperature : 20 °C (temperature of the external environment when air)
I do not understand...Never, the properties of the external environment (air in this case) are defined. Therefore, I cannot to do it when the solid is embedded in water (or an other liquid). Also, when the external environment is bounded (for example when the sphere is embedded in 1 m^3 of water or 1 m^3 of air...), I cannot defined the heat transfert coefficient at the surface of the sphere since COMSOL "forces" me to define it only at the surface of the external environment.
Have you an idea to take into account the properties of the external environment and/or the finite size of this environment ?
Thank you !!
1 Reply Last Post 31 mars 2014, 07:21 UTC−4
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Posted:
1 decade ago
Hello Claire,
In convection, fluid properties (velocity field inclued) are used to calculate the convective coefficient h.
If you add a boundary condition with h fixed, then COMSOL does not need fluid properties.
You can estimate your convective coefficient by calculating the Nusselt's number (which is a function of Reynolds and Prandtl's numbers). Then add it to your boundary limit. That way, you do not need to modelize your fluid volume.
Else, you can modelize your fluid volume and use both HT and CFD physics.
Hope it will help you.
Benjamin
In convection, fluid properties (velocity field inclued) are used to calculate the convective coefficient h.
If you add a boundary condition with h fixed, then COMSOL does not need fluid properties.
You can estimate your convective coefficient by calculating the Nusselt's number (which is a function of Reynolds and Prandtl's numbers). Then add it to your boundary limit. That way, you do not need to modelize your fluid volume.
Else, you can modelize your fluid volume and use both HT and CFD physics.
Hope it will help you.
Benjamin