Questions of the modelling of a light bulb using Heat Transfer Module

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Fangjing Hu

Fangjing Hu

January 29, 2013 11:13am UTC

Questions of the modelling of a light bulb using Heat Transfer Module

Dear all:

Now I am trying to carry out some simulations using heat transfer module. To start with, I downloaded the "free convection in a light bulb" model, which can be obtained from:

www.uk.comsol.com/showroom/documentation/model/303/

After investigated this model, I still have some questions and I will appreciate if can get your help.

1, for the tungsten filament, which is set as the heat source, its surface emissivity is set to be 1 (can be found in surface-to-surface radiation 2). Does that mean it is assumed to be an ideal blackbody radiation?

2. the tungsten is able to emit heat using surface-to-surface radiation, what is the relationship between surface-to-surface radiation and blackbody (or greybody) radiation?

3. From my understanding, the heat emitted by the heat source (tungsten) heats up the inner wall of glass envelope by both radiation and convection. The inner part of the wall will also radiate heat and that's why it is set to surface-to-surface radiation as well (its surface emissivity is set to be 0.8). Is this understanding correct?

4. The glass is set to opaque, does that mean there is no heat radiation can go through the glass from its inner wall to its outer wall? If so, how could the temperature of the outer wall increase, e.g. by conduction?

5. After this process, the outer wall of the glass envelope is heated up. The final temperature of the outer wall can be obtained by considering surface-to-ambient radiation as well as convective cooling. Is this correct?

Sorry for many questions which may stupid to you. But as a newbie of COMSOL, any help from you is highly appreciated. Many thanks in advance.

Regards,

Fangjing

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Ivar Kjelberg

Ivar Kjelberg

January 29, 2013 8:22pm UTC in response to Fangjing Hu

Re: Questions of the modelling of a light bulb using Heat Transfer Module

Hi

I'm not by my workstation, so I can only give some ideas, without managing to check directly on the model:

COMSOL uses global radiative energy, and not wavelength dependent radiative exchanges, "grey" or "black" it's up to you, in the sens the total energy emitted is what you define here.
For me too an emissivity of 1 is total energy is radiated out (chec the help and look at the equations)

The inside gas is normally "transparent", so radiation goes all the way to the bulb glas part, that one, if defined as "opaque", is indeed NOT transmitting further (correct for glass and IR radiation, but visible light goes through, therefore the 0.8 emissivity I suspect). Now the air is also transporting energy by convection (mass transport) and conduction.

The glass, even if opaque has some heat conductivity and then the outer surface is cooled by convection, probably in a standard "engineering" way with a predefined "h[W/m^2/K]" I expect

you need to dig into the doc and the reference guides to get the list of contributive and exclusive BC's and variables, as several heat flux variables add up, and COMSOL gives you acces to many of the individual and global values in the Results section under derived variables, or the plot

--
Good luck
Ivar

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