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## Simulation CFD

Member
Posts: 4
Registered: ‎05-17-2012

# Using Radiation in Natural Convection thermal model

391 Views, 5 Replies
05-17-2012 03:39 PM

Thanks!

Contributor
Posts: 15
Registered: ‎10-17-2011

# Re: Using Radiation in Natural Convection thermal model

05-22-2012 11:43 AM in reply to: bblom1

The radiation is calculated only between solids.  Air and all other fluids are not included in the radiation calculations.  The value of emissivity assigned to your air material is used at boundaries that do not include a solid, like external boundaries.

It appears from your description that your model includes an electronics enclosure that is surrounded by an external air volume.  The emissivity of the air is applied to the boundaries of the external volume.  So an emissivity = 1 is probably too high.  This assumes that your enclosure is surrounded by a black body, and all radiation incident to the external boundaries is absorbed by the external boundaries.

The value of emissivity assigned to the external air volume should be appropriate for your specific environment.  For example, if your enclosure is in a room with green painted walls and ceiling, then the emissivity assigned to the air should be the same as the emissivity of the green paint.

The transmissivity determines the amount of radiation that travels through a transparent solid.  If none of your solids are transparent, then transmissivity = 0.

Product Support
Posts: 465
Registered: ‎08-24-2011

# Re: Using Radiation in Natural Convection thermal model

05-22-2012 11:50 AM in reply to: nmuoio

Nathan is right on the ball!  From your description I would lower the emissivity value of the air to probably 0.3 and assign a fixed temperature to the 4 walls (same temperature as your film coefficient) of your larger air domain, which is typically refered to as a 'bucket'.

-Royce

Royce Abel
Member
Posts: 4
Registered: ‎05-17-2012

# Re: Using Radiation in Natural Convection thermal model

05-22-2012 12:11 PM in reply to: bblom1

Thanks so much this will help a lot! The current emissivity values I have been using for the air in the bucket compared to a radiation off model have shown a delta in temperatures of around 30 degrees C. Would a 0.3 value for the emissivity value of air inside the enclosure be appropriate? I am also comparing using a bucket approach versus applying film coefficients to the outside walls of the enclosure and not modeling the bucket. I am trying to compare both approaches to see if I can get relatively the same results with and with out radiation.

Product Support
Posts: 465
Registered: ‎08-24-2011

# Re: Using Radiation in Natural Convection thermal model

05-22-2012 12:21 PM in reply to: bblom1

It is not advised to have film coefficients on the walls during a radiation analysis and if you mean 'not as a bucket', I would assume that you mean having 5 surfaces with a 0 pressure conditions.  This is also not advise for 2 reasons.  Having that many 0 pressure conditions on a domain of this shape and result expectation can sometime lead to divergences AND radiation is only trasmitted to walls.   Since a 0 pressure is an opening it will not partake in your radiation analysis which will give you some strange temperature results.

In terms of emissivity values, taking a value of 0.3 will probably result in higher temperature than what you have now so it is typically a good go to value to be conservative.  This value can be tricky to define because it is your environment emissivity and will depend on where the electronics will be used.

Royce Abel
Member
Posts: 4
Registered: ‎05-17-2012