Semi-transparent window glass model

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June 15, 2014, 05:04
Semi-transparent window glass model
#1
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sarawut
Join Date: Jul 2013
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I have some questions about CFD simulation because I am a beginner of the CFD field and also FloEFD. I have tried it to solve several problems in the text books, which I found, in most cases, quite accurate in comparing with hand calculation(e.g. conduction, convection, opaque surface radiation) to make sure prior to do a full DSF model. But when use the radiation heat model for the semi-transparent model(simple window glass).

1.I just want to consider only radiation, so I did not put the convection coeff. as a boundary. Is it correct?

2. When I activate the semi-transparent option. Only absorption and refrac coeff. can be specify or there may be a relationship between solid absorption and emissive, but Kirchhoff’s Law.

3.How can the leaving radiant flux becomes invalid or 0 if the solid temperature is not 0.

4.when I check the heat transfers via convection and radiation modes inside floEFD, I've noticed that for convection I can specify the fluid temperature in different domain seperately. In case of radiation, how can I model the heat exchange if the environmental temperature is different between outside and inside the room as in the general setting box allow me to specify only one value as global? Do I have to create my own equation goals?
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 June 23, 2014, 02:51 #3 Senior Member   Boris Marovic Join Date: Jul 2009 Posts: 494 Rep Power: 16 I found what was wrong in your goals. You need to use the "Net volume radiant flow" as goals. If you go in the help and search for that you will find it in the "List of parameters and their definition" on what they mean. Boris

June 26, 2014, 16:34
#4
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sarawut
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Dear Boris,

Thank you very much for your reply! And it is really helpful and I have some ideas to solve my issues. However, some information I gave were not so clear from the beginning, these are;

1. Like you said, the actual behavior is the combinations of three modes. The reason I did was to make sure I can understand (manually checking) the effects of each mode (this case only radiation, get rid of others) before conjugation. Otherwise, it is hard for me to check the results after running if reasonable or not.

2. For glass window applications, there are coatings on clear/tinted glass surfaces (e.g. low-E, solar reflection) for several purposes. That is why I am looking for this boundary condition. So, can you please suggest me what should I do? Perhaps there are some techniques for modeling directly/indirectly.

3. One thing I do not understand is that as long as the object temperature is not 0K, it should radiate the heat wave at the surface either opaque or semi-transparent. But floEFD considers it invalid and I have to use”Net volume radiant flow” as you suggested. I also have read the meaning of each variable, but there are no formulae mentioned how to obtain the answers. It is hard for me to cross check unlike convection and conduction.

4. If I want to simplify the model without creating the interior wall for putting the temperature as boundaries (only glass pane to be modelled), what is the environmental temperature I should set? Regardless of other modes, if the external temperature is Te and the internal temperature is Ti, the glass pane sits between outside and inside of the room, as shown in the attached pics. Or I have no choices, but have to establish a box as an interior boundary.

In case of the radiation task, I have tried as per your comments as shown the attached pic as well (I don’t know if what I put is 100%correct) another thing, I put only 10000nm for you to noticed. Actually it should be 100000nm? The average value of each bandwidth has been shown in the “red box” my problem is if I want to know the overall average value. How can I get it?

Thank you again!
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 G1.jpg (79.1 KB, 31 views) G2.jpg (25.0 KB, 34 views)

 July 1, 2014, 02:46 #6 Senior Member   Boris Marovic Join Date: Jul 2009 Posts: 494 Rep Power: 16 By the way, you can create like a room/box around the one side of the glass, define the box as insulator so it doesn't conduct any heat and apply to interior walls a pressure boundary condition so the fluid can leave the box like in a natural convection case if you want to use fluid at some point and then apply a radiation source on the inside of the box with your ambient temperature. Boris

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