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Temperature drop in Porous Media?

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Old   January 4, 2011, 10:41
Default Temperature drop in Porous Media?
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Sanyo
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Dear All,

At the outset, Wish you all a Very Happy & Prosperous New Year!

I am trying to model a heat exchanger with porous media assumption. It is an HE in the furnace (superheater). I am modeling the flue gas side only. Hence effects of steam have to be approximated. I know the temperature drop in flue gas after passing through the superheater. I am giving the sink value for thermal extraction accordingly. However I am facing two problems due to nonuniformity of the flow.

1. Flue gas temperature is much higher than the expected value.
2. In some cells, where the flow is negligible, temperature of subdomain goes upto "-14" Deg C, which is definitely unrealistic.

Is there any way to limit the minimum temp. value inside the subdomain?

Can I define my sink term to get the exact temperature drop in flue gases? If yes how?

I would appreciate your help.

Thanks in advance,

Regards,

--Sanyo
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Old   January 20, 2011, 03:01
Question So its not possible in CFX?
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Hi all,

Till date no reply, so shall I assume that whatever I am asking is not possible in CFX?

Any help would be highly appreciated.

Regards,

Sanyo
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Old   January 20, 2011, 05:24
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Glenn Horrocks
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Yes, you can set the heat source to give you the intended temperature. It has been a while since I did it but I think you define a source of -c(T-Tset) and a source coefficient of -c, where Tset is the desired temperature and c is a constant large enough to make it work.

This has been asked on the forum before, do a search and hopefully you can find it.
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Old   January 20, 2011, 08:56
Unhappy I want specified temperature drop
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Dear Glenn,

Thanks for your prompt reply. The previous question you are referring was asked by me only. The difference in both problems is that previously I wanted specified temperature throughout the subdomain (Fixed value of temperature like 800Deg C). Now I want specified temperature drop, let us say of 100 Deg C. That I am unable to achieve.

Another problem is that due to a constant value specification of sink term, at some cells of subdomain gas temperature reached to "-14" Deg C, which is impossible in water type of heat exchanger. I wish to limit my temperature values also.

Thanks for your help.

Regards,

Sanyo
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Old   January 20, 2011, 21:21
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Glenn Horrocks
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Oh, you want a specified temp drop, not a specified temp. Sorry, I missed that.

I don't think you can do this with a source. Try an interface, and apply a heat flux on the interface.
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Old   January 21, 2011, 04:29
Default & what about the temperature limit?
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Dear Glenn,

Thank you very much your reply. I will try the interface flux. But what about the temperature limit? Can't I limit the lower & upper temperature values for domain/ subdomain? My simulation is steady state & fluid is air ideal gas.

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Sanyo
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Old   January 21, 2011, 07:29
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You can but it is usually a bad idea as it leads to convergence problems. Better to fix it at the source and stop the spurious temperatures in the first place.

The spurious temperatures are probably caused by your source region and so if you replace it with an interface the problem will go with it.
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Old   January 24, 2011, 10:44
Thumbs up Thanks for your support
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Dear Glenn,

Your suggestions are always extremely helpful. Thanks for your support.

Regards,

Sanyo
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