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porosity of baffle

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Old   March 19, 2002, 23:28
Default porosity of baffle
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raymond
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I'm now trying the simulation of air conditioning system with one buffle area connect one inlet and multi outlets to the studying office. The designed pressure drop between inlet and outlet is 25 Pa. I set the baffles at the outlet of air-condition system (which is the inler to the studying room). I now the pressure drop is determined by alpha and belta value of porosity. But what's the meaning of porosity in baffle property? Furthermore, how can I determine the alpha and belta and porosity of baffle to get the designed condition(pressure drop 25 Pa or nearly unform velocity at outlet)?

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Old   March 27, 2002, 11:38
Default Re: porosity of baffle
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cjtune
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The pressure drop is proportional to your flow speed so you might want to look up fan performance charts and match with heat exchanger pressure loss charts to find a point where you would get 25 Pa nicely.

The alpha and beta coefficients can be obtained by experimental measurement of the heat exchanger (dP vs Q) you plan to use. You could also do a CFD sim for a portion of the of the heat exchanger to get the same thing if you don't have access to experimental measurement rigs.

Also, your heat exchanger has some thickness so why not use porous cells instead?
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Old   April 1, 2002, 21:53
Default Re: porosity of baffle
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Raymond
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Thanks very much. But I still have one question. What's the defination of the porosity? If it useless, why it need to be define in the STAR-CD?

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Old   April 2, 2002, 05:13
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cjtune
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Porosity is like a momentum sink of some kind and can have characteristics according to the direction of the flow (isotropic, orthotropic, etc.). Try reading the methodology manual (Chp 8) and the user guide (Chp 10). It is certainly not useless as it is very widely used to represent air filters, heat exchangers, or other resistances to flow that are very hard to mesh explicitly.
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Old   April 4, 2002, 04:03
Default Re: porosity of baffle
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Carl Fredrik Stein
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Clearly, porous baffles are highly useful in a wide variety of industrial cases. However, reading the Methodology and the User Guide to Star-CD, one gets the impression that the pressure drop over a porous baffle depends on the flow across it, as well as on alpha and beta, but when defining a porous baffle, one must, in addition, specify a third quantity called porosity. I scrutinised the Methodology and User Guide, but I could not find a single place where this quantity was mentioned. Rather than exploring this issue in great detail, I interpreted the porosity as the portion of the area that was open, and the results that came out were reasonable. I suspect that the porosity affects the turbulent quantities, but frankly speaking I do not know. I would, however, very much appreciate a contribution from someone who really does.
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