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Mike23 September 11, 2008 19:55

local age of air in starccm+
 
Hi

I am trying to evaluate the ventilation effectiveness in an enclosed car park with impulse fans in starccm+. (first time working with such kind of problem)

Right now i am trying to determine the age of air and I have seen in this forum an advice to use 1/density for the passive scalar value. But I am not seeing how can this produce a variable with units of time... What should be th field function definition? Do you know any place where I can get any information on this topic?

And, one other thing: what boundary condition do you use for air extraction ducts? Pressure outlets? I mean, in case of fire air inlets are shut down and air extractors work at full power and the manufacturer gives me the volumetric flow rate they can handle. But I am not being able to specify a BC that removes air from my domain...

any help appreciated!

tanks in advance

matej September 12, 2008 03:12

Re: local age of air in starccm+
 
Hi,

well, it is actually not 1/rho, but rho as the source term itself in the age of air scalar transport equation.

the original derivation of the equation could be found here: Li X, Jiang Y. Calculation of age-of-air with velocity field. Paper presented at Post-IAQ 96 Seminar, Beijing, 1996. or maybe some other. I cannot find it right now, but have a look for example here: Zhang Lin, T.T. Chow, C.F. Tsang, K.F. Fong, L.S. Chan CFD study on effect of the air supply location on the performance of the displacement ventilation system Building and Environment, Volume 40, Issue 8, August 2005, Pages 1051-1067

I understand, it is hard to believe it gives you time in seconds and I have to admit I do not remember the argument any more, but it is somewhere in the article.

For the boundary conditions, you should set zero value at the inlet and zero flux at the walls and exhaust (pressure bc,\, I guess). It is actually quite easy, you only need to set up the passive scalar. It only takes some time for the scalar to converge, but as it is passive, I'd suggest run it on a converged velocity field, where you turn off all the solvers except the scalar one. Check for the field and monitor scalar value in some points.

good luck

matej


Mike23 September 17, 2008 23:52

Re: local age of air in starccm+
 
Thank you matej for your answer, it has been quite helpful!

one of my problems was that i was just setting the inlet boundarias value to 0 and didn't know i was supposed to activate the passive scalar option in 'physics conditions' of the region i was studying!

just another question, since it's my first time dealing with this kind of problems:

there's number of papers that use 1 as the value for the source term (Bartak, Cermak et all) wheres others use density(as you suggested) which is 1.18 kg/m^2. Since i didn't see anybody discussing which one would be more suitable, i would like to know, based on your experience, which approach could be better

thanks


matej September 20, 2008 11:16

Re: local age of air in starccm+
 
Hi Mike, I'm glad my post was of some use.

Well, the source was derived to be 1/ density, so it makes sense to use it like this. The true is the density is relatively close to one, but still using 1 instead of 1.18 makes 18 % error to me. On the other hand, the age of air is more of a qualitative measure, so you are not interested in the seconds itself, but more to the maximums and minimums of the field. That's all I could say, really.

wow. I'm writing for 2 minutes still not answering your question. I think I'm becoming a researcher!

matej


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