Hello,
In the help of Wufi (‘Details/Basic Material Data’), the porosity of the material is defined and it is said that it can be estimated from the true density and the bulk density.
Following the formula, it seems that the porosity is the total porosity. But the porosity is also used to estimate the maximum water content (wmax=porosity x 1000). So it is then supposed to be the open porosity.
Can you tell me which of both of the total or the open porosity it is and then which one we have to take for a new material we should add in the material database of Wufi ?
About the cellular glass in the wufi materials database (Holzkirchen), I’m estonished about the value of the porosity : 0,25 : is it not high for a total porosity and very high for an open porosity ?
Thanks in advance,
Brigitte
Porosity of the materials
Re: Porosity of the materials
Hi Brigitte,Brigitte wrote:Can you tell me which of both of the total or the open porosity it is and then which one we have to take for a new material we should add in the material database of Wufi ?
in WUFI we usually do not distinguish between open and total porosity. You are right in pointing out that in these two cases two different kinds of porosity apply; maybe I should update the online-help accordingly.
But what to do is simple: in WUFI the porosity is only used to determine the maximum water content: w_max = porosity * 1000. So you may use any number which results in a plausible w_max. If you have measured or estimated the open porosity in some way, you may use that number. If you know nothing about the porosity of your material simply enter some number which appears to result in an adequate w_max (w_max only must be greater than free saturation).
Under most circumstances the water contents in your materials will not exceed free saturation anyway, so the precise number used for the porosity is irrelevant. You need to care about exact numbers for the porosity only in a situation (like condensation) where free saturation may be exceeded and where it is for some reason important how much water the material can take up (in most design cases, large moisture accumulations will be enough to rule that particular design out, regardless of the details of the accumulation).
Bulk density is only used for converting between mass-specific heat capacity (entered by the user) and volume-specific heat capacity (needed for WUFI's transport equations). You'll usually take this number from tables or estimate it from similar materials. Experience shows that the hygrothermal conditions (as opposed to purely thermal conditions, such as in calculation of energy consumption) are not very sensitive to the thermal parameters (heat capacity and heat conductivity) so it is usually sufficient to use plausible guesses for those numbers. These will thus be your usual sources for the densities: tables or informed guesses.
If you wish to derive an unknown density from a known porosity, you have to use the total porosity.
About the cellular glass in the wufi materials database (Holzkirchen), I’m estonished about the value of the porosity : 0,25 : is it not high for a total porosity and very high for an open porosity ?
I don't know offhand how that value was determined, and right now I can't reach the colleague who collected those data. But I would think that it's very likely just a guess. Cellular glass is considered to be hygrically inert. It has completely closed gas cells (open porosity=0), it does not take up or transport any water (neither liquid nor vapor). It also has no defined moisture storage function, so it is quite irrelevant what the w_max is, but WUFI needs some number in that place, and that number was probably just as good as any other.
Since the moisture storage function for the cellular glass is left undefined in the database, WUFI uses an internally defined default moisture storage function which depends on w_max and thus on the porosity. But WUFI only uses this default function because it needs a well-defined moisture content for all materials, even for materials which cannot take up any water at all. Any water contents reported by WUFI for those materials are thus spurious and should be ignored. So again, the precise number for porosity used in this case does not matter.
Bye,
Thomas