liquid transport coefficients

[jorne]

Dear wufi team

I saw a WUFI-help of liquid transport coefficients, and i read:

there is little capillary conduction above free saturation, the liquid transport coefficients for this moisture region are very low.
Then, How can you explain, that one smaller free water saturation cause more liquid transport coefficient for suction- DWS??, since the liquid transport coefficients for regions that have moisture content above free saturation is very low.

Regards

Jorne

[Thomas]

How can you explain, that one smaller free water saturation cause more liquid transport coefficient for suction- DWS??, since the liquid transport coefficients for regions that have moisture content above free saturation is very low.

I'm not sure whether I understand your question. If you are wondering why some materials with lower free saturation may have higher liquid transport coefficients: this depends on the pore structure of the individual materials in a complex way; there is no simple relationship between free saturation and liquid transport coefficients.

Regards,
Thomas

[jorne]

I said this, because i looked for the formula of Dws(W)= 3,8x(A/Wf)^2..., this formula is in the help of topic:" liquid transport coefficients" .In this formula, exist one relationship among Dws and Wf (free water saturation)???

Regards

[Thomas]

I said this, because i looked for the formula of Dws(W)= 3,8x(A/Wf)^2..., this formula is in the help of topic:" liquid transport coefficients" .In this formula, exist one relationship among Dws and Wf (free water saturation)???

This formula does indeed describe a relationship between Dws and wf, but it also contains the water absorption coefficient A which describes how much water the material absorbs during a given period of time when exposed to liquid water. If the A-value was constant, then your conclusion that smaller wf leads to higher Dws would be correct. However, A depends strongly on wf and Dws, so the condition of A remaining fixed does not apply.

Look at it this way. Let's consider a material with constant liquid transport coefficient Dw for simplicity. When the material is exposed to liquid water, it will absorb an amount m(t) during time t, and experiment shows that this absorbed amount is usually proportional to the square root of time, the constant of proportionality being the A-value:

m(t) = A * sqrt(t).

On the other hand, in the simple case of constant Dw the absorbed amount of water can be computed explicitly, and the result is:

m(t) = 2 * sqrt(Dw/pi) * wf * sqrt(t)

Comparing the two equations shows that the A-value depends on both Dw and wf:

A = 2 * sqrt(Dw/pi) * wf.

Since A and wf can easily be determined experimentally, this provides a convenient way to determine Dw:

Dw = pi * A^2 / (4 * wf^2)

This equation allows to estimate Dw when A and wf are known. However, you also see that A is by no means the same for all materials. Quite to the contrary: it is an integral measure for the effect of both Dw and wf on the water absorption, so it will change more or less strongly when Dw and/or wf change.

In reality, Dw is not constant (it depends strongly on the water content), and the formula derived above for estimating Dw from A and wf is too simple to be useful in practice. The formula you quoted is a more sophisticated version taking an exponential variation of Dw(w) into account.

So if you select materials which happen to have the same A-value, then you should find that in general those materials with smaller wf will have higher Dws. But in general, A is better thought of as being a number which depends on Dw and wf, and thus will vary considerably for different Dw and wf, rendering any comparison moot.

Regards,
Thomas

[jorne]

now , i understand.

thanks for your complet answer.

regards