HW 1 - p 3.24 - Drying a Clay Brick
A wet, clay brick measuring 2 x 4 x 6 in. has an initial uniform moisture content of 12 wt%. At time = 0, the brick is exposed on all sides to air such that the surface moisture content is maintained at 2 wt%. After 5 h, the average moisture content is 8 wt%. Estimate:
(a) The diffusivity of water in the clay in cm^2/s.
(b) The additional time for the average moisture content to reach 4 wt%. All moisture contents are on a dry basis.
(a) The diffusivity of water in the clay in cm^2/s.
(b) The additional time for the average moisture content to reach 4 wt%. All moisture contents are on a dry basis.
posted by Dr. B at 2:09 PM
2 Comments:
You do NOT need to keep 50 terms to get an accurate solution to this problem. In fact, if you use Excel and you do keep 50 terms you will get errors ! I suggest you keep just 10 terms. This worked great for me.
Wow. Everything you said is correct.
The most important point is that CA has units of moles of solutute per volume.
If we assume the volume of the brick does not change as it dries, then your analysis is more accurate than just using wt% as I did in class. Good work. The mass of clay is definitely constant. Then, if the volume of the brick is also constant, then your brick density is constant and your analysis is correct.
I am not sure if the volume of the brick changes as it dries. From what I read on the web, it depends on the composition of the clay. Here is what I read in the class notes of Soils 154 at Iowa State:
"Water tries to work its way between the clay layers in a clay particle. If it succeeds the clay will swell when wetted and shrink as it dries. The stronger the bonding force between clay layers the less likely the soil will exhibit shrink-swell properties."
But, if the volume changes, is it ok to use wt% ? I think NOT. I think your approach is best. Replace CA with wt%/(1-wt%) in the eqns in section 3.3. I will accept either approach on the HW.
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