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Name: Vaheesan
Status: student
Age: 20s
Location: N/A
Country: N/A
Date: 1999-2001


Question:
Is there an equation relating the thermal conductivity and moisture content of sandy soils(Sharp river sand). I am trying to find out the thermal conductivity of sharp river sand with moisture content of 18.84% in J/s/m/degree C.

I would also like to find out how to convert a thermal conductivity value from W/m.K to J/s/m/ degree C.


Replies:
For this type of mixtures, one often conducts an experiment to empirically arrive at a relationship for thermal conductivity as a function of moisture content. Such data are most likely available in the literature; you can search for them using one of the engineering data banks at your library. You may also check text books on "heat transfer in porous (or multiphase) media."

One can also drive equations from first principles but they have to be validated experimentally. The problem you posed is somewhat complicated because not only there is a mixture of two materials but also there is the packing factor which is dependent on the size distribution of sand granular in the mixture.

If you cannot find references, you can set up and conduct a simple experiment. Take a plastic tube, about 20 mm in ID and about 200 mm in length. Fill the tube with the mixture and block the two ends tight with two thin (5-10 mm) cooper disks. Insert a few thermocouples at 20 mm internals (from end to end) into the tube. Steadily heat one of the copper disks at one end and cool the other copper piece at the other end. Moderate heating is sufficient. Plot the measured temperature as a function of distance along the tube length. The slope is q"/k where q" is the heat flux (in Watts per unit area) applied at one end and k is the thermal conductivity sought. You need to estimate q"; this is easy if you apply heating with an electric heater (foil type). The other end of the tube can be immersed in cold or ice water. Consult an introductory text book on Heat Transfer for more information.

Regarding your unit conversion question, W (Watts) is a measure of power which is energy (Jules, or J) per unit time (seconds , s). As such W and J/s are identical; 1 W is one J/s.

AK

Ali Khounsary, Ph.D.
Advanced Photon Source
Argonne National Laboratory



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