Thermal conductivity of soil

A scheme is devised to measure the thermal conductivity of soil by immersing a long electrically heated rod in the ground in a vertical position. For design purposes, the rod is taken as 2.5 cm in diameter with a length of I m. To avoid improper alteration of the soil, the maximum surface temperature of the rod is 55°C while the soil temperature is 10°C. Assuming a soil conductivity of 1.7 W/m °C, what are the power requirements of the electric heater in watts ... 

C How does the condensation or freezing of water vapor in the wall affect the effeclivene.ss of the insulalion in the wall How does the moisture content affect the effective thermal conductivity of soil ... 

Under the conditions described in Example 10.5, what is the average rate of heat loss per unit area from the ground to the air during the 12-h period The thermal conductivity of soil is 0.7 W/m-°C. 

Cables that are laid in the ground need to dissipate heat into their surroundings. The thermal conductivity of soil varies considerably from 0.7 km/W for wet soil e.g. near lakes, coastal areas, high water tables, to 3.0 km/W for very dry soil such as desert sand, see Reference 7. 

Johansen 0. (1977). Thermal Conductivity of Soils. U.S. Army Cold Regions Res. Eng. Laboratory, Translation 637. 

Thermal diffiusivity of soil Thermal conductivity of soil Temperature of the liquid pool Temperature of the soil ... 

For soil, specific studies demonstrate the influence of soil density, moisture content, and soil type (Abu-Hamdeh and Reeder, 2000 Clark, 1966 Quiel, 1975 Schuch, 1980). Thermal conductivity of soil is essentially influenced by density, water content (moisture), and composition (particularly organic content). Schuch (1980) may be cited for the general behaviour of soils "The thermal conductivity of dry soils is small (0.2-0.8W m Y>. ), reaches a maximum by 20 to 30 wt% of water content (2 or 3 W m K ), decreases for higher contents of water, e.g., wet bog, and draws near the value of thermal conductivity of water (0.6 W m K ). This decrease is originated by an increasing porosity in that range and a connected decrease of the heat transfer by the skeleton of the solid parts of the soil . 

FIGURE 9.11 Thermal conductivity of soil (a) thermal conductivity as a function of density at three moisture contents in % (sand), (b) thermal conductivity as a function of density at three moisture contents in % (clay, loam), (c) thermal conductivity of clay loam as a function of organic matter content in %. Data from Abu-Hamdeh and Reeder (2000). 

A significant rise in temperature AT is calculated in a volume of soil at a distance up to 3 from the anode, where is the anode radius. Factors are the thermal conductivity of the soil, k, length of the anode, L, the grounding resistance, Rq, and the current, I. The rise in temperature can be calculated from these parameters [10]. For deep anodes it amounts to ... 

For this example, we will consider the soil surface as a boundary condition with an oscillating temperature, described by a cosine function. The soil will conduct heat from the surface, without flow. The only transport mechanism will be the thermal conduction of the soil matrix. 

A heat pump system utilizes a heat exchanger buried in water-saturated soil as a heat source. The heat exchanger basically consists of a series of vertical plates with height of 30 cm and a width of 10 cm. These plates are effective Is at a uniform temperature of 5°C. The soil can be assumed to have a permeability of 10 0 nr and apparent thermal conductivity of 0.1 W/m-K. The temperature of the saturated soil far from the heat exchanger is 30°C. Assuming natural convective flow and that there is no interference between the flows over the individual plates, find the mean heat transfer rate to a plate. 

These results suggest that sulfur-bonded composites may have uses in civil enginering where thermal insulation is required. General principles indicate that increased voids content or a stable retention of a noncrystalline form by the sulfur would reduce the thermal conductivities of the composites. A composite incorporating potters flint did have a fine void structure, but nonetheless its thermal conductivity was higher than that of sulfur (24) presumably a still larger void content is required. Use of moist fillers such as damp soil produced foaming, but a stable, well distributed system of voids was not produced the bubbles which formed collapsed under the conditions used. 

A 2.54-cm Styrofoam plastic foam with thermal conductivity of ca 0.03 W/(m-K) (0.21 (Btu-in.)/(ft-h- F)) is equivalent to 61 cm of gravel. Any synthetic foam having compressive strength sufficiently high and thermal conductivity sufficiently low is effective. However, the resistance of PS-type foams to water, frost damage, and microorganisms in the soil makes them especially desirable. An interesting and important appHcation of this concept was the use of Styrofoam in the constmction of the Alaska pipeline. In this case, the foam was used to protect the permafrost. 

A 30-m-long, lO-cm-diameter hot-water pipe of a district heating system is buried in the soil 50 cm below the ground surface, as shown in Fig. 3-49. The outer surface temperature of the pipe is 80 C. Taking the surface temperature I of the earth to be 10°C and the thermal conductivity of the soil at that location to be 0.9 W/m "C, determine the rate of heat loss from the pipe. 

Properties The thermal conductivity of the soil is given to be k = 0.9 W/m C Analysis The shape factor for this configuration is given in Table 3-7 to be... 

Arowof 1-m-long and 2.5-cni-dianieter used uranium fuel rods that are still radioactive are buried in the ground parallel to each other witli a cenler-to-center distance of 20 cm at a depth 4.5 m from the ground surface at a location where the thermal conductivity of the. soil is LI W/m "C. If the. surface temperature of the rods and the ground are 175 C and 1 S C, respectively, determine the rate of heat transfer from the fuel rods to the atmosphere through the soil. 

It is expected that nanotubes act as very good thermal conductors, but they are good insulators laterally to the tube axis. Measurements indicate that SWNTs have a room temperature thermal conductivity along its axis of about 3500 W m K higher than that for copper (385 W m K ). Also, SWNT has a room temperature thermal conductivity across its axis (in the radial direction) of around 1.52 W m -K , which is nearly as thermally conductive as soil. The temperature constancy of carbon nanotubes is expected to be up to 2800 °C in vacuum and about 750 °C in air. 

TABLE 9.13 Thermal Conductivity of Some Central European Soils... 

In the design of borehole heat exchangers accurate information on the soil thermal parameters, such as thermal conductivity, heat capacity and temperature, is essential for the design of an economically sized and well-functioning thermal energy store. Especially the soil thermal conductivity is critical, as... 

In the boreholes, which are placed at a parking lot, single U-pipes are installed. There is no back filling, but the holes are filled with ground water. The boreholes are drilled through 65 m of clayey soil and 135 m of shale. The thermal conductivity has been measured with a Thermal Response Test (TRT) to be approx. 2.8 W/m K. 

Thermal property is another critical property for furnace slag. Because of their more porous structure, blast furnace slag aggregates have lower thermal conductivities than conventional aggregates. Their insulating value is of particular advantage in applications such as frost tapers (transition treatments in pavement subgrades between frost-susceptible and nonfrost-susceptible soils) or pavement base courses over frost-susceptible soils. 

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