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86 Analysis and Design of Energy Geostructures

Figure 3.6 Heat transfer modes in soils depending on effective particle diameter, D 10 , and satura-
tion degree, S r . Redrawn after Farouki, O.T., 1986. Thermal Properties of Soils. Series on Rock and Soil
Mechanics, vol. 11. Trans Tech Publications, Clausthal-Zellerfeld.

pores constituting the soil, the type of fluid filling the pores, the degree of satura-
tion of the material and the presence of fluid flow through the pores. An example
of the influence of the effective particle diameter, D 10 (i.e. the size of a sieve
through which the 10% of the grains pass), and the water saturation degree, S r ,on
the dominant modes of heat transfer characterising soils is reported in Fig. 3.6 with
reference to the data reported by Farouki (1986).

3.5 Convection
3.5.1 Physical phenomenon and governing equation

Convection is the mode of heat transfer that characterises fluids in motion with a tem-
perature gradient. This mode of heat transfer is associated with the superposition of two
mechanisms: energy transport by a diffusion motion and by a bulk motion of the fluid.
The former mechanism is the same that characterises conduction and it is the result of
an invisible microscopic random movement of molecules. The latter mechanism is
associated with the so-called advection phenomenon and it is the result of a visible mac-
roscopic movement of fluid molecules as aggregates or a whole. The visible macro-
scopic movement of a fluid is often called convection mass transfer.