In solving for the air temperature along the duct length, the bulk earth temperature te, was assumed and the initial
surface temperature (when j=1) was assumed to be equal to the bulk earth temperature 4
. In the light of a number of
experimental and analytical works 5, 6 and 7
, the bulk earth temperature could be calculated.
On the surface, the average temperature of the ground is determined by the type of the local climate, which can
change year by year, and by balance between several modes of thermal transfer i.e.; the absorption of the incidental
solar radiation, the heat loss of long waves radiation towards the sky, the heat exchange by convection with the
ambient air and the heat loss by evaporation from the soil’s surface. Moreover, with suitable treatments the temperature of the ground surface can be raised or lowered above or below the normal temperature of the ground.
The change in surface temperature also affects the soil temperature at depth.
3.1. Mathematical modeling of the natural ground temperature
Soil temperature undergoes a daily cycle, and an associated cycle with the weather variations in addition to one
annual cycle. These variations are restricted to the layers close to surface. The daily cycles are felt with a penetration
depth of approximately 0.5m under surface and the weather cycles, of approximately 1m 6
.
To evaluate the temperature of the ground, the soil is regarded as a semi-infinite solid. It is expressed according
to the depth and time. When the surface temperatures are known, the soil temperature to a given depth can be
estimated. If these data are not available, weather data can be used in a limited state which indicates the energy
balance at the soil surface 8
.
In order to evaluate the temperature of the ground, several mathematical models were developed such as those of
Morland 8
, Kusuda 9
, and Labs 10. Their models present a solution of the equation of heat transfer of a semi-infinite solid whose variation in the external temperature is sinusoidal.