Page 176 - Biosystems Engineering

Models for Heat Transfer in Heated Substrates 155

Consequently, the soil absorbs heat when ambient temperature is
high and releases heat when temperature decreases. However,
because this situation is completely different in heated substrates, we
focus on temperature distribution and heat flows in a greenhouse
bench with thermal insulation and substrate electric cable heating.
Under these conditions, switching the heating element on consider-
ably increases temperature in the surroundings of the heating ele-
ment, thus generating high-temperature circular-section areas. In the
rest of the substrate, temperatures are distributed into horizontal lay-
ers, with an undulated area that comprises the warmest areas around
the heating element (Fig. 4.3).
While the heating system is switched on, the highest heat flows are
radially distributed around the heating element and are more intense
in the upward direction (Fig. 4.4). The lowest heat flows occur under

Temperature in °C

5 10 15 18 20 22 25 30 40
Insulator Substrate Heating cable
MN

MX

Z X
FIGURE 4.3 Temperature distribution in a cross-section of a heated substrate when
the heating element is switched on.

Heat flux in Wm –2
.151B-11 5 10 25 50 75 100 150 300
Insulator Substrate Heating cable

Y
Z X
FIGURE 4.4 Heat fl ow distribution in a cross-section of a heated substrate while the
heating element is switched on.