Page 115 - Analysis and Design of Energy Geostructures

Heat and mass transfers in the context of energy geostructures 87

The motion of fluids, which involves a movement of the particles bringing
their thermal energy in proximity to each other, is the result of a force. When the
force that causes the motion of the fluid is due entirely to density variations
caused by a nonuniform temperature distribution, that is a temperature gradient,
the convection phenomenon is called free or natural convection. When the force
that causes the motion of the fluid is due to any other cause, the convection phe-
nomenon is termed forced convection. Free convection is an example of the coupling
between heat and mass transfers.
In the analysis of motion of fluids, it is relevant to distinguish problems of internal
flow, external flow and seepage flow. In internal flow problems, the fluid in motion is
completely bounded by a surface (e.g. a heat carrier fluid circulating in a pipe or air
flowing in an underground built environment). In external flow problems, the fluid in
motion is not completely bounded by a surface (e.g. surface air flowing over the
ground). In seepage flow problems, the fluid is in motion across a permeable material
medium (e.g. groundwater flowing within soils).
The physical phenomenon of convection heat transfer can be explained, for exam-
ple with reference to a fluid in motion relative to a bounding surface at a different
temperature, which at a meaningful scale can be considered as a plane wall. This prob-
lem is represented in Fig. 3.7 considering a convection phenomenon that can charac-
terise, for example an energy tunnel. The temperature varies from a value T s at the
surface (e.g. y 5 0) to a temperature T N , T s in the fluid bulk. In this case, heat trans-
fer occurs from the wall to the bulk of the fluid. The opposite is true for T s , T N .At
the surface wall, convection heat transfer occurs because of the sole molecular diffu-
sion mechanism, as the velocity of the fluid is equal to zero. Beyond the surface and
until T s 6¼ T N , the mechanism of bulk motion contributes to the overall convection
heat transfer phenomenon.

Figure 3.7 Heat transfer by convection at the surface of an energy tunnel.