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90 Analysis and Design of Energy Geostructures

Figure 3.9 Comparison of correlations for forced convective heat transfer coefficient with data
from Lee, Y., Choi, M.-S., Yi, S.-T., Kim, J.-K., 2009. Experimental study on the convective heat transfer
coefficient of early-age concrete. Cem. Concr. Compos. 31 (1), 6071. and Guo, L., Guo, L., Zhong,
L., Zhu, Y., 2011. Thermal conductivity and heat transfer coefficient of concrete. J. Wuhan Univ.
Technol. —Mat. Sci. Ed. 26 (4), 791796. Redrawn after Bourne-Webb, P., Freitas, T.B., da Costa
Gonçalves, R., 2016. Thermal and mechanical aspects of the response of embedded retaining walls
used as shallow geothermal heat exchangers. Energy Build. 125, 130 141.

Examples of thermophysical properties as a function of temperature for pure water
as well as for a heat carried fluid composed by a mixture of water and 25% and 50%
of monoethylene glycol (MEG 25 and MEG 50, respectively) are reported in
Tables 3.6, 3.7 and 3.8, respectively.

3.5.3 Remarks about convection
Convection dominates heat transfer when a significant fluid flow is present. Based on
this consideration:
• Either free or forced convection can characterise heat and mass transfer within geo-
materials and reinforced concrete. Convection can generally be neglected in con-
crete. However, it can be significant in geomaterials. Water can be considered to
be the fluid responsible for a significant heat transfer in geomaterials via its
mass flow.