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material. The units used to describe thermal conductivity are Wm-'K-' and, as
wafts are comprised of joules (a basic unit of energy) per second, the units
describe the quantity of energy over time transferred through one metre of
material with a temperature difference of 1K (or 1`C). Thermal conductivity
values of building envelope components are used to calculate potential levels of
heat loss from buildings using 'U' Values, with units Of WM-2 K-1, denoting the
quantity of energy that would be lost through an area of building envelope at a
steady state, where the external temperature was lower then the internal
temperature.
In transient conduction, equation (1) is combined with the principle of
conservation of energy. In three dimensions, this results in the heat conduction
equation (2), which forms the basis for most of the heat conduction analyses
relevant to the thermal probe transient line source methodologies, as described
in chapter 2, the literature review.
a (2aT)+ a(2 aT )+a ('ý aT
ax ax ýy ýy az az
)
+Q* = PC aT
at
Equation (2)
Heat transfer occurs when a temperature gradient exists in or between
materials. The transfer can occur by three distinct means:
Conduction
Convection
Radiation
Often a combination of these can take place simultaneously, such as: in a
porous or granular opaque material, heat transfer through the solid component
may occur by conduction, and through pores containing air by convection and
radiation; or, in a translucent solid, both conduction and radiation may occur
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