The transport of heat through a porous medium in the presence of
exterior forces, generally produced by the Earth's gravitational field
and/or a pressure gradient, is called conduction when the Darcean fluid
is static (motionless), and convection when the Darcean fluid is in
motion. It is customary to use the term convection also to describe the
motion which arises from the density differences due to temperature
gradients within the Darcean fluid. We think that because this last
phenomenon is more general it should be given a specific name; here we
call it thermal flow. In the sense of the above definitions, convection
and thermal flow are two distinct phenomena (they occur together, in
underground combustion for instance), and the convective motion which
arises when a Darcean l'luid is in contact with a source of heat is a
particular case of thermal flow. Thermal flow occurs naturally and is
important in many geophysical and industrial problems, particularly in
oil exploration, and in the petroleum, chemical and nuclear industries
(for instance, in the evaluation of capability of heat-removal from a
hypothetical accident in a nuclear reactor). It can play a part in the
transfer of heat from the deep interior of the Earth to a shallow depth
in the geothermal regions. However, in the field of energy conversion
little attention has yet been paid to the insulating characteristics of
the saturated porous materials introduced in some enclosures (storage
tanks) to decrease the convective and radiative transfer of heat.
