Enhanced heat transfer has evolved into an important component of heat transfer
experimentation and theory. Heat can be transferred from one place to
another by three methods: conduction in solids, convection of uids (liquids or
gases), and radiation through anything that will allow radiation to pass. The
method used to transfer heat is usually the one that is the most e cient. If there
is a temperature di erence in a system, heat will always move from higher to
lower temperatures. The transfer of heat is normally from a high temperature
object to a lower temperature object. Conduction occurs when two object at
di erent temperatures are in contact with each other. Heat ows from the
warmer to the cooler object until they are both at the same temperature. Conduction
is the movement of heat through a substance by the collision of molecules.
At the place where the two object touch, the faster-moving molecules of
the warmer object collide with the slower moving molecules of the cooler
object. As they collide, the faster molecules give up some of their energy to the
slower molecules. The slower molecules gain more thermal energy and collide
with other molecules in the cooler object. This process continues until heat
energy from the warmer object spreads throughout the cooler object. In liquids
and gases, convection is usually the most e cient way to transfer heat. Convection
occurs when warmer areas of a liquid or gas rise to cooler areas in the liquid
or gas. As this happens, cooler liquid or gas takes the place of the warmer areas
which have risen higher. This cycle results in a continuous circulation pattern and
heat is transferred to cooler areas.
The discussion of heat transfer has been structured around some everyday
examples such as the cooling of a hot mug of co ee and the warming of a cold
can of pop. To give an overview of the current state of this important
technology—for the past ten years, representative developments in each
category of enhancement techniques are cited and commented on. Developments
in Heat Transfer emphasizes on experimental and theoretical investigations,
assessment and enhancement techniques aspire to be useful for many
researchers, scientists, engineers and graduate students. Both conduction and
convection require matter to transfer heat. Radiation is a method of heat transfer
that does not rely upon any contact between the heat source and the heated
object. For example, we feel heat from the sun even though we are not touching
it. Heat can be transmitted though empty space by thermal radiation.