2.10 Modes of Heat Transfer
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direction (m
2 ), T 2 –T 1 is the temperature difference across the medium (K), and d is
the distance (m) between points at temperatures T 2 and T 1 .
2.10.2 Convection
Convection is the mode of heat transfer which takes place because of the bulk motion
(observable movement) of fluids. Convective heat transfer can be classified into two
categories: natural convection and forced convection, also known as heat advection.
This can be contrasted with conduction, which is the transfer of energy by vibrations
at a molecular level through a solid. As convection depends on the bulk movement
of a fluid, it can only occur in fluids and multiphase mixtures. The convective heat
loss [72] can be calculated from Eq. (2.3):
convective heat loss(H c ) = k c · A · (T sk − T db )
(2.3)
where k c is the convective coefficient, T sk is the mean weighted skin temperature,
T db is the dry bulb temperature, and A is the surface area of the body.
Many researchers have investigated the natural convection of industrial helmets
and observed large variations between helmets. They have also noticed that there is
a significant effect of vents situated on the top of the helmet. Brühwiler [73] studied
the role of visors in forced convective heat loss in bicycle helmets, using a thermal
manikin head form in a climate chamber. He showed that visor design can help to
optimise thermal comfort via convective heat loss. Depending upon the wind speed,
forced convection can exchange 2–40 W of dry heat [74]. In addition, the sensitivity of
the head region, especially the face, should be considered while considering thermal
influences [75].
2.10.3 Radiation
Radiation is the emittance of heat from the surface of an object which is at a very high
temperature by way of electromagnetic waves. Thermal radiation is caused when heat
from the movement of charged particles within atoms is converted to electromagnetic
radiation.
The waves can pass through the air without imparting much heat to it, and when
they strike an object, their energy is largely transformed into heat [65]. Radiation
can largely be ignored as a mechanism of heat loss from the human body, as it is
very dependent on the temperature of an object (varying as the fourth power of the
temperature). It is basically a means of heat transfer from very hot bodies such as
the sun, radiant heaters or fires. The colour of an object affects the amount of heat
23
direction (m
2 ), T 2 –T 1 is the temperature difference across the medium (K), and d is
the distance (m) between points at temperatures T 2 and T 1 .
2.10.2 Convection
Convection is the mode of heat transfer which takes place because of the bulk motion
(observable movement) of fluids. Convective heat transfer can be classified into two
categories: natural convection and forced convection, also known as heat advection.
This can be contrasted with conduction, which is the transfer of energy by vibrations
at a molecular level through a solid. As convection depends on the bulk movement
of a fluid, it can only occur in fluids and multiphase mixtures. The convective heat
loss [72] can be calculated from Eq. (2.3):
convective heat loss(H c ) = k c · A · (T sk − T db )
(2.3)
where k c is the convective coefficient, T sk is the mean weighted skin temperature,
T db is the dry bulb temperature, and A is the surface area of the body.
Many researchers have investigated the natural convection of industrial helmets
and observed large variations between helmets. They have also noticed that there is
a significant effect of vents situated on the top of the helmet. Brühwiler [73] studied
the role of visors in forced convective heat loss in bicycle helmets, using a thermal
manikin head form in a climate chamber. He showed that visor design can help to
optimise thermal comfort via convective heat loss. Depending upon the wind speed,
forced convection can exchange 2–40 W of dry heat [74]. In addition, the sensitivity of
the head region, especially the face, should be considered while considering thermal
influences [75].
2.10.3 Radiation
Radiation is the emittance of heat from the surface of an object which is at a very high
temperature by way of electromagnetic waves. Thermal radiation is caused when heat
from the movement of charged particles within atoms is converted to electromagnetic
radiation.
The waves can pass through the air without imparting much heat to it, and when
they strike an object, their energy is largely transformed into heat [65]. Radiation
can largely be ignored as a mechanism of heat loss from the human body, as it is
very dependent on the temperature of an object (varying as the fourth power of the
temperature). It is basically a means of heat transfer from very hot bodies such as
the sun, radiant heaters or fires. The colour of an object affects the amount of heat