Vermiculite
Thermal Convection
Convection is the mode of energy transfer between a solid surface and the
adjacent liquid or gas that is in motion, and it involves the combined
effects of conduction and fluid motion. The faster the fluid motion, the
greater the convection heat transfer.
In convection, thermal energy is carried by the large-scale flow of matter. It
can be divided into two types.
In forced convection, the flow is driven by fans, pumps, and the like. A
simple example is a fan that blows air past you in hot surroundings and
cools you by replacing the air heated by your body with cooler air.
In free or natural convection, the flow is driven by buoyant forces: hot fluid
rises and cold fluid sinks because density decreases as temperature
increases. The house in Figure 1.24 is kept warm by natural convection, as
is the pot of water on the stove in Figure 1.25.
Convection is called forced convection if the fluid is forced to flow over
the surface by external means such as a fan, pump, or the wind.
In contrast, convection is called natural (or free) convection if the fluid
motion is caused by buoyancy forces that are induced by density
differences due to the variation of temperature in the fluid (Fig.).
Land Breeze
Radiation
Radiation is the energy emitted by matter in the form of
electromagnetic waves (or photons) as a result of the changes in the
electronic configurations of the atoms or molecules.
Unlike conduction and convection, the transfer of heat by radiation
does not require the presence of an intervening medium. In fact, heat
transfer by radiation is fastest (at the speed of light) and it suffers no
attenuation in a vacuum. This is how the energy of the sun reaches the
earth.
In heat transfer studies we are interested in thermal radiation, which is
the form of radiation emitted by bodies because of their temperature.
All bodies at a temperature above absolute zero emit thermal radiation.
In these examples,
heat is transferred by radiation (Figure 1.28). That is, the hot body
emits electromagnetic waves that are absorbed by the skin.
The rate of heat transfer by radiation also depends on the object’s color.
Black is the most effective, and white is the least effective. On a clear
summer day, black asphalt in a parking lot is hotter than adjacent gray
sidewalk, because black absorbs better than gray (Figure 1.30).
The reverse is also true—black radiates better than gray. Thus, on a clear
summer night, the asphalt is colder than the gray sidewalk, because black
radiates the energy more rapidly than gray. A perfectly black object would
be an ideal radiator and an ideal absorber, as it would capture all the
radiation that falls on it. In contrast, a perfectly white object or a perfect
mirror would reflect all radiation, and a perfectly transparent object would
transmit it all (Figure 1.31).
Such objects would not emit any radiation. Mathematically, the color is
represented by the emissivity 𝜀. A “blackbody” radiator would have an 𝜀 = 1
, whereas a perfect reflector or transmitter would have 𝜀 = 0 . For real
examples, tungsten light bulb filaments have an e of about 0.5, and carbon
black (a material used in printer toner) has an emissivity of about 0.95.
Radiation is a volumetric phenomenon, and all solids, liquids, and gases
emit, absorb, or transmit radiation to varying degrees. However, radiation is
usually considered to be a surface phenomenon for solids that are opaque to
thermal radiation such as metals, wood, and rocks since the radiation
emitted by the interior regions of such material can never reach the surface,
and the radiation incident on such bodies is usually absorbed within a few
microns from the surface.
The radiation emitted by all real surfaces is less than the radiation emitted
by a blackbody at the same temperature, and is expressed as
where is the emissivity of the surface. The property emissivity, whose value
is in the range , is a measure of how closely a surface approximates
a blackbody for which 𝜀 = 1.
