|
Heat Transfer:
Glossary
Glossary
|
| blackbody |
A body with a surface emissivity of 1. Such a
body will emit all of the thermal radiation it can (as described by
theory), and will absorb 100% of the thermal radiation striking it.
Most physical objects have surface emissivities less than 1 and
hence do not have blackbody surface properties. |
| density,
r |
The amount of mass per unit volume. In heat
transfer problems, the density works with the specific heat to
determine how much energy a body can store per unit increase in
temperature. Its units are kg/m3. |
| emissive
power |
The heat per unit time (and per unit area)
emitted by an object. For a blackbody, this is given by the
Stefan-Boltzmann relation s*T4 |
| graybody |
A body that emits only a fraction of the thermal
energy emitted by an equivalent blackbody. By definition, a graybody
has a surface emissitivy less than 1, and a surface reflectivity
greater than zero. |
| heat
flux, q |
The rate of heat flowing past a reference datum.
Its units are W/m2. |
| internal energy, e |
A measure of the internal energy stored within a
material per unit volume. For most heat transfer problems, this
energy consists just of thermal energy. The amount of thermal energy
stored in a body is manifested by its temperature. |
radiation
view
factor, F12 |
The fraction of thermal energy leaving the
surface of object 1 and reaching the surface of object 2, determined
entirely from geometrical considerations. Stated in other words,
F12 is the fraction of object 2 visible from the
surface of object 1, and ranges from zero to 1. This quantity is
also known as the Radiation Shape Factor. Its units are
dimensionless. |
rate of
heat
generation, qgen |
A function of position that describes the rate
of heat generation within a body. Typically, this new heat must be
conducted to the body boundaries and removed via convection and/or
radiation heat transfer. Its units are W/m3. |
| specific
heat, c |
A material property that indicates the amount of
energy a body stores for each degree increase in temperature, on a
per unit mass basis. Its units are J/kg-K. |
Stefan-Boltzmann
constant, s |
Constant of proportionality used in radiation
heat transfer, whose value is 5.669 x 10-8
W/m2-K4. For a blackbody, the heat flux
emitted is given by the product of s and
the absolute temperature to the fourth power. |
| surface
emissitivy, e |
The relative emissive power of a body compared
to that of an ideal blackbody. In other words, the fraction of
thermal radiation emitted compared to the amount emitted if the body
were a blackbody. By definition, a blackbody has a surface
emissivity of 1. The emissivity is also equal to the absorption
coefficient, or the fraction of any thermal energy incident on a
body that is absorbed. |
| thermal
conductivity, k |
A material property that describes the rate at
which heat flows within a body for a given temperature difference.
Its units are W/m-k. |
| thermal
diffusivity, a |
A material property that describes the rate at
which heat diffuses through a body. It is a function of the body's
thermal conductivity and its specific heat. A high thermal
conductivity will increase the body's thermal diffusivity, as heat
will be able to conduct across the body quickly. Conversely, a high
specific heat will lower the body's thermal diffusivity, since heat
is preferentially stored as internal energy within the body instead
of being conducted through it. Its units are
m2/s. |
