Biomedical Engineering Reference
In-Depth Information
Conduction
Convection
Radiation
FIGURE 14.42
The three types of heat transfer: conduction, convection, and radiation.
difference between them over time. The rate of heat transfer between two objects of differ-
ent temperatures depends on several factors:
The temperature difference between the two objects
The total surface area where the two objects are in contact
The efficiency of the insulation that is between the objects
The greater the temperature difference between two objects in contact, the more heat that is
transferred between them in a given time. For example, when you place your hand on a
very hot stove top, you will quickly receive a great heat input from the stove to your hand.
If the stove top is only warm, it will take much longer to receive the same amount of heat
into your hand.
The more surface area in contact between two objects, the more quickly heat is trans-
ferred between them. Stick your finger on an icicle for a minute and it feels cold, but you
will probably not feel too uncomfortable. Strip naked and lay on a block of ice for a minute
and you will most likely be very uncomfortable indeed as the ice absorbs heat from your
body at a very fast rate.
The amount of heat being transferred between two objects of different temperatures can be
slowed by the use of effective insulation. Insulation retards the movement of heat between
them by creating pockets of dead air space that trap the flow of heat or by otherwise slowing
the overall heat transfer rate by adding a low-conductance/high-resistance layer.
The law of heat conduction, also known as Fourier's law, states that the time rate of heat
transfer through a material is proportional to the negative gradient in the temperature and
to the area at right angles to that gradient through which the heat is flowing. We can state
this law in two equivalent forms: the integral form, in which we look at the amount of
energy flowing into or out of a body as a whole, and the differential form, in which we look
at the flows or fluxes of energy locally. The differential form is
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