Classroom Session I
Heat Transfer by conduction and convection
Introduction
1) Distinugishing the two properties (energy is an extensive propoerty while
temperarture is an intensive property)
2) The equation Q = mCp(T2-T1) where m = mass, Cp = heat capacity, and T =
temperature
3) How temperature difference or gradient (T2-T1) leads to heat transfer or energy
flow
4) Units.
5)
Heat Conduction
Conduction is heat transfer by means of molecular agitation within a material without
any motion of the material as a whole. If one end of a metal rod is at a higher
temperature, then energy will be transferred down the rod toward the colder end because
the higher speed particles will collide with the slower ones with a net transfer of energy
to the slower ones. For heat transfer between two plane surfaces, such as heat loss
through the wall of a house, the rate of conduction heat transfer is:
Calculation
= heat transferred in time =
= thermal conductivity of the barrier
= area
= temperature
= thickness of barrier
Points of importance:
Heat transfer area is normal to heat flow. For example, in a pipeline it is the
circumferential area. In fluid flow, the area involved is x-sectional area.
Heat transfer can be studied in terms of Q = (T hot- T cold) / R where R is is
resistance to heat transfer. In this respect it is similar to flow of electrical
charge (current) I =V/R.
What will happen, if there are a series of resistances? Will the laws of
electrical system hold in heat transfer?
Heat Convection
Convection is heat transfer by mass motion of a fluid such as air or water when the
heated fluid is caused to move away from the source of heat, carrying energy with it.
Convection above a hot surface occurs because hot air expands, becomes less dense, and
rises (see Ideal Gas Law). Hot water is likewise less dense than cold water and rises,
causing convection currents which transport energy. Convection is thought to play a
major role in transporting energy from the center of the Sun to the surface, and in
movements of the hot magma beneath the surface of the earth.
It is difficult to quantify the effects of convection since it inherently depends upon small
nonuniformities in an otherwise fairly homogeneous medium. In modeling things like the
cooling of the human body, we usually just lump it in with conduction.
In many industrial applications, heat transfer by conduction and convection are treated
simultaneously and the concept of heat transfer coefficient becomes important. Heat
transfer coefficient incorporates multiple resistances and the net heat transfer rate as a
function of overall temperature difference and an overall heat transfer coefficient which
is the inverse of the overall resistance. The overall resistance is an addition of all the
individual resistances connected in series.
Heat transfer Q/t (BTU/h) = UA∆T where U is the overall heat transfer coefficient and A
is the heat transfer area.
One practical effect of convection is wind chill effect and the concept of multiple
resistances are used in designing thermal protection garments, thermos flasks, home
insulations etc.
Source and Acknowledge: Hyperphysics, Carl R. (Rod) Nave, Hyperhysics website,
©C.R. Nave, 2003, used with permission.