Different examples of heat transfer
Convection heat transfer by hot metal plate.
It is well known that a hot plate of metal will cool faster (figure blow ) when placed in
front of a fan than when exposed to still air, and heat is convected away, the process call
convection heat transfer. For example, we know that the velocity at which the air blows
over the hot plate obviously influences the heat-transfer rate. the temperature gradient is
dependent on the rate at which the fluid carries the heat away; a high velocity produces a
large temperature gradient, and so on. Newton’s law of cooling:
q = hA (Tw – T∞)
Here the heat-transfer rate is related to the overall temperature difference between the wall
and fluid and the surface area A. the quantity h (W/m2.K) is called the convection heat-
transfer coefficient.
Example 1 Air at 20℃ blows over a hot plate 50 by 75 cm maintained at 250℃. The
convection heat-transfer coefficient is 25 W/m 2·℃. Calculate the heat transfer.
Solution. From Newton’s law of cooling
q = hA(Tw-T∞)
= (25)(0.50)(0.75)(250-20)
= 2.156 kW [7356 Btu/h]
Example 2 Assuming that the plate in Ex. 1 is made of carbon steel (1%) 2 cm thick
and that 300 W is lost from the plate surface by radiation, calculate the inside plate
temperature.
Given , the thermal conductivity 43 Kw / M . ℃
Solution. The heat conducted through the plate must be equal to the sum of convection
and radiation heat losses:
Q cond = q conv + q rad
-kA
x
T
= 2.156 + 0.3 = 2.456 kW
T
=
)43)(75.0)(5.0(
)02.0)(2456(−
= -3.05℃ [-5.49℉]
Ti = 250 + 3.05 = 253.05 ℃
1- Heat Loss from a Person
