Convection and radiation
Bruce M. Guenin, Associate Editor, Amkor
Electronics
In last issue's calculation corner,a simple 1-dimensional
conduction calculation was described. This time, we look at the other two
basic heat transfer processes, namely convection and radiation.
In actual applications with multiple heat sources with complicated
geometries, the effect of each of these mechanisms on the heat transfer
process can be quite difficult to calculate. However, for geometries that
can be reasonably approximated by a plane with a low-profile heat source,
the concept of an area-averaged heat transfer coefficient can be used to
estimate the heat loss due to convection and radiation.
The heat, Q, removed from a surface is simply equal to: Q =A x h x
TSA
where A is the surface area, h is the heat transfer coefficient, and
T SA
is the temperature difference between the surface and the ambient air.
Rearranging the terms in this equation provides an expression for the
thermal resistance for heat removal from the surface by convection and
radiation, SA.
In subsequent issues, we shall be applying this equation to more
complicated heat transfer problems.
SA
= TSA/
Q = 1 / A x h
The following graph offers a convenient means of estimating h as a
function of TSA
at various values of the forced air velocity from 0 to 2.5 m/s. These
curves were calculated using values of h from a standard reference for a
75 mm sq. plate and include the effects of radiation. h is most dependent
on TSA
at 0 m/s (natural convection) and least dependent at the higher air
velocities.
Reference G.N. Ellison, Thermal Computations for Electronic
Equipment, Krieger Publishing, Matabar, Florida, 1989.
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