Page 149 - Industrial Ventilation Design Guidebook

4.3 HEAT AND MASS TRANSFER I I 1

FIGURE 4.27 Control volume.

h is normal to assume that the thermal conductivity is constant; hence Eq.
(4.179) gives

where
dT/dt is the derivative of temperature as a function of time; in a steady-
state case it is equal to zero
a — k/pCp, the heat conductivity or thermal diffusivity
f f
H — <&" /C' = heat generation inside a material; for example, for
Joule's heat or a nuclear reaction, <f>'" = heat generation/volume and
C'" = Cpp = heat capacity/volume
For Cartesian coordinates

for cylindrical coordinates

and for spherical coordinates