Page 151 - Industrial Ventilation Design Guidebook

4,3 HEAT AND MASS TRANSFER I i 3

111 the one-dimensional case

The solution of Eq. (4.187) is

with boundary conditions

The logarithmic temperature distribution is

Thus the heat flow per a unit of length $' is

4.3.4 Heat Convection

The mathematical principles of convective heat transfer are complex and
outside the scope of this section. The problems are often so complicated
that theoretical handling is difficult, and full use is made of empirical corre-
lation formulas. These formulas often use different variables depending on
the research methods. Inaccuracy in defining material characteristics, ex-
perimental errors, and geometric deviations produce noticeable deviations
between correlation formulas and practice. Near the validity boundaries of
the equations, or in certain unfavorable cases, the errors can be excessive.
The general forms of the convection equations are given below in a simple
form. More accurate equations can be found from the latest research results
presented in technical journals.
The general equation for the case of forced convection is Nu =/"(Re, Pr).
In the case of free convection it is Nu = /(Gr, Pr).