Page 152 - Industrial Ventilation Design Guidebook

! 14 CHAPTER 4 PHYSICAL FUNDAMENTALS

The characteristic length L denotes the pipe diameter or the hydraulic di-
ameter J hyd = 4A/P (A is the cross-sectional area and P is the wet periphery).
If the cross-section is not circular, or in the case of a plane, the length is mea-
sured in the flow direction.
The temperature changes taking place through the surface of an exo-
thermic body depend on the material characteristics and changes in the pa-
rameters. In formulas involving convection, either the solid surface
temperature or the heat flow from the surface is assumed to be constant.
The temperature 6 defines the material characteristics (c, p, F, etc.). Nor-
mally this temperature is the mixed temperature of the flowing fluid. The
mean temperature of the boundary layer is the average temperature of the
surface temperature and the undisturbed flow (T m = (T p + T 0J/2). Some-
times the boundary layer temperature, which is the average of the mixing
temperature and the surface temperature, is used.

4.3.3.3 Calculation Using the Correlation Formulas
First the dimensionless characteristics such as Re and Pr in forced
convection, or Gr and Pr in free convection, have to be determined. De-
pending on the range of validity of the equations, an appropriate correla-
tion is chosen and the Nu value calculated. The equation defining the
Nusselt number is

given the heat transfer factor a. Using the equation

the heat flow density is determined, which represents a certain temperature
difference or the temperature difference between the wall and the fluid for a
certain heat flow rate.
As an example, for free convective heat transfer from a vertical wall,