Free Convection: Another mode of heat transfer is natural (free) convection. A number of correlations have been developed which are functions of Prandtl number (Pr), Grashof number (Gr), geometry and orientation of the heat transfer surface. See Table 5.2, where Nu is the Nusselt number, L is the characteristic length. The product Gr·Pr is termed as Raleigh number (Ra). Values of a and m are reported in Table 5.3.

For ideal gases β is equal to 1/T. For liquids, this value is calculated from density data. For water, this value is obtained from steam tables. Air properties are calculated at the film temperature that is the average of surface and bulk temperatures.

Module for calculation of heat trandfer for Natural/Free Convection:

do		(Outside diameter of tube, in)
L_t		(Tube length, ft)
L_p		(Plate length, ft)
W_p		(Plate width, ft)
t_a		(Ambient temperature, deg F)
beta		(Coefficient of thermal expansion, per deg R)
visc_f		(Viscosity, lb per ft per hr)
rho_f		(Density, lb per ft³)
k_f		(Thermal conductivity, BTU per hr per ft per deg F)
c_f		(heat capacity, BTU per lb per deg F)
t_s		(Surface temperature of the tube, deg F)

vertical surface horizontal cylinder horizontal flat surface facing upward horizontal flat surface facing downward vertical plates and cylinders