4. 2 Convective heat transfer
T116. governing equation for convective heat transfer is shown as eq. 4.2, and eq. 4.3 is
the equation for latent heat transfer, which will be described in the next section. The heat
transfer coefficient, H, in this equation is dependent on the movement of the adjacent air.
Under outside conditions, wind speed is the primary factor, and the coefficient is expressed
as a function of wind.
Wind speed consists of three directional flows - x, y, and z, and the coefficient is
related to the main directional flow, which is horizontal. The main horizontal wind speed
changes with the logarithmic distance from the surface. It is rather difficult to determine the
height at which wind speed should be taken. However, if the air movement is completely
turbulent, change is negligible beyond the boundary layer. There is no rule for this at the
moment, but traditionally, anemometers are set 3 to 5 m above the ground surface.
Quite a number of experimental data have been obtained from wind tunnel
experiments, and their results are summarized using some non-dimensional numbers such
as Nu, Re, Gr and Pr. However, the situation in natural fields is different from that in a
wind tunnel, and it would be good to be able to show the final relationship between the
coefficient and the wind speed. Figure 4.3 summarizes several of the relationships reported,
although they are used mostly for the outside surface of greenhouses.