5.2 Empirical relations for natural-convective heat transfer        

5.2.1 Natural convection around a flat vertical surface         

Churchill and Chu provided the correlation for average heat transfer coefficient for natural convection for different ranges of Rayleigh number.

Case I:	If RaL < 109
		(5.7)

Case II:	If 10-1 < RaL < 1012
		(5.8)

It should be noted that the eq. 5.7 and 5.8 are also applicable for an inclined surface upto less than inclination from the vertical plane.

The above relations can be used for the vertical cylinder if the boundary layer thickness is quite small as compared to the diameter of the cylinder. The criteria to use the above relation for vertical cylinder is,

(5.9)

where,  is the diameter and  is the height of the cylinder.

5.2.2 Natural convection around a horizontal cylinder          

Churchill and Chu has provided the following expression for natural-convective heat transfer.

(5.10)

Condition of applicability of the eq.5.10: