Convective Heat Transfer in Solar Energy Systems

Abstract

In determining the heat loss by buoyancy-driven convection between the collector plate and the glass cover, when mounted at an angle between 0° and 75° from the horizontal, the following correlation (Hollands et al. 1976) is suggested:

$$Nu = 1.144{\left[ {1 - \frac{{1708}}{{Gr\,\Pr \,\cos \,\beta }}} \right]^ + }\left[ {1 - \frac{{{{\left( {1708\,\sin \,1.8\beta } \right)}^{1.6}}}}{{Gr\,\Pr \,\cos \,\beta }}} \right] + \left[ {\frac{{Gr\,\Pr \cos \,{\beta ^{0.33}}}}{{5830}} - 1} \right]$$

((5.1.1.))

The superscript + denotes a positive value for the quantity in brackets if this quantity is greater than zero and a value of zero otherwise. The characteristic length in determining Gr is taken as the thickness of the air cavity.