Free-convection heat transfer in screened greenhouses

Free-convection heat transfer between heating pipes and air, horizontal screen and air and inner roof surface and air was studied experimentally in twin-span glasshouses, under constant heat flux conditions. Among other results, equation coefficients between Nusselt and Rayleigh numbers are presented for the inner surface of the greenhouse roof, screen surfaces and heating pipes. Special attention is given to the influence of the location of the heating pipes relative to the screen, as well as the presence of a crop, upon the convective heat transfer between different surfaces and air. The equation coefficients obtained in this work are between 2 and 75% greater than the equivalent equation coefficients found for smooth plates and cylinders by other authors. (C) 1998 Silsoe Research Institute.