Mixed, forced and free convection heat transfer at the greenhouse cover

Abstract

A method has been developed to determine experimentally in situ the convective heat transfer coefficients on the inside and outside of the greenhouse cover. The method is based on the energy balance of the greenhouse cover and it was applied to a small experimental polyethylene covered greenhouse. The convective heat transfer coefficients on the inside and outside of the cover were determined as functions of the air-cover temperature difference and the air velocity inside the greenhouse, as well as the wind velocity outside the greenhouse. Also, the nature of the convective heat transfer at the outside and the inside of the greenhouse cover was investigated. At the outside of the cover at moderate wind velocities mixed convection is the prevailing convective heat transfer mechanism. The convection heat transfer on the inside of the greenhouse cover is always pure free convection when the greenhouse vents are closed and the air velocity in the house is low. When the vents are opened the nature of the convection heat transfer depends on the air velocity in the house. Criteria are given to determine whether pure free, mixed or pure forced convection takes place.