View factors involved in radiation exchanges in floating mulches made of non-woven fabrics

Abstract

Non-woven fabrics are plastic fibres assembled and bound to each other to form sheets containing gaps. They are used widely as floating mulches. Owing to the gaps, the microclimate created under the fabric covers is different from that under continuous films. As part of a theoretical analysis of the microclimate under fabric covers, a mathematical formula has been derived of the view factor involved in longwave radiation exchanges in floating mulches of non-woven fabrics. The view factor is related to the geometric relations between two objects and designates the fraction of the radiation leaving one object that reaches the other, provided that the leaving radiation is diffuse. To simplify the analysis, horizontal plane is considered at the top of the canopy, which is named an imaginary surface. The radiation exchanges between various surfaces are divided into two groups, those above the imaginary surface and those beneath it. Only the former is considered in this paper because an existing theory of radiation exchange can be applied to the latter. The view factor, F, between the non-woven fabric and the imaginary surface is analysed. The results indicate that the view factor, F, is independent of the distance between the two surfaces and can be expressed as a function of the porosity of the fabric. The results are also represented diagrammatically.