STUDIES ON THE THERMAL PROPERTIES OF FIBROUS MATERIALS

Abstract

The heat conductivity of fibrous materials is one of the most important properties, and depends on many factors such as volume fraction of fiber and mean free path of air molecules, etc. In this paper, the apparent heat conductivities of fibrous materials are measured in varying volume fraction _??_ and pressure of air, According to the theory, the apparent heat conductivity of fibrous materials ka becomes, where k1 and k2 are the heat conductivities of fiber and air respectively. a and b are determined by the distribution function of fiber direction. With increment of volume fraction or mean free path, the heat conductivity of gas decreases, so the relations between ka and _??_ are rather complicated. A brief discussion on these relations are presented which agree with experimental results.Main results are as follows:(1) The apparent heat conductivity of fibrous materials decreases rapidly at first with increment of volume fraction of fiber. As the volume fraction of fiber increases, the derivative _??_ tends to approach zero.(2) When the air is exhaustal at fixed volume fraction, the apparent heat conductivity decreases to a constant value which depends on the volume fraction of fiber. The smaller the volume fraction, the higher becomes the pressure P0, determined by _??_ max, where _??_ is the mean free path of gas molecules which is proportional to P-1.(3) As shown in (1), the apparent heat conductivity of fiber varies with the volume fraction, so from the following relation the true heat conductivity of fiber can be obtained.(4) From the theoretical results, it is deduced that the heat conductivity of fibrous materials becomes smallar as the fibre diameter becomes thinner.