Research On Thermophysical Properties Of Refractory Fiber Under High Temperature

Refractory fiber mainly containing Al,03 is a new type of heat insulating material under high temperature. It has been widely used with characteristics of being light weight, resisting high temperature and fighting thermal shock. To use this kind of material reasonably, it is very necessary to study its thermophysical properties.Refractory fiber is a kind of porous medium with complex microstructure. It is obvious that its thermophysical properties are concerned with not only material properties and temperature but also its microstructure. The radiation can be ignored under high temperature besides the conduction of fiber and gas among fibers. The reference materials indicate that although such new type of refractory fiber have been studied, theoretical studies were little under high temperature. In this paper, the thermophysical properties have been studied under high temperature from both theoretical analysis and experimental measurement.Fractal theory is used in this paper based on the microstructural characteristics of refractory fiber. The inicrostructure is described with fractal dimension and fiber volume fraction as the major parameters. The model of heat transfer has been established with the equivalent thermal resistance under high temperature. The formula for efficient thermal conductivity is given out as follows:-3 -The formula contains 5 parameters which are thermal conductivities of gas and solid , X3 and X~, fiber volume fraction v, fractal dimension d~ and temperature T. L is scale factor and F is radiant overall constant. The calculative results show that the efficient thermal conductivity rises as temperature rises under a constant fiber volume fraction, and it is proportional to the third power of absolute temperature; it decreases as fiber volume fraction increases under high temperature; it increases as fiber volume fraction increases under low temperature, and it is in agreement with that is described in the literature.According to GB 10294, the measuring apparatus of the efficient thermal conductivity has been designed and made with one dimensional steady-state plate method. The experimental results show that the efficient thermal conductivity increases as temperature rises on the condition of a constant unit weight; it is smaller when fiber direction is vertical to heat flow each other under high temperature; moisture content hardly influences the efficient thermal conductivity at the same unit weight and high temperature.The efficient thermal conductivty has been compared between theoretical calculation and experimental results. It shows that theoretical calculation is almost in agreement with the experimental results. Its relative error is smaller than 18%. It has been shown that the theoretical calculative formula, which is based on fractal description, has higher precision. So it can be used to predict thermal properties of refractory fibers.