Study On Characterization Of Thermal Conductivity Of Multiphase Fibrous Based On The Hot Wire Method

Studies about the thermal transfer process and the means for testing the thermal conductivity of multiphase fiber materials has been the concern of a large number of scholars.The transient hot wire method is tried to test the thermal conductivity of the fiber materials in recent years.The hot wire method is non-steady method.With short testing time and fast testing speed it can greatly reduce test error resulted from water evaporation or air convection.It also can reduce the influence of thermal convection and radiation.So hot wire method can obtain a more accurate thermal conductivity of fiber materials than other methods.From the hot wire testing principle and the characteristics of multiphase fiber materials itself,how to testing the thermal conductivity of high performance and its fabric with hot wire method was discussed in this paper.The thermal transfer mechanism in the material was analyzed.The influence of relationship between the alignment direction of fiber bundles and hot wire on testing results was studied.The relationship of the thermal conductivity of fiber bundles and fabrics was discussed and verified.Using the ANSYS finite element analysis software,this paper built the 3D geometric model of fabric and exerted specific thermal loads and boundary conditions,thus solved the temperature cloud gram and obtained the thermal transfer process in the fabric.At last,the model was validated.The influence of fabric structure parameters on the thermal conductivity of the material was analyzed.And the variation of thermal conductivity with thickness and porosity of the fabric was studied.The thermal conductivity of three fiber materials including single layer fabric,spacer fabric,coated composite fabrics were tested at different temperatures and the thermal transfer mechanism of different kinds of fiber materials were discussed.The influences of temperature on thermal properties of the fiber materials were analyzed.Studies have shown that:(1)Using the hot-wire method to measure the thermal conductivity of fiber bundle,the thermal conductivity on the condition of the direction of fibers arranged parallels to the hot wire is less than on the condition of aligning perpendicular to the hot wire,which results from the difference of contact between the hot wire and fibers when the hot wire is placed with different directions.According to the characteristics of using the hot wire method to test the thermal conductivity of fiber bundle,the calculation model which can be used for predicting the thermal conductivity of fabrics by fiber bundle.It has been proven that the model for plain woven fabric has better applicability.(2)ANSYS software is used to build a three-dimensional thermal model of the fabric.It has been proven that the model calculations and experimental values of the thermal conductivity of the fabric are close.So the model is reliable and can provide better prediction of the thermal conductivity.In addition,the model analyzes the heat transfer process inside the fabric.It indicates the thermal transfer process mainly includes heat conduct when the fabrics are tested by the hot wire method on the normal temperature,and heat radiation and natural convection is negligible.(3)The thermal conductivity of fabrics increases with increasing thickness of the fabric by using the hot wire method,eventually leveling off.This is caused by thinner thickness of the fabric can’t match the requirement of the principle of the hot wire method,which is ideal infinite medium.The error is greater with thinner fabrics.The thermal conductivity of thicker fabrics is closer to the actual value.Experiments show that when the thickness of the glass fabric and amide fabric is about 0.89 mm,thickness requirements have been met and the measured value close the actual value.The required thickness of carbon fabric and basalt fabric are higher than 0.89 mm.In addition,with the increasing of porosity of carbon fabric,the thermal conductivity first increases and then decreases.(4)Carbon fabric has high thermal conductivity and is typical thermally conductive material;Basalt fabric and glass fabric have low thermal conductivity and are good insulating material;organic aramid fabric more significantly affected by temperature,It has low thermal conductivity at low temperatures below 0 ℃,showing good insulation properties at low temperature.But the increasing rate of the thermal conductivity is higher with increasing temperature.Studies has found that,the thermal conductivity of basalt and glass woven fabric remain low at 140℃,and are suitable for heat insulation at high temperature;Amide fabric shows better insulation when testing in the range of-50 ~ 140℃.(5)For the spacer fabric,the air layer can increase the insulation performance of the fabric.When the thickness of air layer of glass fabrics used in this experiment is within 8mm,the insulation of fabric improves with the increasing thickness of air layer.When thickness of air layer rises from 8mm to 10 mm,the insulation of fabrics decrease.