Study On The Characterization Of Thermal Conductivity Of Fibers By Hot Wire Method

Fiber and its composites are required to be stable under the condition of high temperature or ultra-low environment, as fiber is widely applied in the field of industries,especially industrial thermal protection and aerospace. Fiber is involved a large amount of heat transfer problems in the development and application of fiber materials, which has been of particular concern to domestic and foreign researchers. Therefore the heat transfer performance is the most important issue to be considered in the design and production of thermal protective textiles such as bunker clothes, high or low temperature protective clothes.According to the characteristics of textile fiber morphology, the accuracy of the testing results, the feasibility and difficulty of sample processing, the test methods at home and abroad are evaluated. Compared with other test methods, we find that hot wire method has many advantages such as short measuring time, high measuring precision and wide application and therefore can be used as a testing method for measuring thermal conductivity of textile fiber. The testing parameters and the uncertainty of the hot wire thermal conductivity surveying instrument were studied firstly, and then two kind of fiber sample preparation were proposed and optimized. Moreover, the heat transfer mechanism of the internal sample fiber was illustrated in detail. The relation between thermal conductivity and orientation and thickness were analyzed by using numerical simulation.At last, the testing method of thermal conductivity of textile fiber was established.The testing parameters for this instrument are set as the experiment voltage of 1 ~ 3 V and the acquisition time of 0.2 ~ 0.5 s. The repeatability of test results is 2.2%, and the uncertainty of that is 0.681%. It indicates that this instrument has the advantages of good repeatability and high accuracy and can accurately measure thermal conductivity of textile fiber.There are two methods for fiber sample preparation: winding of method and adhesive of method. The thermal conductivity data of the former method is small than that of thelatter. Because there is air gap in the former sample and there are carrier and other materials in the latter sample. The influence of sample parameters(arrangement direction, thickness and fiber fineness) on thermal conductivity of textile fiber was investigated. We find the data gradually increases with the rise of arrangement angles due to the comprehensive effect of two main factors including the thermal contact resistance between the fibers and the anisotropy of thermal conductivity. The thermal conductivity of samples is on the increase when the fineness raise, the reason is that the number is the most for the fiber yarns contacting with hot wire in vertical. The samples with different thickness are formed by a single fiber using different winding numbers. The thermal conductivity of samples with different thickness is fluctuating due to the irregularity of structure.The analysis of the heat transfer mode of the fiber showed that there is two heat transfer mode including fiber heat conduction and air conduction under room temperature and pressure. Evaporation and condensation of water occur under high temperature. The simulated calculation of heat transfer process of fiber sample with COMSOL finite element software show that the speed of heat transfer in vertical is faster than that in parallel and thus the heat transfer performance is better when the hot wire is placed vertically along the fiber axial. The measured thermal conductivities agree with simulated values within a deviation of ±6%.The testing method including fiber sample preparation and test procedures is established, then the sample parameters were standardized and test considerations were illustrated. The evaluation method of the test results of the instrument was discussed.