Abstract:

Objective Thermal protective clothing has attracted much attention because of its unique thermal insulation function and wide application prospects. However, it is difficult to describe the transient heat transfer process in the fabric by physical tests, and the preparation process of thermal protective fabric needs to rely on a large number of thermal protective performance tests. Therefore, the transient heat transfer process of different woven fabric is simulated by finite element method.

Method The transient heat transfer characteristics of six woven fabrics (plain weave, 2　　1-6　　1 twill) of aramid and polyester were studied by finite element simulation, and the temperature nephogram and surface temperature time varying diagram of the fabrics were obtained. From the two dimensions of heat insulation time and heat insulation temperature, five indicators for evaluating the thermal protection performance of fabrics under strong thermal conditions were proposed, namely, the lag time of temperature rise on the lower surface, temperature rise speed of lower surface, stable temperature of upper and lower surfaces, maximum temperature difference and stable temperature difference. The effects of yarn float and heat source intensity on the thermal protection performance of fabrics were studied.

Results The heat flow is transmitted along the yarn float, which causes the temperature of the yarn body on the surface of the fabric to rise, and the temperature of the yarn in the weaving area to rise faster, forming the upper and lower surface temperatures and temperature differences related to the yarn float (Fig. 3). The lag time, maximum temperature difference and stable temperature difference of the initial temperature rise of the lower surface of the six aramid and polyester fabrics from low to high all exist: plain,2　　1,3　　1,4　　1,5　　1,6　　1 twill, showing a positive correlation with the fabric float (Fig. 5, Fig. 6), while the lower surface temperature rise speed and stable temperature show a negative correlation with the fabric float (Fig. 4, Fig. 6). Under the condition of conventional heat source intensity of 0.8 kW/m², single-layer aramid and polyester fabrics can effectively prevent the heat flow lag of about 1.5 and 1.4 s, respectively (Fig. 6). When the heat transfer balance is reached, the upper surface temperature of aramid and polyester fibers is stabilized at about 318.33 and 317.13 K(45.18 and 43.98 ℃), respectively, the lower surface temperature is stabilized at about 306.53 and 307.63 K(33.38 and 34.48 ℃), respectively, and the upper and lower surface temperature difference is stabilized at about 11.8 and 9.5 K respectively. With the increase of heat source intensity, the lag time decreases gradually (Tab. 5). Under the heat source intensity of 4.0 kW/m², the lower surface temperature of aramid and polyester fibers are stabilized at 345.26 and 350.47 K (about 72.11 and 77.32 ℃)(Tab. 6), respectively, which are 37 ℃ higher than the constant physiological temperature of human body.

Conclusion The yarn float will directly affect the heat transfer of the fabric. When other conditions are the same, the thermal insulation and protection performance of the six fabrics from low to high is: plain,2　　1,3　　1,4　　1,5　　1,6　　1 twill. Under strong heat intensity, the single-layer fabric is not enough to delay the heat flow transmission, and the thermal insulation protection ability is limited. It is necessary to increase the protective thickness or add other materials to improve the thermal insulation ability. Through the RPP thermal protection performance test, the test results are in good agreement with the simulation results, and the research results provide guidance for the design of thermal insulation structures.

Key words: woven fabric, anisotropy, yarn float, heat transfer property, thermal protection, numerical simulation