Abstract:

Objective Excessive thermal protection of thermal protective clothing will lead to heat stress, which is detrimental to health and even poses safety risks to firefighters. Reducing the weight and increasing the permeability of firefighting clothing can reduce their heat stress. The purpose of this study was to investigate the effects of fabric panel structure on its thermal protective performance based on numerical simulation, so as to provide a theoretical basis for improving fabric structure design.

Method Both experimental and numerical methods were adopted in this study. The experiments were performed by SET (stored energy tester) with the fabrics used as the outer shell of firefighting clothing, which provided initial and boundary conditions for numerical models. The three dimensional geometric model was developed based on the real fabric structure. On this basis, a fluid-solid conjugated heat transfer model of fabric, air gap and skin was built considering the actual wearing state. The model was validated by the experiment results and a mesh independence test was performed. The validated model was used to carry out parameter studies taking into consideration of the ambient temperature, yarn count and thermal conductivity of yarn as parameters.

Results The simulation results were in correspondence with the testing results. The mesh independence test indicated that the computational results were insensitive to the mesh sizes used in this study. Throughout the entire heat exposure process, the temperature within the air gap beneath the clothing decreased rapidly. The presence of fabric and the air gap significantly contributed to the thermal protection of the skin. Under different ambient temperatures, the skin temperature remained consistent. As the heat exposure progressed, heat continually transferred to the dermis, leading to a continuous increase in dermal heat flux, which plateaued at around 45 s. With increasing peak heat exposure temperature, the surface temperature of the yarn, dermal heat flux, and dermal temperature all increased. Among all the parameters studied in this research, ambient temperature had the most substantial impact on the heat transfer process. At the microscale, yarn count had a minimal impact on skin temperature and heat flux, but this effect was temperature-dependent. Under low heat exposure conditions (758 K, 873 K), increasing yarn count resulted in reduced skin temperature and heat flux. However, as the peak heat exposure temperature rose to 988 K, increasing yarn count led to higher skin temperature and heat flux. An increase in yarn thermal conductivity had a minor effect on skin temperature and heat flux, with limited impact. Treating the yarn layer as a uniform medium resulted in lower yarn surface temperature and heat flux compared to the yarn structure model.

Conclusion To investigate the heat transfer process and thermal protective performance of the fabric used in firefighting clothing, both experiments and numerical simulation were performed in this study. The models were validated by the experimental results and a parameter study was conducted. The effects of ambient temperature, yarn count and yarn thermal conductivity on yarn surface temperature, dermal temperature and dermal heat flux were simulated. The findings of this study indicate that the presence of the fabric and air gap effectively reduces skin temperature. The yarn count in the fabric layer has a complex influence on the heat transfer within the fabric-air gap-skin' system, which varies with changes in ambient temperature. For a single-layer fabric system, the air gap beneath the clothing plays a more crucial role in thermal protection, thereby mitigating the relatively weaker impact caused by variations in the fabric's thermal conductivity. Therefore, the design of the fabric panel structure should take into consideration the heat exposure environment. This approach not only contributes to minimizing the weight of thermal protective clothing but also serves to mitigate the risk of heat stress on firefighters. It ultimately enhances the occupational safety of firefighters in high-temperature environments while ensuring the thermal protection performance of the clothing.

Key words: heat transfer, numerical simulation, fabric structure, firefighter's clothing, low-level radiation