Determining the critical air gap influencing heat transfer mode under firefighter's clothing exposed to different fire conditions

Understanding the heat transfer mechanisms in the air gap under firefighter's uniform is crucial for accurately evaluating its thermal protective performance (TPP). However, existing research presents conflicting views regarding the air gap width (AGW) that natural convection begins and its relationship with the optimal TPP for the human body. To quantitively determine the critical AGW under clothing, experiments were conducted under three types of fire conditions, and the indicators for natural convection begin were calculated. Results indicated that both fire conditions and fabric layers affected the AGW in which natural convection began (9–18 mm). The largest t_2nd (time to second degree burns) was detected for single-layer fabric at 30 mm AGW. However, the t_2nd reached the maximum value at an AGW of 18 mm under 84 kW/m² hybrid heat environment, which was higher than no air gap (50 %) and 30 mm AGW condition (2.5 %), indicating an optimal TPP. Critical AGW also influenced the optimal TPP. The AGW of 18 mm at which multi-layer fabric achieved time-unsteady natural convection was comparable to the optimal TPP. The results of our investigation can provide theoretical reference for the structural design of fire-fighting suits and improve the safety of fire-fighting personnel.