Effect of relative humidity coupled with air gap on heat transfer of flame-resistant fabrics exposed to flash fires

Relative humidity (RH) and air gap size are two characteristic indices of clothing microclimate. The current thermal protective performance (TPP) tests cannot evaluate protective performance of fabrics under a microclimate with high temperature and humidity. In this study, a newly modified TPP test apparatus was applied to investigate the effect of RH in a microclimate on heat transfer of fabrics exposed to flash fires. Air gap heights from 0 to 24 mm with increments of 3 mm were employed. Three microclimates with different RH were preconditioned respectively. The results indicate that the RH in a microclimate significantly improves thermal protection of fabric with various air gaps. Under 35% RH, the TPP obviously increases with the air gap up to 15 mm and then the increment becomes a little lower; under humidified conditions (65% or 95% RH), it becomes higher substantially with air gap height increasing until 12 mm, subsequently it decreases or increases alternatively if the size keeps increasing. The positive effect of air gap with small size on TPP of fabrics is enhanced due to the increase of the vapor in the air; however, the positive effect of air gap with large size is interfered by the RH. It is indicated that around 12–15 mm was a critical air gap size under 35% RH, while about 12 mm was a key air gap size for a damp microclimate with 65% or 95% RH.