Study On The Heat And Moisture Transfer Mechanism In Firefighters’ Protective Clothing Under Low Heat Flux

Generally, firefighters will face on several hazard conditions while firefighting, such as thermal radiation, hot gas convection from a fire, direct contact with hot surfaces and so on. Firefighters’protective clothing （FPC）, as personnel protective equipment, is primarily designed to give an individual a degree of thermal protection from the thermal environment during firefighting. A low performance FPC will affect both fire fighters’safety and work rate. Clothing with better protective performance will result in less injury in each on-duty activities and clothing with better comfort may improve the work rate.During the last few decades, the performance of FPC has captured the attention of many fire engineers and other researchers. Many studies have focused on measuring fire protective performance of FPC, assessing of the risk of burn injury, modeling specific mechanism of heat transfer, evaluating of ergonomics of FPC, managing of heat stress for firefighter and so on. However,when study on the heat transfer in FPC with an air gap layer, the air gap were usually considered located between human skin and comfort layer, few studies were related to the air gap between outer shell and moisture layer. And the exist equipment can not take the effect of the water applied on the outer shell heated surface on the heat transfer into account. Moreover,researches related to the environment relative humidity effect on the heat transfer of FPC are scarce. In the present study, we conducted a nationwide questionnaire of numerous Chinese in-service firefighters to understand the subjective perception to their protective clothing and to identify the deficiencies of FPC in China. Then we measured the temperature on each FPC layer and developed a heat transfer model for the environment-fabric-air gap-skin system to identify the heat and moisture mechanism. Finally, the burn injury time based on the heat transfer model was calculated.A survey was carried out on 1201 firefighters from 119 fire stations, 25 provinces. Results indicated that the current in-service FPC might restrict mobility of firefighters, and the most serious restricted parts were thigh, knee, arm, elbow and ankle. While wearing FPC in work most of the participants felt overheating in summer. And 84.5 percent of the firefighters have exercised heat illnesses, such as heat cramps, heat syncope, heat exhaustion, and heat stroke. Moreover, there isn’t an effective cooling method which could be used in firefighting.We developed a device for multi-layer fabric heat transfer. The device could measure the temperature change on each fabric layer when water inject to the outer shell heated surface in addition to other equipment. Firstly, the thermocouple measurement errors were analyzed based on the apparatus. And a thermocouple measurement error correction model was developed. Then we measured the temperature both on the outer surface and inner one in different conditions. The air gap thickness between comfort layer and water tank was in the range of 0 to 10 mm. The radiation intensity was from 3-10 kW m^-2. The environment relative humidity that the fabric placed was 30% -80%. The water spray was 500 -2000 L h^-1m^-2. Then the heat transfer mechanism in the air gap with different thickness was analyzed based on heat transfer theory. Results indicted that when the radiation intensity was 10 kW m^-2, the tensile strength of outer shell layer was very small and the protection performance lost after 120 s. When the radiation intensity was less than 10 kW m^-2, the FPC material started to pyrolysis after 120 s. The moisture content within the fabric increase linear with the located environment relative humidity. And the temperature on the comfort layer decrease with increase of the located environment relative humidity. When the water spray≥1000 L h^-1m^-2, temperature on the back surface can be controlled at a certain value. The heat transfer mechanism is classⅠandⅡwhen the thickness of air gap in the range of 0-10 mm. That is to say, the convection effect is small; the main mode of heat transfer within the fabric is conductivity in the direction of perpendicular to the fabric.A heat and moisture model for the environment-fabrics-air gap-skin system has been developed based on Whitaker’s model and bioheat transfer model. The model considered the air gap between fabric layers which is more close to the real situation. Then, the simulation results were validated by the experiment data. The simulation temperature is in good agreement with the experimental ones. Then the distribution of each water phase and the effective thermal properties, such as heat capacity, thermal conductivity in the FPC were obtained. Distribution results showed that the free water evaporated completely after 40 second, and the effective heat capacity and the effective thermal conductivity were increase with the temperature. But the presence of moisture within the fabric has little effect on the effective density.Finally, human skin burn injury time has been calculated in different conditions with combination of bioheat transfer equation and Henriques equation to the heat transfer model. The results showed that human skin burn injury time increase linely with the located environment relative humidity and the thickness of air gap which located between human skin and comfort layer. When the outer layer of liquid water volume fraction is above 0.6, water on the out shell surface can effectively extend the time for human skin burn. When the air gap is located between the outer shell layer and the moisture layer, the human skin burn injury time is longer than the air gap located between the skin and comfort layer under the same air gap thickness. When the thickness of air gap between outer shell layer and the moisture layer reaches to 7 mm, and radiation intensity 5 kW m^-2, the human skin burn injury time in more than 800 s.