Heat And Moisture Transfer In Firefighter's Clothing And Prediction Of Skin Steam Burn Under Hot Steam Of Fire Ground

In fire hazards and explosion disasters from industrial manufacture and social life,firefighter usually suffers from multiple thermal hazards,such as flash fire,thermal radiation,hot liquid and steam.Firefighting protective clothing is an indispensable personal protective equipment while firefighter performs emergency rescue.The clothing performance directly determines the life safety of firefighter.However,it is reported that about 65% of skin burn injuries of firefighter are steam burns or scalds,which result from penetration of steam and hot liquid through the clothing.In previous studies,the performance of protective clothing was generally evaluated under dry heat exposure,which could provide protection for firefighter against flame and thermal radiation.But the influence of environmental moisture on thermal protective performance of the clothing was not considered.Therefore,the current protective clothing hardly provides effective steam protective performance.In view of above mentioned problems,a multiple environmental and mul-functional test apparatus is developed to realistically simulate hot steam scene in fire environment.The developed test apparatus is used to assess thermal-wet protective performance of firefighting protective clothing in fire environment and examine the influencing factors of steam protective performance,which contributes to optimizing evaluation apporoach of thermal protective equipment in complex fire environments.Meantime,a heat and moisture transfer model and simulative platform between thermal-humid environment,multilayer fabric,air gap and body skin is developed.On the basis of the developed simulative platform,the mechanism associated with heat and moisture transfer and skin steam burn is analyzed for providing protective guidances for radiative burn and steam burn,and maximuming protective performance of firefighting protective clothing.The main content and conclusions include the following sections:(1)Development on test apparatus of steam protective performance under fire hazardsThe developed test apparatus includes steam generator,steam delivery spout,heat exposure box,specimen transfer component and data acquisition system.The test apparatus firstly characterizes the thermal protective performance of fabric system exposed to thermal radiation and steam.Additionally,the creativity of the test apparatus is capable of controlling radiant heat flux,steam conditions and air gap thickness.Precision and reliability of the test apparatus is validated based on calibration of heat source and measurement of multiple fabric assemblies.The result demonstrates that the test apparatus is capable of evaluating steam protective performance of different fabric systems with various air gaps under complex fire hazards,which remedys defects of the current test apparoaches on the thermal protective performance of fabric system.(2)Study on steam protective performance of firefighting protective clothingSeven fabric systems were selected to examine thermal-wet protective performance of protective clothing and analyze its influencing factors under dry and wet heat exposures based on the developed test apparatus.The results showed that hot steam significantly reduces thermal protective performance of fabric system under radiant heat exposure,depending on fabric thickness and air permeability.Besides,the hot steam greatly increases total heat transfer during heat exposure,while insignificantly reducing heat transfer during cooling.Steam heat transfer accounts for 50.5%-77.7% of total heat transfer.In addition,different layer fabric presents different extent of influence on steam protective performance of fabric system.The insertation of moisture barrier greatly improves steam protective performance that is dertermined by air permeability and steam penetration rate of fabric.The increase of steam protective performance due to thermal liner is achieved by increasing fabric thickness and reducing steam absorption of fabric system.(3)Influence of air gap thickness on steam protective performanceDifferent thicknesses of spacers were made to simulate change of air gap thickness from 0 to 24 mm.The developed test apparatus was employed to analyze the effect of air gap thickness on thermal-wet protective performance of fabric under dry and wet heat exposures,and quantitatively characterize heat and moisture transfer mechanism in the air gap.This helps to develop evaluative and predictive system on thermal protective performance of air gap.It was found that an increase of air gap thickness improves greatly thermal protective performance of fabric system in two test conditions,while impeding cooling effect of skin during cooling for dry heat exposure and having no significant relation with skin heat loss for wet heat exposure.Additionally,the increase of air gap thickness changes heat transfer mode in air gap.For dry heat exposure,the minimum air gap with natural convective heat transfer is between 6 and 9 mm,while the existence of hot steam increases the thickness of air gap with natural convective heat transfer.Thermal energy absorbed by skin during heat exposure shows high correction with time to 2nd degree burn for different air gap thicknesses.This indicates that the thermal energy absorbed by skin can be used to characterize the thermal protective performance of protective clothing with different air gap thicknesses.(4)Combined effects of steam and body sweats on thermal protective performanceAccording to simulative method of body sweats in standard ASTM F2731-11,four kinds of prewet levels were selected to simulate skin sweat level of firefighter in fire environment.The combined effects of ambient steam and body sweats on thermal protective performance were investigated.The moisture distribution and flow direction in fabric system were compared between dry and wet heat exposures.It was found that an increase of moisture content in fabric system enhances thermal protective performance under wet heat exposure,while presenting positive or negative effect on thermal protective performance under dry heat exposure.This is because moisture in fabric system has different roles during total heat exposure,which depends on moisture content and heat exposure duration.Additionally,moisture transfer in fabric system exists bidirectional flow.For dry heat exposure,the amount of moisture evaporation toward outer environment is marginally more than that toward skin-simulant sensor.Total amount of moisture evaporation increases with the initial moisture content in fabric system.For wet heat exposure,the existence of hot steam increases the moisture content within fabric system,while the amount of moisture evaporation toward skin-simulant sensor is greatly more than that toward outer environment.This indicates that the moisture evaporation toward human skin is not only from the initial moisture content of fabric system,but also from steam transfer from the ambience.The standard firefighting protective clothing did not provide effective protection against steam penetration.(5)Development on heat and moisture transfer model and numerical simulative platformHeat transfer model in multilayer fabric system was developed based on thermal physical properties of textile material.Radiative view factor was used to simulate radiant heat transfer from heat source to outer shell.Beer's law and two-flux radiative model was employed to model radiative absorption,transmission,reflection and self-emission.Besides,heat transfer in an air gap with different thicknesses was simulated.Finally,skin bioheat transfer and skin burn integral models were coupled with the clothing model for effectively predicting skin burn time with different fabric systems in dry heat exposure.On the basis of the developed radiant heat transfer model,the coupled model on heat and moisture transfer in fabric system was proposed.The developed model considers the non-transient equilibrium between three phases,the impinging jet flow between steam nozzle and fabric,the steam flow within the fabric induced by the pressure gradient,the dynamic moisture absorption,and the possible phase changes during the process.The model was validated with the experimental results of multiple fabric systems measured by using the developed test apparatus of thermal protective performance,thus developing numerical simulative platform of heat and moisture transfer in firefighting protective clothing.The developed platform was used to analyze the mechanism of heat and moisture transfer and skin burn.The results demonstrated that hot steam greatly reduces radiant and convective/conductive heat transfer in dry heat exposure.This is because hot steam limits the temperature increase in fabric system.For wet heat exposure,moisture phase change on skin surface is the primary reason resulting in skin burn,indicating the occurrence of skin steam burn.For dry heat exposure,radiant heat transfer on skin surface is the main reason causing skin burn.In addition,hot steam in heat exposure presents positive or negative effect on skin burn.When steam pressure and steam temperature is less than a certain level,steam can increase thermal protective performance of fabric system.In contrast,hot steam with larger steam pressure and steam temperature can quicken up skin burn.The research deeply analyzed thermal protective performance of firefighting protective clothing and the reason of skin burn in two heat exposures.It was found that the reduction of steam penetration and steam storage in fabric system greatly increases steam protective performance in wet heat exposure,which is determined by porous structure,moisture absorption and initial moisture content of fabric system.The decrease of radiant and conductive heat transfer in fabric system is the dominant mode to improve thermal protective performance in dry heat exposure that is related to thermal physical properties.Additionally,the increase of fabric thickness and air gap thickness simultaneously increases the thermal protective performance in two heat exposures.Therefore,the research apporoaches and findings obtained by the study provide theoretical foundation and instruction for development of new thermal protective material and scientific selection of thermal protective equipment,which will be of great importance and necessity on safety improvement of fire-fighting operation.