Study Of Heat Transfer Of Wearer Under Thermal Radiation Condition And Prediction Model Of Safety Thermal Exposure Distance

Correct evaluation of safety thermal exposure distance can improve the operational safety of firefighters when exposing to a fire environment.At present,there is lack of systematic study on the safety thermal exposure distance.Its definition,evaluation device and assessment method have no unified standards.Numerical simulation is the main method for evaluating safety thermal exposure distance.However,the process of heat transfer between the clothing and the human is simplified when developing the model.Besides,current studies mainly focus on the fire and environment.Parameters of clothing and human are not paid attention to.Therefore,the study of safety thermal exposure distance carried out from experimental test and model prediction.The evaluation method for the safety thermal exposure distance was presented and the factors influencing the safety thermal exposure distance were initially analyzed.In addition,the prediction model of safety heat exposure distance was developed,considering the heat transfer of the clothing and human.The effects of fire,fabric and air gap thickness on the safety thermal exposure distance were investigated by changing the model parameters of the developed prediction model.The contents and findings of the study were as follows:(1)Experimental study of the safety thermal exposure distanceThe stability of the results was verified by the error analysis of the results.The temperature threshold was selected as the evaluation criterion for determining safety thermal exposure distance.The evaluation method of safety thermal exposure distance was proposed.And the effect of heat temperature and fabric combination on safety thermal exposure distance were investigated based on the above evaluation method.The relationship between heat source distance and safety thermal exposure time was revealed.The results showed that there was a significant positive correlation between heat temperature and safety thermal exposure distance.When the heat temperature increased from 200 °C to 550 °C,the safety heat exposure distance corresponding to the three single-layer fabrics was extended by a minimum of 1.23 times.At the same time,the nine fabric combinations had different safety heat exposure distance due to different thicknesses and weights.The smallest safety thermal exposure distance was A6 and the largest was OS1.The safety thermal exposure time increased as the heat source distance increasing.The mathematical relationship between heat source distance and safety thermal exposure time was obtained.It could provide theoretical support when firefighters determined safety thermal exposure time during fighting against fire.(2)Development and validation of safety thermal exposure distance prediction modelThe radiative heat transfer model from fire to clothing surface was developed by the solid fire model and the geometric relationship between fire and human.A coupled conduction-radiation heat transfer model was built for the interior of garment relying on the porous medium heat transfer mechanism.The coupled conduction-radiation heat transfer model of the air gap was developed through the air gap heat transfer theory.The Pennes biological heat transfer model was used to simulate the heat transfer process in skin.Skin burn injury was predicted by the skin burn integral model.The temperature of skin was used as the reference indicator to verify the validity of the model predictions.The results showed that the model predictions were acceptable for both the small-scale experimental simulation environment and the large-scale fire environment.In the small-scale experimental simulation environment,the average percentage error between the experimental and predicted values was 2.10% for single-layer fabric and 4.79% for multi-layer combination for both conditions without and with air gap,excepted for the heat source distance of5 cm.The average percentage errors for single-layer and multi-layer combination were bigger under two conditions at the heat source distance of 5 cm,respectively.This was mainly due to the convective heat transfer between the heat and fabric surface increased when the heat source distance was small.However,this process was not taken into account during building the model.The percentage error between the model predictions and the results reported in the literature ranged from 4.17% to 33.33% in the large-scale fire environment.This was mainly because the differences between the model predictions and the results reported in the literature on the evaluation criteria used to evaluate the safety thermal exposure distance and the human dressing conditions.(3)Parametric analysis of the safety thermal exposure distance prediction modelThe effect of fire,fabric and the air gap thickness on the safety thermal exposure distance was investigated by changing the parameters of the prediction model.The results showed that fire temperature,fire height/width and fire emissivity had a significant positive correlation with the safety thermal exposure distance.The effect of fire temperature and fire height/width on the safety thermal exposure distance was greater than fire emissivity.Secondly,the influence of fabric radiation parameters on safety thermal exposure distance was related to the heat source distance.When the heat source distance was small,the fabric transmittance had a large effect on safety thermal exposure distance.On the contrary,the fabric absorption rate had a large influence on safety thermal exposure distance.When the thickness of the different fabric layers(Fabric combination A1)was increased from 0.40 mm to 4.60 mm,the minimum decrease in safety thermal exposure distance was 13.27%.The moisture barrier had the greatest impact.For the thermal conductivity,when the thermal conductivity of the fabric increases from 0.04 W/(m K)to0.28 W/(m K),the change in thermal conductivity of the moisture barrier and thermal liner(Fabric combination A1)had twice the effect on the safety thermal exposure distance as the outer fabric.The specific heat of fabric had no significant effect on the safety thermal exposure distance,but it had an effect on the safety thermal exposure time.The specific heat of the thermal liner had the greatest effect.In addition,there was a significant negative correlation between the air gap thickness and the safety heat exposure distance.Appropriately increasing the air gap thickness was effective in reducing the risk of skin burns.This study proposed a method for evaluating safety thermal exposure distance.The prediction model of safety thermal exposure distance for firefighters was developed to consider heat transfer between the clothing and the human body.The mechanisms influencing the safety thermal exposure distance was revealed.It could provide a theoretical basis for the optimal design of thermal protective clothing,which would be important for improving the operational safety of firefighters.