This paper presents a model for heat and moisture transfer through firefighters’ protective clothing (FPC) during radiation exposure. The model, which accounts for air gaps in the FPC as well as heat transfer through human skin, investigates the effect of different initial moisture contents on the thermal insulation performance of FPC. Temperature, water vapor density, and the volume fraction of liquid water profiles were monitored during the simulation, and the heat quantity absorbed by water evaporation was calculated. Then the maximum durations of heat before the wearer acquires first- and second-degree burns were calculated based on the bioheat transfer equation and the Henriques equation. The results show that both the moisture weight in each layer and the total moisture weight increase linearly within a given environmental humidity level. The initial moisture content in FPC samples significantly influenced the maximum water vapor density. The first- and second-degree burn injury time increase 16 sec and 18 sec when the RH increases from 0% to 90%. The total quantity of heat accounted for by water evaporation was about 10% when the relative humidity (RH) is 80%. Finally, a linear relationship was identified between initial moisture content and the human skin burn injury time before suffering first- and second-degree burn injuries.
Firefighters’ protective clothing (FPC) constitutes critically important equipment in firefighting. Typical FPC consists of two parts: an outer shell and an inner linear [
Moisture contained in FPC material usually results from human sweat, penetration of outside water during a rescue, or moisture in the settled environment [
The initial moisture content of FPC material is determined by the relative humidity (RH) of the storage environment, and moisture generally settles in the pores of the fabric before heating. Moisture evaporation mainly occurs during the early stage of heating, which is of critical importance in determining heat and moisture transfer. In this paper, we describe a simulation approach that can characterize the influence of initial moisture content on the heat insulation performance of multilayer FPC materials with air gaps between each layer. The intention of this study is to describe moisture absorption, desorption, and evaporation during the early stage of heating. In order to explain the mechanism by which initial moisture content acts to influence the thermal protective performance of these materials, we also estimate the maximum duration of the temperature before the wearer suffers first- and second-degree burn injuries.
The experimental equipment used in this study consisted of a radiation source, test chamber, constant temperature system, and data acquisition system, as shown in Figure
Schematic diagram and view of experimental chamber.
The samples were cut from the FPC currently in use in China. The FPC material is composed of four layers: the outer shell (OS), the moisture barrier layer (MB), the thermal barrier layer (TB), and the comfort layer (CL). The materials comprising the layers are XDB602, aramid fabric coated with PTFE film, aramid insulation, and cotton shirting, respectively. The dimension of the samples was 230 mm × 230 mm. First, eight samples were set in a drying oven (ZK-1000A, Shanghai Hong Yun experimental equipment factory, China) at a temperature of 100°C for 24 h to remove internal moisture. Then two of the samples were put into programmable constant temperature and humidity testers (PCTHC) for 24 h. The temperature of the PCTHC was set at 20°C, and the humidity was set at
The thermocouples used were identical to those described in He et al.’s paper [
Thermocouples and sample holder.
This study developed a model for coupled heat and moisture transmission in FPC materials during the radiation heating process. The effects of both initial moisture content as determined by RH and the temperature of the storage environment on the FPC samples’ insulation performance were investigated. The burn injury times for human skin under different conditions were also determined. Figure
Sketch of heat and moisture transfer in FPC materials and heat transport in human skin.
We based our simulation model on Chitrphiromsri’s model [ Energy equation is as follows: Mass equations are as follows: Momentum equation for liquid water is as follows:
In (
The conservation equations for each air gap are as follows: Energy equation is as follows: Mass equation is as follows:
The heat transfer in each skin layer was estimated using Pennes’ model as follows [ Initial conditions are as follows: Boundary conditions for the fabric are as follows:
The temperature of the boundary conditions for the fabric is the fitting curve of the average temperature, as measured by the thermocouples under radiation of 5 kw m−2.
We used Henriques’ tissue burn injury model [
Equation (
In this work, differential equations (
Time and step independence are illustrated graphically in Figure
Temperature distributions for time and grid independence tests.
Results of time independence test
Results of grid step independence test
To ensure the accuracy of the experimental results, we first analyzed experimental repeatability. The radiation intensity of the experiment was 5 kW m−2. The sample was dry; the AG thickness between the back surface of the fabric layer and human skin was zero. Other conditions were set as described above. The results are shown in Figure
Temperature change with time at different locations.
In the simulation, the temperature measured at the outer surface and at the back surface of the fabric layer under a radiation intensity of 5 kW m−2 was set as the
Temperature profiles at outer surface and back surface of fabric layer. The fitting equations for
The thermal physical properties of the fabric samples used in the simulations can be reviewed in Chitrphiromsri et al. [
Thermophysical and geometrical properties of fabric [
Property | Outer shell | Moisture barrier | Thermal barrier | Comfort layer |
---|---|---|---|---|
Thickness of the material |
0.31 × 10−3 | 0.5 × 10−3 | 1.1 × 10−3 | 0.29 × 10−3 |
|
||||
Material of composition | XDB602 | Aramid fabric coated with PTFE film | Aramid insulation | Cotton shirting |
|
||||
Density |
677.42 | 210.00 | 77.27 | 413.79 |
|
||||
Thermal conductivity |
0.075 | 0.052 | 0.05 | 0.053 |
|
||||
Volume fraction |
0.334 | 0.186 | 0.115 | 0.15 |
|
||||
Fibre curl |
1.5 | 1.25 | 1 | 1 |
|
||||
Diffusivity of the gas phase in the fabric |
0 × 10−14 | 0 × 10−14 | 0 × 10−14 | 0 × 10−14 |
|
||||
Fiber radius |
1.6 × 10−5 | 1.6 × 10−5 | 1.6 × 10−5 | 1.6 × 10−5 |
|
||||
Darcian permeability coefficient, m2 |
10 × 10−16 | 10 × 10−16 | 10 × 10−16 | 10 × 10−16 |
|
||||
Saturation of the fabric |
0.1 | 0.1 | 0.1 | 0.1 |
|
||||
Proportional constant of liquid water absorption |
0 × 10−4 | 0 × 10−4 | 0 × 10−4 | 0 × 10−4 |
Thermophysical and geometrical properties assigned to skin layers [
Property | Epidermis | Dermis | Subcutaneous tissue | Blood |
---|---|---|---|---|
Density |
1200 | 1200 | 1000 | 1060 |
|
||||
Thermal conductivity |
0.23 | 0.45 | 0.19 | — |
|
||||
|
0.08 × 10−3 | 2 × 10−3 | 10 × 10−3 | — |
|
||||
|
3600 | 3300 | 2300 | 3770 |
|
||||
|
0 | 0.00125 | 0.00125 | — |
Moisture weight in each fiber layer against RH, with fitting line.
The correlation coefficients of the fitting lines for the experimental data describing the outer shell, moisture barrier, thermal linear, and total multilayer material are 0.99205, 0.97086, 0.99226, and 0.99700, respectively. These results demonstrate that the moisture weight in each layer, as well as the total moisture weight, increases linearly with settled environmental RH, as expected. The moisture absorbed by wicking into the thermal barrier is about four times that in the outer shell layer and two times that in the moisture barrier layer. The samples also felt obviously moist when touched after remaining for 24 h in circumstances with RH values exceeding 65%.
We designed an apparatus to simulate the process of heat transfer in FPC samples with different initial moisture contents. The experimental setup and the samples were described in Section
Simulated (continuous lines) and experimental (discrete points) temperature profiles when RH = 65%.
Temperature versus time
Temperature versus thickness
Figure
Temperature profiles in different layers at specific moments in time.
Temperature profile in both fabric and skin layers
Temperature profile at fabric-skin interface
Temperature profile in fabric
Temperature profile in skin
We then analyzed the temperature differences under different initial moisture conditions in the fabric and skin layers, with the results shown in Figures
Figure
Distributions of water vapor density at different times.
RH = 30%
RH = 50%
RH = 65%
RH = 80%
Profiles of water vapor density versus time at a distance of 0.56 × 10-3 m.
In our experiment, the temperature increased when the outer shell of the FPC sample was exposed to heat radiation, which caused the desorption and evaporation of bound water as well as the evaporation of free liquid water located in the fabric pores. To examine further the mechanism of moisture transformation during heating, we analyzed the liquid water volume fraction in the fabric pores at different times, as shown in Figure
Liquid water volume fraction distributions at different times.
RH = 30%
RH = 50%
RH = 65%
RH = 80%
Figure
Heat absorbed by water evaporation against RH, with fitting line.
Figure
We also investigated the effect of the initial fabric moisture content on the maximum duration of heat radiation before the wearer sustains first- and second-degree burn injuries. The tissue burn injury model is based on work by Henriques [
Burning injury time versus RH.
Temperature at skin surface.
We developed a numerical model to investigate the effects of initial moisture content at each fabric layer on heat and moisture transfer in FPC materials. We estimated burn injury times under different initial moisture contents using our model combined with Pennes’ model and Henriques’ tissue burn injury model. We can draw the following conclusions: Initial moisture content in each FPC fabric layer increases linearly as the RH of the storage location increases. We developed a comprehensive model to simulate heat and moisture transfer in FPC, and the simulation results show fairly good agreement with the experimental results. Water vapor is distributed both in the fabric layers and in the air layer, while free liquid water disperses only in the fabric layers. Water vapor density is mainly due to evaporation from free liquid water during the experiment, and it decreases as initial moisture content increases. The total amount of heat absorption attributable to water evaporation is about 10% when the RH is 80%. A linear relationship exists between the initial moisture content and the maximum duration of heat radiation before causing first- and second-degree burn injuries.
Specific heat at constant pressure, J k
Diffusivity of the gas phase in the fabric,
Fiber radius,
Convective heat transfer coefficient, W
Enthalpy per unit mass, J k
Fabric layer
Thermal conductivity, W
Darcian permeability coefficient,
Thickness of the material,
Mass transfer rate from the gaseous phase to the liquid phase, kg
Mass transfer rate from the gaseous phase to the solid phase, kg
Mass transfer rate from the liquid phase to the solid phase, kg
Pressure of liquid water, Pa
Incident radiation heat flux from the radiation onto the outer fabric surface, W
Radiation heat flux from the
Radiation heat flux from the
Total radiation heat flux of the
Saturation of the fabric
Time, s
Temperature,
Velocity of liquid water, m se
Distance, m
Transmissivity of the fabric
Fiber curl
Density, kg
Absorption coefficient
Volume fraction
Radiative extinction coefficient of the fabric,
Dynamic viscosity, kg
Stefan-Boltzmann constant,
Blood perfusion [0.00125
Quantitative measure of the burn damage at the basal layer of human skin
Frequency factor or preexponential factor, se
Activation energy for skin
Universal gas constant,
Initial state
Effective
Liquid water
Bound water
Water vapor
Skin
Blood
Ambient air
Evaporation
Transfer
Human core
Left boundary
Right boundary.
The authors declare that they have no conflicts of interest.
Dongmei Huang carried out the experiment and numerical simulation and drafted and revised the manuscript. Song He participated in the design of the study and performed the statistical analysis and helped to draft the manuscript. All authors read and approved the final manuscript.
This work was supported by Natural Science Foundation of China (no. 51306168) and Zhejiang Provincial Natural Science Foundation of China under Grant no. LY17E060004.