A New Approach to Evaluate Thermal Stress under Flame-resistant

Fabrics Exposed to Flashover

Yehu LU 1, a, Xiaohui LI 1, b, Jun LI 1, 2, c and Daiwei WU 1, d

1 Protective Clothing Research Center, Fashion Institute, Donghua University, Shanghai 200051,

China.

2 Key Laboratory of Clothing Design and Technology (Donghua University), Ministry of Education,

Shanghai 200051, China.

aluyehu@gmail.com, beelxh@126.com, clijun@dhu.edu.cn, ddhwdw88@163.com

Keywords: Thermal stress, Flame-resistant fabric, Flashover, Bio-heat transfer, Air gap

Abstract. Various intensity heat fluxes firefighters encountered will produce thermal stress on skin,

resulting in thermal pain and tissue damage. In this paper, a new approach to evaluate thermal stress

under flashover with short duration was carried out based on plain-stress theory. Instant heat flux

under fabric was calculated so as to determine temperature and thermal stress distribution. The

results obtained were as follows: temperature increased slightly at initial stage and then sharply

increased linearly, moreover, temperature was much higher when sensor directly contacted with

specimen, comparing with that of 6mm air gap; heat flux under fabric quickly reached its maximum,

and higher heat flux was observed as no air gap generated; thermal stress rapidly increased and then

gradually decreased, moreover, higher thermal stress produced without air gap. The newly proposed

method could well distinct heat transfer performance of fabric under different conditions, which

might provide helpful guideline to performance evaluation of thermal protective clothing.

Introduction

Firefighters usually encountered high ambient temperature and radiant heat flux during fire

operation and rescue [1]. Therefore firefighters should be equipped with qualified fire protective

garments to ensure the safety of their lives. However, the higher protection it provided, the less

permeable for body heat and evaporated sweat was the clothing, thus heat storage occurred and

efficiency decreased [2-4]. It was reported that thermal stress was caused by the weight and

insulating properties of protective clothing, environment and exercise performance [5-8]. Therefore,

the modern design philosophy for protective clothing was concentrated on optimized protection and

simultaneously making the wearer more comfortable [9]. The most recent statistics in the United

States reported that 55% of 118 deaths in 2007 were considered as the result of heat stress [10].

Different methods for estimating potential heat stress have been developed in ISO and other

standards by meteorological parameters and physiological variables, including the Wet Bulb Globe

Temperature (WBGT) index, the Required Sweat Rate (SRreq) index and various physiological

measurements, etc [11-13]. However, the WBGT and SRreq indices were not suitable for

firefighters to evaluate heat stress when wearing protective clothing under high radiant heat flux

[14]. Without accurate details of exposure time to a given temperature, it was difficult to determine

whether the physiological strain were the result of physical demands of activities or heat stress

imposed by environment, or a combination of the two. During firefighting activities under different

Advanced Materials Research Vols. 332-334 (2011) pp 1520-1526Online available since 2011/Sep/02 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.332-334.1520

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thermal conditions, the effects of environment and activity should be determined separately [1].

Although a large number of studies have been made [9, 15], no published data existed on

thermoregulatory responses during real life operations, as it was impractical and could be dangerous

to monitor a firefighter with physiological equipment during hazardous activities. Consequently,

study on physiological responses during fire extinguishing activities relied on data collected during

simulations of live fires [8, 16]. Problems existed when monitoring and controlling environment

conditions during fire simulations [17, 18]. Accordingly, the application of heat stress calculated by

physiological indices under simulated conditions was very limited.

Mathematical modeling of thermal responses allowed evaluation of wide performance

limitations in individuals exposed to extreme environment. Some researchers have built heat stress

models applicable for protective clothing to simulate core temperature response [19]. The predicted

heat strain model was also constructed based on heat balance considering sweat evaporation [2, 20].

The validation range on clothing intrinsic thermal insulation was extended to less than 1.0clo [21],

while it was not suitable to evaluate physiological response wearing protective clothing with

thermal insulation more than 2.0clo [22]. Since human skin is the first layer exposing to high heat

flux with short duration, there is no time for the participation of physiological regulation, and thus

the evaluation of thermal stress should be characterized directly by skin thermomechanical response.

Recently, the thermal stress of skin was determined based on plain-stress theory [23]. However,

there was nearly no reports about direct calculation of heat stress in thermal protective clothing.

In this paper, a new methodology to determine thermal stress on skin for firefighters exposed to

high intensity heat flux or flashover was carried out by using numerical methods. It could provide

helpful physiological guideline to the evaluation of thermal protective clothing considering the

thermal stress as protection performance index.

Mathematical model Formulation

Bio-heat Transfer Equations. The prediction of temperature in skin tissue mainly based on Pennes

bio-heat transfer equation [24], given as

( )2

2sk sk sk b b b a m r

T Tc c T T q q

t xρ λ ω ρ

∂ ∂= + − + +

∂ ∂ (1)

where skρ , skc , skλ are the mass density, specific heat and thermal conductivity of skin tissue; bω

is the blood perfusion rate; qm is the metabolic heat generation, qr is the heat source due to external

heating, Ta and T are the temperatures of arterial blood and skin tissue respectively.

In the present study, for simplicity, one-dimensional case was studied, as shown in Fig. 1. Blood

perfusion, thermal conductivity, and heat capacity are assumed to be constant despite of the

temperature rise. Metabolic heat generation was assumed to be zero comparing with external heat

fluxes generated from heating fabrics.

Fig. 1 Structure of multi-layer skin

Advanced Materials Research Vols. 332-334 1521

While exposing to high heat flux, the corresponding boundary conditions are as follows:

( ) 0 / 2sk

Tk q t x L

x

∂+ = = −

∂ (2)

( / 2, ) 37, 0T L t t= > (3)

Initial condition

( ,0) 37T x = (4)

Thermal Stress. During heating, thermally induced mechanical stress arises due to the thermal

denaturation of collagen, resulting in macro scale shrinkage. The stress, temperature and thermal

damage are highly correlated. In fact, skin has a complicated multi-layer structure. As the thermal

properties of the layers have the same order of magnitude, a one-layer continuum model for heat

transfer was assumed. Thermal stress on skin was constructed and the result was given as [23]

( ) ( ) ( ) ( )/2 /2

0 0 0/2 /2

12,

L L

xxL L

E xx t E T T T T dz E T T zdz

L L

λσ λ λ

− −= − − + − + −∫ ∫

(5)

where ( ) ( )2/ 1 , 1E E υ λ υ λ= − = + , E is Young’s modulus, υ is Poisson ratio and λ is

thermal expansion coefficient.

Experimental Procedure and Numerical Method. The determination of temperature

distribution in skin at different time and positions were as follows: A standard TPP testing apparatus

CSI-206 was employed to produce simulated fire environment, and the temperature at back of

testing specimen was recorded with thermal sensor, shown in Fig. 2. The detail description of this

tester was not included here. The test specimen was momentarily exposed to high intense heat flux

about 84kW/m2 for 15s. The copper sensor was contacted with testing sample or located 6mm away

respectively. A kind of Nomex fabric was employed in this study. The detail physical parameters

were as follows: the construction of plain woven fabric, warp density of 253 per 10cm, weft density

of 220 per 10cm, weight of 150g/m2, thickness of 0.36mm, meta-Aramid fabric. In this paper, it was

assumed that copper sensor exhibited a semi infinite behavior, and thus the net heat flux qt exposed

to sensor was calculated as follows:

( ) 4.184n

mC dTq t

K A dtε= × ×

× (6)

where m is mass of copper, C is specific heat capacity of copper, K is conversion coefficient, A

andε are area and absorptivity of copper sensor respectively.

The heat flux data were then introduced to Pennes bio-heat transfer model, and the finite

difference method [25] is used to solve Eq. 1 and determine temperature field in skin tissue.

Subsequently, temperature profiles were used as input to the thermomechanical model given in Eq.

5, and thus the corresponding thermal stress distribution would be obtained. A mathematical

software Matlab7.2 was used to calculate the temperature and thermal stress distribution.

1522 Advanced Textile Materials

For blood, bρ = 1060 kg m−3

and bc = 3770 J kg−1

K−1

were used. The typical thermal physical

parameters for numerical thermal stress analysis are summarized in Table 1 [23, 26], and the

thermal expansion coefficient in all layers are 0.0001/oC.

Fig. 2 Schematic of TPP test

Table 1 Thermal physical properties used in the skin

Thickness

(m)

Specific

heat (J

kg−1

K−1

)

Blood

perfusion

(m3s

−1m

−3)

Thermal

conductivity

(Wm−1

K−1

)

Density

(kgm−3

)

Thermal

expansion

coefficient

(K−1

)

Poisson

ratio

Young’s

modulus

(MPa)

0.0061 3770 0 0.5 1050 0.0001 0.48 0.6

Results and Discussion

Heat Flux Exposed to Skin Surface. Fig. 3 shows the temperature of thermal sensor when the

flame-resistant fabric is exposed to flame and groups of radiant tubes about 84kW/m2 totally with

6mm air gap between fabric and sensor. It is clear in Fig. 3(a) that the temperature increases slightly

during the initial time, and then it rises sharply with the increasing of time. According to the curve

tendency, piecewise linearity method was used to calculate the slope of temperature rise during each

time interval, shown in Fig. 3(b), (c) and (d). It is obviously that the linear regression method can

well describe the temperature rise. The slope is 0.6769, 1.9084 and 3.9956 respectively with very

high R2 >0.96. Similar method was also applied for temperature rise of 6mm air gap. The

temperature gradient was then introduced to Eq. 6, and the heat flux under fabric with 0 and 6mm

air gap was obtained respectively, shown in Fig. 4. The heat flux increases quickly and reaches its

maximum of 44.1 and 62.1kW/m2. It is clear that the final heat flux with no air gap gets higher than

that with 6mm air gap, which means that the heat transferred is much higher when the thermal

sensor contacts with fabric. Fig. 4(a) shows that heat flux quickly gets peak value when thermal

sensor directly contacts with specimen. Comparing with Fig. 4(a), the time of heat flux reaching

maximum prolongs due to existence of 6mm air gap, shown in Fig. 4(b).

The observance of nonlinear increase of temperature versus time during initial stage (shown in

Fig. 3) may be the cause of moisture evaporation and fiber pyrolysis when the fabric is exposed to

high intensity flashover. During this time, dynamic non-steady heat transfer through “flame-heating

fabric-thermal sensor” system works. Subsequently, the specimen is carbonized and combination of

fibers and char forms. The steady heat transfer process emerges, therefore temperature rise curve

presents linearly and constant heat flux under fabric produces (shown in Fig. 4). When thermal

sensor contacts with specimen, heat flux is main transferred by conduction and radiation of fibers,

Advanced Materials Research Vols. 332-334 1523

while the heat transfer style become air conduction and fibers radiation as there is 6mm air gap

between thermal sensor and testing fabric. In one hand, heat flux transferred by conduction greatly

dropped due to low thermal conductivity of air; in other hand, radiation heat transferred decreased

because of reduction of view factor due to air gap increasing. Consequently, heat transfer through

fabric decreases when there is 6mm air gap.

(a)

30

40

50

60

70

80

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Time(s)

Tem

per

ature

(℃)

(b)

T= 0.6769t + 32.855

R2 = 0.9619

32.4

32.8

33.2

33.6

34

34.4

0 0.5 1 1.5 2Time(s)

Tem

per

ature

(℃)

(c)

T = 1.9084t + 30.671

R2 = 0.9821

30

32

34

36

38

40

1.5 2 2.5 3 3.5 4

Time(s)

Tem

per

ature

(℃)

(d)

T = 3.9956t + 21.931

R2 = 0.9985

30

40

50

60

70

80

3 4 5 6 7 8 9 10 11 12 13 14

Time(s)

Tem

per

ature

(℃)

Fig. 3 Temperature rise of thermal sensor versus time with 6mm air gap

(a) sensor contacts with specimen (b) 6mm air gap between specimen and sensor

Fig. 4 Heat flux under fabric at each interval

Thermal Stress on Skin Surface. The heat flux at each interval was considered as boundary

conditions to solve Eq. 1, and temperature distribution was obtained. According to Eq. 5, the

thermal stress distribution can be determined. Fig. 5 shows thermal stress on skin surface under

condition of no air gap and 6mm air gap. It is apparent in Fig. 5 that thermal stress sharply increases

versus time, and then slightly decreases after reaching peak value; moreover, the stronger heat flux

1524 Advanced Textile Materials

is, the higher increment becomes before reaching maximum. Comparing Fig. 5(a) with Fig. 5(b),

time required to reach maximum delayed when there is air gap between fabric and thermal sensor;

thermal stress is also higher when sensor directly contacts with testing specimen.

Thermal-induced mechanical stress arises due to the thermal denaturation of collagen. As

observed above, the final heat flux under fabric without air gap was higher than that with 6mm air

gap, resulting in higher thermal stress, shown in Fig. 5. The newly developed method of thermal

stress evaluation can well distinct heat transfer performance of flame-resistant fabrics exposed to

high intensity flashover with short duration. What deserves attention here is that the mean

mechanical threshold of nociceptors in skin lies in the range of about 0–0.6MPa and mainly

between 0.1–0.2MPa [27]. The numerical results presented above demonstrate that the thermal

stress is 0.048MPa without air gap and 0.038MPa with 6mm air gap, slightly less than 0.1MPa, only

using simple one-layer skin model.

High thermal stress will result in skin shrinkage, protein denaturation, skin pain and even

thermal damage. The thermal stress is a very important index to evaluate thermal protection and

comfort, which is influenced by a number of factors including encountered environment, clothing,

human activity and thermoregulation. In this study, thermal stress only induced by instant flashover

environment was taken into consideration, without including thermoregulation response due to

human activity. The new proposed method can be applied to investigate thermal induced

mechanical stress on skin while wearing thermal protective clothing under various kinds of

condition in the future, considering multi-layer skin tissue.

(a) without air gap (b) with 6mm air gap

Fig. 5 Thermal stress on skin surface

Conclusions

Firefighters usually encountered various high intensity heat fluxes with short duration, which will

results in thermal stress on human skin, leading to thermal pain and tissue damage. In this paper, a

new methodology to calculate thermal stress on skin for firefighters exposed to flashover was

carried out according to plain-stress theory. Piecewise linearity method was employed to determine

heat flux under fabric and then introduced to calculate thermal stress. The results showed that heat

transferred through fabric was higher when thermal sensor directly contacted with specimen, and

thus heat flux under fabric was also much higher, comparing with results under condition of 6mm

air gap. Under higher heat flux, bigger thermal stress and steeper increasing slope was found. The

newly developed evaluation index could well differentiate heat transfer performance of

flame-resistant fabric under different conditions. It might provide helpful guideline to performance

evaluation of thermal protective clothing considering the thermal stress as protection index.

Advanced Materials Research Vols. 332-334 1525

Acknowledgements

This paper was financially supported by Donghua University Ph.D. Thesis innovation funding (NO.

11D10711) and Program for New Century Excellent Talents in University of Ministry of Education

of China.

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Advanced Textile Materials 10.4028/www.scientific.net/AMR.332-334 A New Approach to Evaluate Thermal Stress under Flame-Resistant Fabrics Exposed to Flashover 10.4028/www.scientific.net/AMR.332-334.1520

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