Quantitative assessment of the thermal stored energy in protective clothing under low-level radiant heat exposure

Abstract

In addition to direct thermal energy from a heating source, a large amount of thermal energy stored in clothing will continuously discharge to the skin after exposure. Therefore, thermal protective clothing may have a dual effect on human skin in reality. An experimental investigation was conducted to study the energy storage within 15 different combinations of clothing layers exposed to low heat fluxes ranging from 2.5 kW/m² to 8.5 kW/m². The energy storage process, the distribution of energy storage, and variables critically impacting energy storage, including fabric layers, air gap under clothing, thermal resistance and heat source intensity were discussed. It is demonstrated that the weight and thickness of the fabric are dominating factors affecting energy storage. For a multilayer fabric system, 36–57% of the total amount of energy is stored in the outer shell. The neighboring layer proves to be very important for the energy storage in an individual fabric. The air gap that exists between the fabric and the skin exerts an influence on the energy storage within fabric layers. In addition, a linear correlation is observed between the energy storage and the total thermal resistance of a fabric system. The research findings will be brought to researchers to better understand the mechanism and factors associated with energy storage and help develop new fabric combinations in order to minimize heat transmission to the skin.

Keywords

protective clothing stored energy radiation thermal hazard air gap thermal resistance

Thermal protective clothing is designed to mitigate burn injuries and reduce the risk of death in case of exposure to unpredictable thermal environments. These thermal environments are generally classified into three categories: routine, hazardous, and emergency.¹ An enormous amount of research effort goes into emergency conditions that are characterized by high heat flux varied between 20 and 160 kW/m².^2–4 The failure of protection observed in emergency conditions should be partly due to thermal degradation or thermal shrinkage of protective clothing that may destroy the integrity of the clothing.^5,6 However, it is reported that skin burn commonly occurs in the low-level thermal radiation ranging from 5 to 20 kW/m².^7,8 Contrary to emergency conditions, exposures to low-level thermal radiation are generally not sufficient to degrade the protective clothing. There is considerable interest, therefore, in understanding the phenomena of skin burns in these low-level heat exposures. One potential source is probably the thermal stored energy in protective clothing as a result of prolonged exposure to low heat flux conditions.^9,10 When exposed to heat, protective clothing definitely serves as a barrier to protect the skin from potential environmental threats, but simultaneously it stores a large amount of thermal energy due to the heat capacity of the fabric. The energy-stored clothing becomes a passive heating source as soon as the exposure ends, releasing the thermal energy to the skin.^11,12 The discharge of stored energy can significantly lower the thermal protection expected from the clothing, and even result in more serious burn injuries.^9,13 Therefore, it can be concluded that thermal protective clothing may have a dual effect on human skin in reality. To date, there is little agreement on what quantity of energy is actually stored and how does the energy discharge affect the severity of the injury. The knowledge of the dual effect of thermal protective clothing will provide new insights into the development of novel protective materials in order to reduce the heat transmission to the skin, and help develop new test methods to evaluate the overall thermal protective performance of clothing.

To evaluate the thermal protective performance of clothing associated with thermal stored energy, some work has been carried out. Neal et al. firstly developed a laboratory test method to provide information on the potential for burn injury as a result of prolonged exposure to low-level radiation.¹⁴ This test method attempted to simulate a wearer suddenly compressing a heated clothing against his body due to either a flexing action of his limbs or the action of compressing his garment against a structural wall or other fixed surface. The initial step of this method was to preheat the testing fabric. And at the end of exposure, a thermal sensor was placed in direct contact with the internal surface of the fabric to measure the transmitted heat flux through the preheated fabric. Barker et al. later used this method to examine the effects of critical variables on the thermal protection levels of fabric systems containing thermal stored energy.⁹ However, Neal and colleague's method did not consider the transmitted energy through the fabric during exposure when estimating the time to different levels of burn injury. To fill up this deficiency, Song et al. developed a new testing procedure named ‘stored energy approach’.¹⁵ This method provides procedures for evaluating a skin burn injury that is caused by the combination of the transmitted energy during exposure and the discharged stored energy after exposure. Some researchers used this test method to compare the results obtained by the stored energy approach and the conventional approach that ignores the energy discharge.^16–18 Based on these studies, a ‘stored energy test’ (SET) apparatus is introduced in the newly developed ASTM F2731-11 standard.¹⁹ Using the SET apparatus, Barker et al.¹⁰ and Su et al.²⁰ studied the heat transmission and thermal energy storage in fabrics exposed to low-level radiant heat. However, it is noteworthy that neither Neal and colleague's test method nor the so-called SET method measures the actual amount of energy storage within fabrics. These methods only evaluate the part of stored energy that is transmitted through the preheated fabric to the skin after exposure, but do not consider the energy part discharging to the surrounding environment. Therefore, there is still a lack of studies conducting quantitative assessments for the actual energy storage within fabrics.

The investigation of energy storage in clothing layers is very important in terms of the dual effect of protective clothing. The location and amount of energy storage in protective clothing may affect not only the transmitted energy through clothing during exposure, but also the energy discharge to the skin afterwards. Up until now, it is not firmly known which fabric layers contain how much energy storage and what are the factors affecting the energy storage. This paper sets the stage for a series of studies in understanding the dual effect of thermal protective clothing associated with the thermal stored energy. The amount of energy storage within the fabric system exposed to radiant heat fluxes was determined. The energy storing process, the distribution of energy storage within fabrics, and the influencing factors (i.e. fabric layer, air gap under clothing, thermal resistance of clothing, and heat intensity of exposure) were investigated. The research findings will be brought to researchers to better understand the mechanism and factors associated with the energy storage, and help develop new fabric combinations in order to improve the overall thermal protective performance of clothing.

Experimental methods

Materials

Fabrics selected for experiments are commercially available and commonly used in the thermal protective clothing field. Table 1 displays the basic technical descriptions of materials. A series of experiments was conducted on a group of fabric systems constructed by these materials. Fabric systems with different layers were used to represent different levels of thermal protection. The single layer and two-layer fabric systems represent the actual protective garments worn by industrial workers and military personnel, while the multilayer fabric systems were representative of those used in the firefighter protective clothing, typically consisting of an outer shell, a moisture barrier, and a thermal liner. Fabric layers in double layer or multilayer fabric systems were contacted together, and therefore there was no air gap between the layers. For multilayer fabric systems, two outer shells, two moisture barriers, and three thermal liners differed mainly by weights were chosen for analysis. Details of fabric system constructions and physical properties are listed in Table 2.

Table 1.

Basic technical descriptions of the fabrics

Fabric code	Fabric layer	Fiber content	Structural features	Mass (g/m²)	Specific heat at 20 ℃ (J/ (kg℃))
OS1	Outer shell 1	100% Nomex®IIIA	Twill	205.3	1.30
OS2	Outer shell 2	100% Nomex®IIIA	Plain	252.3	1.30
MB1	Moisture barrier 1	80% Nomex®/20% Kelvar®	Water thorn felt with PTFE	109.8	1.08
MB2	Moisture barrier 2	80% Nomex®/20% Kelvar®	Water thorn felt with PTFE	163.6	1.08
TL1	Thermal liner 1	100% X-Fiper® (meta-aramid)	Needle-punched nonwoven with meta-aramid woven face cloth	196.9	1.11
TL2	Thermal liner 2	100% X-Fiper® (meta-aramid)	Needle-punched nonwoven with meta-aramid woven face cloth	275.0	1.11
TL3	Thermal liner 3	100% X-Fiper® (meta-aramid)	Two-layer needle-punched nonwoven with meta-aramid woven face cloth	428.6	1.11

PTFE: polytetrafluoroethylene

Table 2.

Structural features and physical properties of fabric systems

Assembly code	Assembly construction^a	Thickness (mm)	Mass (g/m²)	Density (kg/m³)	Air permeability (cm³/s/cm²)
S1	OS1	0.60	205.3	342.2	7.6
D1	OS1+TL1	1.73	402.2	232.5	7.3
D2	OS1+TL2	2.47	480.3	194.5	6.4
A1	OS1+MB1+TL1	2.48	512.0	206.5	0.3
A2	OS1+MB2+TL1	2.59	565.8	218.5	0.0
A3	OS2+MB1+TL1	2.53	559.0	220.9	0.2
A4	OS2+MB2+TL1	2.88	612.8	212.8	0.0
B1	OS1+MB1+TL2	3.14	590.1	187.9	0.0
B2	OS1+MB2+TL2	3.37	643.9	191.1	0.0
B3	OS2+MB1+TL2	3.24	637.1	196.6	0.0
B4	OS2+MB2+TL2	3.48	690.9	198.5	0.0
C1	OS1+MB1+TL3	5.10	743.7	145.8	0.0
C2	OS1+MB2+TL3	5.31	797.5	150.2	0.0
C3	OS2+MB1+TL3	5.24	790.7	150.9	0.0
C4	OS2+MB2+TL3	5.51	844.5	153.3	0.0

refer to Table 1 for fabric details

Test apparatus

A stored energy tester/SET (MTN-P292-08, Measurement Technology Northwest, USA), used to perform the laboratory simulation of low-level radiant heat exposure, was employed in this study. The apparatus mainly consisted of a heating source, data collection sensor, specimen holder, transfer tray, and a compressor, as illustrated in Figure 1. Heat exposure was delivered by a black ceramic thermal flux source as specified in ASTM F2731-11.¹⁹ The ceramic heating source was positioned vertically and electronically regulated to deliver the specified thermal flux. According to the researches by Song et al.⁸ and Mell,²¹ the radiant exposures were chosen to simulate three levels: 2.5 kW/m², 6.3 kW/m² and 8.5 kW/m², representing the most common fire ground conditions.²² The heat exposure of 8.5 kW/m² was especially designed based on ASTM F2731-11. Specimens were sandwiched unrestrained between the upper and lower mounting plates of a specimen holder in a vertical position, facing to the heating source. A water-cooled Schmidt–Boelter thermopile type thermal sensor was positioned at the back of the specimen to record the heat flux through the fabric. The rise in heat flux of the water-cooled sensor was recorded on a high speed strip chart recorder so that a resolution of 0.1 s was possible. The specimen was separated from the sensor by the upper mounting plate of the specimen holder, creating a 6.4 mm separation to simulate the air gap entrapped between the protective clothing and the skin.¹⁹ Alternatively, samples were tested in a direct contact configuration,^18,23 in which the specimen was placed against the sensor. A transfer tray was designed to transfer the combined specimen and sensor between the heating source and the compressor.

Figure 1.

Test protocol for determining the energy storage within a multilayer fabric system with an air gap.

Test protocol

Test specimens shall have dimensions of 152 mm × 152 mm and were preconditioned for at least 24 hours at a standard climatic condition of 20 ± 2 ℃ and 65 ± 5% relative humidity prior to the test. In order to determine how much thermal energy is being stored within fabric layers, a set of thermocouples should be installed at fabric locations to measure the temperature distribution through the textile assembly. Type-K thermocouples (Omega Engineering, USA; accuracy: ± 0.5 ℃) with a wire diameter of 0.127 mm were used. Thermocouples locations and their attachment to protective fabrics were respectively determined according to Keiser's study²⁴ and the research of NISTIR 6400.²⁵ For the single layer fabric, thermocouples were placed in the center of the outer face and backside of the fabric. For the double layer and multilayer fabric system, thermocouples were placed between each of the layers. And another two thermocouples were placed onto the outer face of the outer shell and the backside of the thermal liner. The location of the thermocouples for a multilayer fabric system is shown in Figure 1.

After the preparation of the fabric system with thermocouples, the sample was exposed to the heating source for 600 s. At the end of exposure, the sample was removed from the heating source with the immediate activation of the transfer tray, and was cooled naturally without radiation for the remaining 600 s. The test protocol is shown schematically in Figure 1. The temperature data of thermocouples and the heat flux readings from the sensor were recorded throughout the test. A total of three replicates of each fabric system were tested.

Calculation of energy storage

During exposure, energy storage in clothing should be the ‘sensible’ heat, which occurs due to the temperature change of the clothing. This study referred to the heat supply engineering^26,27 for calculating the sensible energy storage of a medium, as shown in Equation (1). There, the fabric system with N layers is discretized into a number of layer elements, and the amount of energy storage is calculated as the sum of the energy storage in each layer element.

Q_{st} (t) = \sum_{i = 1}^{N} \sum_{j = 1}^{n} {ρ_{i}}_{, j} {c_{i,}}_{j} Δ {x_{i,}}_{j} (T_{i, j} (t) - {T_{i, j}}_{, 0})

(1)

where Q_st(t) refers to the accumulated energy storage in the fabric system during the exposure time of t in kJ/m². The subscript j refers to the element in layer i, n is the total number of elements in this layer, N is total number of layers in a fabric system, ρ is the density of the layer element in kg/m³, c is the specific heat capacity in kJ/(kg·℃),Δx is the internodal distance in m, and T(t) and T₀ refer to the temperature of the layer element at time t and its initial temperature in ℃, respectively.

Using Equation (1), it should be assumed that heat transfer in fabric layers is one-dimensional as most of the heat transfer models are in the protective clothing field.^11,21,28 In the quantification of energy storage, Equation (1) requires temperature distribution along the thickness of each fabric layer. However, for experimental analysis temperatures on the inner and outer surfaces of the fabric layers were measured only. Therefore simplified method was implemented to obtain the approximate temperature distribution of each fabric layer. Except for the unsteady-state heat transfer, quasi-stable heat transfer in fabric systems should be observed for such a long thermal exposure of 600 s.²⁹ In the process of quasi-stable heat transfer, temperature gradient within a layer does not vary with location, indicating that the temperature in a specific fabric layer is distributed linearly. Conversely, in the process of unsteady heat transfer temperature gradient within a layer does small changes with location.^21,28 For ease of calculation, the temperature gradient is approximated to be invariant with respect to the location of a material throughout the test. On the basis of this approximation, Equation (1) can be transferred into Equation (2) as:

Q_{st} (t) = \sum_{i = 1}^{N} ρ_{i} c_{i} L_{i} \frac{Δ T_{1 i} (t) + Δ T_{2 i} (t)}{2}

(2)

where L refers to the fabric layer thickness in m, and ΔT₁(t) and ΔT₂(t) are the temperature fluctuations in duration of t on the inner and outer surfaces of a fabric layer in K, respectively.

Q_st is defined to determine the amount of energy storage, while q_st is used to determine the energy storage rate, as shown in Equation (3):

q_{st} (t) = \sum_{i = 1}^{N} ρ_{i} c_{i} L_{i} \frac{Δ T_{1 i} (t) + Δ T_{2 i} (t)}{2 Δ t}

(3)

where Δt refers to the sampling time interval in s.

Equations (2) and (3) can be used to calculate the energy storage either within the entire fabric system or within an individual fabric layer. The basic physical parameters of fabrics are displayed in Table 1 and Table 2. Fabric specific heat capacity was measured by using the differential scanning calorimeter according to ASTM E1269-11.³⁰ The specific heat capacity is taken to be a linear function of temperature only,³¹ shown in Equation (4) as:

c (T) = c_{0} + k (T - T_{0})

(4)

where c(T) refers to the specific heat capacity at the temperature of T, c₀ is the specific heat capacity at constant temperature, and k is the linear coefficient with temperature. The specific heat of the fabric at 20 ℃ was shown in Table 1. And the coefficients of k for the outer shell, the moisture barrier and the thermal liner are 0.00135, 0.00068, and 0.00214, respectively.

Results and discussion

Energy storage process

The energy storage rate (q_st) and accumulated energy storage (Q_st) of the fabric system B1 with an air gap exposed to 8.5 kW/m² are taken as examples to illustrate the energy storage process, as shown in Figure 2. It can be seen that the energy storage in fabric layers is highly dependent on time. In general, each fabric layer is storing the energy when exposed to the heat (≤600 s), whereas it is releasing the heat after exposure (>600 s). For the entire fabric system, energy storage process can be divided into three phases.

(1) Growth phase. This phase occurs in the initial stage of thermal exposure with the duration of 150 s or so. This phase is characterized by a positive q_st and a continuously increasing Q_st. It can be seen that q_st is relatively high when the exposure starts to activate, and tends to decline as the exposure goes on. During exposure, a portion of thermal energy from the heater is stored inside the fabric, raising the fabric temperature. And therefore, the temperature gradient developed between the fabric and the ambient decreases, reducing the energy storing rate of the fabric.²⁸ Note that there are some fluctuations observed in q_st during this stage. This is because fabric layers may contain tiny amounts of water during preconditioning, which would be later subjected to evaporation and take the heat when exposed to high temperatures.

(2) Steady-state phase. The second phase, indicative of zero in q_st and a constant value in Q_st, lasts for a relatively long time, specifically for the next 450 s of exposure. In this stage, the fabric system could no longer store the heat due to the achievement of equilibrium in heat transfer.²⁹ The occurrence of this phase demonstrates that the energy storage capability of a fabric system is limited for a fixed heat exposure.

(3). Decline phase. The third phase is observed after the exposure, during which q_st inverts to the −y axis and Q_st decreases greatly. During this phase, the fabric system starts to release the energy storage to the environment, indicating the occurrence of fabric cooling. As the temperature of the fabric drops, there is a decline in the temperature gradient developed between the fabric system and the ambience, decreasing the energy discharge rate.

Figure 2.

Energy storage process of the system B1 with an air gap when subjected to a radiative heat flux of 8.5 kW/m²: (a) energy storage rate; (b) accumulated energy storage.

As can be clearly seen from Figure 2(b), the accumulated energy storage of the fabric system researches the highest value during the steady-state phase. This value reflects the final level of energy storage within the fabric system when exposed to a fixed thermal exposure, and it can be regarded as an indicator of energy storage performance of the fabric system. In the following texts, values observed in this steady-state phase are used to discuss the influencing factors on energy storage.

Effect of fabric layers on energy storage

Energy storages within fabric systems with or without air gaps under three radiant heat exposures are described in Table 3. It can be seen that the test results have a high repeatability: for all 15 fabric systems, the standard deviation of repeated measurements ranges 0.01–6.7 kJ/m², so that the ratio of the standard deviation to the mean value never exceeds 4%. In the condition of 8.5 kW/m², the single layer S1 with an air gap stores 60.7 kJ/m² of heat, which represents only 40–48% and 25–40% of that within a double layer and a multilayer fabric system, respectively. It demonstrates that the single layer or the thin fabric system stores less thermal energy than multilayer and thick fabric systems. Figure 3 and Figure 4 respectively show the effects of fabric system's weight and thickness on energy storage. Regardless of exposure intensity, strongly positive linear relationships are observed between the energy storage and the fabric weight with R² > 0.98, and between the energy storage and the fabric thickness with R² > 0.87. It demonstrates that both the weight and thickness of the fabric system are the dominant factors affecting energy storage, which are somehow consistent with the results obtained from the heat supply engineering.²⁶ The sensible energy storage performance of a building wall that is defined as a thermal mass is highly dependent on the weight and thickness of the wall.³²

Table 3.

Energy storage within fabric systems

Fabric system^a	Energy storage(kJ/m²) (SD)
	8.5 kW/m²		6.3 kW/m²		2.5 kW/m²
	0 mm	6.4 mm	0 mm	6.4 mm	0 mm	6.4 mm
S1	45.1 (1.6)	60.7 (0.6)	33.8 (0.7)	44.1 (1.1)	18.1 (0.7)	23.4 (0.8)
D1	109.2 (3.5)	126.2 (2.6)	83.5 (2.9)	93.7 (2.3)	41.2 (1.4)	45.8 (1.3)
D2	123.6 (3.4)	151.9 (3.2)	96.7 (3.2)	115.7 (2.2)	47.2 (1.7)	56.1 (1.8)
A1	126.9 (4.7)	150.2 (3.2)	102.8 (4.0)	119.0 (3.1)	49.4 (1.9)	55.9 (1.7)
A2	136.3 (2.2)	164.4 (5.2)	107.0 (2.6)	125.7 (2.4)	52.0 (0.8)	60.7 (1.9)
A3	140.0 (1.2)	167.9 (6.0)	111.0 (2.4)	129.9 (1.2)	53.6 (0.1)	62.4 (1.7)
A4	150.4 (2.6)	180.1 (5.1)	119.9 (1.0)	140.2 (3.2)	57.2 (0.5)	67.9 (2.0)
B1	146.4 (4.3)	172.0 (2.6)	118.1 (3.8)	138.8 (1.7)	56.6 (1.4)	64.8 (2.1)
B2	160.8 (5.5)	187.0 (3.1)	121.2 (4.3)	137.9 (3.1)	59.0 (2.0)	65.8 (2.0)
B3	167.5 (5.8)	189.5 (2.2)	132.5 (4.8)	143.0 (3.3)	61.9 (1.8)	71.3 (2.2)
B4	179.9 (4.8)	204.8 (4.1)	139.3 (3.9)	154.2 (2.2)	67.6 (1.1)	74.3 (2.0)
C1	194.6 (6.7)	216.0 (6.0)	152.6 (4.2)	162.4 (0.9)	74.1 (1.4)	78.1 (2.9)
C2	205.5 (0.6)	228.8 (3.9)	158.1 (0.1)	176.2 (1.5)	78.4 (2.2)	84.1 (1.5)
C3	208.9 (1.2)	231.8 (5.7)	158.9 (5.1)	173.9 (1.8)	78.2 (2.3)	83.6 (2.9)
C4	224.5 (6.3)	245.7 (2.6)	170.3 (2.9)	184.1 (2.3)	87.5 (2.4)	88.5 (3.5)

refer to Table 2 for fabric details

SD: standard deviation.

Figure 3.

Effect of fabric weight on energy storage in fabric systems with air gaps.

Figure 4.

Effect of fabric thickness on energy storage in fabric systems with air gaps.

The method to investigate the energy storage has proved practicable to determine the energy storage within an individual fabric layer. The distributions of energy storage within different layers of the fabric system with air gaps exposed to 8.5 kW/m² radiant heat are described in Figure 5. Every fabric layer in the single, double or the multilayer fabric system can store the heat; however, the energy storage varies among layers. The energy storage distribution is more complicated for the multilayer fabric system. The outer shell there generally stores the most heat, which corresponds to 36–57% of the total amount of energy storage. The reason for this is that the outer shell is settled as the first layer facing to the heating source directly and has the character of high specific heat capacity. The outer shell with this high energy storage has the potential to release more heat after exposure. And therefore, it is suggested that for burn prevention, human skin should avoid being in immediate contact with the surface of the outer shell after exposure. The thermal liner, although is close to the sensor, still stores 23–47% of the total energy storage. This is because the thermal liner used in this study mostly has higher fabric weight compared with the outer shell or the thermal liner. Note that the energy storage within the thermal liner would be more likely to release to human skin after exposure because this layer is located next to the skin. The moisture barrier, however, only stores a small percentage (14–26%) of energy storage due to the small weight of this layer.

Figure 5.

Distributions of energy storage in different layers of the fabric system with air gaps exposed to 8.5 kW/m².

In this study, the two outer shells shown in Table 1 differ only by fabric weight, and so do the two moisture barriers and three thermal liners. The fabric systems constructed by theses fabric layers should mainly differ by the weight of the fabric system. Therefore, to further discuss the effect of fabric layers the energy storage in the outer shell (OS1 and OS2), in the moisture barrier (MB1 and MB2) and in the thermal liner (TL1, TL2 and TL3) that vary as a function of fabric system's weight under 8.5 kW/m² thermal exposure are separately summarized in Figure 6. In Figure 6(a), the energy storage within the same outer shell fabric goes up linearly with the increase of fabric system's weight. The energy storage within OS1 observed for the single layer fabric system S1 is 60.7 ± 0.6 kJ/m², and it increases by 36% for the multilayer fabric system C2 (82.4 ± 1.2 kJ/m²). The increased weight of the fabric systems with the same outer shell can only result from the weight improvement of the moisture barrier and the thermal liner, which provides more thermal resistance of fabric layers closer to the sensor^4,8 and increases more thermal energy stored within the outer shell. However, the overall trends of energy storage exhibited by the thermal liner are quite opposite to the outer shell's, and they decrease as the weight of the fabric system increases (Figure 6(c)). The energy storage within TL1 obtained for the fabric system D1 is 53.7 ± 1.1 kJ/m², but it reduces by 22% for a heavier fabric system A4 (41.7 ± 0.6 kJ/m²). For a fixed thermal liner, the increase of fabric system weight should be due only to the additional weight on the outer shell and the moisture barrier. This would result in an improvement on the thermal insulation of fabric layers within reach of the heating source. Therefore, the temperature of the thermal liner is lowered and the energy storage within it is reduced. For the moisture barrier, the variation of energy storage becomes more complicated as the fabric system's weight changes. It depends on the location of the added weight on fabric layers. For instance, when the fabric system changes from A2 to A4, the energy storage within MB2 is declined by 4% because the fabric weight is merely added on the outer shell. However, when the fabric system changes from B2 to C2, the energy storage within MB2 is increased by 5% owing to the improved weight on the thermal liner.

Figure 6.

The energy storage in the outer shell (Figure 3(a)), in the moisture barrier (Figure 3(b)) and in the thermal liner (Figure 3(b)) that vary as a function of fabric system's weight under 8.5 kW/m² thermal exposure. (a) Outer shell, (b) Moisture barrier, (c) Thermal liner.

The above analyses demonstrate that the energy storage in an individual fabric is highly dependent on the material properties of this particular layer (e.g. fabric weight, thickness and specific heat capacity), and properties of the neighboring fabric layers. For a multilayer fabric system, improving the thermal insulation of the thermal liner that is closest to human skin, such as by increasing this layer's weight can lead to not only the increase of energy storage within this layer but also the increases within the outer fabric layers. However, if the thermal insulation is added on the outer shell or the moisture barrier only, energy storage within the thermal liner is reduced.

Effect of air gap on energy storage

To quantitatively analyze the effect of air gap, an index named the changing rate on energy storage (θ) is introduced. The calculation is shown in Equation (5) as:

θ = \frac{Q_{1} - Q_{0}}{Q_{0}} \times 100 %

(5)

where Q₁ and Q₀ refer to the energy storage of the fabric system with and without an air gap, respectively.

The energy storage changing rates for different fabric layers and fabric systems under 8.5 kW/m² thermal exposure are displayed in Figure 7. All θ are positive, indicating that the air gap that exists between the fabric system and the sensor can improve the energy storage within each fabric layer. Many previous studies have placed emphasis on the positive effect of the air gap on burn injury preventions,^28,33,34 because air is a good insulator that can provide more resistance to heat transfer. From the viewpoint of energy conservation, this study reveals that the positive effect of the air gap can be explained as the air gap increases the energy storage within fabric layers and therefore decreases the heat transmission to the skin. The observed θ for the thermal liner in a multilayer fabric system ranges 23.7–54.5%, which is obviously higher than those for the moisture barrier (2.6–18.6%) and the outer shell (0.8–11.2%). This higher θ demonstrates that the introduced air gap mainly leads to the increase of energy storage within the thermal liner but has less impact on the energy storage within the outer shell as well as within the moisture barrier. Energy storage essentially reflects fabric temperature fluctuations as Equation (1) shows. Therefore, it can be inferred that the introduced air gap causes temperature rises in each fabric layer, especially in the thermal liner (i.e. owing to the introduced air gap the temperature of the thermal liner for fabric system B1 increases from 148.8 ℃ to 183.0 ℃ after exposure to 8.5 kW/m², while the temperatures of the moisture barrier and the outer shell increase from 226.7 ℃ to 244.9 ℃, and from 252.6 ℃ to 265.3 ℃, respectively). The thermal liner has an average temperature elevation of 34.2 ℃, which is much higher than the moisture barrier (18.2 ℃) and the outer shell (12.7 ℃).

Figure 7.

The energy storage changing rates for different fabric layers and fabric systems exposed to 8.5 kW/m².

Effect of thermal resistance on energy storage

The thermal resistance of the fabric system is usually measured by a guarded hot plate according to ASTM F1868-11.³⁵ This conventional method is performed in the normal environment with the air temperature in the range 20–35 ℃. However, this study was conducted under radiant heat exposures. The practical thermal resistance in high-temperature environments are different from the values tested in normal environments.^29,36 Therefore, the practical thermal resistance of the fabric system should be determined first in this section.

The total thermal resistance (R_T, ℃·m²/W) indicates the thermal insulation from the skin surface to the environment (including all clothing layers, and the boundary air layer), which is calculated as the sum of the following resistances:²⁶

R_{T} = \frac{1}{h_{1}} + \sum_{i}^{N} R_{cli} + \frac{1}{h_{2}}

(6)

where R_cli refers to the thermal resistance of the fabric layer i in ℃·m²/W. h₁ refers to the combined radiation and convection coefficient at the outer clothing surface in W/(m²·℃), and h₂ refers to the inner clothing surface's coefficient of heat transfer in W/(m²·℃).

The thermal resistance of the fabric system (R_cl, ℃·m²/W) indicates the thermal insulation from the inner clothing surface to the outer clothing surface. It should be noted that R_cl only includes the thermal insulation of the fabrics, but contains no information about the thermal insulation of the boundary air layer as shown in Equation (7):

R_{cl} = \sum_{i}^{N} {R_{cl}}_{i} = \frac{T_{out} - T_{in}}{q}

(7)

where T_out and T_in are the outside surface temperature of the outmost fabric layer and the backside surface temperature of the innermost fabric layer in ℃, respectively. q refers to the heat loss of the skin in W/m² during the quasi-stable heat transfer process.

The results of thermal resistances of clothing systems (R_cl) with and without air gaps under 8.5 kW/m² thermal exposure are compared in Figure 8. In configurations of no air gaps, thermal resistances of the single layer, double layer and multilayer fabric systems are 0.48, 4.0–5.2, 6.2–11.7 ( × 10⁻²℃·m²/W), respectively. As air gaps are introduced, their thermal resistances are decreasing to 0.24, 3.8–4.4, 5.7–11.3 ( × 10⁻²℃·m²/W), respectively. Song et al.⁸ have measured the thermal resistances of fabric systems with different layers by using the guarded hot plate, giving the values of 1.2, 7.5, 17.5–28.0 ( × 10⁻²℃·m²/W) to the single layer, double layer and multilayer fabric system, respectively. The guarded hot plate experiment is normally performed to measure the heat resistance of the fabric with no air gaps under clothing. It can be concluded that the thermal resistances in normal environments are usually about 1.5–4.5 times of those under thermal radiations. This is primarily due to the fact that the heat transfer coefficient, reflecting the heat radiation and conduction/convection within the fabric, increases with the rising of the temperature.³⁷ And the thermal resistance decreases with the rising of the heat transfer coefficient. The phenomenon that the thermal resistance of the fabric system with an air gap is smaller than that of the fabric system without an air gap should be attributed to its high fabric temperature. However, the preceding texts has shown that the fabric system with an air gap stores more thermal energy during exposure, which seems to indicate that the energy storage is not in direct proportion to the own thermal resistance of the fabric system.

Figure 8.

Comparison of thermal resistance of fabric systems with and without air gaps under exposure of 8.5 kW/m².

The total thermal resistance (R_T), however, contains the thermal boundary resistances, reflecting the total degree of thermal insulation from the body surface to the ambience. The effect of total thermal resistance on energy storage is summarized in Figure 9. A positive linear relationship is observed between them with an adjusted R² value of 0.967. Contrary to the situation observed in R_cl, R_T of the fabric system with an air gap (red dots in Figure 9) is greater than that of the fabric system without an air gap (blue square dots). This is primarily caused by the improved thermal resistance due to the introduction of enclosed air layer under clothing. For the no air gap configuration, heat is directly transferred from the fabric system to the sensor through heat conduction, resulting in a high coefficient of heat transfer.²⁸ But for the air gap configuration, thermal energy is transferred by radiation and conduction/convection across the air gap. This study demonstrates that the thermal energy storage is increased as the total thermal resistance increases. R_cl can be also calculated from the thickness of fabric and its thermal conductivity by Equation (8), and thus the increase of thermal energy storage can be achieved by decreasing the boundary layers' heat transfer coefficients and the thermal conductivity of fabric or by increasing the thickness of the fabric.

R_{cl} = \frac{L}{k}

(8)

where k and L are the coefficient of thermal conductivity and the thickness of the fabric layer, respectively.

Figure 9.

Effect of total thermal resistance on energy storage of fabric systems under thermal exposure of 8.5 kW/m².

Effect of heat source intensity on energy storage

The results of energy storage under different levels of radiant heat exposures are shown in Table 3. It can be seen that the energy storage is decreasing with the increase of the thermal radiation from 2.5 to 8.5 kW/m² regardless of the air gap. This should be attributed to the fact that the fabric temperature is rising as the increase of external radiant fluxes.³⁶ Previous studies have reported that fabric temperature has a linear relationship with the intensity of low-level thermal radiation.²⁹ Therefore, one might reasonably assume as a linear regression with the effect of heat source intensity on energy storage, as follows in Equation (9):

Q_{st} = a \times HL + b

(9)

where HL refers to the thermal flux generated by the heating source in kW/m², and a and b are the coefficients of the slope and the intercept for linear regression analyses, respectively.

The linear correlation between Q_st and HL is summarized in Table 4. According to the coefficients of the slope, it is clear that the energy storage within the multilayer fabric system climbs much higher compared with that within the single layer or double layer fabric systems. Besides, compared with no air gap configurations, the energy storage in fabric systems with air gaps would rise more while increasing the thermal flux.

Table 4.

Linear fitting results for the relationships between thermal flux and energy storage within fabric systems

Fabric system^a	without air gap (0 mm)				with air gap(6.4 mm)
Fabric system^a	a	b	R	Adj.R²	a	b	R	Adj.R²
S1	4.46	6.63	0.996	0.985	6.14	7.34	0.998	0.994
D1	11.30	12.78	0.999	0.996	13.31	11.84	0.991	0.985
D2	12.77	15.50	0.993	0.990	15.94	15.97	0.999	0.995
A1	13.05	17.83	0.982	0.968	15.81	17.18	0.998	0.992
A2	14.10	17.13	0.998	0.994	17.26	17.40	0.998	0.991
A3	14.48	18.06	0.989	0.981	17.60	18.55	0.994	0.976
A4	15.65	18.94	0.973	0.971	18.75	21.29	0.981	0.976
B1	15.10	19.92	0.98	0.982	18.05	21.14	0.998	0.993
B2	16.89	16.26	0.969	0.962	20.07	14.49	0.966	0.956
B3	17.71	18.59	0.995	0.991	19.61	21.51	0.999	0.968
B4	18.72	20.97	0.997	0.989	21.67	19.47	0.989	0.931
C1	20.15	24.25	0.987	0.981	22.89	20.18	0.998	0.991
C2	21.16	25.33	0.995	0.99	24.13	23.91	0.989	0.981
C3	21.72	23.40	0.996	0.994	24.59	21.33	0.984	0.974
C4	22.73	29.68	0.990	0.984	26.09	22.28	0.995	0.992

refer to Table 2 for fabric details

Conclusions

This paper reported an experimental investigation on energy storage within different layers of fabric systems used for thermal protective clothing. The temperature distribution through the fabric system was measured and used to determine the energy storage within fabric layers. The results show that the energy storage process of a fabric system involves three phases, including the growth phase, the steady-state phase, and the decline phase. With the increase of fabric weight or thickness, the amount of energy storage within the fabric system will increase linearly. The energy storage in an individual fabric is not only dependent on material properties of that particular layer, but mainly on properties of the neighboring layers. The improvement of thermal insulation for the thermal liner in a multilayer fabric system, such as by increasing the weight of this layer, increases the energy storage within all fabric layers. However, if the thermal insulation is added on the outer shell or the moisture barrier only, it will reduce the energy storage in the thermal liner. The introduction of an air gap under clothing improves the energy storage within all fabric layers, especially within the thermal liner that locates closer to the skin. Energy storage in a fabric system is not in direct proportion to the thermal resistance of the fabric system, but shows a direct linear correlation to the total thermal resistance. In addition, a well linear relationship is observed between the intensity of the heat source and the energy storage in the fabric system.

Owing to the dual effect of protective clothing, the improvement of clothing should be achieved by considering both the performance during and after exposure. However, it should be noted that the energy storage within protective clothing can be discharged after exposure, and the energy discharge to the skin is expected to be as little as possible. So far, however, there has been little discussion about the relationship between the energy storage and the energy discharge to the skin. The results obtained here are available to be used in further studies to examine whether higher energy storage within the fabric causes greater energy discharge to the skin after exposure.³⁸ These results could be brought to research institutions to develop new fabric combinations in order to minimize the heat transmission to the skin. With all this knowledge, more efforts should be made also to investigate how the energy storage as well as the energy discharge of a fabric influences the time to different levels of burn injury.

Footnotes

Acknowledgements

This research was conducted at the Protective Clothing Research Center, College of Fashion and Design, Donghua University, China.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Nature Science Foundation (Grant NO. 51576038), the Fundamental Research Funds for the Central Universities (Grant NO. 17D110714), Shanghai Municipal Natural Science Foundation (Grant NO. 17ZR1400500), and Natural Science Research Project for Colleges and Universities in Jiangsu Province (Grant NO. 17KJB540003).

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