Three-dimensional heat and moisture transfer analysis for thermal protection of firefighters’ gloves with phase change materials

Abstract

Transient three-dimensional (3D) heat and moisture transfer simulations were conducted to analyze the thermal performances of the entire phase change material (PCM) integrated firefighters’ gloves. PCM was broken down into several segments to cover the back and palm of the hand but to avoid finger joints to keep hand functions. Parametric studies were performed to explore the effects of PCM melting temperatures, PCM locations in the glove, and PCM layer thicknesses on the overall thermal performance improvement of firefighters’ gloves. The study found that PCM segments could extend the time for hand skin surfaces (areas covered or not covered by PCM) to reach second-degree burn injury (60°C) by 1.5–2 times compared to conventional firefighters’ gloves without PCM. Moreover, PCM segments could help mitigate the temperature increase on hand skin and glove surface after fire exposure.

1. Introduction

Firefighters are exposed to a wide range of thermal conditions, including flashover conditions in which the air temperature may reach 300°C-1000°C with a thermal radiation flux of 15–120 kW/m², and hazardous conditions that may have air temperatures up to 250°C with a thermal radiation flux of 1–10 kW/m² [1]. In addition, the glove is one of the thinnest/weakest component of firefighter turnout gear compared to jackets, suits, and pants due to the need for hand dexterity and hand grip strength to complete tasks. Typical firefighters’ gloves have four layers: the inner thermal lining, a flame-retardant thermal barrier, a moisture barrier, and a fire-proof leather outer shell [2]. The current structural firefighters’ gloves must satisfy the minimum requirement of a thermal protective performance (TPP) rating of 35, equating to 17.5 seconds until second-degree burns occur in a flashover situation, to comply with NFPA 1971 (Standard on Protective Ensembles for Structural Fire Fighting and Proximity Fire Fighting) [3]. However, exposure to a high-temperature environment could be much longer than a few seconds when firefighters are conducting rescue tasks at a fire scene. In 2021, around 32% of firefighter injuries in U.S. were from fireground (i.e., 19,200 injuries) [4]. Thermal burn and thermal stress were among the major fireground injuries, which accounted for approximately 10% of total fireground injuries [4]. Hence, to maintain protection for much longer exposures to extreme heat without causing burn injuries, it is critical to enhance the thermal protection performance of these gloves.

Firefighters need gloves that meet heat resistance criteria while still fitting correctly and enabling dexterity. The current advanced textile technology, such as special leather materials, elastomeric pillars on fire-resistant fabrics, nanocomposite technology, etc. [5–8], can improve the material insulation performance. However, the improvement is purely based on the delay of conductive heat transfer. The use of phase change material (PCM) can help mitigate the limitation from conduction and significantly enhance the insulation performance of gloves. PCM needs to absorb large amount of latent heat of fusion to change the phase (e.g., melt), while it maintains at a constant temperature. Applying this phenomenon of PCM to extreme heat environment, it can absorb the heat for melting, and therefore, significantly reduce the thermal transport rate to hand skin for efficient temperature control and thermal protection. The PCM can utilize its large latent heat to achieve thermoregulation. Additionally, adding a thin PCM layer to firefighters’ gloves allows for smooth hand movements without adding bulk.

Some researchers have investigated microencapsulated PCMs or PCM packs for thermo-regulation in everyday clothing or protective garments for thermal control [9,10]. Microencapsulated PCM is PCM housed in capsules ranging in size from nano- to micro-scale, while a PCM pack is PCM in a macro-scale package. Pause (2003) explored a new approach to incorporate PCM into nonwoven protective garments [11]. The PCM was directly incorporated into a polymer film, then laminated to a nonwoven fabric system [11]. It was found that using PCM can delay the skin temperature increase, reducing the moisture build-up in the microclimate, which improves the thermo-physiological wearing comfort of nonwoven protective garments [11]. Lu et al. [12] developed a novel personal cooling system by incorporating PCM packs and ventilation fans in clothes. These studies found that incorporating PCMs with melting points in the range of 15°C-35°C in garments could help improve human thermal comfort when wearing protective garments and in daily life [9–12].

Unlike everyday clothing and protective garments, firefighters’ personal protective equipment (PPE) is exposed to extremely high-temperature conditions. Hence, it is critical to enhance the thermal insulation performance of PPE to protect firefighters. Some researchers have experimentally investigated thermal protection by integrating microencapsulated PCMs or PCM layers in firefighters’ protective clothing (FFPC) for thermal protection under harsh conditions. It was found that using microencapsulated PCMs could reduce the temperature rise behind the innermost layer by around 5°C-8.5°C and increase the thermal protection of FFPC samples by approximately 40% under the radiant heat exposure at 5–40 kW/m² [13,14]. The PCM with a melting point range of 47°C-53°C had the highest heat protection index under such heat flux exposure [15]. Moreover, the effect of PCM was more pronounced when the PCM (melting point of 50°C) coated layer was placed in the inner layer of firefighters’ protective clothing [13]. However, it was found that the PCM did not perform effectively once the environment reached flashover conditions (air temperature of 300°C-1000°C) [14,16]. Further parametric studies are required for PCM to improve its performance under extreme heat conditions.

For the above reasons, numerical simulation work becomes critical to provide time and cost-effective systematic investigations. Several numerical studies related to using PCM for thermal protection in the FFPC ensemble have been reported. For example, Hu et al. [17], Fonseca et al. [18,19], and Zhang et al. [20] have performed 1D simulations for parametric studies on the effects of PCM mass, position, melting temperature, and latent heat on the firefighter clothing thermal performances under various heat intensity conditions (from 5 to 84 kW/m²; low heat intensity to flashover condition). Moreover, Fonseca et al. [19] have also modeled the thermal transport performance of firefighter clothing after fire exposure. The heat stored in PCM can extend the time that a firefighter is protected in the fire scene before a second-degree burn occurs. However, the accumulated energy in PCM will be eventually released towards the environment and the firefighter’s body after coming out of the fire scene, making the post-fire exposure behavior of PCMs critical. Few researchers have conducted preliminary studies on PCMs for their post-fire exposure behaviors [19].

In an earlier study, the authors conducted 1D numerical simulations to investigate the improvement of firefighters’ gloves thermal protective performance when incorporated with a thin PCM layer [21]. This study found that (a) the optimum melting point of PCM was in the range of 80°C-140°C for the hand’s thermal protection, (b) the location of the PCM layer should be close to the inner glove’s surface for high-heat situations, and (c) the addition of a 0.5–1.0-mm PCM layer could increase the time for the skin to reach second-degree burn temperature by 2–4 times when compared to the conventional firefighters’ gloves without PCM [21].

To better protect firefighters from burn injuries and thermal stress at a fire scene, the authors propose incorporating PCM into the glove to break the ceiling of the thermal protective performance of current commercial firefighters’ gloves. Unlike firefighting clothing, a glove is small in volume and asymmetric in geometry, so the actual thermal performance in a 3D glove would be different from the 1D simulation results. Currently, there are no studies on the entire glove system to explore how PCM can be incorporated in an actual glove. 3D models are not available that accurately simulate the thermal performance of the entire glove and hand system. Therefore, this work is the first 3D numerical simulation to explore (1) the effects of different variables (i.e., PCM melting point, position, and thickness) on the thermal insulation performance of the entire firefighters’ gloves, and (2) the heat and moisture (including sweat and spray water) transport phenomena in the whole glove on the hand under flashover and hazardous conditions. The numerical modeling and simulations presented here will guide future experimental design and testing.

2. Mathematical modeling and numerical solutions

2.1. Model design – hand model and glove composition

The 3D heat and moisture transfer simulations were conducted using COMSOL Multiphysics software (COMSOL, Inc., Burlington, MA 01803, USA). The 3D model geometry, as shown in Figure 1, involved a human skin portion and a glove structure. Human skin includes subcutis, dermis, and epidermis, and a typical structural firefighter glove comprises a base layer (inner thermal lining), thermal barrier, moisture barrier, and outer shell. The PCM can be capsulated and integrated between these layers in the glove structure. In this study, PCM was placed between the base layer and thermal barrier (above base layer: ABL) and between the moisture barrier and outer shell (beneath outer shell: BOS) to explore the effect of PCM location on glove thermal performance. Considering hand dexterity, one piece of PCM to cover the entire glove would not be appropriate. Hence, PCM was broken into several segments to cover the back and palm of the hand while avoiding the finger joints to maintain hand function, as shown in Figure 1c. Commercial bio-based PCMs (fatty acids) were used in the simulations due to their non-toxic nature [22]. Three different melting points of PCMs were studied, including melting points of 70°C, 100°C, and 150°C. The melting temperature selection was based on the optimum melting temperature range identified from the authors’ previous 1D simulation work [21].

Figure 1.

Geometry of the 3D glove-hand model: (a) Overall view of the 3D glove-hand model; (b-1) and (b-2) each glove and skin layer in the model (b-1: PCM located ABL in glove, b-2: PCM located BOS in glove); (c) the positions of PCM segments in the glove (blue parts indicate PCM segments). Air gaps existed between each glove layer as well as between hand and glove in the simulation model.

2.2. Heat transfer simulation

Heat transfer simulation explored the thermal protection improvement of firefighters’ gloves by applying PCM and the amount of time it took for hand skin surface to reach skin second-degree burn injury (~60°C [1]) when exposing in the fire scene. The model involved bioheat transfer physics. The 3D heat diffusion (energy) equation with blood circulation effect, as expressed in Equation (1), was applied to simulate the thermal transport in glove structure as well as hand skin [21,23,24]:

$${\rho \text{C}}_{\text{p}}\frac{\partial \text{T}}{\partial \text{t}}=\text{k}{\nabla }^2\text{T}+{\dot{\text{Q}}}_{\text{bio}}$$ (1)

where ρ is density, C_p is specific heat, k is thermal conductivity, T is temperature, t is time, and ${\dot{\text{Q}}}_{\text{bio}}$ is the bioheat source term indicating heat transfer by blood circulation (which was only used in the dermis and subcutaneous layers of the skin, set at zero in the epidermis layer and glove structure). The bioheat source term can be expressed as [21,23,24]:

$${\dot{\text{Q}}}_{\text{bio}}={\rho }_{\text{b}}{\text{C}}_{\text{p},\text{b}}{\omega }_{\text{b}}\left({\text{T}}_{\text{b}}-\text{T}\right)+{\dot{\text{Q}}}_{\text{met}}$$ (2)

Here, ρ_b is density of blood, C_p,b is specific heat of blood, ω_b is the rate of blood perfusion in skin, T_b is blood temperature, and ${\dot{\text{Q}}}_{\text{met}}$ is metabolic heat source [21,23,24].

For PCM, the equivalent heat capacity method was adopted for the phase change simulations in COMSOL. Latent heat of fusion of PCM was integrated in the specific heat, as expressed in Equation (3), for the phase changing process [25]:

$${\text{C}}_{\text{p}}=\frac{1}{\rho }\left[{\rho }_{\text{s}}{\text{C}}_{\text{p},\text{s}}+\left({\rho }_{\text{l}}{\text{C}}_{\text{p},\text{l}}-{\rho }_{\text{s}}{\text{C}}_{\text{p},\text{s}}\right)\text{LF}\left(\text{T}\right)\right]+\text{LH}\frac{\partial \text{G}}{\partial \text{T}}$$ (3)

where C_p,s and C_p,l are the specific heats of solid- and liquid-state PCM, respectively; ρ_s and ρ_l are the densities of solid- and liquid-state PCM, respectively; LH is the latent heat of fusion of PCM; $\frac{\partial \text{G}}{\partial \text{T}}$ is the Gaussian function used to account for the latent heat during phase change; and LF(T) is the liquid fraction (ranging from 0 to 1) used to determine the change between solid (0) and liquid (1) phases of PCM. The natural convection in molten PCM was negligible in this study [25].

Parameters and thermal properties of human skin layers, glove materials, and PCM are displayed in Table 1. The glove layers were modelled as porous materials, while human hand skin and PCM were modelled as non-porous materials. Three different melting points (i.e., 70°C, 100°C and 150°C) of PCMs were simulated in the 3D model to explore the effect of melting points on the thermal performance of PCM-integrated firefighters’ gloves. The latent heat of fusion of PCM was assumed to be 200 kJ/kg based on the average latent heat of commercial bio-based PCMs [22].

Table 1.

Parameters and thermal properties of human skin and glove layers [22,26–31].

Layer	Thickness (mm)	Density (kg/m3)	Heat Capacity (J/kg·K)	Thermal Conductivity (W/m·K)	Porosity (unitless)
Subcutis	3.885	1109	2344	0.293	---
Dermis	1.125	1109	3773	0.582	---
Epidermis	0.075	1109	3968	0.628	---
Base layer (Insulated cotton lining)	1	134	1865	0.05	0.885
Thermal barrier (Fire retardant cotton)	1	520	1300	0.1	0.885
Moisture barrier (PTFE#)	0.1	2285	1030	0.25	0.814
Outer shell (Kevlar fiber)	1.5	567	1300	0.8	0.666
PCM layer (Bio-based fatty acids)	0.5–1	1000	2000	0.2	---

^#PTFE: Polytetrafluoroethylene

Initial conditions:

The initial temperature of the glove layers was assumed to be room temperature (25°C). The initial temperature distributions in skin layers are shown in Table 2. The subcutis is close to the body, so it was assumed to be equal to internal body temperature (37°C) [21,24]. The epidermis faces the outside environment, lowering skin temperature to 34°C [21,24]. There is always temperature gradient from body core to the skin surface even when gloves and clothes are worn. Hence, the skin surface temperature was slightly lower than that of the body core temperature [24]. The temperature distribution in hand skin is nearly linear [21,24].

Table 2.

Initial conditions of hand skin and glove layers for heat transfer simulation [21,24].

	Subcutis	Dermis	Epidermis	All Glove Layers
Temperature (°C)	37	35	34	25

Boundary conditions:

The outer surface of the gloves’ outer shell was set at a heat flux of 83 kW/m² or 8.3 kW/m² to mimic flashover or hazardous conditions, respectively [1]. The temperature at the interface between the hand subcutis and internal muscles/bones was set at a constant 37°C (the same as the internal body temperature) [21,24]. Moreover, there are air gaps between each glove layer and between the hand skin and glove. These air gaps were assumed to have 0.1-mm thickness for perfectly fitting gloves based on the surface roughness of textile fabrics [32,33].

2.3. Heat and moisture transfer simulation

The Heat Transfer in Porous Media module and Transport of Diluted Species in Porous Media module were coupled in COMSOL Multiphysics to simulate the heat and moisture transport phenomena in firefighters’ gloves. Hand sweating and external water spray from a hose would cause moisture to transfer into the glove. Therefore, this study explores the effect of moisture on the thermal performance of PCM-integrated firefighters’ gloves in the cooling period after exposure to the fire scene.

The glove was first heated/exposed under flashover conditions for 4 seconds and then cooled down until 60 seconds after the exposure (mimicking when a firefighter gets out of the fire scene) [31]. Two cooling processes were studied—i.e., air- and water-cooled processes. The air-cooled process is for the situation when the firefighter does not use the water hose. Therefore, the glove was just cooled in ambient air. Thus, only hand sweating was considered in the simulation. The water-cooled process is for the situation where a firefighter uses a water hose. Both hand sweating and external water spray from the hose were considered in the simulation.

The governing equation for 3D moisture transfer is expressed as Equation (4) [31]:

$${ϵ}_{\text{p}}\frac{\partial \text{c}}{\partial \text{t}}={\text{D}}_{\text{eff}}{\nabla }^2\text{c}$$ (4)

where c is the moisture (water vapor) concentration (mol/m³); ${ϵ}_{\text{p}}$ is the porosity of glove fabric (refer to Table 1); and ${\text{D}}_{\text{eff}}$ is the effective diffusion coefficient in porous medium, which is calculated by [30]:

$${\text{D}}_{\text{eff}}=\frac{{ϵ}_{\text{p}}}{\tau }\text{D}\left(\text{T}\right)$$ (5)

Here, $\tau$ is fabric tortuosity ($\tau ={{ϵ}_{\text{p}}}^{-\frac{1}{3}}$) and $\text{D}\left(\text{T}\right)$ is the diffusion coefficient of water vapor (moisture) in air, which is a function of temperature as [34]:

$$\text{D}\left(\text{T}\right)=1.87\times {10}^{-10}\frac{{\text{T}}^{2.072}}{{\text{P}}_{\text{atm}}}$$ (6)

where ${\text{P}}_{\text{atm}}$ is the atmosphere pressure (atm).

The heat diffusion equation is modified into Equation (7), adding the term of enthalpy of vaporization due to moisture mass transfer in the glove structure (${\text{Q}}_{\text{vap}}$):

$${\rho \text{C}}_{\text{p}}\frac{\partial \text{T}}{\partial \text{t}}=\text{k}{\nabla }^2\text{T}+{\text{Q}}_{\text{bio}}+{\text{Q}}_{\text{vap}}$$ (7)

The moisture bonded with textile fibers or in pores would evaporate and condense during the temperature change, leading to the latent heat of vaporization accompanying moisture mass transfer. Hence, ${\text{Q}}_{\text{vap}}$ was defined as [34]:

$${\text{Q}}_{\text{vap}}=-{\text{D}}_{\text{eff}}{\nabla }^2\text{c}\bullet \Delta {\text{H}}_{\text{vap}}$$ (8)

where $\Delta {\text{H}}_{\text{vap}}$ is the latent heat of vaporization of water.

Initial conditions:

The initial temperature distributions in the glove and hand skin are the same as in the heat transfer simulation (refer to Table 2). The initial moisture content in the glove structure was assumed to be 0.816 mol/m³ based on the relative humidity of 60% [31,34].

Boundary conditions:

For heat transfer, the flashover condition with a heat flux of 83 kW/m² was applied at the outer surface of the glove’s outer shell during the first 4-second heating process. Then, natural convection and radiation cooling were applied to the outer shell during the cooling process afterward.

The component of natural convection heat flux (${\text{Q}}_{\text{conv}}^{”}$) was calculated by [27]:

$${\text{Q}}_{\text{conv}}^{”}={\text{h}}_{\text{conv}}({\text{T}}_{\text{s}}-{\text{T}}_{\infty })$$ (9)

Here, T_s is glove outer surface temperature; T_∞ is environment temperature out of fire scene (25°C) [27]; and ${\text{h}}_{\text{conv}}$ is the convective heat transfer coefficient, which was evaluated based on the Nusselt number (Nu) correlation on a vertical plate surface recommended by Churchill and Chu [31]:

$$\text{Nu}={\left\{0.825+\frac{0.387{\text{Ra}}^{\frac{1}{6}}}{{\left[1+{\left(0.492/\text{Pr}\right)}^{\frac{9}{16}}\right]}^{\frac{8}{27}}}\right\}}^2$$ (10)

where Ra is the Rayleigh number ($\text{Ra}=\frac{\text{g}\beta \left({\text{T}}_{\text{s}}-{\text{T}}_{\infty }\right){\text{L}}^3}{\nu \alpha }$) and Pr is the Prandtl number. Both air- and water-cooled processes were considered for natural convection cooling.

The component of radiation heat flux (${\text{Q}}_{\text{rad}}^{”}$) was expressed as [26]:

$${\text{Q}}_{\text{rad}}^{”}={\text{h}}_{\text{rad}}({\text{T}}_{\text{s}}-{\text{T}}_{\infty })$$ (11)

where ${\text{h}}_{\text{rad}}$ is the radiation heat transfer coefficient—i.e., ${\text{h}}_{\text{rad}}=\epsilon \sigma ({\text{T}}_{\text{s}}^2+{\text{T}}_{\infty }^2)({\text{T}}_{\text{s}}+{\text{T}}_{\infty })$ [27].

Table 3 displays the properties of air and water and other parameters to calculate the natural convection and radiation cooling boundary conditions using Equations (9)–(11).

Table 3.

Parameters to calculate natural convection and radiation heat transfer coefficients [26,31].

	Air (at 750 K)	Water (at 300 K)
Kinematic viscosity (ν: m 2 /s)	7.637×10−5	8.58×10−7
Thermal diffusivity (α: m 2 /s)	1.09×10−4	1.47×10−7
Thermal expansion coefficient (β: K−1)	1.33×10−3	2.761×10−4
Prandtl number (Pr)	0.702	5.83
Length of glove (L: mm)	200
Emissivity of glove outer shell material (ε)	0.725
Stefan-Boltzmann constant (σ: W/m2·K4)	5.67×10−8

Note: The Pr number is a dimensionless number that compares the transport between momentum and thermal diffusivities, i.e., Pr = ν/α.

For moisture transfer, the water vapor would transport through glove fabric layers (the porous materials) but would not be able to pass through the PCM layer (non-porous material). At the hand skin surface (surface of epidermis), the human hand sweating flux was defined as 0.014 mol/m²·s, calculated based on the maximum human sweating rate of 15 g/min·m² [35–37]. On the outer surface of the glove’s outer shell, the moisture mass flux ($\dot{\text{m}}$) was defined as [31]:

$$\text{In}\text{the}\text{heating}\text{process},\dot{\text{m}}={\text{h}}_{\text{air}}\left({\text{c}}_{\text{amb},\text{air}}-\text{c}\right)={\text{h}}_{\text{air}}\left(0-\text{c}\right)$$ (12)

$$\text{In}\text{The}\text{air}-\text{cooled}\text{process},\dot{\text{m}}={\text{h}}_{\text{air}}\left({\text{c}}_{\text{amb},\text{air}}-\text{c}\right)={\text{h}}_{\text{air}}\left(0.816-\text{c}\right)$$ (13)

The term h_air is the convective mass transfer coefficient in ambient air, with the value of 0.021 m/s [31]. In the heating process, due to the extremely high heat flux to mimic flashover conditions, the moisture content in ambient air (${\text{c}}_{\text{amb},\text{air}}$) was assumed to be negligible (zero). In the air-cooled process, the moisture content in ambient air (${\text{c}}_{\text{amb},\text{air}}$) was assumed to be at a relative humidity of 60% (0.816 mol/m³) [31,34].

For the water-cooled process, the moisture mass flux on the outer surface of the glove was based on external water spray from a hose. This work assumed ten times the hand sweating rate, leading to 0.14 mol/m²·s.

2.4. Numerical solution reading points and mesh for modeling

The numerical solutions were calculated through solving the governing equations (heat diffusion equation, Equation (1), and coupled heat and moisture transfer equations, Equations (4) and (7)) along with the initial and boundary conditions in COMSOL Multiphysics. The built-in time-dependent solver with backward differentiation formula (BDF) of COMSOL was utilized to obtain the temperature and moisture concentration solutions with respect to time. Based on the temperature data, the time for hand skin surface to reach second-degree burn injury (60°C) could be determined. The flow chart for the numerical solution procedure is given in Figure 2.

Figure 2.

Flow chart for numerical solution procedure.

To export the temperature and moisture concentration profiles from COMSOL program and explore the temperature control performance of PCM-integrated firefighters’ gloves, we created nine probes at different positions in the glove geometry and on the hand skin, as shown in Figure 3. Probes 1–6 were located on the human skin surface to collect the skin temperature and seek the time to reach the second-degree burn injury in this study. Probe 2 was the location of the finger joint, which was not covered by the PCM segment. Probes 1–3 were on the finger, while Probes 4–6 were on the palm. Probe 7 was on the inner surface of the PCM encapsulation, while Probe 8 was on the outer surface of the PCM encapsulation. Probes 7 and 8 provided information on the phase change status of the PCM. These two probes were absent for the glove with no PCM integrated. Probe 9 was located at the outer surface of the glove’s outer shell.

Figure 3.

Probe locations in the model to export temperature and moisture concentration profiles after simulation: (a) Probes 1–6 measure hand skin surface temperatures; (b) Probes 7 and 8 measure the data on the inner and outer surfaces of the PCM segment, respectively; (c) Probe 9 measures the temperature on the outer surface of the outer shell of the glove.

Figure 4a shows the mesh structure in the glove-hand model. Free tetrahedral elements were used to create the mesh structure for the model. The normal element size in COMSOL was chosen, leading to 310,486 domain elements, 114,470 boundary elements, and 8,710 edge elements. The meshing size was found sufficient for the heat and moisture transfer simulations based on a mesh independence study for the entire glove-hand model shown in Figure 4b. Fine element size provided similar simulation results (see temperature profiles in Figure 4b), but its computational time was much longer compared to the normal element size.

Figure 4.

(a) Mesh structure in firefighters’ gloves model created by free tetrahedral elements; (b) Mesh independence study on the 3D entire glove-hand model under high-heat condition (the temperature data were exported from Probe 5 – the hand skin surface temperature at the center of palm).

2.5. Parametric evaluation of PCM in firefighters’ gloves

A parametric study was conducted to evaluate how PCM influenced the overall thermal protective performance of firefighters’ gloves. The parameters include PCM melting point, PCM location, and PCM thickness. This study aimed to identify the optimum PCM property, configuration, and geometry to achieve a better thermal protective performance of glove.

3. Results and Discussion

3.1. Heat transfer analysis for PCM-integrated firefighters’ gloves

Figure 5 displays the phase transition process of PCM segments in firefighters’ gloves under exposure at a flashover condition (heat flux at 83 kW/m²). The PCM segments had a melting temperature of 70°C, were 1 mm thick, and were located above the base layer (ABL). The blue color in Figures 5a–d represents the solid state (0) of PCM, while the red color represents the liquid state (1) of PCM. Most of the PCM was still in the solid state at the early heating stage, as shown in Figure 5a. However, as time went on, more and more PCM turned into a liquid state by absorbing a large amount of heat during the heating process. Because the heat flux was applied at the outer surface of the glove, the PCM melted from outside to inside. During the melting process, the PCM could absorb the heat but maintain a relatively constant temperature, which helped to achieve efficient temperature control for hand skin protection. Hence, the glove’s outer surface temperature rose rapidly beyond 1000°C. However, the inside glove layers still maintained below 44°C (the threshold temperature of pain [1]) after 25 seconds of heating at 83 kW/m², as shown in Figures 5e and f.

Figure 5.

Phase transition process and temperature profiles in PCM-integrated firefighters’ gloves under exposure at the flashover condition (heating process at a heat flux of 83 kW/m²): (a)-(d) Phase transition process of PCM in glove. Blue indicates the solid state (0) of PCM, red indicates the liquid state (1) of PCM; (e) and (f) are temperature (°C) profiles of PCM-integrated firefighters’ gloves after 10 seconds and 25 seconds of exposure at the flashover condition, respectively.

The PCM segments had a melting temperature of 70°C, were 1 mm thick, and were located above the base layer (ABL). Temperature profiles on the hand skin surface, PCM segment, and glove outer surface are displayed in Figure 6. This figure compares the temperatures between conventional firefighters’ gloves and PCM-integrated firefighters’ glove-protected hands to explore the thermal protection improvement by PCM. The times for hand skin (skin area covered by PCM, locations of Probes 1, 3–6) to reach second-degree burn injury temperature (~60°C) could be extended by more than two times compared to conventional firefighters’ gloves (no PCM), as shown in Figures 6a and c. Even for the hand surface areas (such as the finger joints) that were not directly covered by PCM segments, the time to reach 60°C was around 1.5 times longer than that for conventional firefighters’ gloves (comparing temperatures at the location of Probe 2). Under flashover conditions, the times for hand skin temperature to reach 60°C were extended from 15.5 seconds (baseline; control) to 26.5–36.5 seconds (refer to Probes 2 and 5 in Figure 6a) when PCM segments were integrated into conventional firefighters’ gloves. Under the hazardous condition, the times for hand skin temperature to reach 60°C could be extended from 63–66 seconds (baseline; control) and to 101–129 seconds (refer to Probes 2 and 5 in Figure 6c) by integrating the PCM segments. The simulation results found that the glove PCM segments could enhance the entire hand’s thermal protection by absorbing large amounts of heat during the phase-changing process, even for the areas not covered by PCM. Moreover, the temperature increase rate on the glove’s outer shell was slowed down through PCM segments, as shown in Figures 6b and d. Temperatures on the glove’s outer surface could be reduced by 50°C-100°C after 30- and 115-second exposure to flashover and hazardous conditions, respectively, when integrating PCM in firefighters’ gloves.

Figure 6.

Temperature profiles on the hand skin surface, embedded PCM, and glove outer surface (refer to Figure 3 regarding the locations of Probes 1–9): (a) Hand skin surface temperatures under the flashover condition (heat flux of 83 kW/m²); (b) embedded PCM and glove outer surface temperatures under the flashover condition (heat flux of 83 kW/m²); (c) hand skin surface temperatures under the hazardous condition (heat flux of 8.3 kW/m²); (d) embedded PCM and glove outer surface temperatures under the hazardous condition (heat flux of 8.3 kW/m²).

Figure 7 shows a parametric study on the effects of PCM melting point, PCM location, and PCM thickness on the thermal protective performance enhancement of firefighters’ gloves. As with the baseline (control group), the times for the hand skin surface to reach second-degree burn injury under conventional firefighters’ gloves (no PCM involved) were 15.5 seconds and 63–66 seconds under flashover and hazardous conditions, respectively.

Figure 7.

Times for the hand skin surface to reach second-degree burn injury (~60°C): (a) and (b) for locations of Probe 5 and Probe 2, respectively, under the flashover condition; (c) and (d) for locations of Probe 5 and Probe 2, respectively, under the hazardous condition.

PCM melting temperature effect:

Three melting temperatures of PCMs were studied, including 70°C, 100°C, and 150°C. The melting points of 70°C and 100°C showed better thermal protective performance in gloves than the 150°C melting point PCM did (Figure 7). And the 100°C melting point PCM could help the hand skin (area covered or not covered by PCM segments) hold the longest time to reach second-degree burn injury. The performance of 70°C melting point PCM was close to that of 100°C melting point PCM—sometimes only having a 1–2 second difference. The time to reach second-degree burn injury could be shortened by up to 7 seconds when using a 150°C instead of a 100°C melting temperature PCM. The effect of PCM melting temperature was more profound under hazardous conditions.

PCM location effect:

Under flashover conditions (heat flux of 83 kW/m²), the skin area covered by PCM (Probe 5) was protected better when PCM was located inside, above the base layer (ABL), compared to that located beneath the outer shell (BOS). The PCM could melt very fast when located close to the outer environment under the high heat flux. It would quickly convert to the liquid state and lose the benefits of the phase change function. Hence, moving the PCM segment toward the inside of the glove (close to the hand) could reduce the high heat flux effect, and therefore, extend the melting (phase change) time for better hand protection. It was found that the time to reach second-degree burn injury could be extended by 1–3 seconds when PCM was located ABL, compared to the location of BOS.

Under hazardous conditions (heat flux of 8.3 kW/m²), the skin area covered by PCM (Probe 5) was protected better when PCM was located beneath the outer shell (BOS), compared to that located close to hand (ABL). The PCM could melt slower when the outside heat flux was lower, maintaining a longer time at a solid state. Thus, the phase change function was not efficient. A more efficient phase change function could be achieved when PCM was located close to the outer environment. The time to second-degree burn injury could be extended by 3–10 seconds by moving PCM from ABL to BOS in a hazardous condition. These phenomena were consistent with those obtained from previous 1D simulation results [20]. For the area not covered by PCM segments (e.g., location of Probe 2), the time to second-degree burn injury could be extended by 1.5–6 seconds when PCM segments were located BOS, compared to the location of ABL, for both flashover and hazardous conditions.

PCM thickness effect:

As shown in Figure 7, the thicker PCM had better thermal protection performance than the control group (firefighters’ gloves with no PCM). A 0.5-mm-thick PCM segment could help extend the thermal protection time by 5–10 seconds and 17–36 seconds under flashover and hazardous conditions, respectively; a 1-mm-thick PCM segment could extend the thermal protection time by 10–20 seconds and 35–70 seconds under flashover and hazardous conditions, respectively.

3.2. Heat and moisture transfer analysis during the cooling period

The study on post-fire exposure behaviors of firefighters’ gloves is also important. It examines how the stored heat in gloves from fire scene releases and how the released heat can affect the human hand. The heat released after fire exposure could accompany hand sweating and external spray of water from a hose. Thus, moisture transfer in gloves was considered in this study to explore how moisture could influence hand skin temperature variations. In this study, the PCM segments had a melting temperature of 70°C, were 1 mm thick, and were located between the base layer and thermal barrier (i.e., ABL). Both air- and water-cooled processes were investigated in this work.

Figure 8 shows the temperature profiles on the hand skin surface, embedded PCM, and glove outer surface for the air-cooled process after fire exposure. The moisture content in the environment and hand sweating factors were considered in this simulation. The cooling period occurred after a 4-second exposure under the flashover condition (heat flux of 83 kW/m²). From Figure 8a, the hand skin temperature did continue to increase even after the 4-second heating process because the stored heat in the glove dissipated towards the surroundings when firefighters came out of the fire scene. Hence, the skin temperature would increase slightly before decreasing due to thermal inertia. Results showed that the hand skin temperature protected by the glove with PCM segments increased more slowly than that protected by the glove without PCM. Even for the area not covered by PCM segments (e.g., Probe 2), the temperature rise was much slower compared to the glove with no PCM. Figure 8b displays the inner surface (Probe 7) and outer surface (Probe 8) of the encapsulated PCM segment in the glove. The portion of the PCM closest to the external environment (Probe 8) had undergone phase change. Thus, it maintained the PCM outer surface temperature around 70°C, effectively maintaining the temperature beneath the PCM segments at a lower temperature level compared to the glove with no PCM. Figure 8c shows the temperature at the outer surface of the glove’s outer shell. The PCM segments could also help reduce the glove’s outer surface temperature compared to the glove with no PCM. Therefore, the PCM segments could help improve the temperature control performance of firefighters’ gloves even after fire exposure.

Figure 8.

Temperature profiles at the (a) hand skin surface, (b) embedded PCM, and (c) glove outer surface during the cooling period after fire exposure, considering the effects of hand sweating and moisture content in the environment. The glove was cooled by ambient air only.

Figure 9 considered the external water spray from a hose, which could provide a more efficient cooling process than ambient air. Hence, the temperatures of the hand skin, embedded PCM segment, and glove’s outer shell surface were much lower than those from air cooling only. The effect of PCM segment protection was insignificant as the temperatures reduced faster for water cooling. Nevertheless, PCM segments could still help mitigate the peak temperature increase after fire exposure (Figure 9a).

Figure 9.

Temperature profiles at the (a) hand skin surface, (b) embedded PCM, and (c) glove outer surface during the cooling period after fire exposure, considering the effects of hand sweating, the moisture content in the environment, and external water spray from a hose. The glove was cooled with water spray.

Figure 10 displays the moisture concentration on the hand skin at finger joint (Probe 2) during the cooling period. For the air-cooled process (Figure 10a), the moisture concentration was from hand sweating and humidity in ambient air (i.e., relative humidity of 60%). Because PCM segments are non-permeable material, the embedded PCM could affect the permeability of the glove. The hand skin with PCM-integrated glove protection showed higher moisture concentration than conventional firefighters’ gloves with no PCM because the PCM segments slowed down the moisture dissipation of hand sweat towards the outside. The increase of moisture concentration on hand skin was not significant, resulting in only a 1–2 mol/m³ increase. Similar phenomena were observed for the water-cooled process (Figure 10b). The major moisture transfer direction was from the outside to the inside because the water spray was from the glove’s outer surface. The PCM segments slowed down the moisture transport from the external spray water, resulting in a lower concentration on the hand skin.

Figure 10.

Moisture concentration on the hand skin surface at finger joint (Probe 2) during the cooling period: (a) The glove was cooled by ambient air; (b) the glove was cooled by water spray.

More detailed moisture concentration distributions in gloves were displayed in Figures 11 and 12 for PCM-integrated and conventional firefighters’ gloves, respectively. The PCM segment is a non-porous material, which does not allow water vapor to pass through directly. Hence, moisture could be accumulated on one side of the PCM segment, as shown in Figure 11. However, because PCM segments were used instead of a whole piece of PCM in glove, moisture from hand sweating could still dissipate outside through the areas not covered by PCM to maintain the permeability of firefighters’ gloves. The moisture concentration gradients could be observed around PCM segments in Figures 11a–d, indicating the moisture transport direction in the glove to dissipate the hand sweat.

Figure 11.

A 3D illustration of moisture concentration distribution in PCM-integrated firefighters’ gloves: (a) View at the cut plane of the inner surface of PCM segments after 4-second fire exposure; (b) view at the cut plant of the outer surface of PCM segments after 4-second fire exposure; (c) view at the cut plane of the inner surface of PCM segments after 60-second air cooling period; (d) view at the cut plant of the outer surface of PCM segments after 60-second air cooling period; (e) 3D view of the entire glove after 60-second water cooling period.

Figure 12.

3D vision of moisture concentration distribution in conventional firefighters’ gloves (with no PCM): View at the cut plane of the interface between the base layer (inner thermal lining) and thermal barrier after (a) 4-second fire exposure and (b) 60-second air cooling period; (c) 3D view of the entire glove after a 60-second water cooling period.

The primary moisture transfer direction was from the outer surface to the inside of the glove when using external water spray from the hose for cooling (water-cooled process). Hence, the moisture concentration was higher on the glove’s outer surface, and lower inside, especially in the palm area, due to the large PCM segment covered, as shown in Figure 11e. For conventional firefighters’ gloves, moisture concentration distribution was more uniform throughout the entire glove. There was no accumulated moisture in the glove, as shown in Figure 12.

4. Limitation and validation of the simulation model

Limitation:

The 3D simulation model predicts the overall thermal and moisture transport performance in firefighters’ gloves. Due to the limitation of establishment for complex geometry, it is challenging to create an exact hand geometry in COMSOL. COMSOL used rectangular configurations to generate the glove-hand model, which involves a 90° angle around fingers and the hand instead of smooth curved surfaces. As a result, it could create some degree of simulation inaccuracy around these angles. Nevertheless, these angles do not affect the overall simulation results for the entire hand and glove point-of-view, resulting in reasonable numerical predictions. Moreover, the hand skin was assumed to be nonporous material in the simulation. The hand sweating rate was applied as the boundary condition at the surface of epidermis. Although the hand skin was considered to be nonporous material in the model, the thermal properties of hand skin used in the simulation were the effective properties considering the effects of pores in the skin. Hence, the heat transfer predictions in skin were reasonable. For the moisture transfer simulation, the main focus was on the water vapor transport in glove. We were not studying the moisture transfer in hand skin. Thus, the nonporous assumption for the hand skin would not affect the overall moisture transport phenomena in the glove. The hand sweating rate applied at the surface of the epidermis was based on the maximum skin sweating rate of the human body. The actual hand sweating rate may change based on different situations. Moreover, a fixed water spray rate was assumed at the glove’s outer surface in the model. The actual water spray rate from an external hose may also vary in different situations. Thus, the absolute values from the simulation results may be different from actual situations, but the comparisons and trends of the results should be the same as those from the actual situations. Therefore, the numerical simulations can provide valuable information for the design of PCM-integrated structural firefighters’ gloves in the future.

Validation:

Experimental measurements have been conducted to verify the numerical simulation model [38]. The studies were conducted in a furnace environment under hazardous (furnace temperature of 200°C with 10 kW/m² radiation heat flux) and lower bound of flashover (furnace temperature of 300°C with 15 kW/m² radiation heat flux) conditions [38]. To better observe the PCM behavior and ensure safety, the testing environmental temperatures were not set at upper bound of flashover conditions. Commercial structural firefighting glove was used and cut into a 4” × 4” glove sample for testing. A 1-mm thick PCM layer (commercial bio-based PCM with melting point of 68°C) was inserted above base layer (ABL) in the glove sample. The glove sample materials along with the embedded PCM layer were attached on an insulation block to mimic a gloved hand (hand protected by glove), as shown in Figure 13. The temperatures on the insulation block surface were measured by three thermocouples (TCs), representing the hand skin surface temperatures.

Figure 13.

Glove sample materials along with the embedded PCM layer adhered/mounted on an insulation block to mimic gloved hand [38].

The numerical simulations were compared to the testing results under the same heat flux condition and PCM-integrated glove configuration, i.e., 1-mm thick bio-based PCM layer (with melting point around 70°C) located ABL in glove. Figure 14 displayed the temperature profiles on hand skin surface from both numerical simulation and experimental testing. The temperatures at Probe 5 (numerical results; refer to Figure 3a) and temperatures at TC 2 (experimental results) were used for the comparison in Figure 14, because they all represented the center of the palm. The results were consistent with each other. The differences in temperature profiles between simulation and experiment were less than 15%. It was noted that better agreement between numerical and experimental studies occurred under higher heat flux condition. The experimental results showed that the optimum PCM melting temperature range was around 70°C-100°C, the PCM location was better toward environment in a glove under hazardous condition, and overall, the PCM could extend the thermal protection time of firefighters’ gloves (the time for hand skin to reach second degree burn injury) by 1.5–2 times [38], which consistent with the numerical simulation results in this study.

Figure 14.

Comparison of temperatures on hand surface between numerical simulation and experimental testing. (a) Under hazardous condition (furnace temperature of 200°C with 10 kW/m² radiation heat flux); (b) under lower bound of flashover condition (furnace temperature of 300°C with 15 kW/m² radiation heat flux). Note that “SIM” represents numerical simulation results (Probe 5) and “EXP” represents experimental testing results (TC 2).

5. Conclusion

As demonstrated in this study, the use of PCM could help remarkably improve the thermal protective performance of firefighters’ gloves. Considering the hand dexterity and permeability of the glove, PCM was broken into several segments to be embedded in the glove’s palm, back, and finger areas, but while avoiding all the finger joint areas. This study found that the PCM segments could extend the time for the hand skin surface to reach second-degree burn injury by more than two times compared to a conventional glove with no PCM. Furthermore, even for the areas not covered by PCM (such as joints), the PCM segments could still protect them to extend the time to reach second-degree burn injury by around 1.5 times.

The parametric study found that the PCM with a melting temperature range of 70°C-100°C provided the best thermal protective performance for hand skin. The location of PCM segments depends on the condition of the fire scene. PCM provides better thermal protective performance when located close to the hand in the glove under flashover conditions. Further, PCM offers better thermal protection when located close to the environment in a glove under hazardous conditions.

A thicker PCM layer in the glove could provide better thermal protection for firefighters’ hands. With only 0.5-mm-thick PCM segments, the glove thermal protection time could be extended by up to a half-minute (around 30 seconds) compared to conventional firefighters’ gloves (with no PCM) under hazardous conditions. When the thickness of PCM segments increases to 1 mm, the glove thermal protection time could be extended by more than 1 minute under hazardous conditions (compared to the glove with no PCM). Nevertheless, the PCM layer could not be too thick to compromise hand function. It requires a trade-off between hand thermal protection and dexterity. Thus, the optimum PCM thickness needs to be further explored considering both glove thermal protection and hand dexterity aspects in the future.

Moreover, PCM segments could block the moisture transfer to some degree and cause accumulation of moisture in some regions of the glove (such as at the PCM segment surface). Thus, the PCM segments’ integration reduced the glove’s overall moisture dissipation rate. However, the moisture could still move through the area not covered by PCM segments, maintaining the permeability of firefighters’ gloves.