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Types of thermal exposures faced by firefighters

A great deal of research has acknowledged that firefighters can be exposed to various thermal exposures including radiant heat, flame, hot surfaces, molten metal substances, hot liquids, and/or steam, depending upon job activities [69,76-83]. On the basis of thermal exposure, there may be differences in the modes of thermal energy transfer (convection/radiation/conduction) from fire hazards to firefighters [76,84-87]. Therefore, it is necessary to scientifically understand each type of thermal exposure along with its corresponding mode of thermal energy transfer toward firefighters.

Radiant heat: According to the discussion in Section 2.2, it is clear that luminous flame generates from the combustion of various organic substances present in a fire hazard. It has also been observed that the gaseous molecules vibrate within the flame and these molecular vibrations cause an emission of thermal energy [88,89]. In this context, the emissivity (e) of the flame is dependent upon the temperature and nature of the flame [66]. In particular, it has been found that the emissivity exponentially depends on the flame thickness. As a uniform flame possesses a constant thickness throughout its body, it can be inferred that the uniform flame has an overall constant emissivity. In this context, it is notable that the generation of a uniform flame in a fire hazard is rare. This is because the temperature surrounding the flame is always lower than the temperature at the flame body; as a result, nonuniform flame predominates. This nonuniform flame has variable thickness throughout its body and therefore variable emissivity (e). It has been observed that the emissivity of a nonuniform flame can be 0 < e < 1. Sometimes, the emissivity in the center of a nonuniform flame body can be as strong as the emissivity of a black body (e = 1) [the black body can be defined as an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence]. Here, the overall emissivity of a nonuniform flame can be mathematically represented by Eq. (2.3). Depending upon the overall emissivity, a large amount of thermal energy emits from a flame (uniform and nonuniform) in the form of electromagnetic waves. These electromagnetic waves help to transfer the thermal energy from the flame to its surrounding.

where ef = emissivity of the flame, L = length of the flame (cm), Tf = absolute flame temperature (°C), a = 237.7K^/gB ?0 • d0, K0 = 3^(1 — V)50/51, gB = amount (g) of combustion products per amount (g) of combustible substances, S0 = initial density of combustible substances (g/cm3), S1 = density of burned combustible substances (g/cm3), d0 = initial average diameter of the particle of combustible substances (cm), and V=fraction of volatile matter and moisture in combustible substances.

The earlier discussion proves that electromagnetic waves are responsible for transferring thermal energy from a flame to its surroundings [90,91], and infers that radiation is the prime mode of thermal energy transfer in this environment. As clothed firefighters extinguish the flame, they need to manage the thermal energy (radiant heat) present in the flame’s surroundings [24,29,92,93]. The thermal energy transfer from the flame to the firefighters’ clothing in the form of radiant heat can be denoted by Eq. (2.4) [88,89].

where Q = net thermal energy transfer (kW/m2), a = Stefan Boltzmann constant (W m—2 K—4), A = surface area of the clothing exposed (m2), es = the effective emissivity of the flame surface (dimensionless), ef = emissivity of the flame at temperature Tf (°C) [ie, the ratio of flame’s emissive power at Tf (°C) to that of a black body at Tf (°C)], affs = absorptivity of clothing fabric at Tf (°C) for incident waves from a black body at temperature Ts (°C) [ie, the ratio of the absorptive power at Tf (°C) for incident waves from a black body at Ts (°C) to that of a black body at Tf (°C) for incident waves from a black body at Ts (°C)].

Flame: Firefighters may also encounter a direct flame while rescuing fire victims/ property. In this flame exposure environment, hot gaseous molecules of combustible substances move towards clothed firefighters [94,95]. When these gaseous molecules with a certain temperature (Ta) come into contact with the clothing at a leading edge (A), the molecules achieve thermal equilibrium with the surface temperature (Ts, where Ts < Ta) of the clothing (Fig. 2.2). In turn, these molecules retard the temperature of the molecules present in the adjoining gaseous layer, which again retard the temperature of the molecules present in the next adjoining layer. This situation eventually develops a temperature gradient within the gaseous molecules imposed on the clothing. The region of the gases in which this temperature gradient exists is called the thermal boundary layer, and its thickness (Sx) can be expressed as

where v=velocity of the gaseous molecules (m/s); p = density of the gaseous molecules (g/cm3); p = viscosity of the gaseous molecules (m2/s); and x=distance from the leading edge A (m).

Thermal boundary layer on the clothing under flame exposure

Fig. 2.2 Thermal boundary layer on the clothing under flame exposure.

In the thermal boundary layer shown in Fig. 2.2, the convective thermal energy transfer coefficient (h) from the hot gaseous molecules to the clothing is shown in Eq. (2.6). The boundary layer can be divided into two regions—laminar and turbulent. In the laminar thermal boundary layer, the hot gaseous molecules are highly ordered, and it is possible to identify streamlines along which the hot gaseous molecules move. In the turbulent thermal boundary layer, the hot gaseous molecules are highly irregular and have three-dimensional random motion. As a consequence, the temperature gradient is high in the turbulent thermal boundary layer in comparison to the laminar thermal boundary layer. Eventually, the convective thermal energy transfer coefficient becomes high in the turbulent thermal boundary layer in comparison to the laminar thermal boundary layer based on Eq. (2.6).

where kf = thermal conductivity of the gaseous molecules (W/(m K)).

Thermal energy is imposed on the clothing via moving hot gaseous molecules under the flame exposure. In other words, convection is the primary mode of thermal energy transfer from flame to clothing [96-99]. Here, it is also notable that the flame comprises the carbon particles of the combustible substances and that the sizes of the carbon particles of different combustible substances are nearly the same, about 12 x 106 inches in diameter. Based on the size, it was calculated that these carbon particles can generate the radiant heat equivalent of 5% of black body radiation at a particular temperature [66]. It seems that some amount of thermal energy can also transfer through radiation from a flame surface to clothing. In summary, a combination of convective and radiant modes of thermal energy transfer occurs from flame to clothing [75,100-102]. Moreover, the characteristics of the flame change depending upon the composition of the combustible substances present in the fire hazard, resulting in variation in the ratio of convective to radiant mode.

Hot surfaces: In a structural fire hazard, thermal energy may transmit through many noncombustible solid substances (eg, iron or steel furniture/windows/doors) when these substances come in contact with radiant heat and/or flame [103]. The transmission rate of thermal energy depends on the thermal conductivity/resistivity, density, and heat capacity of these substances. After transmitting some amount of thermal energy, the molecules at the heated end of the substances start to vibrate. These vibrated molecules generate kinetic energy to agitate the atoms of these molecules, which results in oscillation of the neighboring molecules. This elevates the temperature of these substances.

At the time of rescuing fire victims and/or property, firefighters need to set aside or break hot substances. Hence, they come into contact with these substances’ hot surfaces [104-106]. During a hot surface exposure, a direct physical connection establishes between the hot surface and firefighters’ clothing [76] (Fig. 2.3). Here, the clothing becomes compressed between the hot surface and a firefighter’s body due to the applied force by the body and the counterforce from the hot surface.

Thermal energy transfer under hot surface exposure

Fig. 2.3 Thermal energy transfer under hot surface exposure.

According to Fig. 2.3, it is clear that two homogeneous and isotropic materials— the hot surface and the compressed clothing—come into contact with each other under hot surface exposure. As the temperature (Ts) of the hot surface is greater than the temperature (Tci) of the outer surface of the clothing (in direct contact with the hot surface), the thermal energy always transfers from the hot surface towards the outer surface of the clothing. In a steady-state condition (Ts and Tc> are constant), the conductive transfer of the thermal energy (Q0) depends upon thermal contact resistance (R0) between the hot surface and the clothing’s outer surface (Eq. 2.7) [76,77]. Theoretically, the contact resistance is dependent upon the surface roughness [66]. If the surface roughness of the hot surface and/or clothing is higher, it will hold more dead air between the hot surface and clothing. As dead air acts as an insulator, the thermal contact resistance will be higher; in turn, less transfer of thermal energy occurs from the hot surface to the clothing. Furthermore, the conductive thermal energy (Q00) transfer occurs in steady-state from the outer surface to the inner surface of the clothing (in contact with the firefighters’ body), depending upon the thermal conductivity (k) of the clothing, constant temperature difference between outer (Tc0) and inner (Tc00) surfaces of the clothing, thickness (T) of the fabric material used in the clothing, and area (A) of the clothing (Eq. 2.8). Additionally, the temperature of the inner surface of the clothing (Tc00) is greater than the temperature of the firefighter’s body (Tb); hence, the thermal energy would transfer from the inner surface to the body. In a steady-state condition (Tc00 and Tb are constant), the conductive transfer of thermal energy (Q000) depends upon thermal contact resistance (R00) between the clothing’s inner surface and the firefighter’s body (Eq. 2.9).

Molten metal substances: Molecular movement starts within a noncombustible solid substance (eg, iron or steel furniture/window) in the presence of a flame and/or radiant heat; consequently, the temperature of the substance increases. Due to this raised temperature, the molecular movement within the substance becomes rapid (Fig. 2.4). In this condition, the thermo-physical properties of the substance (thermal conductivity, thermal expansion, and elasticity) can change depending upon the temperature [107,108]. In the case of steel, it has been found that thermal conductivity decreases, thermal expansion increases, and elasticity decreases with the rise in temperature [109]. In this condition, the solid steel becomes viscous in nature, and may start to flow. Molten steel may drip and fix on clothed firefighters, which results in a thermal energy transfer from the drops of molten steel to the clothing. Here, the thermal energy transfer depends upon the temperature difference as well as contact resistance between the drops of fixed molten steel and the clothing. It seems that the conductive mode of thermal energy transfer predominates in the molten metal exposure [110,111].

Molecular behavior of a noncombustible solid substance in the presence of radiant heat and flame

Fig. 2.4 Molecular behavior of a noncombustible solid substance in the presence of radiant heat and flame.

Hot liquids: In the presence of flame and/or radiant heat, the temperature of semicombustible (eg, cooking oil) and/or noncombustible (eg, water) liquids gradually increase, and the liquids eventually become hot. This causes higher molecular movements in the liquids than in their normal condition (room temperature). In this condition, the liquids contain an excess of thermal energy. However, other properties of these liquids (eg, viscosity, thermal conductivity) affect their molecular movement, causing the amount of thermal energy within the liquids to vary (Fig. 2.5) [112].

Molecular behavior of a noncombustible liquid substance in the presence of radiant heat and flame

Fig. 2.5 Molecular behavior of a noncombustible liquid substance in the presence of radiant heat and flame.

Several researchers suggested that a clothed firefighter may come into contact with noncombustible or semicombustible hot liquids (eg, hot water, hot cooking oil) [22,76,113]. On contact, the surface of a liquid acts as an interface between that liquid and the outer surface of thermal protective clothing. At the point of interface, a fixed contact angle (the tangent the liquid surface makes with the clothing) is formed (at a particular temperature and pressure) depending upon the surface tension of the liquid and the outer surface properties of the clothing. If the surface tension of the hot liquid and the outer surface smoothness of the clothing is low, the contact angle remains low (<90°). In this low contact angle situation, the liquid absorption level of the outer surface of the clothing becomes high. As clothing is typically a porous media that comprises high-energy solid outer surfaces, most molecular hot liquids achieve complete saturation upon contact with the outer surface of clothing. After soaking the outer surface, the hot liquid penetrates through the clothing and comes into contact with firefighters.

Steam: Mandal et al. [76,113] suggested that hot water present in any fire hazard may convert into steam; additionally, the water used by firefighters (to extinguish the fire) may also convert into steam [81-83,114]. This steam diffuses to its surrounding environment according to Fick’s law shown in Eq. (2.10), where J = steam diffusion flux per unit area per unit of time (mol/m2 s); D = steam diffusion coefficient or steam diffusivity in dimension of length2 time-1 (m2/s) (D is proportional to the squared velocity of the steam particles, which depends on the temperature and size of the steam particles); Ф = steam concentration, or dimension of amount of steam per unit volume (mol/m3); and x = distance from the steam source (m). Eventually, clothed firefighters get exposed to steam. After contact, the steam may penetrate through the clothing depending upon the diffusion resistance of the clothing and may come into contact with firefighters (Eq. 2.11). While passing through the clothing, some amount of steam may condense into hot water and come into contact with firefighters’ bodies.

where R = diffusion resistance; L = thickness of the clothing (m); A = area of the clothing (m2); and p = density of the clothing (g/m3).

 
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