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Assessment of thermal and evaporative resistance

Many researchers evaluated the thermal and evaporative resistance of clothing, using the equipment/methods described in various standards (eg, ASTMF 1291, ISO 15831, EN 342, ASTM F 1720, ASTM F 2370, ISO 9920) or by employing their own customized instruments/procedures [440,441,447]. These studies have identified that fabric features (eg, fiber types, weaves, design, weight, thickness, porosity), clothing attributes (eg, fit, design, construction), and/or ambient environmental variables (air, temperature, relative humidity) mainly affect heat and/or moisture/water vapor transfer (convective/conductive/radiative/diffusive) through clothing, which ultimately affect the thermal and evaporative resistance of the clothing.

Lotens and Hevenith [440] studied the thermal resistance of clothing having a layered structure and different thickness/weights. It was found that thermal resistance is mainly dependent upon ambient air velocity and clothing weight. Generally, an increase in air velocity can decrease thermal resistance, with a lesser effect on heavy clothing than on light clothing. It was also determined that layered structure clothing may trap larger amounts of air within its structure, which can enhance the thermal resistance of this clothing [447]. Researchers also found that thermal resistance is affected by the tightness of the fit of clothing and the clothing area factor (ratio of clothed body surface to the nude body surface). Here, highly tight-fitting clothing has a lower clothing area factor than loose-fitting clothing, which ultimately lowers the thermal conductivity of the tight-fitting clothing and increases its thermal resistance. Bouskill et al. [447] analyzed the thermal resistance of air-impermeable and -permeable clothing having layered structures. Here, air-impermeable clothing had a one-layered structure and air-permeable clothing had a three-layered structure. In this context, it was found that air-permeable clothing allows for transfer of the relatively cool air from the environment toward the manikin; as a consequence, the thermal resistance of this clothing becomes lower. In this study, it was also found that air layers trapped in between the clothing and manikin body (clothing microenvironment) plays an important role in clothing insulation; any exchange of air between the trapped air layers and the cooler ambient environment results in a change in thermal insulation. It was concluded that the exchange of air increases the heat transfer from the manikin surface to the ambient environment; consequently, the thermal resistance of the clothing decreases. In addition, the movement of the manikin allows the exchange of air and affects thermal resistance. If the speed of the movement is high, it will help to exchange a high amount of air between the clothing microenvironment and the ambient environment; eventually, the thermal resistance of the clothing decreases. Chen, Fan, and Zhang [441] corroborated that the thermal resistance of clothing is mainly evaluated in dry conditions. However, clothing can be wet due to perspiration from wearers’ bodies, and this accumulated water in clothing has a significant effect on thermal resistance. In this study, the thermal resistance of slightly wet (due to low perspiration) and highly wet (due to high perspiration) clothing are compared and analyzed. It has been found that the thermal resistance of wet clothing is significantly lower than the thermal resistance of dry clothing, and the thermal resistance of wet clothing can vary between 2% and 8% depending upon the accumulated water within the clothing. This lower thermal insulation can provide a chilling effect to wearers. Chen, Fan, and Zhang [441] challenged many other studies (McCullough, Jones, and Tamura [428]; Mecheels and Umbach [429,430]), where THL through clothing was considered the combination of thermal and evaporative resistance, and thermal resistance was evaluated only in dry clothing conditions. Chen, Fan, and Zhang [441] suggested that clothing’s thermal resistance should be calculated in both dry and wet conditions in order to accurately evaluate the THL through clothing. Xu et al. [448] studied the thermal resistance of liquid cooling garments. It was stated that only a portion of total liquid cooling garments can actually reduce thermal resistance by perfusate circulating within the garments. Here, the perfusate inlet temperature is lower than both manikin and ambient temperatures. As a consequence, perfusate helps to absorb heat from both the manikin and ambient environments; this situation lowers the thermal resistance of the garments. In this study, it was also found that the placement of an outer clothing layer on a garment may enhance the thermal resistance of the garment. Contextually, Bogerd, Psikuta, Daanen, and Rossi [449] studied the cooling garments with a manikin. They found that the manikin overestimates the cooling effect due to the lack of vasoconstriction simulation. The human subjects had vasoconstriction in the skin, which limited the cooling effect of the cooling vests. Qian and Fan [450] evaluated the thermal resistance of clothing under various ambient air velocities and walking speeds of manikins. It has been found that ambient air velocity significantly affects the thermal resistance of clothing in combination with walking. Here, it was evident that thermal resistance decreases with increasing air velocity and walking speed. This is because increased air velocity and walking speed enhance the transfer of heat from the manikin body to its ambient environment, which helps to reduce the thermal resistance of clothing. The effect of walking speed on the total thermal resistance of a clothing system was equivalent to 180% of air velocity. Fan and Tsang [451] discussed the thermal resistance of a tracksuit. They confirmed that fabric properties such as porosity and movement of the manikin have significant effect on thermal resistance. It was evident that highly porous fabric-based clothing may allow for transfer of convective heat from a manikin’s body to its ambient environment and can lower the thermal resistance of clothing.

Ho et al. [392] investigated the impact of clothing design on the thermal resistance of clothing. In order to do so, they chose 10 short sleeve T-shirts of varying opening styles and mesh styles. They found that the design has significant effect on thermal resistance in standing as well as walking conditions of the manikin. It was evident that the thermal resistance of all of the T-shirts was much lower in the walking condition of the manikin than the standing condition of the manikin. This is because more natural convection (ventilative cooling) occurs in the walking condition in between the clothing and manikin body, which ultimately lowers thermal resistance. It was also found that the thermal resistance of a T-shirt with more openings or comprising mesh fabrics at two vertical side panels along the side seams is significantly lower than a T-shirt with less opening or one comprising no mesh fabrics; this was evident in both standing and walking conditions of the manikin. Additionally, it was found that the presence of mesh fabrics at the center back or center front (either horizontally or vertically) of the manikin body does not have much effect on thermal resistance. This is because the mesh fabrics at these locations tend to lay on the manikin’s surface due to garment draping, and do not allow for transfer of heat from the manikin’s body to the ambient environment (less ventilative cooling). In the walking condition, the drape of the garment changed rapidly, which allowed the mesh fabrics to gain more contact with the manikin’s surface. Furthermore, Ho et al. [392] concluded that the thermal resistance of a T-shirt gradually increases with increased T-shirt size in standing or no wind conditions. This increasing trend of thermal resistance continued even in walking and windy conditions. This study showed that adding fullness to the T-shirt design to create a flared drape can significantly reduce the thermal resistance of T-shirts under walking or windy conditions. The reduction of thermal resistance can further be enhanced by creating small apertures in the T-shirt design for added fullness.

Zhou et al. [452] compared the thermal resistance of permeable and impermeable clothing in dry and wet conditions of the manikin. It was found that water condensation occurs within clothing in wet conditions, and this condensation process affects thermal resistance. Due to water condensation, the thermal resistance of wet clothing is much higher than the thermal resistance of dry clothing. It has been found that condensation occurs more in impermeable clothing than in permeable clothing; as a consequence, the difference between the thermal resistance in dry and wet conditions is greater for impermeable clothing than for permeable clothing. Wu, Fan, and Yu [453] evaluated the thermal resistance of various clothing under different postures (standing, sedentary, supine) of the sweating thermal manikin Walter. In this study, it was evident that the thermal resistance of clothing is significantly higher in the sedentary posture than the standing posture. Here, the radiative heat transfer coefficient from manikin body to the ambient environment was lower in the sedentary posture than the standing posture due to a reduction in the radiative body surface area in the sedentary posture; this lower radiative heat transfer coefficient increases the thermal resistance of the clothing. The sedentary posture also creates a cavity over the horizontal knees and thighs of the manikin. As a consequence, natural convection reduces over the manikin body in the sedentary posture versus the standing posture; this reduction in natural convection enhances the thermal resistance of the clothing. Furthermore, it was evident that thermal resistance is significantly higher in the supine posture than the standing posture. This is because the manikin remains flat on a wooden bed in the supine posture, and this wooden bed enhances thermal resistance. In this study, it was evident that thicker clothing always has a higher thermal resistance than thinner clothing in all postures. However, the thickness of the clothing may be significantly reduced in the supine posture due to compression provided by the manikin body; this reduction in thickness significantly reduces the thermal resistance of the clothing.

Holmer [454] investigated the thermal resistance of protective clothing under hot environments. It has been found that thermal resistance is dependent upon the emis- sivity of clothing. If the emissivity of clothing is higher in a hot environment, it can reduce the thermal resistance of clothing. The emissivity is dependent upon the material and surface structure of the fabric used to manufacture the clothing. Here, a polished surface emits much less radiation at a given temperature (10-20%) in comparison with painted, matte, varnished, or dark surfaces (80-100%). As a consequence, polished surface-based fabric causes less radiant heat-load on wearers under hot environments; eventually, the thermal resistance of this clothing is indirectly reduced. However, a polished surface may enhance the evaporative resistance of clothing, which can be detrimental to wearers. Similarly, Oliveira, Gasper, and Quintela [437] evaluated the thermal resistance of cold-weather protective clothing ensembles, both in static conditions and considering the effects of body movements. The results from this study showed that the dynamic thermal resistance of this clothing was always lower than the corresponding static resistance. This means that an effective reduction in thermal resistance should always be expected in the presence of any kind of movement. It was evident that a reduction in thermal resistance mainly occurs in thick and layered clothing. Here, the thermal resistance of clothing was highly dependent upon the layered structure of the chosen fabric, fabric weight and thickness, and clothing area factor. Brien et al. [455] evaluated the thermal resistance of clothing designed to provide protection from chemical, biological, radiological, nuclear, and explosive hazards. In this study, it was inferred that the thermal resistance of protective clothing is highly dependent upon the wind velocity present in the clothing’s surroundings. Here, wind speed has greater effect on clothing with high thermal resistance in normal ambient conditions, and an increase in wind speed significantly reduces the thermal resistance of this clothing. However, the effect of wind speed is significantly lower on low-permeable clothing than high-permeable clothing. It should be noted that the protective clothing used in this study needs a high thermal resistance to provide protection from outside thermal hazards; thus, there must be a balance on thermal resistance that can provide optimum protection to wearers as well as effectively transfer their metabolic heat to the surrounding environment [24,29].

McCullough [456] investigated the evaporative resistance of clothing using a standing sweating thermal manikin. This evaluation was carried out in a residential building and vehicles. It was identified that the heating, ventilating, and air conditioning system of a building or vehicle have a significant effect on the evaporative resistance of clothing. It can be concluded that the consideration of ambient environmental conditions is highly significant when evaluating evaporative resistance. Wang et al. [457] studied the intrinsic evaporative resistance of multilayered winter clothing ensembles. In this study, various individual clothing articles (underwear, garment, and jackets) were used to construct multilayer clothing ensembles. It was identified that the intrinsic evaporative resistance of the individual clothing articles is dependent upon clothing area factors (ratio of clothed body surface to the nude body surface); and the clothing area factor of thicker fabric-based clothing is much higher than for thinner fabric-based clothing. As a consequence, the evaporative resistance of thick and thin clothing is very different. It was also identified that the evaporative resistance of a clothing ensemble has a linear relationship to the combined evaporative resistance of the individual clothing used in the ensemble.

Wu, Fan, and Yu [453] evaluated the evaporative resistance of various clothing under different postures (standing, sedentary, supine) of the sweating thermal manikin Walter. They identified a high correlation of the evaporative resistance in standing and sedentary postures. It was observed that the evaporative resistance of the sedentary posture was about 20-97% higher than that of the standing posture. In this context, Havenith, Heus, and Lotens [458] mentioned that the evaporative resistance of the sedentary posture is about 16-38% higher than that of the standing posture. It seems that Wu and Havenith’s study provided a different range of difference of evaporative resistance between the sedentary and standing postures; nevertheless, Wu’s study may be more realistic because they used the sweating thermal fabric manikin Walter, a highly reliable testing method. Wu, Fan, and Yu [453] also stated that the evaporative resistance for the manikin’s supine posture was significantly higher than that of the standing or sedentary posture. This is because a large amount of water condensation occurs (within the clothing, in between the clothing and the bed, etc.,) in the supine posture, and this water condensation enhances evaporative resistance. In this study, it was found that evaporative resistance in nonisothermal conditions is generally lower than in isothermal conditions in both standing and sedentary postures. This is because the natural convection induced by the temperature gradient is much lower in isothermal conditions than in nonisothermal conditions. It was also observed that a higher temperature gradient in between the manikin body and the ambient environment contributes to the accumulation of water within the manikin’s clothing; this situation drastically changes the evaporative resistance. In fact, the moisture accumulation in the isothermal condition was around 1% or close to zero, which is substantially lower than that in the nonisothermal condition. In the nonisothermal condition, some moisture vapor accumulated in the clothing ensemble instead of fully evaporating. The evaporative resistance measured in the nonisothermal condition, therefore, is apparently substantially lower than that of the isothermal condition.

Wang et al. [459] analyzed clothing’s evaporative resistance on different local body parts. The individual and interactive effects of air and manikin’s body movements on localized evaporative resistance were examined using a strict protocol. Localized evaporative resistance was measured on the sweating thermal manikin at three different air velocities (0.13,0.48, and 0.7 m/s) and three diverse walking speeds (0, 0.96, and 1.17 m/s). This study showed that wind speed has a distinct effect on localized clothing’s evaporative resistance. In contrast, walking speed had a larger effect on evaporative resistance of limbs (eg, thigh, forearm) versus the torso (eg, back, waist). In addition, the combined effect of body and air movement on localized clothing’s evaporative resistance demonstrated that walking has more influence on body extremities than the torso. This study concluded that localized clothing’s evaporative resistance is important for providing better comfort to wearers.

Zuo and McCullough [460] extensively studied the evaporative resistance of a variety of permeable and impermeable protective clothing ensembles used in certain sports, such as football, baseball, soccer, and tennis. It was observed that the evaporative resistance of these clothing ensembles depends upon the moisture permeability characteristics and wicking properties of the fabric materials used in the clothing, and the amount of skin surface covered by the fabric. In this context, it is notable that the fiber content of the fabric has little effect on moisture permeability; instead, the moisture permeability is usually dependent upon fabric structure and type of surface finishes used on the fabric. Generally, the moisture permeability of fabric with more open structures is higher than fabric with less open structures. In this study, it was found that the permeability index of impermeable and permeable clothing varies between 0 and 0.5; clothing with a high permeability index possesses lower evaporative resistance. In addition, if an article of clothing covers more body parts than others, generally it has high evaporative resistance.

Endrusick, Gonzalez, and Gonzalez [461] researched the evaporative resistance of US military chemical and biological protective clothing. They corroborated that the thicker, multilayered, and impermeable nature of the fabrics used in protective clothing are mainly responsible for evaporative resistance. It was identified that evaporative resistance can be proportionately decreased by decreasing the thickness of the fabrics and/or by increasing the permeability of the fabrics. Gao and Holmer [462] studied the evaporative resistance of impermeable protective clothing with respect to time on the manikin body. In this study, the impermeable protective clothing was used in combination with cotton underwear. The researchers identified that the evaporative resistance of the impermeable clothing is different in the initial, transient, and steady-state of moisture vapor transfer through the clothing. It has been found that evaporative resistance is more than two times higher in the initial phases of moisture vapor transfer than in the steady-state phase. Here, the moisture content increased exponentially with time in the clothing ensemble; on the contrary, mass loss directly from the wet manikin skin decreased exponentially with respect to time. Candas, Broede, and Havenith [463] explored the evaporative resistance of various protective clothing (more or less permeable or impermeable coveralls) in combination with single-layer dry and wet underwear using a static (no body movement) standing manikin with 34°C skin temperature. This study investigated the impact of clothing attributes, wet/dry underwear, and ambient environmental conditions on evaporative resistance. It was found that the evaporative resistance of the coveralls was very different under the same testing conditions; this is because the permeability of the coveralls was different. In this case, the impermeable coveralls showed the highest evaporative resistance—more than other more or less permeable coveralls. Additionally, it was observed that the evaporative resistance of the permeable or impermeable coveralls is very different in combination with dry and wet underwear. It was found that in the initial phase (when the coverall immediately comes into contact with the wet underwear), the coveralls absorb water and their evaporative resistance decreases; however, after some time, the evaporative resistance increases due to condensation of water inside the coveralls. This phenomenon was more prominent in the impermeable coveralls, and the evaporative resistance of the impermeable coveralls varied at three different temperatures (10, 20, and 34°C) with constant ambient air velocity (0.5 m/s) and water vapor pressure (1 kPa). Holmer [454] analyzed the evaporative resistance of protective clothing. He reported that the evaporative resistance of this clothing is dependent upon its permeability and thickness; generally, the evaporative resistance of impermeable and thick clothing ensembles is higher than for permeable and thin clothing ensembles. He explained that in impermeable or less permeable clothing, the saturation of the clothing microclimate and condensation within clothing ensembles occurs very quickly. This condensation occurs more effectively inside the outer layer and moisture barrier present in protective clothing, especially in temperate and warm ambient environments. Eventually, the heat is liberated due to condensation and raises the local temperature within the clothing. This increasing local temperature increases the evaporative resistance of the protective clothing. Richards et al. [464] investigated the effect of moisture and underwear on the evaporative resistance of cold weather protective coveralls in transient and steady-state conditions. The clothing materials used in this study had a range of different properties; the underwear clothes were hygroscopic, hydrophilic, or hydrophobic; and the coveralls had different permeability and thermal insulation values. It was observed that the evaporative resistance of tight-fitting clothing is more dependent upon the hygroscopicity of underwear than that of loose-fitting clothing. This study established that moisture absorption and desorption occurs very frequently in transient conditions; as a consequence, the evaporative resistance of clothing varies continuously. As no moisture absorption and desorption occurs in the steady-state condition, it was observed that evaporative resistance does not change very frequently in this condition.

 
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