This study aims to investigate the characteristics of heat transfer and its mechanism for styling a design garment differently, and to improve thermal comfort caused by clothing styling design, a computational fluid dynamics (CFD) approach has been used to perform numerical investigations of fluid flow and heat transfer across a clothing air gap. Relationships between the heat transfer from the body to clothing (computed by heat transfer equations) and the air gap were examined by body heat loss of different styles of garments. Also, the clothing temperature distribution of different garments was obtained and compared. Computed results reveal that the air gap can play a central role in the heat transfer from the body to the surface of different style garments. When the air gap is small enough, namely about 5 mm in the chest and bust of the X-type of clothing, the conductive heat flux can transfer through the air gap and reach the cloth surface easily, which will bring about the increase of temperature on the clothing surface. The decreasing air gap distance from 50 mm (O-type) to 10 mm (X-type) increases the convective heat flux by up to 25% on the waist. However, the airspeed will increase to greater numbers while the air gap decreases to narrow channels, and it will bring about fierce forced convection heat flux. So the heat transfer must be considered in the process of garment design, and the air gap should be kept at a suitable level. These findings can be used to improve the clothing’s thermal comfort or optimize the cloth structure.
The rapid development of science and technology has brought about increasing requirements on the comfort of clothing, which makes possible the further development of new thermal comfort technologies in the recent years. One of the important parts of garment comfort is the microclimate, defined as the climate in the air gap between the skin and the clothing. The position and thickness of the air gap affect the garment’s thermal comfort to a great extent . A garment of a different style has a different air gap distribution. Thus, the allowance for garment ease should be considered carefully to generate a reasonable thermal microenvironment.
Some preliminary studies provide useful background to the present work. Udayraj  has reviewed the research on heat and mass transfer through thermal protective clothing and holds that the air gap is one of the most critical parts of the analysis of thermal protection. Sawcyn  has investigated the heat transfer through horizontal air spaces in bench-top tests with various types of air gap widths. Bhattacharjee  has developed a mathematical model to predict the conductive and radiative heat transfer through fabric and validated it with the experimental thermal resistance values. Ghazy  used the finite-volume method (FVM) to solve the energy equations, and the model was used to analyze the effect of dynamic heat transfer through fabric-air-skin system. He et al  have studied the mechanism of heat transfer through the air gap of the fire protective clothing simulated by several parallel plates, and insist that the skin surface temperature was increased with the air gap until the thickness reached 7 mm. Udayraj  studied the heat transfer through air gaps with the CFD model, comparing assumptions such as constant thermo-physical properties; neglecting convection showed that these assumptions can lead to significant deviation in the heat transfer prediction.
Recently, some full-scale heat transfer simulations have been conducted. Tian  has developed a 3D finite-volume model to simulate the transient heat transfer through a flame manikin. The model was validated by comparing it with physical experiments. Then they  studied 3D heat transfer of thermal protective clothing based on computational fluid dynamics. And they used the real shape of a human body with 12 mm of uniform air space under the garment. Choudhary  developed a numerical model of a thermal manikin wearing an air ventilation clothing with actual dimensions and shape. The authors argued that the area-weighted average torso heat flux increased as the fan airflow rate increased.
To the best of our knowledge, few reports comprehensively describe how the 3D air gap affects the heat transfer on the body surface and temperature on the garment surface. The influence of different clothing styles on the microclimate has also not been clear up to now. In this paper, we present a detailed numerical simulation of the real 3D body-air-cloth microclimate heat transfer system, which takes into consideration the real body and clothing and the air gap between them. The combined effect of conductive, convective, and radiative heat transfer across the clothing microclimates was investigated.
2.1. Geometric models
In the present work, heat transfer through clothing involving an air gap is analyzed, and the temperature distribution on the garment’s surface is predicted. From a heat transfer and air gap point of view, the X-type of garment is fitted on a human body and has the narrowest air gap; the O-type of garment may trap most of the air gap under clothing, and the heat transfer characteristics of the A-type of garment are similar to those of H-type clothing. So the X-, H-, and O-types of garments are selected and simulated. A naked female body established by CLO Standalone (CLO Virtual Fashion Inc, Korea) is used here, and three different types of feature-aligned garments, namely, the H-, X-, and O-types, are constructed by Bezier-spline curves [11, 12]. The procedure for creating different types of clothing surfaces is as follows. The body feature points on the key girths were located by a horizontal cut plane and comparison of curvature. Then the corresponding garment feature points were obtained by adding distance ease. After that, the Bezier-spline curves were created from points, and the garment surfaces were constructed from these curves. Basic dimensions of the human body and clothing are shown in Table 1.
|Bust girth (cm)||Waist girth (cm)||Hip girth (cm)||Garment length (cm)|
|H-type of model||92||93||94||66|
|X-type of model||88||68||94||66|
|O-type of model||92||104||94||66|
A 3D H-type model garment on the mannequin model, which is used for heat transfer simulation, is shown in Figure 2 (a). Three different styles of virtual garments are generated and tried on. The garment surfaces were flattened and merged by 3D modeling software. Also, 2D patterns are generated and compared in Figure 1 (c). Ansys Icem is used, and nonuniform hexahedral unstructured meshes are generated with fine mesh near boundary walls to capture the physics of flow properly . Only half-geometry meshes are generated due to the symmetric configuration. The final mesh sizes used for three different type of computations are 2.9, 2.8, and 2.6 million cells, respectively. Three styles of upper garment meshes are shown in Figure 1 (b).
2.2. Mathematical modeling and Boundary conditions
Coupled Navier-Stokes and radiative transfer equations were used to simulate heat transfer and fluid flow through fabric-air-skin system involving air circulation. Three-dimensional continuity, momentum, and energy Reynolds-averaged Navier-Stokes (RANS) equations [13, 14] with a realizable k-ɛ model in tensor form were considered for modeling heat transfer and air flow in the air gap in case of an unsteady state and incompressible turbulent. The radiation heat flux inside the air gap is calculated by radiative transfer equations (RTE), which, with the DO radiation model are expressed as :
where, Ip is the equivalent emission of the particles and κp, γp are the equivalent absorption coefficient and scattering coefficient, respectively.
Owing to the complexity of heat transfer in the air gap, some assumptions are adopted to simplify the numerical model: the flow inside the air gap can be characterized as a turbulent, weakly compressible flow containing both forced convection due to the airflow from the clothing hem and natural convection due to buoyancy and heat radiation due to the temperature gradient. The body posture is assumed to be calm, and the body movement is assumed to be zero, especially for the girth of the chest. The airflow within the fabric occurs in a laminar regime with low velocity. The bulk density is set to 0.9388 kg/m3 . Gravity is considered to be acting in the vertically downward direction. The gravitational acceleration vector is 9.81 m/s2.
In order to study the natural and forced convection, the colder external environment, namely 283 K, is modeled by considering the natural air flow blowing vertically from the hemline with a velocity of 0.5 m/s. The surface around the neck and arm is considered an outlet. The body’s skin is regarded as a conductive, convective wall with an emissivity of 0.98 . Due to the consideration of garment wearing comfort and fabric properties, the cloth is considered a smooth surface, and the fabric is twill woven fabric (with a component of 97% cotton and 3% Spandex, weight of 245 g/m2, and 0.42 mm of thickness). Detailed properties can be found in Table 2.
|Medium||Boundary types||Physical properties||Heat transfer||Heat radiation|
|Inlet||air||Characteristics inflow||pair = 101,325 pa,
ρair = 1.165 kg/m3,
vair = 0.5m/s
|Tair = 283.15K
Cp_air = 1,004 J/kg·K
TCair = 0.026 W/m·K
|Ɛair = 0.02|
|Outlet||air||Back pressure outlet||pout = 101,325pa||--||--|
|Body||skin||Heat transfer wall||ρskin = 860 kg/m3||Tskin = 310.15K = 37°C
Cp_skin = 5021J/kg·K
TCskin = 0.030 W/m·K
|Ɛskin = 0.98|
|Fabric||cotton 97% + Spandex 3%||Adiabatic wall||ρfabric = 245g/m2||Cp_fabric = 1150 J/kg·K
TCfabric = 0.059W/m·K
|Ɛfabric = 0.9|
2.3. Verification of the numerical method
A widely used 3D thermal protective performance test, e.g., the ASTM D 4,108 apparatus, is simulated to validate the numerical method, as shown in Figure 2. In this bench top test, the cloth to be tested is assembled on a holder on top of the fixed heat source. The heat flux from the heat source transfers through the cloth and air gap and then reaches the sensor’s surface on the top. Here, the air gap distance is set to 6.4 mm, and the heat flux is constant at 80 kW/m2. These data keep the same as Sawcyn’s [3, 16] experimental data and Udayraj’s  simulation conditions.
The temperature rise and total heat flux obtained by experiment sensors and simulation probe are compared in Figure 3 (a) (b). As shown there, the simulated temperature rise and heat flux are very close to experimental and public data. These curves fit well with each other, and the differences are within the acceptable error range. That is to say, the numerical method has the ability to describe heat transfer accurately in the following investigation.
3. Results and discussion
3.1. Air gap distribution
The air gap present between fabric and human skin plays an important role in heat transfer. The 3D air gap thickness under the clothing is obtained by aligning the naked body and clothing in Geomagic Qualify 2,013 (Geomagic, Inc, USA). Figure 4 shows the distance ease allowance of the H-, X-, and O-types of body model. As the figure shows, the air gap is unevenly distributed over the body due to the specific geometry of the human body shape, so the small distance of the H-type of clothing appears around the shoulder, neck, and chest. The small air gap thickness of the X-type of clothing appears on the shoulder, neck, under-chest, armpit and abdomen and is lower than 3 mm because of the close-fitting clothing. For the O-type, the distance is relatively small on the parts above the bust and is big on the parts under the bust.
We aim to analyze the air gap distance in detail, and we obtained the air gap distance distribution on key body girth along the horizontal axis, i.e., the x-coordinate.. The hemline is considered to be y = 0 cm; then the hip, waist, bust, and shoulder circumference are located at y = 2 cm, 22 cm, 38 cm, and 49 cm. The cross-section and contour line, as well as the Cartesian coordinates, are shown in Figure 1(d).
The study comparison of three types of garment air gap distributions on key girth is shown in Figure 5. The horizontal axis refers to the X-coordinate distance, and the vertical axis refers to the air gap distance. As shown, the X-type of garment has a narrow air gap of less than 5 mm on the chest girth, and the H-type garment has a distance of less than 8 mm on the whole chest girth. However, the air gap distance of the O-type of garment is between 8 mm and 12 mm. As for the bust girth, the X-type of garment yields a distance of 7 to 9 mm, while the H-type and O-type of garments have a distance of 10 mm to 13 mm in the area of x < 0.04 m. However, this value will fall to a lower level of about 5 mm near the area of 0.08 m < x < 0.10 m because of the breast region. The air gap distributions on waist girth are very different from others, as can be seen, and the X-type of clothing has a narrow air gap of about 10 mm, while the H-type of clothing has a distance of 20 mm to 60 mm on the front and back side, and the O-type of clothing has a distance of 38 mm to 70 mm. These differences are created from the clothing structure on the waist girth. For the hip girth, the air gap distance is 10 mm to 15 mm for the X-type and is nearly 20 mm to 30 mm for the H-type and O-type of clothing.
3.2. Heat transfer characteristic
Figure 6 shows the total heat flux loss of body skin. It depicts that the surface near the hemline lost the most heat flux because of forced convection. The areas around the neckline and bust, even the abdomen of the X-type of model, have a relatively higher heat loss, for the clothing fitts the body surface well in these regions. Relatively speaking, the clothing of the X-type of model has the maximum heat flux of the three models on account of the narrow space between the body and the clothing.
In order to analyze the heat transfer characteristics under different types of clothing, the total heat flux along the body contour lines on the XOZ cross-section is obtained for three types of apparel models, as shown in Figure 7. It is thus clear that the heat flux distributions display nonuniformity and a wave-like appearance in every cross-section. Figure 7(a) shows that the X-type of model yields the highest heat flux on the chest circumference and the O-type has a lower value than the H-type of clothes in the same location. The reason is that the air gap of the X-type of model is slightly narrow on the chest, so the conductive heat flux can transfer from the air gap and reach the cloth surface near the chest region easily. Figure 7(b) shows that every heat transfer curve on the bust circumference has a lot of peaks and troughs. The maximum value appears at the end of the curves, near the armpit region, where the clothing is next to the body surface with a small air-gap distance (lower than 5 mm). The reason could be that the airflow will speed up when it encounters a shrink section, and the acceleration of the air will bring about high heat flux loss near the area. Figure 7(c) makes it clear that the X-type of model is higher than the other two on the waist back side and is lower than others on the front side owing to the strong forced convective heat transfer in the air gaps. The heat transfer of the H-type of model and the O-type of model stays at a low level around the waist girth due to the big enough. Figure 7(d) shows that the heat transfer of hip girth presents a different scene, as the X-type of model is bigger than others mainly due to having the strongest forced convection from the hemline. Meanwhile, the curve of O-type of model is slightly lower the H-type of model because of the relatively loose air gap (more than 20 mm). From the analysis above, it can be easily concluded that the three-dimensional heat flux loss is negatively related to the air gap distance to a large extent.
3.3. Temperature distribution on a garment surface
The flow contours of temperature and total heat flux of different types of cloth are investigated as follows. As can be seen in Figure 8, the high-temperature regions of X-type clothing are in the shoulders, neck, chest, armpit, and waist parts owing to the close range of body skin and cloth surface. The high temperature regions of the H-type of model are limited to the shoulders, neck, and chest, while the high temperature regions of the O-type are just the neck and mastoid areas. This phenomenon can be explained as follows: the cloth surface of the shoulder, neck, and chest areas close to body with a small air gap, so the conduction, convection, and radiation heat flux can reach the cloth surface easily; this will result in the increasing of temperature on the cloth surface. The other parts of the cloth have a big air gap distance, so the surface temperature remains at a low level.
Figure 9 shows the cloth temperature on the XOZ cross-section of the chest, bust, waist, and hip under different calculations. Figure 9(a) shows that the X-type clothing has the highest temperature on the whole, but the H-type model obtains the highest temperature in the front side between x = 0.08 m and 0.14 m, whereas the O-type has the biggest numbers in the back side between x = 0 m and 0.03m. This result is a little different from our daily knowledge. The reason can be explained by hydromechanics and heat transfer theory as the idea that quicker air flow brings higher heat flux, which results in high temperature on the clothing surface. Figure 9(b) (c) shows that the X-type has a higher temperature on bust girth than others, and then the temperature curve has several ups and downs near the bust and waist girth. This is due to the narrow air gap and the turbulent flow between the body and cloth which brings uneven heat conduction and convection. Figure 10(d) shows that the X-type has the highest temperature both in the front and back sides of the hip girth, and the other curves show a wave type with a low numerical level due to the forced inflow cooling air.
From the above analysis, it can be seen that the heat loss around the neck line, bust, and armpit area is higher than in other regions for all three types of clothing. So the heat transfer must be considered for the design of a garment and the air gap should be kept at a suitable level, namely, more than 3 mm in the neck and bust regions. Furthermore, in the design process of a tight-fitting garment, the waist and abandon regions need to be considered at the same time.
4. Concluding Remarks
The primary goal of this study is to investigate several important heat transfer issues of three different types of air gap models for clothing. The numerical method has been validated by available bench-top test experimental data, and good agreement between the two has been found. This CFD model is then used to determine the effect of heat transfer through the air gap. The heat transfer characteristics and airflow mechanism have been found and are discussed. The influence of different clothing styles on the microclimate has been investigated. Some useful suggestions are given for garment design and thermal wearing comfort.
The heat transfer between the body and clothing is related to the air gap distance. When the air gap is relatively small, namely, about 5 mm in the chest and bust girth, the conductive heat transfer can transfer through the narrow air gap and reach the cloth surface easily; therefore, it will bring about the increase of temperature on the clothing surface. When the air gap distance becomes big enough, namely 50 mm on the waist girth of the H-type and O-type of garment, the conductive heat transfer becomes small and the convective heat transfer becomes the main part of heat flux.
The analysis of airflow mechanism shows that the airspeed will be accelerated to high numbers when the air gap decreasing from loose to a narrow channel. And the acceleration of airflow results in the increase of forced convection, and then the temperature on the clothing surface will rise due to heat flux injection. A comparison of different types of clothing shows that the X-type of model has the highest and most complex heat loss and temperature change and the H-type and O-type of models gain much slower airflow and lower heat transfer on the whole.
The analysis of high temperature regions on clothing and high heat flux regions on body skin shows that the upper part of the garment that includes the neck, bust, and armpit region includes the main areas correlated with thermal comfort. So the areas of the shoulders, chest, bust, and armhole should be considered carefully in the garment design process, and the air gap distance should be kept at a suitable level for a tight-fitting garment.
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© 2022 Yijie Zhang et al., published by De Gruyter
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