Effect of underwear on microclimate heat transfer in clothing based on computational fluid dynamics simulation

Abstract

In order to study the influence of underwear on microclimate heat transfer among different age groups, this study measured the temperature of the microclimate layer corresponding to the main parts of the human body or the key parts that affect average skin temperature. A computational fluid dynamics numerical model was then used to simulate the influence of underwear on heat transfer between the human body and the microenvironment and to explore the physical phenomenon. The results obtained show that underwear has a great influence on the average temperature of the microclimatic air layer, especially the air layer at the upper arm, forearm, and thigh. The findings of this study provide fundamental knowledge to improve the thermal comfort of underwear.

Keywords

underwear heat transfer microclimate air layer CFD

Human beings have been studying thermal comfort for a long time. From factors affecting thermal comfort to indexes evaluating thermal comfort, from architectural physics to human physiology and psychology, scholars at home and abroad have undertaken considerable research. As an important physiological parameter of the human body, skin temperature is closely related to the human body’s thermal sensation and thermal comfort, and can reflect the human body’s state of thermal equilibrium. In addition, many factors that affect human thermal comfort mostly influence human thermal balance and thermal sensation by heat exchange between the skin and the environment, and these factors can be expressed as a function of skin temperature.^1–7 Therefore, in order to study human thermal comfort more carefully and accurately, it is necessary to study this physiological parameter further. The influence of various kinds of clothing or clothing styles on human thermal comfort has also been studied. For example, Liu,⁸ Zhai et al.,⁹ Fu,¹⁰ and other researchers carried out simulation research on the distribution law of the microclimate’s temperature field and water vapor pressure field between the human body and clothing (including protective clothing), and found the main influencing factors of the thermal response of the dressed human body by combining experimental measurements, thereby laying a theoretical foundation for later scholars’ research on the thermal response of the human body. Wang et al.¹¹ and Quintela et al.¹² obtained the heat transfer coefficient by analyzing heat transfer between a manikin and the environment. Famworth,¹³ Stuar et al.,¹⁴ Lotens et al.,¹⁵ Berger,¹⁶ and Nordon et al.¹⁷ conducted numerical simulation research on microclimate heat and moisture transfer through a heat conduction–radiation model, convection model, and heat–moisture coupling model, respectively, and established a mathematical model for heat and moisture transfer inside clothing.

Many scholars have conducted simulation research mainly on the heat and moisture distribution law of clothing–human microclimate, heat transfer between the human body and the environment, human body thermal response parameters, and human body heat and moisture comfort in special environments. However, most research has ignored the influence of underwear on human thermal comfort or has considered it only in terms of thermal resistance (thermal resistance of air).^18–24 This will have had a certain influence on the accuracy of the research findings and means that the research was not rigorous enough, since underwear is an important item of clothing. The fabric of underwear and the complex structure between fabrics influence the heat and moisture transfer or microclimate state. At the same time, as clothing that is next to human skin, underwear absorbs sweat when the body perspires, which greatly changes many properties of the fabric, such as moisture dissipation performance, thermal insulation performance, and so on, thus greatly affecting the thermal and moisture comfort of the underwear. Therefore, the underwear shouldn't be ignored. However, because the air in the microclimate is always changing, including air motion, air thickness, and air temperature, underwear is part of a more complex environment, which makes it more difficult to study the heat transfer effect of underwear on the microclimate. In order to overcome this difficulty, this study measured the temperature of the microclimate layer corresponding to the main parts of the human body or the key parts that affect the average temperature of the skin, and then used computational fluid dynamics (CFD) to simulate the effect of underwear on heat transfer between the human body and the microenvironment.

Methods

Experimental participants

In order to cover the population comprehensively at various stages, 15 healthy participants were selected and divided into three groups: group A (aged 18–24 years old), group B (aged 25–50 years old), and group C (aged 51–60 years old). Those tested in each group were recorded as A1 … A5, B1 … B5, and C1 … C5. The tolerance and sensitivity of the participants in each group to the thermal environment were basically the same. The specific parameters of human body shape are shown in Table 1. The subjects could not perform any strenuous activities before the experiment.

Table 1.

Body-shape parameters of participants

Participant	Age	Breast (cm)	Hip (cm)	Waist (cm)	BMI	Human surface area (m²)
A1	18	85	91	76	21.9	1.8
A2	20	89	98	73	22.9	1.9
A3	21	88	100	79	22.2	1.9
A4	23	85	88	80	21.1	1.8
A5	24	87	90	72	22.2	1.9
B1	28	93	90	83	20.1	1.8
B2	31	95	98	79	24.4	1.9
B3	38	88	95	88	23.5	1.9
B4	43	93	94	89	22.8	1.9
B5	49	99	98	92	25.0	1.9
C1	51	100	99	86	25.3	1.9
C2	53	98	89	85	21.1	1.7
C3	53	93	92	83	21.2	1.9
C4	56	103	98	82	25.2	1.9
C5	60	105	102	98	29.1	1.9

BMI = weight (kg)÷Height ^2(m); human surface area = 0.0057 × height + 0.121 × weight + 0.0882.

BMI: body mass index.

Materials

Using the same fabric (see Table 2 for fabric parameters) and the same sewing process, three pieces of tight short-sleeved underwear for the upper body and underwear with the same style were made according to the average body shape of each group of testers. The styles are shown in Figures 1 and 2. Coats and trousers with the same brand and the same style were bought according to the average body size of the three groups, as shown in Figure 3.

Figure 1.

Style structure of male upper-body underwear.

Figure 2.

Style structure of male underwear.

Figure 3.

Style structure of coat and pants.

Table 2.

Parameters and styles of experimental clothing

Fabric	Textile construction	Fabric weight (g/m²)	Longitudinal density/number of coils/5 cm	Horizontal dense/number of coils/5 cm	Moisture permeability (g/m²·h)	CLO value (clo)	Thickness (mm)	Heat transfer coefficient
98% cotton, 2% spandex	1 + 1rib	152.6	112	84	76.314	0.355	0.73	18.14

Experimental environment

The experiment was carried out in a climate chamber (l = 5 m, w = 6.4 m, d = 3.5 m). The climate chamber was ventilated via an upper inlet and a lower return air vent, and its structure is shown in Figure 4. The humidity was 40%, and wind speed was 0.1 m/s.

Figure 4.

Structure of the climate chamber.

Experimental procedure

Before the experiment, the participants sat quietly for 30 minutes outside the climate chamber wearing the test clothing and then went into the climate room for testing. The temperature of the climate chamber was set to 23℃, 30℃, and 40℃. The experiments were performed at the same time on different days, and the experimental duration was 60 minutes.

The laboratory was divided into two parts: subjects wearing neither upper-body underwear nor underwear, and subjects wearing upper-body underwear and underwear.

In order to record the temperature of the microclimate more accurately, the temperature at different positions in the microclimate layer corresponding to key parts of the human body was measured to indicate the temperature of the microclimate (i.e. the temperature at different distances from the underwear or skin was measured in the microclimate). By analyzing the process and characteristics of microclimatic temperature changes, the influence of the underwear on heat transfer between the human body and the microenvironment is discussed.

According to some literature, the upper arm, forearm, hand, back, chest, abdomen, thigh, lower leg (shank), and foot are important body parts that affect skin temperature. The weight coefficients of the forehead are 0.06, 0.08, 0.06, 0.05, 0.12, 0.12, 0.19, 0.13, and 0.07, respectively.^25–31 The formula for average skin temperature (ST) is:

\begin{matrix} ST = 0.0 6^{*} HT + 0.0 8^{*} UAT + 0.0 6^{*} FAT + 0.0 5^{*} HaT \\ + 0 . 12^{*} BT + 0 . 12^{*} CT + 0 . 12^{*} AT + 0 . 19^{*} TT \\ + 0 . 13^{*} ShT + 0.0 7^{*} FT \end{matrix}

(1)

where HT is the head temperature, UAT is the upper-arm temperature, FAT is the forearm temperature, HaT is the hand temperature, BT is the back temperature, CT is the chest temperature, AT is the abdomen temperature, TT is the thigh temperature; ShT is the shank temperature, and FT is the temperature of the feet.

Therefore, the experiment used a temperature measuring system (Maxim, San Jose, CA; the system is mainly composed of DS1922L button-type wireless sensors and data-processing software) to measure the temperature of the microclimate layer corresponding to the forehead, upper arm, forearm, hand, back, chest, abdomen, thigh, shank, and foot, as shown in Figure 5. Because the forehead, hand, and foot were not in the microclimate formed by the underwear and coat, the microclimate thickness was zero. Therefore, the microclimatic temperature of the forehead, hand, and foot was not measured.

Figure 5.

Human body surface testing point.

Determination of air thickness

A three-dimensional human-body scanner was used to measure the average air thickness at the measuring points of the three groups wearing underwear. Because the underwear was a tight garment made according to the average body shapes of the three study groups, when the ease allowances of the underwear were lower than those of the outer garment, the air thickness measured was similar for the outer garment scanned with and without underwear being worn because the outer garment did not compress the underwear. Therefore, the air layer between the underwear and the human body could be ignored.³² The measured air thickness is shown in Table 3.

Table 3.

Air thickness at different parts of the human body

Parts	Air thickness for group A (mm)	Air thickness for group B (mm)	Air thickness for group C (mm)
Head	–	–	–
Upper arm	8.3	7.7	5.6
Forearm	15.1	13.9	10.3
Hand	–	–	–
Back	1.5	1.6	0.9
Chest	0.8	0.3	0.7
Abdomen	2.6	1.1	0.6
Thigh	20.2	19.8	20.3
Lower leg (shank)	4.9	5.6	5.3
Foot	–	–	–

Because the surface of the human body is not a regular curved surface, the air thickness between the human body part and the garment is different when the body is dressed. For example, the air thickness at the shoulder, back, chest, and buttocks is smaller or even zero. The thickness of the air layer at the waist, thigh, and crotch is larger.^33,34 Moreover, the air thickness formed by different age groups is obviously different because body shape changes greatly as people get older.

Results

The results from the microclimate measurement with and without underwear at different temperatures for different parts of body are summarized in Tables 4–9. Tables 10–12 and Figure 6 present the average microclimatic temperature. In addition, we calculated the average temperature difference of the microclimate layer corresponding to each body part for the three experimental groups both with and without underwear, as shown in Table 13 and Figure 7.

Figure 6.

Comparison of the average temperature of the microclimate with and without underwear in different external environment temperatures.

Figure 7.

Difference diagram.

Table 4.

Average temperature of the air layer at different environment temperatures corresponding to each body part in group A when underwear is not worn

Parts	Environment temperature 23℃								Environment temperature 30℃								Environment temperature 40℃
Parts	1 mm	4 mm	6 mm	8 mm	10 mm	12 mm	15 mm	18 mm	1 mm	4 mm	6 mm	8 mm	10 mm	12 mm	15 mm	18 mm	1 mm	4 mm	6 mm	8 mm	10 mm	12 mm	15 mm	18 mm
Head	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Upper arm	32.5℃	30.3℃	28.6℃	25.7℃	–	–	–	–	34.3℃	33.7℃	32.1℃	30.2℃	–	–	–	–	35.4℃	34.3℃	33.7℃	36.6℃	–	–	–	–
Forearm	32.8℃	29.1℃	30.3℃	29.2℃	27.6℃	25.5℃	24.3℃	–	35.0℃	33.3℃	33.5℃	30.8℃	29.6℃	28.9℃	28.3℃	–	35.6℃	34.8℃	34.1℃	32.7℃	30.3℃	31.7℃	35.6℃	–
Hand	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Back	33.3℃	–	–	–	–	–	–	–	34.3℃	–	–	–	–	–	–	–	35.5℃	–	–	–	–	–	–	–
Chest	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Abdomen	33.8℃	–	–	–	–	–	–	–	35.1℃	–	–	–	–	–	–	–	36.8℃	–	–	–	–	–	–	–
Thigh	32.6℃	31.8℃	30.1℃	28.6℃	27.1℃	25.8℃	25.3℃	24.8℃	33.7℃	31.9℃	30.8℃	29.8℃	30.1℃	29.6℃	29.5℃	29.4℃	34.8℃	34.7℃	33.5℃	32.9℃	33.5℃	34.7℃	34.8℃	35.3℃
Shank	30.4℃	26.8℃	–	–	–	–	–	–	32.6℃	31.2℃	–	–	–	–	–	–	34.9℃	35.5℃	–	–	–	–	–	–
Foot	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–

The average temperature is the air-layer temperature value at 1, 4, 6, 8, 10, 12, 15, and 18 mm from the skin of each part of the human body. – means not participating in the measurement or the air thickness does not meet the measurement requirements.

Table 5.

Average temperature of the air layer at different environment temperatures corresponding to each body part in group B when underwear is not worn

Parts	Environment temperature 23℃								Environment temperature 30℃								Environment temperature 40℃
Parts	1 mm	4 mm	6 mm	8 mm	10 mm	12 mm	15 mm	18 mm	1 mm	4 mm	6 mm	8 mm	10 mm	12 mm	15 mm	18 mm	1 mm	4 mm	6 mm	8 mm	10 mm	12 mm	15 mm	18 mm
Head	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Upper arm	32.7℃	29.3℃	26.0℃	–	–	–	–	–	34.9℃	33.4℃	30.5℃	–	–	–	–	–	35.1℃	33.5℃	34.9℃	–	–	–	–	–
Forearm	32.1℃	31.8℃	31.2℃	27.9℃	26.4℃	25.9℃	–	–	35.7℃	34.1℃	33.8℃	31.2℃	29.6℃	27.1℃	–	–	36.0℃	35.1℃	34.4℃	32.6℃	31.5℃	33.3℃	–	–
Hand	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Back	33.9℃	–	–	–	–	–	–	–	34.2℃	–	–	–	–	–	–	–	36.4℃	–	–	–	–	–	–	–
Chest	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Abdomen	33.2℃	–	–	–	–	–	–	–	34.6℃	–	–	–	–	–	–	–	36.7℃	–	–	–	–	–	–	–
Thigh	33.4℃	32.3℃	31.6℃	29.3℃	27.5℃	25.8℃	24.8℃	23.9℃	34.1℃	32.0℃	31.6℃	29.1℃	28.3℃	27.2℃	27.6℃	29.2℃	35.2℃	34.9℃	32.7℃	31.9℃	32.6℃	33.1℃	33.7℃	35.9℃
Shank	30.8℃	25.5℃	–	–	–	–	–	–	33.3℃	31.8℃	–	–	–	–	–	–	34.1℃	35.2℃	–	–	–	–	–	–
Foot	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–

The average temperature is the air-layer temperature at 1, 4, 6, 8, 10, 12, 15, and 18 mm from the skin of each part of the human body. – means not participating in the measurement or the air thickness did not meet the measurement requirements.

Table 6.

Average temperature of the air layer at different environment temperatures corresponding to each body part in group C when underwear is not worn

Parts	Environment temperature 23℃								Environment temperature 30℃								Environment temperature 40℃
Parts	1 mm	4 mm	6 mm	8 mm	10 mm	12 mm	15 mm	18 mm	1 mm	4 mm	6 mm	8 mm	10 mm	12 mm	15 mm	18 mm	1 mm	4 mm	6 mm	8 mm	10 mm	12 mm	15 mm	18 mm
Head	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Upper arm	31.1℃	24.3℃	–	–	–	–	–	–	35.3℃	32.6℃	–	–	–	–	–	–	35.7℃	34.1℃	–	–	–	–	–	–
Forearm	30.5℃	28.8℃	26.7℃	24.9℃	23.9℃	–	–	–	34.9℃	33.3℃	32.5℃	28.2℃	27.3℃	–	–	–	36.3℃	35.6℃	33.8℃	31.9℃	34.6℃	–	–	–
Hand	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Back	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Chest	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Abdomen	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Thigh	30.6℃	30.2℃	29.5℃	28.1℃	26.2℃	24.7℃	23.2℃	22.8℃	33.5℃	32.2℃	30.1℃	28.5℃	26.5℃	25.3℃	26.9℃	28.1℃	35.6℃	35.1℃	34.3℃	33.4℃	32.8℃	31.6℃	33.3℃	35.1℃
Shank	29.7℃	24.5℃	–	–	–	–	–	–	33.3℃	30.9℃	–	–	–	–	–	–	35.1℃	35.9℃	–	–	–	–	–	–
Foot	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–

Table 7.

Average temperature of the air layer at different environment temperatures corresponding to each body part in group A when underwear is worn

Parts	Environment temperature 23℃								Environment temperature 30℃								Environment temperature 40℃
Parts	1 mm	4 mm	6 mm	8 mm	10 mm	12 mm	15 mm	18 mm	1 mm	4 mm	6 mm	8 mm	10 mm	12 mm	15 mm	18 mm	1 mm	4 mm	6 mm	8 mm	10 mm	12 mm	15 mm	18 mm
Head	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Upper arm	33.1℃	31.3℃	29.4℃	25.1℃	–	–	–	–	34.2℃	33.7℃	32.6℃	30.5℃	–	–	–	–	36.3℃	35.4℃	34.2℃	37.2℃	–	–	–	–
Forearm	33.4℃	31.2℃	30.5℃	20.1℃	27.9℃	27.4℃	24.3℃	–	35.6℃	34.7℃	33.8℃	31.2℃	30.1℃	29.2℃	28.9℃	–	36.0℃	35.4℃	34.9℃	33.6℃	33.1℃	32.3℃	36.1℃	–
Hand	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Back	34.1℃	–	–	–	–	–	–	–	35.3℃	–	–	–	–	–	–	–	36.5℃	–	–	–	–	–	–	–
Chest	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Abdomen	33.8℃	–	–	–	–	–	–	–	35.3℃	–	–	–	–	–	–	–	36.7℃	–	–	–	–	–	–	–
Thigh	33.0℃	32.1℃	31.0℃	29.3℃	28.5℃	26.4℃	25.8℃	24.9℃	35.1℃	33.4℃	32.6℃	30.1℃	29.7℃	29.4℃	29.3℃	29.8℃	36.1℃	35.7℃	34.4℃	33.1℃	32.8℃	35.2℃	35.3℃	35.5℃
Shank	31.6℃	25.6℃	–	–	–	–	–	–	33.3℃	32.1℃	–	–	–	–	–	–	35.3℃	36.1℃	–	–	–	–	–	–
Foot	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–

Table 8.

Average temperature of the air layer at different environment temperatures corresponding to each body part in group B when underwear is worn

Parts	Environment temperature 23℃								Environment temperature 30℃								Environment temperature 40℃
Parts	1 mm	4 mm	6 mm	8 mm	10 mm	12 mm	15 mm	18 mm	1 mm	4 mm	6 mm	8 mm	10 mm	12 mm	15 mm	18 mm	1 mm	4 mm	6 mm	8 mm	10 mm	12 mm	15 mm	18 mm
Head	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Upper arm	34.3℃	32.1℃	25.1℃	–	–	–	–	–	35.1℃	33.8℃	31.2℃	–	–	–	–	–	36.1℃	35.5℃	35.1℃	–	–	–	–	–
Forearm	32.7℃	31.8℃	31.5℃	28.2℃	27.3℃	26.2℃	–	–	35.9℃	34.7℃	33.7℃	32.1℃	30.3℃	29.5℃	–	–	36.2℃	35.5℃	34.8℃	33.1℃	31.8℃	34.9℃	–	–
Hand	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Back	34.0℃	–	–	–	–	–	–	–	34.8℃	–	–	–	–	–	–	–	36.5℃	–	–	–	–	–	–	–
Chest	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Abdomen	33.9℃	–	–	–	–	–	–	–	35.1℃	–	–	–	–	–	–	–	36.7℃	–	–	–	–	–	–	–
Thigh	33.2℃	32.1℃	31.3℃	29.1℃	28.4℃	26.5℃	25.8℃	24.8℃	35.3℃	33.7℃	32.9℃	30.5℃	29.4℃	29.0℃	29.4℃	29.7℃	36.5℃	35.9℃	34.5℃	33.0℃	32.7℃	35.7℃	35.4℃	35.6℃
Shank	32.1℃	25.9℃	–	–	–	–	–	–	33.8℃	32.9℃	–	–	–	–	–	–	36.0℃	36.6℃	–	–	–	–	–	–
Foot	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–

Table 9.

Average temperature of the air layer at different environment temperatures corresponding to each body part in group C when underwear is worn

Parts	Environment temperature 23℃								Environment temperature 30℃								Environment temperature 40℃
Parts	1 mm	4 mm	6 mm	8 mm	10 mm	12 mm	15 mm	18 mm	1 mm	4 mm	6 mm	8 mm	10 mm	12 mm	15 mm	18 mm	1 mm	4 mm	6 mm	8 mm	10 mm	12 mm	15 mm	18 mm
Head	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Upper arm	31.3℃	24.3℃	–	–	–	–	–	–	35.6℃	32.7℃	–	–	–	–	–	–	36.1℃	35.0℃	–	–	–	–	–	–
Forearm	30.4℃	28.9℃	26.4℃	24.5℃	23.8℃	–	–	–	35.0℃	33.7℃	32.9℃	28.1℃	27.5℃	–	–	–	36.6℃	35.8℃	34.1℃	31.7℃	34.8℃	–	–	–
Hand	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Back	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Chest	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Abdomen		–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Thigh	30.8℃	30.3℃	29.4℃	28.3℃	26.2℃	24.8℃	23.1℃	22.6℃	33.7℃	32.3℃	30.1℃	28.4℃	26.4℃	25.4℃	27.0℃	28.2℃	35.8℃	35.4℃	34.4℃	33.4℃	33.0℃	31.9℃	33.6℃	35.7℃
Shank	29.6℃	24.3℃	–	–	–	–	–	–	33.4℃	30.8℃	–	–	–	–	–	–	35.3℃	35.8℃	–	–	–	–	–	–
Foot	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–

Table 10.

Comparison of the average temperature of the microclimate with underwear and without underwear in the external environment at 23℃

	Average air temperature without underwear (℃)			Average air temperature with underwear (℃)
	A	B	C	A	B	C
Upper arm	29.3	29.3	27.7	29.7	30.5	27.8
Forearm	28.4	29.2	27.0	27.8	29.6	26.8
Hand	33.3	33.9	–	34.1	34.0	–
Back	33.8	33.2	–	33.8	33.9	–
Chest	28.3	28.6	26.9	28.9	28.9	26.9
Abdomen	28.6	28.2	27.1	28.6	29.0	27.0

The temperatures shown are the average temperatures of the air layers corresponding to the parts of the human body.

Table 11.

Comparison of the average temperature of the microclimate with underwear and without underwear in the external environment at 30℃

	Average air temperature without underwear (℃)			Average air temperature with underwear (℃)
	A	B	C	A	B	C
Upper arm	32.6	32.9	32.6	32.8	33.4	34.2
Forearm	31.3	31.9	30.3	31.9	32.7	31.4
Hand	34.3	34.2	–	35.3	34.8	–
Back	35.1	34.6	–	35.3	35.1	–
Chest	30.6	29.9	28.9	32.2	31.2	28.9
Abdomen	31.9	32.6	32.1	32.7	33.4	32.1

The temperatures shown are the average temperatures of the air layers corresponding to the parts of the human body.

Table 12.

Comparison of the average temperature of the microclimate with underwear and without underwear in the external environment at 40℃

	Average air temperature without underwear (℃)			Average air temperature with underwear (℃)
	A	B	C	A	B	C
Upper arm	35.0	34.5	34.9	35.8	35.6	35.6
Forearm	33.5	33.8	34.4	34.5	34.4	34.6
Hand	35.5	36.4	–	36.4	36.5	–
Back	36.8	36.7	–	36.7	36.7	–
Chest	34.3	33.8	33.9	34.8	34.9	34.2
Abdomen	35.2	34.7	35.5	35.7	36.3	35.6

The temperatures shown are the average temperatures of the air layers corresponding to the parts of the human body.

Table 13.

Average temperature difference of microclimate layer corresponding to each part of the three groups of experimenters with and without underwear

	NW-W difference (ET 23℃)			NW-W difference (ET 30℃)			NW-W difference (ET 40℃)
	A	B	C	A	B	C	A	B	C
Upper arm	0.4	1.2	0.1	0.2	0.5	1.6	0.8	1.1	0.7
Forearm	−0.6	0.4	−0.2	0.6	0.8	1.1	1	0.6	0.2
Back	0.8	0.1	–	1	0.6	–	0.9	0.1	–
Abdomen	0	0.7	–	0.2	0.5	–	−0.1	0	–
Thigh	0.6	0.3	0	1.6	1.3	0	0.5	1.1	0.3
Shank	0	0.8	−0.1	0.8	0.8	0	0.5	1.6	0.1

NW-W difference (ET 23℃): microclimate mean temperature difference with and without underwear at 23℃; NW-W difference (ET 30℃): microclimate mean temperature difference with and without underwear at 30℃; NW-W difference (ET 4℃): microclimate mean temperature difference with and without underwear at 40℃.

Discussion of experimental results

The tables and figures show a number of important findings. First, for all three age groups, the microclimate has a greater impact on the temperature of the microclimate’s air layer at the forearm and thigh when underwear is worn. So, in order to improve thermal comfort, the design of underwear should focus mainly on the forearm and thigh. At the same time, the temperature of the microclimate layer at the abdomen and back is obviously higher than that of other parts of the microclimate layer. So, attention should also be paid to the fabric characteristics of these two parts when designing underwear.

Second, when the air thickness becomes small, the temperature at 1 cm in the same part gradually increases with the increase in external environment temperature, but as the distance increases, the temperature begins to decrease and then increases, for example the microclimate corresponding to the thigh. If the air thickness is large, the temperature at the same position is different because air convection is generated, which affects the temperature. For example, when the upper arm is at 30℃, the temperature of the microclimate layer decreases with the increase in distance.

Third, the temperature of young and middle-aged men is similar in the same position of the air layer at each body part, but the temperature of the elderly is lower than that of young and middle-aged men because the metabolism of the elderly is slower and the body generates less heat.

Fourth, in high-temperature environments, the temperature of the air layer on or near the surface of the coat or trousers is greatly affected.

Fifth, underwear has an obvious effect on the microclimatic temperature, especially for the elderly. At the same time, it can be seen that the microclimatic temperature corresponding to some parts of underwear is higher than that without underwear, such as the microclimate layer corresponding to the upper arm, because the heat transfer coefficient of underwear is greater than that of air.

Sixth, the greater the air thickness, the greater the influence of underwear on microclimate will be.

In short, underwear has a significant effect on the temperature fluctuation of microclimate.

Discussion of CFD simulation analysis

Thermal comfort is greatly affected by environmental conditions and differences in human body parts. Even under the same environmental conditions, the skin temperature and thermal comfort of human body parts will show certain differences, and a CFD model can avoid such problems. A CFD model can be used to predict the heat exchange of the air layer. Here, the influence of underwear on the microclimate inside clothing is systematically discussed from a three-dimensional perspective, and then the heat transfer mechanism of the underwear–human body–microenvironment system is studied. Using 3Dmax and CLO3D to build clothing models, grid division was carried out using the CFD preprocessor Gambit to generate calculation nodes. The ANSYS Fluent of the CFD software was used to establish the model’s control equations, initial and boundary conditions, SST model, radiation model, and solution method. The CFD post-processing software Tecplot was used to process simulation results.

Basic equation

Since fluid flow is governed by the law of conservation in physics and the control equation is a mathematical description of the law of conservation, the CFD simulation model is established according to the law of mass conservation, the law of momentum conservation, and the law of energy conservation.

Mass conservation equation

Any flow problem must satisfy the law of mass conservation. This refers to the increase of mass in fluid microelements in unit time and the net mass flows into the microelements in this time interval. According to this law, the mass conservation equation can be obtained as flow:

\frac{\partial ρ}{\partial t} + \frac{\partial (ρ u)}{\partial x} + \frac{\partial (ρ υ)}{\partial y} + \frac{\partial (ρ ω)}{\partial z} = 0

(2)

where ρ is air density (kg/m³); u, υ, and ω are velocity vectors of fluid in x, y, and z directions, respectively (m/s); and t is time (s).

Momentum conservation equation

Any flow system must satisfy the momentum conservation equation. This means that the rate of change of momentum flowing in a microelement with respect to time is equal to the sum of various external forces acting on the microelement. According to this law, scalar forms of momentum conservation equations in x, y, and z directions can be derived:

\begin{matrix} \frac{\partial (ρ u)}{\partial t} + div (ρ uU) = \frac{\partial p}{\partial x} + \frac{\partial τ_{xx}}{\partial x} + \frac{\partial τ_{yx}}{\partial y} + \frac{\partial τ_{zx}}{\partial z} + F_{x} \end{matrix}

(3)

\begin{matrix} \frac{\partial (ρ υ)}{\partial t} + div (ρ υ U) = \frac{\partial p}{\partial y} + \frac{\partial τ_{xy}}{\partial y} + \frac{\partial τ_{yy}}{\partial y} + \frac{\partial τ_{zy}}{\partial y} + F_{y} \end{matrix}

(4)

\begin{matrix} \frac{\partial (ρ ω)}{\partial t} + div (ρ ω U) = \frac{\partial p}{\partial z} + \frac{\partial τ_{xz}}{\partial z} + \frac{\partial τ_{yz}}{\partial z} + \frac{\partial τ_{zz}}{\partial z} + F_{z} \end{matrix}

(5)

where p is the pressure on the fluid microelement (Pa); U is resultant velocity (m/s); τ_xx, τ_xy, and τ_xz are components of viscous stress τ acting on the microelement due to molecular viscosity action (Pa); and F_x, F_y, and F_z are the physical force on the microelement. If the physical force is only gravity and the z-axis is vertical upward, F_x = 0, F_y = 0, and F_z = –ρg.

Energy-conservation equation

Energy conservation is the basic law that the flow system of heat exchange must satisfy. The increase in the rate of energy in the microelement body is equal to the net heat flow into the microelement body plus the work done to the microelement body by physical force and surface force. This law is actually the first law of thermodynamics.

\frac{\partial (ρ T)}{\partial t} + div (ρ UT) = div (\frac{k}{c_{p}} gradT) + S_{T}

(6)

where c_p is the specific heat capacity (J/(kg·K)); S_T is the internal heat source of fluid and the part where fluid mechanical energy is converted into heat energy due to viscous action, sometimes referred to as viscous dissipative phase for short (J); T is temperature (K); and k is the heat transfer coefficient of the fluid (W/(m·K)).

Species mass-conservation equation

In a species system, there may be mass exchange, or there may be many kinds of species, and each component must follow the law of species mass conservation. For a certain system, the law of species mass conservation can be expressed as: the rate of change of the mass of a chemical component in the system with respect to time is equal to the sum of the net diffusion flow through the interface of the system and the productivity of the component produced by chemical reaction. According to the law of conservation of component mass, the equation of conservation of component mass for component r can be written as:

\frac{\partial (ρ c_{r})}{\partial t} + div (ρ U c_{r}) = div (D_{r} grad (ρ c_{r})) + Sr

where, c_r is the volume concentration of component r; ρc_r is the mass concentration of component r; D_r is the diffusion coefficient of component r; Sr is the rate of formation of component r.

Establishment of human-body model

Using the average body-shape data of three groups of personnel, three average body-shape models for CFD simulation were established through 3Dmax, as shown in Figure 8.

Figure 8.

Human body model for computational fluid dynamics simulation.

Figure 9.

Schematic of temperature in air layer at the upper arm when underwear is not worn.

Heat-exchange model

CFD calculation requires a fluid and heat transfer model. When calculating convection, the ideal gas equation is chosen to describe the component concentration and to solve natural convection. Radiation calculation selects the surface radiation heat transfer model. Combined with the research of some scholars, the wind speed is <0.5 m/s under the thickness of the air layer in the clothing within 20 mm. So, the simulation of the microenvironment in the clothing is generally a low flow speed and a low Reynolds number flow state. At the same time, due to the precision requirement for the near wall surface, the SST model is a more suitable turbulence model for the solution process of the microenvironment in the clothing. So, the SST model was selected in this experiment.^35–37

Set boundary conditions

The boundary conditions are provided by the experimental conditions. The wall temperature, air temperature, and wind speed of the climate chamber remained the same as the experimental conditions. After setting the model and boundary conditions and determining the under-relaxation factor, initialization was carried out and iterative calculation was started until a sensible heat exchange between the human body and environment was obtained by convergence, as shown in Table 14.

Table 14.

Set boundary conditions

Area	Name	Category
Air layer	Upper boundary of air layer (Head)	Opening
	Down boundary of air layer (Feet)	Opening
	Skin/outer surface of underwear	Wall
	Inner surface of clothing	Interface
	Radial boundary	Symmetry
Clothing	Apparel/underwear surface	Interface
	Outer surface of clothing	Wall
	Upper and down boundaries of clothing (the positions of clothing openings)	Wall
	Apparel radial boundary	Symmetry

CFD simulation results and discussion

CFD was used to simulate the temperatures of the microclimate layer corresponding to the upper arm, forearm, and thigh of the three groups of subjects, as shown in Figures 9–14.

Figure 10.

Schematic of temperature in air layer at the upper arm when underwear is worn.

Figure 11.

Schematic of temperature in air layer at the forearm when underwear is not worn.

Figure 12.

Schematic of temperature in air layer at the forearm when underwear is worn.

Figure 13.

Schematic of temperature in air layer at the thigh when underwear is not worn.

Figure 14.

Schematic of temperature in air layer at the thigh when underwear is worn.

The results shows that in the two states of wearing and not wearing underwear, the changing amplitude of the air-layer temperature of the forearm and thigh was greatly affected by the external environment temperature, mainly due to the large air thickness at the forearm and thigh, resulting in air convection leading to temperature instability. In addition, the air-layer temperature was higher when wearing underwear than when not wearing underwear because the thermal resistance of the fabric is lower than that of air, and more heat generated by human body can be transferred to the air layer through underwear. With the increase in thickness of the air layer, the temperature of the air layer is constantly changing. For example, for an air layer with a thickness of 12 mm, the temperature of the air layer decreases first and then increases with the increase in thickness. The average temperature of the air layer close to the external environment is closer to the external environment temperature. Underwear has a greater impact on young and middle-aged people because they have higher levels of vitality and metabolism, and their bodies generate a large amount of heat. With close-fitting underwear, excess heat can be quickly transferred to the microclimate inside the clothes.

Conclusions

This study aimed to analyze how underwear influences heat transfer in the microclimate. We used CFD to simulate the process, combining this with experimental results from the human body. The results show that underwear has a great influence on the average temperature of the microclimatic air layer. Fluctuations in the microclimatic temperature show some obvious discrepancies at different parts of the human body and at different ages. Furthermore, the thickness of the air layer also influences the temperature of the microclimate. In summary, compared to other methods, CFD can overcome some of these difficulties to show microscopic changes in microclimate, providing a more intuitive way to help people understand the changes in microclimatic temperature.

Footnotes

Declaration of conflicting interests

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

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Financial support was received from the Fundamental Research Funds for the Central Universities (223 + 2019 + G-08). This study was also financially supported by funds from the Collaborative Innovation Center of Modern Clothing Technology, Minjiang University (No. MJKFFZ201708), National key research and development plan “science and technology in Winter Olympic Games” (2019YFF0302100) and special funds from Fuzhou Department of Science & Technology (No. 2017-G-112).

ORCID iDs

Pengpeng Cheng

Daoling Chen

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