Advances in research on air layer heat transfer performance under clothing

For single-layer clothing, the air layer is only the micro air layer between the surface of the body's skin and the inner surface of the garment, and for multi-layer garment system, the air layer is also including the air layer between the garment layer and the layer. [1-2]. Since the thermal conductivity of the air is much smaller than the thermal conductivity of the fibers, the stationary air has good thermal insulation performance. In the style design of protective clothing, the air layer is effectively increased, which can improve the heat preservation properties of cold clothing, and can also enhance the thermal protection performance of high temperature protective clothing. Therefore, the clothes air layer is widely focused on Chinese foreign scholars and conducted in-depth research.

This article is based on the loading air layer as the research object, which introduces the heat transfer mechanism of the clothes air layer and the test method of heat transfer performance and its advantages. It affects six factors affecting the performance of heat transfer, and The trend is a two-point look.

First, the air layer heat transfer mechanism

comprehensively analyzes the heat transfer mechanism of the air layer under the sheath, which helps to establish accurate The thermal transfer model of clothing, understanding each factor (thickness, volume, etc.) for the heat transfer performance of the air layer. The heat transfer method of the air layer includes conduction, convection and radiation, and the flow state of the air determines the air layer conduction or convective heat transfer method, the geometric shape of the air layer affects the heat exchange coefficient of the radiation.

1. Transmission or convection heat transfer

The flow state of air is divided into stationary, laminar flow and turbulence, and in different flow conditions, the heat transfer method of the air layer is different. In the heat transfer, the flow state of the air can be determined according to the Rayleigh number RA [3], ie

formula: RA is the number of quarters; ?? is the air Layer thickness, m; g is gravity acceleration, m / s2; T is the temperature difference of the air layer boundary, k; beta; is a volume expansion coefficient, that is, the absolute temperature of the air, K-1; alpha; is air heat Diffusion coefficient, M2 / S; V is aerodynamic viscosity, M2 / s.

Document [4] By studying the air flow state in the bottom heating horizontal rectangular cavity, when the length and width of the rectangular cavity are much larger than the thickness, if Ra Le; 1700, the air in the rectangular cavity is in a stationary state. If 1 700 lt; Ra le; 5 Times; 104, air is in the laminar flow state (rectangular ring flow); if Ra gt; 5 Times; 104, air is in a turbulent state. In the thermal insulation performance test between the fabric system, the air layer between the hot plate and the fabric is similar to a horizontal rectangular cavity heated by a bottom. Therefore, scholars [1, 5] are designed for hot flat tests, and the fabric layer-air layer rdquo; thermal transfer model, generally multi-use RA u003d 1700 as a basis for determining whether the air layer is naturally convective. If the air is in a stationary state, according to Fourier conducting the lawEstablish the air layer heat transfer model of the clothes; if the air is in the laminar flow or turbulent state, the heat transfer model can be established according to Newton Cooling Formula.

1.2 Radiation heat transfer

The main components of air are nitrogen and oxygen, and both gas does not radiate, and air can be regarded as polar molecules such as CO2 and water vapor. For the transparency, according to Stephen Bolzman's law, it is found that the main factors affecting its radiation heat transfer performance is the viewing coefficient between clothing and skin two surfaces. For heat transfer models established by hot plate test [6], the fabric and the hot plate are in parallel and the surface area is equal to 1; the heat transfer model established for hot cylindrical test [6], inner cylinder The skin layer, the outer cylinder is a fabric layer, and its viewing coefficient is the ratio of the inner circular radius and the outer radius; for the warmer dummy [7] test, due to human body surface form [8], clothing style [9] and The influence of fabric mechanics [10], the distribution of air layer under the clothing is uniform, and it is necessary to take the view coefficient according to the angle of relational settlement.

In order to study the influence of the heat transfer performance of the garment system, scholars A large number of experimental and model studies have been conducted for each influencing factors, and the research methods can be summarized as hot flat test, fake people test and numerical simulation.

2.1 Hot Tablet Test

Hot Tablet Test For the study of thermal transfer performance of fabric system (skin layer-air layer-fabric layer-external environment). The test device is generally an improvement to the existing fabric heat transfer performance testing device (such as a sweat plate instrument), mainly including: a hot plate that can simulate the temperature of the skin, prevent heat insulation heat insulation sheet from the side and bottom dispersions, can simulate Different thickness of clothing spacers, temperature sensors, heat flow sensors, and artificial climate boxes. In order to prevent the fabric from falling down from the weight, it will be used to make a mesh shape using the strong nylon yarn on the partition plate to hold up the fabric to ensure the accuracy of the thickness of the air layer under the clothes, as shown in Figure 1. .

The method can simulate the thermal resistance of the fabric system by simulating different thicknesses, quantitatively analyze the relationship between the thickness of the clothes and the thermal resistance of the fabric system, and can Establish a linear regression equation between the thickness of the clothes and the thermal resistance of the fabric system [11]. Further, a partition plate having different thicknesses can also be added to the layer between the fabric layer and the layers to investigate the size of the air layer of each layer and its air layer in the position of the fabric system for the heat transfer performance of multilayer fabric system [12] . However, the flat panel test is only analyzed on the fabric level, and the air layer is closed, which does not match the actual situation, and therefore, the study on the fabric level does not comprehensively reflect the space of the air layer. Thermal performance still needs to consider the impact of other factors.

2.2 Dummy Test

Dummy Tests for Research Apparel Systems (Skin Layer - Underwater Air Layer - Garment Layer - External Environment) Relationship between the volume and thickness of the air layer under the clothing. Test systems include: 3D human body measuring system and dummy system.

Scholars [13-15] measures the volume and thickness of the air layer under the clothing of the three-dimensional body measurement system, and the step is to three-dimensionally scanned the nude morphology and dressing form of the human body, obtain two forms. Point cloud data, then encapsulate, repair, and fill holes to obtain two forms of three-dimensional curved surface models, and finally, 3D comparisons, and 2D comparison, obtaining the thickness and volume of the clothes. The dummy system can be applied to test clothing system heat insulation performance, such as a warm fake person, combustion fake person [16]. In order to ensure the effectiveness of the experimental study, the human body model in the three-dimensional scan should be the same or similar to the dummy body type of the test clothing insulation performance to ensure that the air layer is studied in the same housing before and after. At the same time, since the clothing needs to be taken away from the human body model and the dummy, it will cause the air layer morphology in the clothes. Therefore, Wang et al. [17] proposes to build a three-dimensional body measurement system with the dummy system under the same operating platform to avoid ldquo; secondary wear away rdquo; resulting measurement error, improves the accuracy of the test.

Scholars [18-20] have been used to study the relationship between the air layer volume or thickness and the thermal insulation performance of the clothing, and established a related linear regression equation. Because the dummy test is based on the discussion on the clothing, its research results are more practical. However, the outer air layer obtained by the three-dimensional body measurement method is the spatial form between the outer surface of the clothing to the human surface, and ignores the disturbance of the fabric thickness for the air layer of the clothes. Therefore, the measurement method is only suitable for a thin layer of fabric. Clothing, not suitable for clothing with thicker fabrics or multi-layer fabrics.

To exclude disturbance of fabric thickness, Chen et al [21] measures the thickness of the presuppost (50 GF / CM2) state using KES-F (Kawabata Evaluation System for Fabric), and comes from it The lower air layer is subtracted in the thickness of the air layer to obtain a more realized air layer. However, in the dressing state, the pressure of the fabric of the clothing is not the same, and the thickness is different. Therefore, this method does not better reflect the true distribution of the air layer of the thick clothing. To measure the air layer between the outer air layer and the clothing layer between the multilayer fabric combination garment and the layer, McQuerry et al [22] will be three-layer fire service (from the outer to the outermost layer, the waterproof air gas gas layer and insulation) Layer) Split is three-piece single-layer clothing, and then measure the clothes air layer of the single layer of clothing one by one. However, affected by the performance and gravity of the fabric, the air layer of the three-piece single-layer clothing and the three-layer fire service is very different. For example, the clothing of the waterproof gas-free layer is testedThe lower air layer has a thickness of 37.2 mm, while the heat insulating layer is 39.7 mm, and the thickness of the outer garment of the outer garment is small in the inner layer, and the measurement has an error. Therefore, the outer air layer of the clothing that measures the thickness of the fabric or multilayer fabric is still difficult to measure, and the research on dummy test is more concentrated on the single-layer thin clothing level.

2.3 Numerical Simulation

With the development of computer simulation modeling technology, numerical models have been applied to various fields. Scholars have studied a variety of factors (thickness, volume, etc.) for the effects of air layer insulation properties in the clothes by constructing a numerical thermal transfer model. The research step is to construct ldquo; skin layer-clothing air layer-fabric layer / clothing layer - external environment rDQUO; heat transfer model, and then calculate the heat transfer model based on the experimental research and verification model Reliability, finally, the effect of each factor is used to analyze the heat insulation performance of the clothes under the construction of heat transfer model.

Testing and dummy testing can only qualitatively or quantitatively study the effects of various factors for the heat insulation performance of the clothes, and the numerical simulation is further interpreted on this, and the heat transfer of the air layer is further interpreted. mechanism. Therefore, scholars have established a large number of numerical thermal transfer models for the air layer (Table 1). As can be seen from Table 1, the heat transfer model has been developed from one-dimensionally three-dimensional, single-layer air layer to a plurality of air layers, a single heat transfer manner to a plurality of heat transfer methods.

Third, heat transfer performance influencing factors

Three test methods, scholars The heat transfer performance of the clothes air layer is mainly analyzed from the thickness, volume, position, direction, non-uniform morphology and motion state, and provide comprehensive theoretical guidance for the style design of cold clothing and thermal protection clothing.

3.1 Thickness and volume

The thickness of the air layer is the main factor affecting the form of air flow. When the thickness of the air layer is gradually increased from zero, since the air buoyancy can not obey the viscous resistance in a stationary state, the heat transfer mode is conducted and radiated. According to Fourier conductive law, the thickness is increased, and the insulation performance is increased. When the thickness exceeds a certain value, the air buoyancy overcomes viscous resistance, air turns to laminar flow state, heat transfer mode is convection and radiation, and the convection rate increases the heat transfer rate, so the thickness increases, the heat insulation performance is no longer increased. And even decreased. Therefore, during the increase in thickness, thermal insulation performance will have a maximum value, i.e., the best air layer [31]. Accurately determine the optimal air layer threshold, which is of great significance for design functionality or protective clothing.

Document [11, 23, 32-35] Based on the hot plate test method, the best air layer of the normal temperature environment and the fire environment is determined, as shown in Table 2. Contrast found that the best air layer in the fire environment is generally more than normal temperature environmentsSmall, the reason is that the temperature of the fire is high, that is, the temperature difference between the skin and the garment is large. According to the formula (1), if the? T is large, the RA is fixed, and the air layer of natural convection occurs. The thickness will be small, so the best air layer in the fire environment is smaller.

In addition, scholars have also studied the impact of moisture for the optimum air layer threshold for fire protection services in a fire environment. Lu et al [32] tested the optimum air layer, Li et al. [33] with ultrasonic nebulizers to humidify the air layer with ultrasonic nebulizers. 3 optimal air layers under relative humidity (35%, 65% and 95%). Comparative Torvi et al. [23] The best air layer measured under a donor flage conditions, and the results showed that the aqueous content of the fabric or the relative humidity of the microenvironment of the fabric is increased, the optimum air layer threshold increases.

The air layer volume of the underwater can be obtained by a vacuum method [36] or a three-dimensional body scan method. Document [18-20] The effect of air layer volume on clothing thermal resistance was studied by a dummy test. The study found that the effects of volume and thickness were similar to the heat transfer performance of the air layer. Duan Xiyuan et al [20] establishes a linear regression equation of volume and clothing thermal resistance in conjunction with ordinary underwear and fever underwear, as shown in the following formula.

Y is the overall thermal resistance of underwear, ° C MIDDOT; M2 / W-1; X is between underwear and dummy Air layer volume, CM3.

3.2 Position

For clothing of multilayer fabric, the air layer is not only between the skin and the garment, but also between the garment layer and the layer. Scholars [12, 37-39] mainly investigated the position of the air layer on the thermal protection performance of clothing in multi-storey fire clothing.

Huang et al. [37] constructed a heat transfer model of 4 layers of fire fabric system, and the fabric system is the outermost layer, waterproof gas-free layer, heat insulating layer, and comfort layer, including the insulation layer from the outer to the outer to the outside. Connected to the comfort layer, there is no air layer, and the remaining positions are present in the air layer. The comparison of the air layer is located between the outermost layer and the waterproof gas-free layer and the heat insulation performance between the heat insulating layer and the skin. As a result, when the air layer is in the outermost layer and the waterproof layer, the thermal insulation performance of the fabric is better. That is, the air layer is located close to the outermost layer of the fabric, and the heat insulation performance is better. WANG et al [38] and Fu et al. [39] use TPP (Thermal Protective Performance) and the RPP (Radiant Protective Performance) device capable of simulating sweating, the location of the air layer is studied for clothing heat insulation performance. Impact. WANG and FThe test results of U are contrary to Huang's research results. They found that the air layer is located between the comfort layer and the skin layer or between the heat insulating layer and the waterproof gas-free layer is better, that is, the air layer is located close to the skin position, insulation Better performance. Analysis, this may be due to air layer direction (Huang (vertical direction), WANG and FU (horizontal direction)) and heat source strength (Huang (5 kW / m2), WANG (84 kW / m2), Fu (2 ~ 10 The difference between KW / M2))) results in the difference in test results.

3.3 Direction

The fabric heat plate apparatus can study the heat transfer performance of the air layer in the horizontal direction. However, when people are standing or walking, the air layer under the clothes is vertical. The literature [5, 30, 40] studied the heat transfer performance of the air layer in the vertical direction.

In a normal temperature environment, SMITH [40] tested the threshold of natural convection of the natural convection of the natural convection in the vertical direction of the vertical direction (thickness of 5 to 30 mm) using an improved thermal plate device, less than the literature [ 11-12] Test results in the horizontal direction. Yoshio et al. [5] and udayraj et al. [30] use numerical models to study the heat transfer performance of the air layer in the horizontal direction and the vertical direction, and the simulation results also show that the heat transfer rate in the vertical direction is larger, and it is easier to achieve the second level. burn.

In summary, the air layer in the vertical direction is more likely to occur natural convection, accelerating the heat transfer rate of the air layer, so the air layer heat transfer performance in the vertical direction is better. In addition, due to different working environment, there are still a variety of tilt angles in the underwater air layer, and the thermal transfer performance device capable of testing the plurality of tilt angles can be developed, and the optimum air layer of each tilt angle should be measured. Protective clothing provides theoretical reference.

3.4 Non-uniform form

The above studies assume that the air layer is uniform, and the fabric and the skin are kept parallel, and the actual outer air layer is non-uniform. Document [41-43] The thermal transfer performance of the air layer under non-uniform clothes was studied based on hot flat test and dummy test.

Sun et al. [41] Test 5 clothing thermal resistance using male warm body dummy and female warmer dummy, and the results have found that the thermal resistance of women's warm body dummy is even greater. The study believes that this is due to the smaller female dummy, under the same apparel, the air layer of female dummy's clothing is larger, and the female dummy is more complex, and the air layer in the non-uniform clothes can be stuck more. Still air, increase the thermal resistance of clothing. Similarly, Tannie et al. [42] compares the air distribution and thermal protection performance of men and women's hot protective clothing. As a result, it is found that the second-stage burn area of female hot protective garments is greater, and the thermal protection performance is worse.

The study believes that this is due to complex female body surface, causing a large number of people and clothing.The contact area and the local large air layer thickness, the direct contact of the skin and the garment accelerate the transfer of the external heat, resulting in more burn the skin, and the local larger air layer thickness will cause natural convection in the air layer, increase The heat transfer rate, therefore, the protective performance of women's protective equipment is worse.

To more detailedly exploring the influence of non-uniform formation of air layer heat transfer performance, EMEL et al. [43] sets a heated cylindrical torso device of the air layer in a vertical direction, non-uniform clothes, and analyzes folding The effect of size, folding quantity, and clothing and skin between clothing thermal resistance. As a result, it was found that the temperature of the air layer of 50 mm, and the upper end of the fabric surface and the lower end of the surface of the fabric were large, which was 1.77 ° C, and for the non-uniform form, the temperature difference between the upper end and the lower end of the fabric was smaller, 0.62 ° C, indicating that the non-uniform form is bound to the floating force of the air, and natural convection is not generated in the air layer under the clothes.

Although the non-uniform air layer hinders the convection of the air, EMEL, etc. Direct contact between clothing and skin. The direct contact of the skin and the clothing increases the thermal conductivity of the human body to the outside, so the thermal resistance of the air layer in the non-uniform clothes is lower. At the same time, since EMEL did not have a unified setting of the experimental parameters, for example, in the study of the contact area for the heat insulation performance of the clothes, the air layer is large, and the average outer air layer thickness will be A non-uniform clothes smaller than the contact area. Therefore, the reduction in air layer heat insulation performance under non-uniform clothes may be due to increased contact area, or may be due to the reduction in the thickness of the air layer under the average, and the experimental results cannot be quantified for a single variable. Therefore, the experimental scheme should be optimized in the future research, and the impact of non-uniform forms on the heat transfer performance of the air layer under the same parameters.

3.5 Motion Status

The human body will cause the air layer to produce forced convection, thereby reducing the thermal resistance of clothing. Therefore, the morphological changes of the air layer under different motion states and the thermal insulation performance are very important. The measurement method of the air layer dynamic change is the first to decompose the movement of the continuous change into a number of human body postures, and then measure the clothes air layer of each posture by a three-dimensional scanner. The map, that is, the dynamic air layer [44-45].

Document [46-47] By improving the experimental device, install a power system for the hot plate, so that the upper and lower reciprocating movements are fixed, the fabric surface is fixed, and the dynamic air layer is simplified into the clothes air. Layer thickness do periodic sinusoidal changes, as shown in the following formula.

(4) In the formula: y is the thickness of the air layer changed over time, mm; Y0 is the thickness of the air layer, mm; y0;? Y is the air layer thicknessThe amplitude, mm; f is the frequency of vibration, R / S.

Ghali et al. [46], in the normal temperature environment, the dynamic air layer for the heat dissipation power of skin, wherein the air layer has a thickness of 38.1 mm, an amplitude of 6.35 mm, and a rotational speed of 25 r / min. As a result, it was found that the heat dissipation power of the hot plate was exhibited in a sinusoidal up and down fluctuation by the effect of the heat dissipation power of the thermal air layer. XIN et al [47] studied the effect of dynamic air layers for skin lifting lines in a fire environment. The results found that the three types of vibration frequencies were less different than that of the surface temperature rise curve, and the temperature rise curve did not show significant sinusoidal fluctuation. Compared with the test results of Ghali and XIN, the difference is that Ghali is tested under normal temperature conditions, the dynamic air layer is mainly caused by heat transfer, so the heat dissipation is sine fluctuation; and XIN is exposed to the flame, the back of the fabric The air layer is mainly radiated heat transfer, which has a small impact on the flow heat transfer, so the temperature fluctuation of the sensor surface temperature is not obvious.

Ghazy et al. [48] constructed one-dimensional ldquo; fabric layer-air layer rdquo; thermal transfer model Considering the two heat transfer methods of conducting and radiation, the convection heat is ignored. Therefore, the model has achieved good simulation results only in the range of small thickness of the air layer. UDayraj et al [30] constructed three-dimensional ldquo, aid, conduction, radiation phase coupling, a fabric layer-air layer rDQUO; thermal conductivity, a heat transfer model, and a dynamic mesh technology to simulate the air layer Change, its simulation results maintain good consistency between the experimental results of XIN. The model shows that the radiation heat transfer is mainly role in the stream conditions, accounting for more than 80% of the total heat transfer, the transmission and the flow of flow heat, and the dynamic air layer can enhance the convection heat transfer of the underwater air layer.

In actual motion, the dynamic changes of the air layer under the clothes are more complicated. Ghaddar et al. [49] builds ldquo; arm - sleeve rdquo; dynamic heat transfer model. The model divides the outer air layer in the process of the arm swing into two stages: the first stage, the sleeve is stationary, the arm swing in the sleeve, the shape of the air layer is deformated over time; the second stage, the sleeves and The arm is swing together, and the air layer in the clothes remains unchanged. This study describes the motion of the air layer in the clothes, and the simulation results are more practical.

For the analysis of the above six factors, in designing protective garment style, the work environment (cold environment, high temperature environment, fire environment, etc.) and working conditions should be taken in the design of the protective garment style (cool, sitting, walking) Wait for the starting point, analyze the temperature difference between the upper surface of the air layer (inner surface) and the lower surface (skin surface), the angle, shape, and motion of the air layerIn state, it is determined that the critical threshold of natural convection occurs within the air layer, thereby optimizing the release of protective clothing, improves the cold / thermal protection of clothing.

Fourth, Research Prospect

The air layer of the clothes is an important constituent affecting the thermal insulation performance of clothing. This paper summarizes three research methods for exploring the impact of thermal transfer performance of clothing systems for clothing systems, and expounds the advantages and disadvantages of various research methods. According to the above research methods, the effects of six factors on the performance of clothing heat transfer are analyzed by one by one. Based on the current research status, this paper provides the following prospects for existing test methods and research content:

1) The influence of the air layer of the non-uniform clothes for the heat transfer of clothing system. At this stage, for the study of the air layer, no matter the thermal plate experiment, or the numerical heat transfer model, the air layer is mostly uniform, and the actual clothing air layer is non-uniform. If the thickness of the uniformity of air layer is directly applied to the results of the effect of thermal insulation performance, it is directly applied to the thermal insulation performance of the clothing system in real dress, and the deviation of the prediction results will cause the prediction. Therefore, in the future, the influence of geometric characteristics such as wavelength frequency, ribble, and contact area ratio, etc., one by one, based on the average thickness of the same air layer, etc., and establishes a non-uniform air layer fabric or garment system. The heat transfer model is more comprehensive to explain the heat transfer mechanism of the non-uniform air layer, providing protection for accurate characterization or predicting clothing insulation performance.

2) Measurement Method for Developing Air Layer of Thick Clothing and Multi - layer Clothing. Since the three-dimensional body measurement method has ignored the impact of the fabric surface of the clothing surface for the air layer of the clothing, the air layer of the thick clothing is unable to measure, and the distribution of air layer under the clothing of thick clothing cannot be analyzed and insulated from clothing. Performance effect. In the future, it is possible to consider the method of combining the pressure dummy system and the three-dimensional body measurement system. By measuring the thickness of the body by measuring the pressure of each portion of the human body, the thickness of the portion is subtracted from the air layer by reverse engineering software. To get the air layer distribution of the thick clothing, it is studied its impact on the thermal insulation performance of clothing. In addition, in multilayer clothing systems, many studies show that the air layer between clothing layers and layers is also the main factor affecting clothing insulation performance. At present, the study of such air layers is only through the thermal plate experiment and model simulation indirectly analyzes the thermal insulation performance of such air layers. If a test method measures the air layer between the clothing layer and the layers in the real dress, It will improve the thermal transfer performance of air layer under clothing clothing, which is very important for the development of cold / heat protection multi-layer clothing systems.