Effect of posture positions on the evaporative resistance and thermal insulation of clothing

Evaporative resistance and thermal insulation of clothing are important parameters in the design and engineering of thermal environments and functional clothing. Past work on the measurement of evaporative resistance of clothing was, however, limited to the standing posture with or without body motion. Information on the evaporative resistance of clothing when the wearer is in a sedentary or supine posture and how it is related to that when the wearer is in a standing posture is lacking. This paper presents original data on the effect of postures on the evaporative resistance of clothing, thermal insulation and permeability index, based on the measurements under three postures, viz. standing, sedentary and supine, using the sweating fabric manikin-Walter. Regression models are also established to relate the evaporative resistance and thermal insulation of clothing under sedentary and supine postures to those under the standing posture. The study further shows that the apparent evaporated resistances of standing and sedentary postures measured in the non-isothermal condition are much lower than those in the isothermal condition. The apparent evaporative resistances measured using the mass loss method are generally lower than those measured using the heat loss method due to moisture absorption or condensation within clothing. STATEMENT OF RELEVANCE: The thermal insulation and evaporative resistance values of clothing ensembles under different postures are essential data for the ergonomics design of thermal environments (e.g. indoors or a vehicle's interior environment) and functional clothing. They are also necessary for the prediction of thermal comfort or duration of exposure in different environmental conditions.     

A quasi-physical model for predicting the thermal insulation and moisture vapour resistance of clothing

Based on the improved understanding of the effects of wind and walking motion on the thermal insulation and moisture vapour resistance of clothing induced by air ventilation in the clothing system, a new model has been derived based on fundamental mechanisms of heat and mass transfer, which include conduction, diffusion, radiation and natural convection, wind penetration and air ventilation. The model predicts thermal insulation of clothing under body movement and windy conditions from the thermal insulation of clothing measured when the person is standing in the still air. The effects of clothing characteristics such as fabric air permeability, garment style, garment fitting and construction have been considered in the model through the key prediction parameters. With the new model, an improved prediction accuracy is achieved with a percentage of fit being as high as 0.96.     

The effect of added fullness and ventilation holes in T-shirt design on thermal comfort

This paper reports on an experimental investigation on the effect of added fullness and ventilation holes in T-shirt design on clothing comfort measured in terms of thermal insulation and moisture vapour resistance. Four T-shirts in four different sizes (S, M, L, XL) were cut under the traditional sizing method while another (F-1) was cut with specially added fullness to create a 'flared' drape. A thermal manikin 'Walter' was used to measure the thermal insulation and moisture vapour resistance of the T-shirts in a chamber with controlled temperature, relative humidity and air velocity. The tests included four conditions: manikin standing still in the no-wind and windy conditions and walking in the no-wind and windy condition. It was found that adding fullness in the T-shirt design (F-1) to create the 'flared' drape can significantly reduce the T-shirt's thermal insulation and moisture vapour resistance under walking or windy conditions. Heat and moisture transmission through the T-shirt can be further enhanced by creating small apertures on the front and back of the T-shirt with specially added fullness. STATEMENT OF RELEVANCE: The thermal comfort of the human body is one of the key issues in the study of ergonomics. When doing exercise, a human body will generate heat, which will eventually result in sweating. If heat and moisture are not released effectively from the body, heat stress may occur and the person's performance will be negatively affected. Therefore, contemporary athletic T-shirts are designed to improve the heat and moisture transfer from the wearer. Through special cutting, such athletic T-shirts can be designed to improve the ventilation of the wearer.     

Assessment of cold stress in terms of required clothing insulation—IREQ

A heat exchange model has been developed, by which the thermal stress associated with work in cold environments can be evaluated. Based on measurements of air temperature, mean radiant temperature, humidity and air velocity and measurements or estimates of activity level (energy metabolism) the model calculates a clothing insulation (IREQ) required to maintain body heat balance. IREQ may be regarded as an index of cold stress, and the value for IREQ specifies the insulation to be provided by clothing under given conditions, in addition to the insulation of the boundary air layer. IREQ, hence, may serve as a guideline for selection of appropriate clothing in cold environments. Basic insulation values of clothing (I_cI) measured with thermal manikins can be used for this purpose, but need to be corrected to account for the effect of body motion, posture, wind penetration and moisture absorption before a comparison is made with IREQ.     

Measurement technique for the thermal properties of thin-film diaphragms embedded in calorimetric flow sensors

In diaphragm-based micromachined calorimetric flow sensors, convective heat transfer through the test fluid competes with the spurious heat shunt induced by the thin-film diaphragm where heating and temperature sensing elements are embedded. Consequently, accurate knowledge of thermal conductivity, thermal diffusivity, and emissivity of the diaphragm is mandatory for design, simulation, optimization, and characterization of such devices. However, these parameters can differ considerably from those stated for bulk material and they typically depend on the production process. We developed a novel technique to extract the thermal thin-film properties directly from measurements carried out on calorimetric flow sensors. Here, the heat transfer frequency response from the heater to the spatially separated temperature sensors is measured and compared to a theoretically obtained relationship arising from an extensive two-dimensional analytical model. The model covers the heat generation by the resistive heater, the heat conduction within the diaphragm, the radiation loss at the diaphragm’s surface, and the heat sink caused by the supporting silicon frame. This contribution summarizes the analytical heat transfer analysis in the microstructure and its verification by a computer numerical model, the measurement setup, and the associated thermal parameter extraction procedure. Furthermore, we report on measurement results for the thermal conductivity, thermal diffusivity, and effective emissivity obtained from calorimetric flow sensor specimens featuring dielectric thin-film diaphragms made of plasma enhanced chemical vapor deposition silicon nitride.     

Size effect on heat transfer in micro gas sensors

Gas gap is usually used as an important thermal insulation in micro gas sensors to reduce the heating power. The heat transport through the gap consists of two parts, heat conduction by air and thermal radiation between surfaces. It is usually regarded that thermal radiation through the gap is negligible compared with conductive heat transfer by air. This work investigates the heat transport by thermal radiation and heat conduction through a broad size range of gas gaps from one nanometer to dozens of micrometers. The result shows that thermal radiation is the major way of heat transfer when the gap is less than 20 nm, which will result in unexpected high energy consumption in the process of minimization. The equivalent thermal conductivity of thermal radiation is computed and a partition map is depicted to demonstrate the relative importance of radiation and conduction on different gap scales under dissimilar surface temperatures. A practical gas sensor heated by a micro hotplate (MHP) is thermally analyzed. The calculation shows that extra energy consumption comes forth as the gap distance reduces to several tens of nanometers.     

Simulation of scaling effects of thermal emission from non-isothermal pixels with the typical three-dimensional structure

This paper suggests approaches to simulate scaling effects of thermal emission from non-isothermal pixels with a typical three-dimensional structure. We simulate effective emissivities of various V-shaped valleys by using our Monte Carlo method under the isothermal assumption. An analytical formula of the effective emissivities is derived based on photon rebounding between surfaces in the valleys. The comparison between the simulated and the analytically modelled effective emissivities shows that the analytical formula is highly accurate. After simulating effective emissivities of the V-shaped valley under several non-isothermal conditions, we find that the structure and component temperature difference of a pixel cause the scaling effects of thermal emission of the pixel. The results, therefore, prove that Planck's law has to be corrected for remote sensing to estimate land surface temperature with high accuracy.     

A review of heat transfer phenomena and the impact of moisture on firefighters' clothing and protection

Protective clothing with high insulation properties helps to keep the wearer safe from flames and other types of hazards. Such protection presents some drawbacks since it hinders movement and decreases comfort, in particular due to heat stress. In fact, sweating causes the accumulation of moisture which directly influences firefighters' performance, decreasing protection due to the increase in radiant heat flux. Vaporisation and condensation of hot moisture also induces skin burn. To evaluate the heat protection of protective clothing, Henrique's equation is used to predict the time leading to second-degree burn. The influence of moisture on protection is complex, i.e. at low radiant heat flux, an increase in moisture content increases protection, and also changes thermal properties. Better understanding of heat and mass transfer in protective clothing is required to develop enhanced protection and to prevent burn injuries.

Practitioner Summary: This paper aims to contribute to a better understanding of heat and mass transfer inside firefighters' protective clothing to enhance safety. The focus is on the influence of moisture content and the prevention of steam burn.     

Thermal dispersion effects on non-Darcy convection over a cone

The similarity solution for non-Darcy mixed convection about an isothermal vertical cone with fixed apex half angle, pointing downwards in a fluid saturated porous medium with uniform free stream velocity is obtained. The effect of thermal dispersion is studied in both the aiding and opposing flows. Flow separation is observed when the forced and free convection act in opposite directions. It is interesting to note that when buoyancy effects are neglected, the similarity solution exists for all realistic power law variations of the wall temperature, and for uniform free stream velocity, a closed form solution is possible for the isothermal wall temperature and uniform free stream. The heat transfer is enhanced due to the thermal dispersion effects.     

Effects of heat source/sink,radiation and work done by deformation on flow and heat transfer of a viscoelastic fluid over a stretching sheet

This paper presents a study of the flow and heat transfer of an incompressible homogeneous second-grade fluid over a non-isothermal stretching sheet. The governing partial differential equations are converted into ordinary differential equations by a similarity transformation. The effects of viscous dissipation, work due to deformation, internal heat generation/absorption and thermal radiation are considered in the energy equation, and the variations of dimensionless surface temperature and dimensionless surface temperature gradient as well as the heat transfer characteristics with various physical parameters are graphed and tabulated. Two cases are studied, namely, (i) a sheet with prescribed surface temperature (PST case) and (ii) a sheet with prescribed heat flux (PHF case).     

Effect of superficial insulation on roller-compacted concrete dams in cold regions

Superficial insulation is often used to prevent cracking of concrete dams located in cold regions. In this study, surface temperatures with and without heat insulation during the overwintering period are calculated. Using the material properties of roller-compacted concrete (RCC) as bases, we simulate and analyse the temperature field and thermal stress of certain RCC gravity dams in cold regions. The simulation and analysis are performed by three-dimensional finite element relocating mesh method under the following conditions: under the absence of heat insulation, and with the application of a 5 or 8 cm polystyrene slab for heat conservation. Moreover, the effects of superficial insulation and different thicknesses on the temperature field and thermal stress are analysed. Results show that superficial insulation can considerably increase the superficial temperature of RCC dams in cold regions, thereby decreasing superficial temperature difference and maximal tensile stress. These conditions prevent surface cracks from forming.     

基于恒温差的热膜风速测量方法研究

风速测量是流体流速测量领域中的重要组成部分,对于保证产品质量、提高生产效率以及节约能源都具有非常重要的意义。针对热扩散原理进行了深入的研究,确定了在恒温差状态下风速和加热功率的关系,通过恒温差控制电路改变供电电流补充气流带走的热量,该测量方法消除了环境温度变化的影响。实测结果表明：本恒温差风速测量方法有效地增加了测量范围、提高了测量精度,为风速测量提供了新方法。     

Effects of slip on sheet-driven flow and heat transfer of a non-Newtonian fluid past a stretching sheet

The flow and heat transfer of an electrically conducting non-Newtonian fluid due to a stretching surface subject to partial slip is considered. The constitutive equation of the non-Newtonian fluid is modeled by that for a third grade fluid. The heat transfer analysis has been carried out for two heating processes, namely, (i) with prescribed surface temperature (PST-case) and (ii) prescribed surface heat flux (PHFcase) in presence of a uniform heat source or sink. Suitable similarity transformations are used to reduce the resulting highly nonlinear partial differential equations into ordinary differential equations. The issue of paucity of boundary conditions is addressed and an effective second order numerical scheme has been adopted to solve the obtained differential equations. The important finding in this communication is the combined effects of the partial slip, magnetic field, heat source (sink) parameter and the third grade fluid parameters on the velocity, skin friction coefficient and the temperature field. It is interesting to find that slip decreases the momentum boundary layer thickness and increases the thermal boundary layer thickness, whereas the third grade fluid parameter has an opposite effect on the thermal and velocity boundary layers.     

Effect of physical activity and air velocity on the thermal insulation of clothing

Abstract

Intrinsic thermal clothing insulation and surface air insulation were measured on human subjects by the use of indirect calorimetry. Four male clothing ensembles (0-1-1 -8 clo) and three female clothing ensembles (0-2-1-2 clo) were investigated. Using the standing position as a reference, the influence of sitting, bicycling (40r.p.m., 20 W), walking (3-75 km hour^?1) and of light packing work on the thermal insulation was studied. The influence of an air velocity of 11ms^?1 on thermal insulation during the standing and walking conditions was investigated. The results showed that: (i) intrinsic clothing insulation was maximal in the standing position. It was reduced by 8-18% in the seated position and by 30-50% during bicycling and walking. An air velocity of 11ms^?1 did not influence the intrinsic clothing insulation during walking, but decreased it by 18% in the standing position; (ii) surface air insulation varied with activity and air velocity, but not with clothing. It was increased by up to 25% in the seated position, reduced by 7-26% during bicycling and by 30-50% during walking. An air velocity of 11 ms-1 reduced the surface air insulation by 50% in the standing position and 30% during walking.     

Soil moisture retrieval from MODIS data in Northern China Plain using thermal inertia model

Soil moisture plays an important role in surface energy balances, regional runoff, potential drought and crop yield. Early detection of potential drought or flood is important for the local government and people to take actions to protect their crop. Traditionally measurement of soil moisture is a time‐consuming job and only limited samples could be collected. Many problems would be results from extending those point measurements to 2D space, especially for a regional area with heterogeneous soil characteristics. The emergency of remote‐sensing technology makes it possible to rapidly monitor soil moisture on a regional scale. Thermal inertia represents the ability of a material to conduct and store heat, and in the context of planetary science, it is a measure of the subsurface's ability to store heat during the day and reradiate it during the night. One major application of thermal inertia is to monitor soil moisture. In this paper, a thermal inertia model was developed to be suitable in situations whether or not the satellite overpass time coincides with the local maximum and minimum temperature time. Besides, the sensibilities of thermal inertia with surface albedo and the surface temperature difference were discussed. It shows that the surface temperature difference has more effects on the thermal inertia than the surface albedo. When the temperature difference is less than 10 Kelvin degrees, 1 Kelvin degree error of temperature difference will lead to a big fluctuation of thermal inertia. When the temperature difference is more than 10 Kelvin degrees, 1 Kelvin degree error of temperature difference will cause a small change of thermal inertia. The temperature difference should be larger than 10 Kelvin degrees when the thermal inertia model is selected to derive soil moisture or other applications. Based on this thermal inertia model, the soil moisture map was obtained for North China Plain. It shows that the averaged difference between the soil moisture values derived from MODIS data and in situ measured soil moisture data is 4.32%. This model is promising for monitoring soil moisture on a large regional scale.     

Evaporative heat transfer characteristics of industrial safety helmets

Thermal discomfort is one of the major complaints from the wearers of industrial safety helmets. While studies have been reported on dry heat transfer (conduction, convection and radiation) in safety helmets, the investigation of wet heat dissipating (evaporation) properties has not been found in the literature. To evaluate experimentally the evaporative heat transfer characteristics of industrial safety helmets, a method was developed to simulate sweating of a human head on a thermal head manikin, and to use this manikin to assess the wet heat transfer of five industrial safety helmets. A thermal head manikin was covered with a form-fitting cotton stocking to simulate 'skin'. The skin was wetted with distilled water to simulate 'sweating'. A form-fitting perforated polyethylene film was used to cover the wetted stocking to control the skin wettedness at two levels, 0.64 and 1.0. Experiments were conducted in a climatic chamber, under the following conditions: the ambient temperature = head manikin surface temperature = 34 +/- 0.5 degrees C; ambient relative humidity 30% and 60%. Also, the effects of wind and a simulated solar heat load were investigated. The five helmets showed statistically significant difference in evaporative heat transfer under the experimental conditions. Skin wettedness, ambient humidity, wind and solar heat showed significant effects on evaporative heat transfer. These effects were different for the different helmets.     

3D quantification of microclimate volume in layered clothing for the prediction of clothing insulation

Garment fit and resultant air volume is a crucial factor in thermal insulation, and yet, it has been difficult to quantify the air volume of clothing microclimate and relate it to the thermal insulation value just using the information on the size of clothing pattern without actual 3D volume measurement in wear condition. As earlier methods for the computation of air volume in clothing microclimate, vacuum over suit and circumference model have been used. However, these methods have inevitable disadvantages in terms of cost or accuracy due to the limitations of measurement equipment. In this paper, the phase-shifting moiré topography was introduced as one of the 3D scanning tools to measure the air volume of clothing microclimate quantitatively. The purpose of this research is to adopt a non-contact image scanning technology, phase-shifting moiré topography, to ascertain relationship between air volume and insulation value of layered clothing systems in wear situations where the 2D fabric creates new conditions in 3D spaces. The insulation of vests over shirts as a layered clothing system was measured with a thermal manikin in the environmental condition of 20 degrees C, 65% RH and air velocity of 0.79 m/s. As the pattern size increased, the insulation of the clothing system was increased. But beyond a certain limit, the insulation started to decrease due to convection and ventilation, which is more apparent when only the vest was worn over the torso of manikin. The relationship between clothing air volume and insulation was difficult to predict with a single vest due to the extreme openings which induced active ventilation. But when the vest was worn over the shirt, the effects of thickness of the fabrics on insulation were less pronounced compared with that of air volume. In conclusion, phase-shifting moiré topography was one of the efficient and accurate ways of quantifying air volume and its distribution across the clothing microclimate. It is also noted that air volume becomes more crucial factor in predicting thermal insulation when clothing is layered.     

Analytical solution of micro-/nanoscale convective liquid flows in tubes and slits

Analytical solutions examining heat transport in micro-/nanoscale liquid flows were developed. Using the energy equation coupled with fully developed velocity, we solved developing temperature profiles with axial conduction and viscous dissipation terms. A comprehensive literature review provided the published range of velocity slip and temperature jump conditions. While molecular simulations and experiments present constant slip and jump values for a specific liquid/surface couple independent of confinement size, non-dimensional forms of these boundary conditions were found appropriate to calculate non-equilibrium as a function of flow height. Although slip and jump conditions are specific for each liquid/surface couple and hard to obtain, we proposed modeling of the slip and jump as a function of the surface wetting, in order to create a general, easy to measure methodology. We further developed possible correlations to calculate jump using the slip value of the corresponding surface and tested in the results. Fully developed Nu showed strong dependence on slip and jump. Heat transfer stopped when slip and jump coefficients became higher than a certain value. Strong variation of Nu in the thermal development length was observed for low slip and jump cases, while an almost constant Nu in the flow direction was found for high slip and jump coefficients. Variation of temperature profiles was found to dominate the heat transfer through the constant temperature surface while surface and liquid temperatures became equal at heat transfer lengths comparable with confinement sizes for no-dissipation cases. In case of non-negligible heat dissipation, viscous heating dominated the Nu value by enhancing the heating while decreasing the heat removal in cooling cases. Implementation of proposed procedure on a micro-channel convection problem from a micro-fluidics application showed the dominant effect of the model defining the slip and jump relationship. Direct use of kinetic gas theory resulted in an increase of Nu by an increase in non-equilibrium, while models developed from published liquid slip and jump values produced an opposite behavior.     

Spectral and Haar wavelet collocation method for the solution of heat generation and viscous dissipation in micro-polar nanofluid for MHD stagnation point flow

The aim and significance of paper presents, the semi-numerical investigation of magnetohydrodynamic flow of micropolar nanofluid with stagnation point is carried out under the influence of viscous dissipation and heat generation. The micropolar nanofluids are electrically conducting non-Newtonian fluids. The important applications of these fluids are observed in many research areas viz. bioengineering, biofuels and biomedical sectors etc. The appropriate similarity transformations are used to transform the governing equations into system of coupled nonlinear ordinary differential equations and are solved by using shifted Chebyshev collocation method and Haar wavelet collocation method. The variations in velocity, angular velocity, temperature and concentration profiles under the impact of various physical parameters, characterizing the flow field are discussed and are presented via graphs and tables. Temperature enhancement occurs with increment in each parameter except for Prandtl number. The concentration near the surface decreases with increment in the values of parameters and gradually it increases, except for Prandtl number and Schmidt number. The reverse trend of heat transfer occurs​ near a surface, when the dominance of stream velocity over stretching velocity is observed.     

Study of emissivity scaling and relativity of homogeneity of surface temperature

Three definitions of emissivity for non-isothermal pixels are evaluated, and their differences are analysed. On the basis of the experimental data and numerical simulation, a generalized formula is established to scale for land surface emissivity, reflectance, and other variables between pixel scale and sub-pixel scale. Finally, it is pointed out that isothermal concept of land surface temperature and homogeneous concept of some other parameters in one spatial scale are relative according to humans' capability of measurement, and that the heterogeneity in a spatial scale is absolute in the natural world.