Advanced modelling of the transport phenomena across horizontal clothing microclimates with natural convection

Abstract

The ability of clothing to provide protection against external environments is critical for wearer's safety and thermal comfort. It is a function of several factors, such as external environmental conditions, clothing properties and activity level. These factors determine the characteristics of the different microclimates existing inside the clothing which, ultimately, have a key role in the transport processes occurring across clothing. As an effort to understand the effect of transport phenomena in clothing microclimates on the overall heat transport across clothing structures, a numerical approach was used to study the buoyancy-driven heat transfer across horizontal air layers trapped inside air impermeable clothing. The study included both the internal flow occurring inside the microclimate and the external flow occurring outside the clothing layer, in order to analyze the interdependency of these flows in the way heat is transported to/from the body. Two-dimensional simulations were conducted considering different values of microclimate thickness (8, 25 and 52 mm), external air temperature (10, 20 and 30 °C), external air velocity (0.5, 1 and 3 m s(-1)) and emissivity of the clothing inner surface (0.05 and 0.95), which implied Rayleigh numbers in the microclimate spanning 4 orders of magnitude (9 × 10(2)-3 × 10(5)). The convective heat transfer coefficients obtained along the clothing were found to strongly depend on the transport phenomena in the microclimate, in particular when natural convection is the most important transport mechanism. In such scenario, convective coefficients were found to vary in wavy-like manner, depending on the position of the flow vortices in the microclimate. These observations clearly differ from data in the literature for the case of air flow over flat-heated surfaces with constant temperature (which shows monotonic variations of the convective heat transfer coefficients, along the length of the surface). The flow patterns and temperature fields in the microclimates were found to strongly depend on the characteristics of the external boundary layer forming along the clothing and on the distribution of temperature in the clothing. The local heat transfer rates obtained in the microclimate are in marked contrast with those found in the literature for enclosures with constant-temperature active walls. These results stress the importance of coupling the calculation of the internal and the external flows and of the heat transfer convective and radiative components, when analyzing the way heat is transported to/from the body.

Effects of clothing and fibres properties on the heat and mass transport, for different body heat/sweat releases

“…5a and b, respectively). This dependency is consistent with results reported in literature [17,31], though, in addition to previous works, the approach implemented in this work also takes into account the coupling between heat and mass transfer along the clothing structure. For instance, at the end of the exercise and post-exercise phases (phase I and II, Fig.…”

“…The aforementioned boundary conditions are also used in other field of application like artificial microclimates to study the interaction between the building ventilation and personalised airflow systems, with the thermal plume around human body [25][26][27][28]. In clothing applications, those boundary conditions are used to study various design parameters concerning how products properties should be selected to minimise the wind chill effect [19,20,22,29], and how the microclimates inside clothing affect the transport rates from the body [30][31][32][33]. However, the available literature still lacks detailed information about parameters of multilayer clothing that affect its thermal performance and water distribution, when the user performs physical activity implying different heat/sweat release.…”

Validation of a novel 3D scanning method for determination of the air gap in clothing

“…An air gap provides the bulk of the thermal resistance (specific thermal resistance of air is 38 mK/W, and of a typical fabric 24 mK/W [1]) and contributes significantly to the evaporative resistance (2.3 m 2 Pa/W per 1 mm of air layer thickness [2]) of clothing ensembles. For larger air gaps (>8 mm) [3], the local heat and water vapour exchange may be increased by the onset of natural convection [4,5]. Additionally, when the surfaces of the fabrics and/or the surface of the skin are in contact, the direct exchange of liquid water will occur.…”

Numerical simulation of the transport phenomena in tilted clothing microclimates

“…natural convection) and substantially alter the way heat is transported to/from the body. Recent literature ([ 2 - 4 ]) report relevant changes in the local heat transport along the skin, in horizontal clothing microclimates, stressing the need for analyses of other geometrical arrangements occurring within clothing.…”