On the determination of parameters required for numerical studies of heat and mass transfer through textiles – Methodologies and experimental procedures

Abstract

a b s t r a c tThe aim of this study was to develop methodologies and experimental procedures to determine textile parameters required in numerical approaches of heat and mass transfer through textiles. Privileging techniques usually available in textile/clothing laboratories, experimental approaches were defined that allow to estimate all required parameters, while taking into consideration water presence in the fibres and hence the effect of fibres hygroscopic properties. Numerical models usually require values of textile thickness, fibre fraction, and tortuosity, as well as knowledge of the boundary conditions, e.g. convective heat and mass transfer coefficients. To calculate these parameters, thickness, weight, and volume of textile samples were measured, whereas convective and textile evaporative resistances were determined by indirect measurements. Results were obtained for four distinct textile samples (made of wool, cotton, and a mixture of materials), of different hydrophilic nature. When the obtained parameters were incorporated in a numerical model and numerical predictions of temperature and humidity were compared with experimental data obtained during measurements of fabric evaporative resistance, it was shown that the predictions were accurate; this lends support to the developed methods and approaches.Since an uncertainty in the measured parameters can compromise the accuracy of numerical predictions, a sensitivity analysis was conducted to study the influence of deviations in the model input parameters and of assumptions regarding water presence in the fibres, on the predictions of heat and mass transfer rates. The results show that a deviation in textile weight and thickness as well as the assumption of no water retained in fibres during their characterisation, have a significant effect on the predicted heat transfer and water distribution over time. Moreover, a deviation in the total evaporative resistance, convective evaporative resistance, and sorption rate factor has great influence on the heat flux obtained in the initial period of testing.

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

“…To determine these parameters, the experimental procedures reported in Neves et al [24] were used. More properties of the gaseous phase such as the gas specific heat (Cp c ), gas pressure (p c ), air partial pressure (p a ), water vapour partial pressure (p v ), dry air density (q a ), and gas density (q c ) are described by Eqs.…”

“…(2.17)) using values of gas fraction and tortuosity based on the specific characteristics of the textile, in order to obtain a realistic estimate of effective diffusivity. For that purpose, the gas fraction and textile tortuosity were obtained via experimental procedures [24]. The gas fraction was related with the textile variables obtained under constant temperature and humidity conditions: the fraction of fibre, the fraction of bounded water (via isothermal sorption curve, [40]) and the effective density of the textile [24].…”

“…The gas fraction was related with the textile variables obtained under constant temperature and humidity conditions: the fraction of fibre, the fraction of bounded water (via isothermal sorption curve, [40]) and the effective density of the textile [24]. The tortuosity was also obtained under equilibrium conditions, however, through tests of evaporative resistance [24].…”

“…The present model was implemented and its numerical predictions were compared to experimental temperature and humidity data obtained during evaporative resistance tests [24]. Following validation (Section 3.1), the model was used (Section 3.2) to study the effect of several properties of multilayer clothing, for different heat/sweat release (mimicking different exercise intensity).…”

“…As in experimental procedures, the skin boundary conditions of heat and of mass are often set constant in numerical studies of heat and mass transfer through clothing [15][16][17][18][19][20][21][22][23][24]. Neves et al [15] studied the thermal performance of multilayer clothing assuming constant temperature and sweat rate on the skin, whereas Wu and Fan [18] defined the mass boundary condition as the concentration of water saturation, when studying a multilayer assembly exposed to a cold ambient.…”