No abstract

In extreme cold weather clothing ensemble, multiple layers of high bulk nonwovens are used to provide thermal insulation to the wearer. In this work, the effect of layering sequence in multi-layered high bulk thermal bonded nonwoven assembly on its thermal resistance is evaluated experimentally under sub-zero temperatures. Two multi-layered nonwovens, one made up of 1.4 denier solid (1.4 D S), 6 denier hollow (6 D H) and 15 denier hollow (15 D H) and the second made up of 3 denier hollow (3 D H), 6 denier hollow (6 D H) and 15 denier hollow (15 D H) polyester fibres were studied. The experiments were performed in a climatic chamber in the temperature range of 310 K to 210 K. Numerical simulations were carried out assuming heat transfer through the nonwovens as one-dimensional coupled conduction-radiation. The numerical methodology was developed using theoretical relations available in the literature to estimate the steady-state temperature profiles through the nonwoven layers and were validated using experimental data. The concurrence of experimental and numerical temperature profiles justifies the numerical methodology adopted in this work. Thermal resistance provided by the high bulk nonwoven increases with a decrease in ambient temperature. It is found that the thermal conductivity of nonwoven layers decreases from inner-to outer layers at a given ambient temperature. The heat flux through nonwoven layers, overall thermal conductivity and the thermal resistance of multi-layer nonwoven are independent of layering sequence if the convective heat transfer is extremely low.

In this work, temperature-dependent effective thermal conductivity and specific heat of high bulk thermally bonded polyester nonwovens (composed of 4 fibres viz. 1.4 denier solid, 3 denier hollow, 6 denier hollow and 15 denier hollow individually) are estimated for the sub-zero condition where the temperature drops up to 213 K (−60 °C). Experiments are performed in a custom made climatic chamber for measurement of the transient sensor temperature data placed at a different location inside the fibre web. The test chamber can achieve and maintain the temperature in the range of 213 K - 310 K. The properties are estimated using theoretical relations available in the literature. Heat transfer through the fibre web is assumed to be a one-dimensional coupled conduction-radiation problem. Effective thermal conductivity is calculated at a different ambient temperature ranging from 213 K to 273 K and specific heat is calculated at an ambient temperature of 213 K. Experimental temperature profiles compares well with numerical temperature profiles and thus justifies the numerical methodology adopted in this work. It can be seen from the results that effective thermal conductivity and specific heat decreases with a decrease in temperature which is favourable in providing more insulation in an extremely cold climate.

Prior knowledge of the survival time of a person clothed with an extreme cold-weather clothing would be useful in designing an appropriate fabric ensemble. The survival time may depend on ambient temperature, fabric insulation and metabolic heat and can be evaluated by designing a suitable experiment. To study the effect of different parameters on the survival time of humans, one such experiment is designed in an in-house developed guarded hot plate instrument placed inside a cold chamber capable of maintaining the ambient temperature between 210 K to 310 K. The experiments were performed at three different sub-zero temperatures, three different fabric insulation and selecting three metabolic heats. The transient temperature profiles at different locations of the fabric assembly were measured continuously. Further, a one-dimensional heat transfer model was developed to carry out the numerical simulation. The experimental and numerical transient temperature profiles across the fabric assembly agree well. The correlations developed for theoretically predicted survival times compare well with actual experimental data. The parametric analysis has shown that the ambient temperature is the most influencing parameter affecting the survival time followed by fabric insulation. The metabolic heat does not have a significant effect on survival time.