Effect of ambient temperature, metabolic heat and clothing insulation on the required external active heating for the survival of humans exposed to sub-zero temperatures

Active heating in extremely cold weather clothing would be useful to save the life of an injured or sick person due to low metabolic rate. A fully dressed thermal manikin is useful in evaluating the performance of the active heating ensemble. The thermal manikin tests are time-consuming and could be done only after making the complete ensemble. An experiment that is designed to evaluate the performance of an active heating fabric fixed to a set of layers of extreme cold climate clothing using a guarded hot plate instrument would save a lot of time as it skips the processes of garmenting and subsequent evaluation on the thermal manikin. In this work, a guarded hot plate placed inside a cold chamber is used to investigate the external heating required for the prolonged comfort of the wearer at various sub-zero ambient temperatures, clothing insulation and metabolic heat. A plain woven active heating fabric sandwiched between a knitted fabric and nonwoven of different insulation/thickness is used to perform experiments. The external heat required to keep the test plate (representing the human skin) at 307 +/- 1 K (34 +/- 1 degrees C) is measured at different experimental conditions. Further, a one-dimensional numerical model based on coupled conduction-radiation heat transfer with heat generation was developed and validated with experimental results. The experimental temperature profiles match well with the numerical temperature profiles at different locations, justifying the numerical methodology adopted in this work. This suggests that the developed numerical model would also be useful to estimate the required active heating as per the wearer's requirement and an available power source. Both the experimental and numerical results suggest that the external active heating required to maintain test plate (skin) temperatures within the human skin comfortable range is a function of ambient temperature, metabolic heat and clothing (nonwoven) insulation.