Performance of hybrid PCM-FAN vest with deferred fan operation in transient heat flows from active human in hot dry environment.

Abstract

Hybrid cooling vests containing phase change material (PCM) and ventilation fans cool the human trunk while permitting the evaporation of the accumulated sweat in hot dry climates. However, during the period before enough sweat is captured by the inner clothing, introducing hot ambient air to the vest causes heating of the body and faster melting of the PCM, thus decreasing its cooling duration. The aim of this study is to determine the appropriate period after which operating the ventilation fans results in improving hybrid vest performance compared to continuous fan operation. A model is developed for transient heat and moisture transfer through clothing layers incorporating PCM packets and ventilation fans including the period for sweat buildup in the inner fabric. During fan operation and in presence of the PCM, the hybrid vest model was validated by experiments performed on a heated plate. The model was then integrated with a validated bioheat model to simulate a human working in hot dry conditions at metabolic rates of 3, 4.5 and 6 MET. It was found that the transient period of sweat capture by the inner fabric was significant of about 18.3, 14.3 and 10.3 min at 3, 4.5 and 6 MET, respectively. Results showed that operating the fans in that period caused undesired heating effects and increased the PCM melted fraction by 19.2percent, 17.8percent and 16.4percent at 3, 4.5 and 6 MET, respectively. Operating the fan after that period enhanced the torso heat losses and improved comfort and sensation levels.