Mass transfer due to perspired moisture in a clothing system is critical for the understanding of thermo-physiology and thermal protection of a clothed body. Previous studies usually investigated moisture transfer without considering the effect of liquid sweating or external heat hazards. To understand the mechanisms of sweat evaporation, accumulation and dripping with continuous sweating under radiant heat, a multi-phase experiment was designed with a sweating Torso. The concept of clothed wettedness was proposed to understand sweat evaporation of the clothed body. Results showed that the evaporation rate of the clothed body increased with increasing perspiration rate and the rate increase can be explained by the material properties (e.g., material composition, hydrophilicity and evaporative resistance ([Formula: see text])), which affected the sweat accumulation ability. Results also demonstrated a dual relationship of [Formula: see text] with the evaporation rate of the clothed body. Firstly, the evaporation rate was increased for greater [Formula: see text] due to the higher moisture accumulation. Secondly, when [Formula: see text] exceeded a certain value, the evaporation rate decreased with greater [Formula: see text] due to the reduction in the mass transfer coefficient. For radiant heat exposure, evaporated sweat may condense on the skin surface, decreasing the evaporation rate and increasing the dripping rate. The sweat transfer process was also investigated in detail by the combined analysis of the sweat transfer rate and the evaporative cooling efficiency. This study provides insights into how continuous liquid sweat transfers and evaporates in the clothed body and its interaction with clothing material and environment radiant heat, contributing to the understanding of thermo-physiological burden and thermal protection of the clothed body with intensive activities.
Apparent evaporative cooling efficiency has typically been determined by applying a pre-wetted fabric "skin" on a dummy ("manikin") simulating human thermal physiology, to understand the effective cooling components of body perspiration in clothing systems. This procedure is only a very rough approximation of real life, as the pre-wetted fabric does not have the capacity to continuously push extra moisture into the clothing layers, which would happen with continuous sweating of the body. In this study, a sweating torso was used to mimic different sweating situations (100, 175 and 250 g/h) with twelve single-layer (SI) and multilayer (MU) clothing systems to understand the effect of continuous sweating and its interaction with clothing materials on cooling efficiency. Our experiments revealed that evaporative cooling efficiency is affected differently by continuous sweating when compared to pre-wetted fabric "skin" approaches. With continuous sweating, up to 15 % (24 W•m -2 ) cooling power came from the so-called "heat pipe effect" and/or wet conduction and 24 % (44 W•m -2 ) evaporative latent heat was gained from the environment. It was found that the increase of perspired moisture can affect evaporative cooling efficiency for hydrophilic materials in dual ways. For each 75g/h sweat rate increase, the in-plane moisture transfer can raise the evaporative cooling efficiency of SIs at least 3-12 % and the transplanar moisture transfer may reduce the evaporative cooling efficiency by at least 2-7 % and 2-9 % for SIs and MUs, respectively. For hydrophobic materials, the evaporative cooling efficiency was less affected by different levels of perspiration due to low wicking. Results also showed the negative correlation of evaporative cooling efficiency of hydrophilic materials with fabric evaporative resistance and thickness. Our study contributes to the understanding of the effective sweat cooling power and evaporative latent heat from environment for the clothed human body with continuous sweating. It also provides insight into the interaction between liquid and water vapor transport, and material design for optimizing the evaporative cooling.
Perspired moisture plays a crucial role in the thermal physiology and protection of the human body wearing thermal protective clothing. Until now, the role of continuous sweating on heat transfer, when simultaneously considering internal and external heat sources, has not been well-investigated. To bridge this gap, a sweating torso manikin with 12 thermal protective fabric systems and a radiant heat panel were applied to mimic firefighting. The results demonstrated how the effect of radiant heat on heat dissipation interacted with amount of perspired moisture and material properties. A dual effect of perspired moisture was demonstrated. For hydrophilic materials, sweating induced evaporative cooling but also increased radiant heat gain. For hydrophilic station uniforms, the increment of radiant heat gain due to perspired moisture was about 11% of the increase of heat dissipation. On the other hand, perspired moisture can increase evaporative cooling and decrease radiant heat gain for hydrophobic materials. In addition to fabric thermal resistance ( Rct) and evaporative resistance ( Ret), material hydrophilicity and hydrophobicity, emissivity and thickness are important when assessing metabolic heat dissipation and radiant heat gain with profuse sweating under radiant heat. The results provide experimental evidence that Rct and Ret, the general indicators of the clothing thermo-physiological effect, have limitations in characterizing thermal comfort and heat strain during active liquid sweating in radiant heat. This paper offers a more complete insight into clothing thermal characteristics and human thermal behaviors under radiant heat, contributing to the accurate evaluation of thermal stress for occupational and general individuals.
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