Simulation of coupled transient heat and water vapor transfer in porous fiber membrane with different fiber orientations and porosity

The transfer process of heat and water vapor in a porous fiber membrane was investigated through the simulation of a 3D model for optimizing the configuration design. 3D models with different fiber orientations and porosity were constructed by the parameter input method, and the accuracy of the model was validated by the coefficient of determination (R2) between the apparent velocity of the model and the air permeability of the membrane. The permeability of 3D model was used to reflect the discrepancy in fiber orientation of the model. The influences of fiber orientation and porosity on heat and water vapor transfer were surveyed by the coupled physics of heat transfer and dilute substance transfer. Since there was no temperature difference in the entire domain, heat conduction (10−9 W/m2) and moisture convection (10−14 mol·m−2·s−1) were faint in the model. With the diffusion of water vapor in the moisture, the heat convection flux and water vapor diffusion flux gradually decreased and reached equilibrium. When the permeability was increased by adjusting the fiber orientation (from 1.002 to 1.200 m2), the heat convection flux and water vapor diffusion flux followed a similar growth pattern due to the coupling effect of heat transfer and water vapor transfer. The R2 for the heat convection flux and water vapor transmission rate of the simulations and experiments with different porosity (44.87, 47.64 and 50.15%) were 0.999 and 0.923, respectively, which demonstrated the validation of the simulation in heat and water vapor transfer.

Thermal and moisture behavior of a multi-layered assembly in a pneumatic compression device

The objective of this research was to determine the effect of multiple layers of materials typical of those used in air pneumatic compression devices (which require air impermeable layers to function) on thermal and water vapor resistance. The experimental set-up included: (a) single layers of two next-to-skin knit fabrics in both relaxed and extended conditions, (b) two layers of silicone, and (c) a multi-layered assembly comprised of a next-to-skin fabric and two layers of silicone. Structural properties (thickness, mass) dominated thermal resistance of the multi-layered assembly, and the silicone layers rendered this assembly impermeable to water vapor as expected. Results confirmed the need for some form of 'ventilation' to facilitate water vapor transfer from a potential user’s skin to the environment. By creating 18 circular vents across the silicone layers (each vent 314 mm2), which formed ventilation of ∼2% of total surface area, the water vapor resistance of the multi-layered assembly dropped significantly from very high (but non-measurable) to below ∼300 m2 Pa/W, although ventilation did not improve the thermal resistance of the multi-layer arrangements. Results of this research will enable manufacturers of pneumatic compression devices to develop devices comprised of a multiple layer arrangements i.e. a knit fabric next-to-skin layer and silicone layers with optimized vents across the silicone layers, so that the user can continue the compression treatment with an acceptable microenvironment.

Water vapor transport to a semi-infinite material with simultaneous varying surface relative humidity and temperature

The water vapor transfer between the indoor air and hygroscopic finishing materials is of importance for the moisture balance of the room. Most protocols for determining the effect are based on isothermal conditions and cycling relative humidity in the form of square wave or sinusoidal functions. A new analytical solution for a material exposed to a both time varying surface relative humidity and temperature is presented in the paper. The time varying temperature inside the material is assumed to follow the surface temperature throughout the material layer since the reaction time for temperature changes in a reasonable thin surface material is rather short compared with the one for moisture changes. The semi-infinite approach is justified by the fact that the penetration depth for moisture variations are very limited for diurnal variations. The analytical approach and solution are presented in the paper

Effect of Air Gap Entrapped in Firefighter Protective Clothing on Thermal Resistance and Evaporative Resistance

Heat and water vapor transfer behavior of thermal protective clothing is greatly influenced by the air gap entrapped in multilayer fabric system. In this study, a sweating hot plate method was used to investigate the effect of air gap position and size on thermal resistance and evaporative resistance of firefighter clothing under a range of ambient temperature and humidity. Results indicated that the presence of air gap in multilayer fabric system decreased heat and water vapor transfer abilities under normal wear. Moreover, the air gap position slightly influenced the thermal and evaporative performances of the firefighter clothing. In this study, the multilayer fabric system obtained the highest thermal resistance, when the air space was located at position B. Furthermore, the effect of ambient temperature on heat and water vapor transfer properties of the multilayer fabric system was also investigated in the presence of a specific air gap. It was indicated that ambient temperature did not influence the evaporative resistance of thermal protective clothing. A thermographic image was used to test the surface temperature of multilayer fabric system when an air gap was incorporated. These results suggested that a certain air gap entrapped in thermal protective clothing system could affect wear comfort.