Research Papers

Impact of Interstitial Mass Transport Resistance on Water Vapor Diffusion Through Fabric Layers

[+] Author and Article Information
Anshul Sharma

Department of Mechanical Engineering,
University of Texas,
Austin, TX 78712

Sandra K. S. Boetcher

Department of Mechanical Engineering,
Embry-Riddle Aeronautical University,
Daytona Beach, FL 32114

Walid A. Aissa

Department of Mechanical Power,
High Institute of Energy,
South Valley University,
Aswan, Egypt 81528

Matthew J. Traum

Department of Mechanical Engineering,
Milwaukee School of Engineering,
Milwaukee, WI 53202

Manuscript received June 5, 2011; final manuscript received December 10, 2011; published online October 12, 2012. Assoc. Editor: S. A. Sherif.

J. Thermal Sci. Eng. Appl 4(4), 041001 (Oct 12, 2012) (9 pages) doi:10.1115/1.4005733 History: Received June 05, 2011; Revised December 10, 2011

Textiles maintain wearer comfort by allowing evaporated sweat to permeate through, providing thermal management and keeping skin dry. For single layers, resistance to mass transport is relatively straightforward. However, when textiles are layered, water vapor transport becomes more complex because diffusing molecules must traverse interstitial spaces between layers. Interstitial mass transport resistances of significant magnitude can reduce rates of water vapor transport through layered textile stacks. The prevailing textile mass transport resistance interrogation method is ASTM F1868: “Standard Test Method for Thermal and Evaporative Resistance of Clothing Materials Using a Sweating Hot Plate.” Four improvements to ASTM F1868 are recommended: (1) gravimetric mass transport measurement, (2) evaluating transport using the Stefan flow model, (3) correct accounting for apparatus mass transport resistances, and (4) recognizing and measuring interstitial mass transport resistances. These improvements were implemented and evaluated by running tests using Southern Mills Defender™ 750 fabric, the calibration standard used for ASTM F1868, on a new gravimetric experimental apparatus. For a single layer of calibration fabric, the gravimetric approach is consistent with the prescribed result from ASTM F1868; however, for stacks of two or more calibration fabric layers, the gravimetric approach does not agree with the prescribed ASTM F1868 result due to interstitial mass transport resistance between fabric layers.

Copyright © 2012 by ASME
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Fig. 1

Textiles are made from repeating woven thread patters (left), which produce void spaces (right). These voids act as pores that readily conduit evaporated water vapor through fabrics.

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Fig. 2

Twenty optical micrographs captured the fine pore structure of a Southern Mills Defender™ 750 calibration fabric sample (similar to the left image). The resulting images were then each manually postprocessed using NIH ImageJ to isolate the pores. Porosity (with standard deviation) for the fabric was calculated using the white/black pixel ratio of the processed images.

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Fig. 3

The ASTM F1868 testing protocol assumes a linear relationship between number of fabric layers and total resistance to mass transport presented by a fabric stack, which implies that resistances to mass transport in the interstitial spaces between fabric layers are inconsequential. Testing layered ASTM calibration fabrics using a gravimetric technique shows that interstitial air gaps and tortuosity are nontrivial mass transport resistance components of fabric stacks.

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Fig. 4

(Top) A schematic (not to scale) of the evaporation apparatus for measuring resistance to water vapor transport from porous barriers and textiles. Shown are important temperature measurement locations. (Bottom) The apparatus shown running with one layer of Southern Mills Defender™ 750 calibration fabric—the textile material.

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Fig. 5

Raw experimental data for the evaporation chamber temperature (top) and reduction in apparatus mass from water evaporation (bottom) from one characteristic run (two fabric layers). Three regimes are apparent: (1) warm-up, (2) steady temperature with linear mass depletion rate, and (3) steady temperature with nonlinear mass depletion rate. Reported results are averaged data from the first 10 min of the regime of steady temperature with linear mass depletion rate.

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Fig. 6

A comparison of the prescribed ASTM F1868 calibration stack resistance model to measured gravimetric results shows the discontinuity at N = 2 and subsequent disagreement between prescribed and measured results. This mismatch arises from interstitial mass transport resistances between fabric layers, which are not accounted for in the ASTM F1868 calibration standard.

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Fig. 7

A constant value of 23.6 s/m agrees with all measurements of interstitial mass transport resistance presented by stacks of calibration fabric with two or more layers




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