Type of Document Master's Thesis Author Alvarez, Juan C. Author's Email Address firstname.lastname@example.org URN etd-71798-14046 Title Evaluation of Moisture Diffusion Theories in Porous Materials Degree Master of Science Department Mechanical Engineering Advisory Committee
Advisor Name Title Thomas, William C. Committee Chair Dancey, Clinton L. Committee Member Ellis, Michael W. Committee Member Keywords
- moisture diffusion
Date of Defense 1998-08-07 Availability unrestricted AbstractMoisture transport in building materials is directly responsible for structural
damage, as well as poor indoor air quality. For these reasons, the need to
understand transfer mechanisms and predict moisture transport through building
materials has increased over the last couple of decades.
Although moisture diffusion phenomenon in the isothermal regime has been
studied and explained extensively, there is no universally accepted model for
predicting the moisture diffusion in a nonisothermal situation. Several
diffusion models in the form of "Fick's Law" including ones based on gradients
of water-vapor pressure, chemical potential of water, moisture concentration and
activated moisture molecules have been proposed for predicting moisture diffusion
through porous materials. However, the lack of reliable experimental results,
resulting from the complexity of arranging accurate and repeatable measurement
techniques and slow moisture movement, has prevented any model from being universally
accepted. The present research addresses this modeling problem by evaluating
current diffusion models through a series of experiments performed on oriented
strand board (OSB), which is a wood-based material.
The present experimental apparatus, developed over the last three years, was
designed for the specific purpose of studying and developing an accurate method
to measure moisture transfer properties in porous materials under nonisothermal
conditions. The apparatus consists of a system of two environmental chambers
capable of achieving a wide range of temperatures and relative humidities.
Temperature and relative humidity can be independently controlled to within ±0.05°C and ±0.10 per cent R.H. of the set points. This apparatus is an alternative to the
ASTM "cup method" which is limited to isothermal conditions and discrete relative
humidities that correspond to those for various saturated salt-in-water solutions.
Unlike the cup method, the relative humidity within the chambers is controlled by
the direct removal and injection of distilled water. The system has forced
recirculating flow which reduces the time to reach steady state. The new forced,
direct control measurement procedure is denoted "ASHRAE FDC".
The results obtained from the ASHRAE FDC experiments, show that moisture diffusion
under nonisothermal conditions is governed by the gradient of the water-vapor pressure.
The moisture transfer must cease when the diffusion potential is the same on both sides
of the material for the validation of the diffusion model. The results show that the
water-vapor pressure model meets this necessary and sufficient condition. Furthermore,
a plot of the diffusion flux versus vapor-pressure difference was linear, within
measurement uncertainty bounds. This observation infers that the permeability is
approximately constant over the range of temperatures and humidities used in the
During the ASHRAE FDC experimental procedure a small difference in the static pressure
between the chambers was found. This pressure difference which was also observed in
ASTM cup tests, is believed to be caused by concurrent air diffusion. The bulk flow
of air governed by Darcy's equation balances the diffusion of air in the opposite
direction as a result of the gradient in the partial pressure of (dry) air. The air
permeability of an OSB specimen was measured and the results presented.
The operation and accuracy of the apparatus was validated by comparing results from
a series of isothermal tests to previously published results. The results obtained
from the isothermal test allowed the permeability to be compared to results obtained
from cup tests during the present investigation and to those previously published
using the same method. Good agreement was found between the new data from both FDC
and cup experiments and previously published results.
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28.8 Modem 56K Modem ISDN (64 Kb) ISDN (128 Kb) Higher-speed Access Appendix_A.pdf 27.24 Kb 00:00:07 00:00:03 00:00:03 00:00:01 < 00:00:01 Appendix_B.pdf 32.37 Kb 00:00:08 00:00:04 00:00:04 00:00:02 < 00:00:01 Appendix_C.pdf 18.18 Kb 00:00:05 00:00:02 00:00:02 00:00:01 < 00:00:01 CHAPT3.PDF 178.32 Kb 00:00:49 00:00:25 00:00:22 00:00:11 < 00:00:01 Chapter2.pdf 98.26 Kb 00:00:27 00:00:14 00:00:12 00:00:06 < 00:00:01 Conc_Refe.pdf 15.10 Kb 00:00:04 00:00:02 00:00:01 < 00:00:01 < 00:00:01 Intro.pdf 82.23 Kb 00:00:22 00:00:11 00:00:10 00:00:05 < 00:00:01 Vita.pdf 4.45 Kb 00:00:01 < 00:00:01 < 00:00:01 < 00:00:01 < 00:00:01
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