Moisture Transport Mechanisms during the Drying of Granular Porous Media
The theoretical foundations of heat, mass and momentum transport in granular porous media are reviewed, and the process of drying a layer of wet sand with warm, dry air is considered. The gas-phase momentum transport problem is examined from the point of
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ABSTRACT The theoretical foundations of heat, mass and momentum transport in granular porous media are reviewed, and the process of drying a layer of wet sand with warm, dry air is considered. The gasphase momentum transport problem is examined from the point of view of the species momentum equation, and the analysis supports prior approximations of the momentum transport process. The liquid-phase moisture transport process is then re-examined in the light of past theoretical and experimental studies, and we conclude that the traditional concepts of relative permeability and capillary pressure saturation relations must be modified in order to obtain agreement between theory and experiment. 1.
INTRODUCTION
When a layer of wet sand is dried by passing warm dry air over the sand, as illustrated in Fig. 1, one encounters the basic components of most dry-
~Dry Air~
Gravity
FIGURE 1 Basic Drying Process
R. Toei et al. (eds.), Drying ’85 © Springer-Verlag Berlin Heidelberg 1985
21
author [2] provides evidence of the existence of a "temperature jump" at the interface. The need for a solid, theoretical analysis of the interfacial transport process will become apparent when the heat transfer process is examined in detail. While the drying rate may be constant during the early stages of drying, the moisture distribution within the porous medium can take on a variety of forms depending on the nature of the capillary pressure-saturation curve. The internal moisture distribution during the constant rate period has been studied extensively by Chen [3] who found that the liquid-phase motion owing to capillary action is always quasi-steady. For a capillary pressuresaturation curve of the type shown in Fig. 2, Chen's work indicates that the saturation distribution can be calculated by the laws of hydrostatics provided the saturation is everywhere greater than the irreducible saturation, S0 . The fractional saturation used here is defined by S
=
y 1j!l3pl3 + lj!/pl > pl3(l-1j!cr)
-13
=-
!_ K •(V
13 -
n13
~13
(2)
(3)
can be expressed as a function of the saturation and the temperature, Eq. 2 can be expressed as
1
= nl3
>\ r;~ ~s + (a
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