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Research Papers

High Intensity Drying in Porous Materials

[+] Author and Article Information
V. K. Chaitanya Pakala

Department of Mechanical Engineering,  University of Wyoming, Laramie, WY 82071-3295vpakala@uwyo.edu

O. A. Plumb1

Department of Mechanical Engineering,  University of Wyoming, Laramie, WY 82071-3295gplumb@uwyo.edu

1

Corresponding author.

J. Thermal Sci. Eng. Appl 4(2), 021010 (Apr 20, 2012) (8 pages) doi:10.1115/1.4006275 History: Received September 23, 2011; Revised February 16, 2012; Published April 19, 2012; Online April 20, 2012

High intensity drying is used to characterize those situations for which the drying medium is sufficiently above the saturation temperature of water to preclude the existence of a two-phase zone. In the present work, three models are applied to high intensity drying of porous materials. The three models are: (1) a traditional one-equation model that assumes local thermal equilibrium (LTE); (2) a two-equation model that utilizes lumped capacitance assumption to predict the heat transfer to the solid phase; and (3) a two-equation model that utilizes a more precise quasi-analytical approach to more accurately characterize the conduction in the solid phase. In addition, the relationship between pressure and the drying conditions and material properties is examined since elevated pressure that can occur during high intensity drying is potentially destructive. An implicit finite difference scheme is utilized to determine the drying rate in a porous medium saturated with water and undergoing the phase change process. The case for low local Reynolds number is considered, hence Nusselt number is assumed constant. Results illustrate that the one-equation model does not yield accurate results when the thermophysical properties characterized by the volume weighted ratio of thermal diffusivities, C > 10 (within 5% error). Hence, a two-equation model is suggested. In addition, consistent with the established “rule of thumb,” for Biot number, Biv  < 0.1, the traditional two-equation model which makes the lumped capacitance assumption for the solid phase compares well with a two-equation model that more accurately predicts the time dependent diffusion in the solid phase using Duhamel’s theorem. The peak pressures observed during drying for a range of Darcy number and surface heat flux are presented.

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Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Schematic of the temperature distribution

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Figure 2

Numerical scheme

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Figure 3

Initial condition (vapor phase)

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Figure 4

Comparison of models for refractory material

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Figure 5

Effect of C = 3 for refractory material

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Figure 6

Comparison of models for grains

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Figure 7

Effect of Biv for grains

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Figure 8

Effect of saturation pressure as a function of permeability and wall heat flux

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Figure 9

Front location as a function of wall heat flux

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Figure 10

Residual plot (two-equation lumped model)

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