Call number:
ZSP-201-85/9
In:
CRREL Report, 85-9
Description / Table of Contents:
Large temperature gradients applied to a snow cover drive water vapor upwards and result in rapid recrystallization of snow crystals. The same temperature gradients create gradients of air density that can cause flows of air through the snow cover. The formalism necessary to describe these flows I developed heroin an effort to include the convection of vapor in the understanding of snow metamorphism. The theory of convection through porous media is extended here to include the transport of water vapor, which is important because of its latent heat. Results are presented in terms of a Lewis number, defined as the ratio of thermal to mass diffusivities. For Lewis numbers greater than 1.0 phase change intensifies convection, and for Lewis numbers less than 1.0 phase change retards convection. Two boundary conditions of special interest in the study of snow, a constant heat flux bottom and a permeable top are investigated. Their influence on the transfer of heat is quantified, and it is found that heat transfer can be described as a linear function of the driving force for convection. Convection in sloped layers is quantified, and explained in a physically consistent manner. The effect of a permeable top on convection at low Rayleigh numbers is derived. Experiments are performed to measure the effects of convection on heat transfer through glass beads and snow. The model results using constant flux boundary conditions are confirmed by the experiments. Experiments show that convection can occur in snow, and that convection behaves in a manner consistent with our theoretical understanding of the phenomenon. Some uncertainty exists about the permeability and thermal conductivity of snow and hence it is uncertain if thermal convection would occur for a given temperature gradient, density and thickness. Also, for a given convective intensity, there is much uncertainty about how much the rate of snow metamorphism is increased.
Type of Medium:
Series available for loan
Pages:
vi, 70 Seiten
,
Illustrationen
Series Statement:
CRREL Report 85-9
URL:
https://apps.dtic.mil/dtic/tr/fulltext/u2/a157577.pdf
URL:
https://hdl.handle.net/11681/9395
Language:
English
Note:
CONTENTS
Abstract
Preface
Nomenclature
Introduction
Snow metamorphism
Mass transfer by diffusion in snow
Heat transfer
Background-porous media
Structure of thermal convection
Rayleigh number
Onset problem
Heat transfer attributable to thermal convection
Layering and slope effects
Studies of convection through snow
Modeling
Equation of motion
Energy equation
Finite difference methods
Numerical solution
Verification of the model
Modeling results
Effects of constant flux and permeable boundaries on convection in horizontal layers
Effects of phase change on convection
Convection in sloped layers
Experiments
Introduction
Experimental apparatus
Experimental results and discussion
Glass beads
Snow
Applications and conclusions
Onset of Benard convection in seasonal snow covers
Applications to snow metamorphism
Summary
Recommendations
Literature cited
Appendix A: Derivation of fmite difference formulae
Appendix B: Computer programs
Appendix C: Sample calculations
Location:
AWI Archive
Branch Library:
AWI Library
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