Transport Processes in Wood

This book has a similar subject content to the author's previous Flow in Wood but with substantial updating due to the abundance of research in the wood science field since 1971. Several different concepts have been introduced, particularly in regard to wood-moisture relation­ ships. The role of water potential in the equilibria between wood and its humid and moist environments is considered. Two theories are introduced to explain the nonisothermal transport of bound water in the steady and unsteady states. As in the former text, the wood-. structure relationship is emphasized . . The author is especially grateful to Dr. C. Skaar for his careful and critical review of much of the manuscript and for the productive dis­ cussions of many of the concepts. Dr. T. E. Timell, the series editor, rendered major assistance in the preparation of Chap. 2 and in his editing of the manuscript. The author wishes to thank Dr. W. A. Cote, Mr. A. C. Day, and Mr. J. J. McKeon for providing electron micro­ graphs, Mr. G. A. Snyder for his photography of much of the art work, Dr. C. H. de Zeeuw for his advice in the field of wood anatomy, and Ms. Mary M. Siau for her careful rendition of the art work. Apprecia­ tion is extended to Miss Judy A. Barton and Mrs. Stephanie V. Micale for their work in typing and checking the manuscript. Mr. J. A.

Contenu

1 Basic Wood-Moisture Relationships.- 1.1 Introduction.- 1.2 Saturated Vapor Pressure.- 1.3 Relative Humidity.- 1.3.1 Use of the Psychrometric Chart.- 1.3.2 Measurement of Relative Humidity.- 1.3.3 Control of Relative Humidity.- 1.4 Equilibrium Moisture Content and the Sorption Isotherm.- 1.5 The Effect of Changes in Pressure and Temperature on Relative Humidity.- 1.6 Specific Gravity and Density.- 1.7 Specific Gravity of the Cell Wall and Porosity of Wood.- 1.8 Swelling and Shrinkage of the Cell Wall.- 1.9 Swelling and Shrinkage of Wood.- 2 Wood Structure and Chemical Composition.- 2.1 Introduction.- 2.2 The Cell Wall.- 2.3 Structure of Softwoods.- 2.4 Types of Pit Pairs.- 2.5 Softwood Pitting.- 2.6 Microscopic Studies of Flow in Softwoods.- 2.7 Structure of Hardwoods.- 2.8 Hardwood Pitting.- 2.9 Microscopic Studies of Flow in Hardwoods.- 2.10 Chemical Composition of Normal Wood.- 2.10.1 Cellulose.- 2.10.2 Hemicelluloses.- 2.10.2.1 Introduction.- 2.10.2.2 Softwood Hemicelluloses.- 2.10.2.3 Hardwood Hemicelluloses.- 2.10.3 Lignins.- 2.11 Chemical Composition of Reaction Wood.- 2.11.1 Introduction.- 2.11.2 Compression Wood.- 2.11.3 Tension Wood.- 2.12 Topochemistry of Wood.- 3 Permeability.- 3.1 Introduction.- 3.2 Darcy's Law.- 3.3 Kinds of Flow.- 3.4 Specific Permeability.- 3.5 Poiseuille's Law of Viscous Flow.- 3.6 Turbulent Flow.- 3.7 Nonlinear Flow Due to Kinetic-Energy Losses at the Entrance of a Short Capillary.- 3.8 Knudsen Diffusion or Slip Flow.- 3.9 Corrections for Short Capillaries.- 3.10 Permeability Models Applicable to Wood.- 3.10.1 Simple Parallel Capillary Model.- 3.10.2 Petty Model for Conductances in Series.- 3.10.3 Comstock Model for Softwoods.- 3.10.4 Characterization of Wood Structure from Permeability Measurements.- 3.11 Measurement of Liquid Permeability.- 3.12 Measurement of Gas Permeability.- 3.13 The Effect of Drying on Wood Permeability.- 3.14 Treatments to Increase Permeability.- 3.15 The Effect of Moisture Content on Permeability.- 3.16 The Influence of Specimen Length on Permeability.- 3.17 Permeability of the Cell Wall.- 3.18 Zones of Widely Differing Permeabilities in Wood.- 3.19 General Permeability Variation with Species.- 4 Capillary and Water Potential.- 4.1 Surface Tension.- 4.2 Capillary Tension and Pressure.- 4.3 Mercury Porosimetry.- 4.4 Influence of Capillary Forces on the Pressure Impregnation of Woods with Liquids.- 4.5 Collapse in Wood.- 4.6 Pit Aspiration.- 4.7 The Relationship Between Water Potential and Moisture Movement.- 4.8 Notes on Water Potential. Equilibrium Moisture Content, and Fiber Saturation Point of Wood.- 5 Thermal Conductivity.- 5.1 Fourier's Law.- 5.2 Empirical Equations for Thermal Conductivity.- 5.3 Conductivity Model.- 5.4 Resistance and Resistivity; Conductance and Conductivity.- 5.5 Derivation of Theoretical Transverse Conductivity Equation.- 5.6 Derivation of Theoretical Longitudinal Conductivity Equation.- 5.7 R and U Values; Convection and Radiation.- 5.8 Application to Electrical Resistivity Calculations.- 5.9 Application to Dielectric Constant Calculations.- 6 Steady-State Moisture Movement.- 6.1 Fick's First Law Under Isothermal Conditions.- 6.2 Bound-Water Diffusion Coefficient of Cell-Wall Substance.- 6.3 The Combined Effect of Moisture Content and Temperature on the Diffusion Coefficient of Cell-Wall Substance.- 6.4 Water-Vapor Diffusion Coefficient of Air in the Lumens.- 6.5 The Transverse Moisture Diffusion Model.- 6.6 The Importance of Pit Pairs in Water-Vapor Diffusion.- 6.7 Longitudinal Moisture Diffusion Model.- 6.8 Nonisothermal Moisture Movement.- 6.9 Measurement of Diffusion Coefficients by Steady-State Method.- 7 Unsteady-State Transport.- 7.1 Derivation of Unsteady-State Equations for Heat and Moisture Flow.- 7.2 Derivation of Unsteady-State Equations for Gaseous Flow in Parallel-Sided Bodies.- 7.3 Graphical and Analytical Solutions of Diffusion-Differential Equations with Constant Coefficients.- 7.3.1 Solutions of Equations for Parallel-Sided Bodies.- 7.3.2 Solutions of Equations for Cylinders.- 7.3.3 Simultaneous Diffusion in Different Flow Directions.- 7.3.4 Significance of Flow in Different Directions.- 7.3.5 Special Considerations Relating to the Heating of Wood.- 7.4 Relative Values of Diffusion Coefficients.- 7.5 Retention.- 7.6 Unsteady-State Transport of Liquids.- 7.6.1 Parallel-Sided Bodies, Permeability Assumed Constant with Length.- 7.6.2 Parallel-Sided Bodies with Permeability Decreasing with Length (Bramhall Model).- 7.6.3 Cylindrical Specimens.- 7.6.4 Square and Rectangular Specimens.- 7.7 Unsteady-State Transport of Moisture Under Noniso-thermal Conditions.- 7.8 Heat Transfer Through Massive Walls.- References.- Symbols and Abbreviations.