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Electrical heating of green logs using Joule’s effect: a comprehensive computational model used to find a suitable electrode design

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Abstract

In order to insure an optimal process, logs must be heated prior to peeling. This operation requires considerable time under steaming or boiling. Electrical heating of logs using Joule’s effect is a way to reduce dramatically the heating time. A comprehensive computational model has been developed to investigate the possibilities offered by this method. This model accounts for the strongly coupled and non-linear effects that exist between heat, mass and electricity transfers within the log. Several simulations are presented. At first, the problem due to the presence of both sapwood and heartwood in the log is analysed. Then, some solutions, namely in the design of the electrodes, are proposed to address this problem. The main conclusion of this work is that electrical heating alone is not able to guarantee a uniform temperature field. Finally, a severe heating of the sapwood part followed by a waiting period which enables heating of the heartwood part by conduction is recommended. This procedure is able to cut the classical heating time by a factor of ten.

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Abbreviations

c :

Molar concentration (mol m-3)

\(\overline{\overline {\mathbf{D}}}\) :

Diffusivity tensor (m2 s-1)

E :

Electrical field (V m-1)

g :

Gravitational acceleration (m s-2)

h :

Specific enthalpy (J kg-1)

h s :

Differential heat of sorption (J kg-1)

j :

Current density (A m-2)

J :

Flux expression (kg m-2 s-1) or (J m-2 s-1)

k h :

Heat transfer coefficient (W m-2 K-1)

k m :

Mass transfer coefficient (m s-1)

\(\overline{\overline {\mathbf{K}}}\) :

Absolute permeability tensor (m2)

\(\overline{\overline {\mathbf{k}}}\) :

Relative permeability tensor

M :

Molar mass (kg mol-1)

n :

Unit normal vector

P :

Pressure (Pa)

R :

Gas constant (J mol-1 K-1)

T :

Temperature (K)

t :

Time (s)

U :

Electrical potential (V)

X :

Moisture content of wood (dry basis) (kg kg-1)

\(\overline{\overline {\mathbf{\Lambda }}} \) :

Thermal conductivity tensor (W m-1 K-1)

\(\overline{\overline {\mathbf{\gamma }}} \) :

Electrical conductivity tensor (Ω-1 m-1)

μ:

Dynamic viscosity (Pa s)

ρ:

Density (kg m-3)

ϕ:

Porosity or volume fraction (m3 m-3)

Φ:

Volumetric heat source (W m-3)

χ:

Depth scalar (m)

ω:

Mass fraction

a:

Air

b:

Bound water

c:

Capillary

eff:

Effective property

FSP:

Fibre saturation point

g:

Gas phase

ℓ:

Liquid phase

s:

Solid phase

v:

Vapour

∞:

Value outside the boundary layer in the free stream

v :

Vectors are denoted by bold letters (or vi in index notation)

\(\overline{\overline {\mathbf{v}}}\) :

Second order tensors are denoted by bold letters with double bar

v̄ :

A simple bar denotes a global average over the representative elementary volume (REV)

v̄ i :

A simple bar with a superscript denotes an intrinsic average over the corresponding phase included in the representative elementary volume (REV)

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Acknowledgements

Financial support for this work has been obtained from the Ministry of Agriculture, through a research project entitled “Contribution au développement d’une chauffe électrique rapide de bois vert de Douglas en vue de son déroulage” convention DERF n° 01.40.32/98, coordinated by Remy Marchal, ENSAM Cluny, France.

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  1. Laboratoire d’Etude et de Recherche sur le Matériau Bois, UMR 1093 INRA/ENGREF, Université Henri Poincaré Nancy I, 14 rue Girardet, 54042, Nancy Cedex, France

    Patrick Perré

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Perré, P. Electrical heating of green logs using Joule’s effect: a comprehensive computational model used to find a suitable electrode design. Wood Sci Technol 38, 429–449 (2004). https://doi.org/10.1007/s00226-004-0240-x

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