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Influence of heartwood-sapwood proportions on the drying kinetics of a board

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Summary

Several properties have been measured for pinewood samples, corresponding to the region within the heartwood or the sapwood, with the aim of treating wood as a heterogeneous medium when modelling the drying process. A two driving-force model is used to compare a homogeneous and a heterogeneous approach to the drying of a pinewood board. The main difference between the two theoretical descriptions is a specific boundary condition for the inter-region, which may generate jumps of the moisture content fields. Experimental and numerical results are presented and some comments are made concerning the use of the heterogeneous model.

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Abbreviations

A:

surface area of the inter-region m2

C:

vapour mass-fraction kg/kg of humid air

Cp :

heat capacity J/kg/K

D w :

effective transport coefficient relative to a moisture gradient m2/s

D T :

effective transport coefficient relative to a temperature gradient m2/s

e:

thickness of the board m

f :

restraint factor for the gas phase diffusion

Fm :

mass-flux kg/m2/s

hC :

convective mass-transfer coefficient m/s

hT :

convective heat-transfer coefficient W/m2/K

H:

heat of vaporization J/kg

HB :

heat of desorption J/kg

\(\vec J_B\) :

flux density for the bound water flow kg/m2/s

\(\vec J\) :

flux density for the free water flow kg/m2/s

k:

intrinsic permeability m2

kr :

relative permeability

KB :

bound water evaporation rate per unit volume kg/m3/s

K:

liquid water evaporation rate per unit volume kg/m3/s

\(\vec n\) :

external vector

Pc :

Capillary pressure Pa

RH∞ :

relative humidity of the drying air%

S:

liquid saturation

T:

temperature K

T∞ :

dry bulb temperature of the drying air K

T∞h :

wet bulb temperature of the drying air K

t:

time s, h

V:

volume m3

W:

moisture content dry base

x:

space coordinate m

λ:

thermal conductivity W/m/K

ρ:

density kg/m3

Φ:

volume fraction%

μ:

viscosity N.s/m2

B:

relative to the bound water

eq:

related to an equilibrium stage (sorption isotherms)

exp:

related to an experiment (Figs. 7, 8, 11)

cri:

related to the relative permeability

FSP:

related to the fibre saturation point

G:

relative to the gas phase

hw:

relative to the heartwood

ini:

related to the initial state, fresh cut moisture content

irr:

relative to the end of the capillary flow

L:

relative to the liquid, free water

mat:

relative to the material (sum of all the active phases)

S:

relative to the dry bone state

sw:

relative to the sapwood

surf:

related to the product exposed surface

T:

relative to a temperature gradient

W:

relative to a moisture gradient

∞:

relative to the drying air

dash :

average value relative to the whole piece

{}:

homogeneous modelling

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Authors and Affiliations

  1. Laboratoire Energétique et Phénomènes de Transfert L.E.P.T. — ENSAM, UA CNRS 873 Esplanade des Arts et Métiers, 33405, Talence Cedex, France

    P. Bonneau & J.-R. Puiggali

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  1. P. Bonneau
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  2. J.-R. Puiggali
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Bonneau, P., Puiggali, JR. Influence of heartwood-sapwood proportions on the drying kinetics of a board. Wood Sci.Technol. 28, 67–85 (1993). https://doi.org/10.1007/BF00193878

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