Thermophysical characterisation of tropical wood used as building materials: With respect to the basal density

Construction and Building

MATERIALS

Construction and Building Materials 20 (2006) 929–938

www.elsevier.com/locate/conbuildmat

Thermophysical characterisation of tropical wood used as building materials: With respect to the basal density P.S. Ngohe-Ekam a, P. Meukam b

a,*,1

, G. Menguy b, P. Girard

c

a Laboratoire dÕEnerge´tique, Ecole Nationale Supe´rieure Polytechnique, P.O. Box 8390, Yaounde´, Cameroon Laboratoire dÕEtudes Thermiques et Solaires, Universite´ Claude Bernard, Lyon I. 43 Bd du 11 novembre 1918, 69622 Villeurbanne, France c Laboratoire Energie – Environnement, Cirad-Foreˆt, 73 rue J.F. Bre´ton, BP 5035, 34090 Montpellier, France

Received 30 May 2004; received in revised form 10 February 2005; accepted 30 June 2005 Available online 19 August 2005

Abstract An experimental study has been carried out to determine thermophysical properties of tropical wood. Five species, covering a wide range of densities of most of the wood used in Central Africa, has been chosen. These properties which characterise the thermally insulating materials, are related to basal density in order to help predict the thermophysical properties of any tropical wood as soon as its basal density is known. Steady-state and unsteady state methods were used to measure thermal conductivity and thermal diffusivity, respectively. Specific heat and thermal effusivity were then calculated. The influences of moisture content and the principal cutting plan on the thermophysical properties of tropical wood were examined. Higher conductivity, diffusivity and effusivity in the axial direction were observed, as well as the non-directional dependence character of the specific heat. It was also observed that thermal conductivity of tropical wood increases with infradensity both in the axial and the transverse directions. Finally, it was shown that conductivity and effusivity increase and thermal diffusivity decreases with the increase of moisture content.  2005 Elsevier Ltd. All rights reserved. Keywords: Tropical wood; Thermal conductivity; Thermal diffusivity; Infradensity; Effusivity

1. Introduction The cost of building materials in developing countries is often exorbitant, particularly when most of the materials have to be imported. On the other hand, the use of expensive and inappropriate materials is improper in the tropical regions. Timber is available locally throughout every country in Africa and has always been widely used as a building material for temporary and permanent buildings. Wooden board materials such as solid wood, plywood and fibre building board can cover *

Corresponding author. Fax: +237 222 45 47. E-mail addresses: [email protected] (P.S. Ngohe-Ekam), [email protected] (P. Meukam). 1 The Abdus Salam International Centre for Theoretical Physics, Strada Costiera, 11-34014 Trieste, Italy. 0950-0618/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2005.06.017

walls, doors and windows of timber structures. Wood has the advantages of relative tooling costs compared to those for competitive materials, a high strength to weight ratio, an excellent insulation and unique aesthetic properties [1]. The materials used in the structure should be given careful consideration. Because of the low thermal conductivity and moderate density of wood, the thermal diffusivity of wood, which is a measure of how quickly a material can absorb heat from its surroundings, is much lower than that of other structural materials, such as metal, brick, and stone. The durability of woods is often a function of water, but that does not mean wood can never get wet. Quite the contrary, wood and water live happily together. Wood is hygroscopic material, which means it naturally takes on and gives off water to balance its surrounding environment. The durability of each timber species depends

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Nomenclature b Cp D e f Kg LV Mv Pvs T R Re S P Pv Ta

thermal effusivity (J m1 K1 s1/2) specific heat (J kg1 C1) air-vapour mass diffusion coefficient (m2 s1) thickness of sample (m) resistant factor to gas diffusion in porous media water vapour permeability (kg m1 Pa1 s1) water latent heat of vaporisation (J kg1) molar mass of vapour (kg mol1) pressure of saturating vapour (Pa) temperature of the sample (C) constant of perfect gases resistance of the heater (X) area of surface perpendicular to the flux direction (m2) total pressure of the gaseous phase (Pa) pressure of vapor (Pa) ambient temperature of laboratory (C)

on the surrounding environment and the building construction [2]. Thermal conductivity, thermal diffusivity and specific heat are three important engineering properties of a material related to heat transfer characteristics. Each of the three properties can be measured by several well-established methods, but measuring any two of them would lead to the third through the relation k a¼ ; ð1Þ qC p where k, a, Cp and q are, respectively, thermal diffusivity, thermal conductivity, specific heat and density. Thermal effusivity b characterises how fast the superficial temperature of a material rises. From thermal conductivity, b can be calculated when heat capacity and density are available, using the following relation pffiffiffiffiffiffiffiffiffiffiffi b ¼ kqC p . ð2Þ Methods of measuring thermal conductivity can be classified into two broad categories: steady and transient-state heat transfer. Many authors usually utilise one of the two methods for this physical property. The thermal conductivity of borage Seeds has been measured by the transient technique using line heat source [3]. Transient thermal conductivity methods have benefited from rapid nature of the testing [4]. Density (q) and heat capacity (Cp) are measured in order to calculate thermal conductivity. Omar Douzane et al. [5] conducted a method, which employs periodic signals, for measurement of the thermal diffusivity and effusivity of building materials. An experimental study has been carried out in order to present the influence of density on the thermophys-

Tb Th Tc U Xv

inside temperature of box (C) temperature of hot surface of sample (C) temperature of cool surface of sample (C) voltage (V) vapour mass fraction into pore

Greek symbols a thermal diffusivity (m2 s1) q density (kg m3) x water content (%) mg kinematic viscosity (m2 s1) Subscripts eq equivalent g gaseous phase v vapour s saturation

ical properties of tropical woods [6]. As density varies with the moisture content, it was difficult to examine efficiently the effect of this parameter. In order to avoid this difficulty, basal density, which only depends on the species of wood according to Chesseron [7] and Guitard [8], is used in this paper. The moisture content of woods can modify their thermal performance. Thus a study of the influence of moisture content on equivalent thermal conductivity and thermal diffusivity is presented.

2. Materials and method 2.1. Description of material The samples were taken from the following five species of tropical wood:  Two high-density woods: Tali and Bilinga.  Two medium density woods: Sappily and Sipo.  One low-density wood: Ayous. Wood is a porous material, which is constituted with at least two phases. The fibrous nature of wood strongly influences how it is used. Wood can safely absorb large quantities of water without attaining moisture levels that will be inviting fungal decay. The characteristics and arrangements of these fibrous cells affect properties such as dimensional stability. It is necessary to specify the significance of the different thermal conductivities obtained experimentally in this work.

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2.2. True thermal conductivity and apparent thermal conductivity According to Degiovanni and Moyne [9], relation (3) can be used to determine thermal conductivity. Uniform pressure and hygroscopicity are assumed in the experiment, and a very short experimental time is considered. oX v K ¼ k þ qg fDLv ; ð3Þ oT where f is the resistant factor to gas diffusion in porous media, k is the thermal conductivity (W m1 C1), qg is the vapour density (kg m3), D is the air-vapour mass diffusion coefficient (m2 s1), Lv is the water latent heat of vaporisation (J kg1). The thermal conductivity k, obtained directly from the FourrierÕs equation on a homogeneous material equivalent to the porous material, is called ‘‘true thermal conductivity’’. K is called ‘‘apparent thermal conductivity’’ [9] because it considers in addition to k, an evaporating and diffusing term. Yet, for macroscopically immobile air in the material (i.e., steady-state particular case or samples with adiabatic edges), and neglecting the influence of temperature variation on latent heat Lv, it is experimentally possible to obtained the equivalent thermal conductivity keq given by  1  fD P vs fD P vs mg Mv Mv   k eq ¼ k þ Lv 1 þ RT RT P P Kg dP vs ; ð4Þ  dT where Kg is the gaseous material permeability (kg m1 Pa1 s1), Mv is the molar mass of vapour

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(kg), Pvs is the pressure of saturating vapour (Pa), mg is the kinematic viscosity (m2 s1). Using the steady-state method, an experimental study has been carried out in order to measure the equivalent thermal conductivity keq. Degiovanni [9] has defined the parameters in Eqs. (3) and (4) in details. keq is the equivalent thermal conductivity and is measured, in this work, using the steady-state method. 2.3. Thermal conductivity measurement Thermal conductivity measurements were conducted using the box method. This method is based on steady-state heat transfer. The apparatus is a isothermal enclose with an exchanger at its base, which contains water with glycol maintained at low temperature by a cryostat. Each box has a heater and there are two environments above and below the sample (Fig. 1). The experimental setup consists of:  An isothermal vacuum A which is cooled by a heat exchanger R fed by a fluid temperature regulator, the cryostat HAAKE D 3 G.  A plywood made box B: isolated inside by ‘‘styrodur’’; it has an opened face where the sample E is placed.  A heat emitter C: this is a heating film fed by an autotransformer. It enables to create a hot atmosphere above the sample.  Temperature sensors D: they are platinum resistors for surface temperature and atmosphere temperature measurement.  A measurement station, which receives among others, the connecting wires coming from the sensors.

Fig. 1. Schematic of the apparatus for thermal conductivity measurement.

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The dimensions of a sample were 27 cm · 27 cm. The samples thickness varied between 3 and 5 cm. As the surroundings are at the same temperature as the upper boxes, the power supplied by the electrical resistance mainly goes through the sample. When the steady-state is reached through the sample, the balance equation at the unit time is given by V 2 k eq S ðT C  T F Þ þ C 1 ðT B  T A Þ. ¼ e R

ð5Þ

Eq. (5) takes into account the three following fluxes 2

 JouleÕs effect produced by the heating element: UR .  Global heat losses through the box B: C1(Tb  Ta).  Heat conduction transfer through the sample: k eq S ðT h  T c Þ. e The expression of the thermal equivalent conductivity keq is deduced from Eq. (5) as follows:  2  e U  C 1 ðT b  T a Þ ; ð6Þ k eq ¼ S ðT h  T c Þ R where C1 is the over all heat transfer coefficient through the box B. The average value of C1 is 0.16 W C1 [10]. Temperatures were measured using platinum resistances and results were recorded for the calculation of equivalent thermal conductivity of each sample. Tests were conducted under temperatures between 17 and 25 C.

Fig. 2. Schematic view of box for measurement of thermal diffusivity.

Elsewhere Sacadura [12] relates that if two materials whose temperatures are T1 and T2, and thermal effusivities b1 and b2, are put in perfect contact, their contact surface will have an equilibrium temperature given by: T eq ¼

b1 T 1 þ b2 T 2 . b1 þ b2

ð9Þ

If b1 is higher than b2, the equilibrium temperature is nearer to T1. The thermal effusivity then shows the ability of a material to impose its temperature to the other when they are in physical contact. 2.6. Relationship between wet density qx and basal density qi

2.4. Thermal diffusivity measurements For the thermal diffusivity estimation, an unsteady state method (DegiovanniÕs model using a very short signal) is used. Several sensors are used to detect temperatureÕs variations. These variations are sent to an ANC, ARDETEM, which also amplifies the signal (i.e., the difference between actual and initial temperature) before orienting it towards the temperature curves recorder. One side of the sample received uniform short impulse energy from a constant flux radiant source whose power was 500 W (Fig. 2). With the theoretical model of Degiovanni [11] the thermal diffusivity is evaluated using the response of the other side of the sample.

The wet density qx of a sample is the ratio of its weight (kg) and its volume (m3) at a given moisture content x (%). Wet density can be related to water content as follows [13]:

2.5. Specific heat and thermal effusivity calculations

where q0 is the density at 0% water content and Cr is the retractibility coefficient of the sample, defined by the radio Rxmax . Rmax is given by: sat

Specific heat Cp and thermal effusivity b were calculated using Eqs. (7) and (8), respectively. In these relations, the values of thermal conductivity keq and thermal diffusivity a were obtained experimentally as described in Sections 2.3 and 2.4. Cp ¼

k eq ; qa

k eq b ¼ pffiffiffi . a

ð7Þ ð8Þ

 In the hygroscopic domain, (x 6 xsat): qx ¼ q0

1þx . 1 þ Crx

ð10Þ

 Out of the hygroscopic domain (x > xsat): qx ¼ q0

Rmax ¼

1þx ; 1 þ C r xsat

V sat  V 0 ; V0

ð11Þ

ð12Þ

where Vsat and V0 are the volumes of the sample at xsat and at 0% moisture content, respectively. The basal density (also called infradensity) qi of a sample is the ratio between the mass M0 (kg) of the sample at 0% moisture content and its saturation volume

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Table 1 Saturation water content, retractibility coefficient and infradensity of the tested specie (mean values) Species 2

xsat (10 ) Cr qi (kg m3)

Tali

Bilinga

Sappily

Sipo

Ayous

32.3 0.386 776

31.8 0.300 692

32.3 0.356 559

31.1 0.361 499

24.8 0.150 345

Table 2 Saturation water content, retractibility coefficient and infradensity of TaliÕs tested samples (leading to the mean values for Tali in Table 1) Direction 2

xsat (10 ) Cr qi (kg m3)

Radial

Tangential

Axial

Mean value

30 0.370 784

39 0.360 764

28 0.430 782

32.3 0.386 776

0.9

0 % water content

0.8

12 % water content

0.7

20 % water content

0.6

40 % water content

Conductivity (W/m.K)

a: axial direction

60 % water content

0.5 0.4 0.3 0.2 0.1 0 0.3

0.4

0.5

0.6

0.7

0.8

Infradensity (kg.m-3) 0 % water content

0.9

b: transverse direction

12 % water content

0.8 Thermal Conductivity (W/m.K)

20 % water content

0.7 40 % water content

0.6

60 % water content

0.5 0.4 0.3 0.2 0.1 0 0.3

0.4

0.5

0.6

0.7

0.8

-3

Infradensity (kg.m )

Fig. 3. (a) and (b) Variation of equivalent thermal conductivity of tropical wood with infradensity.

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Vsat (m3). Chesseron [7] and Guitard [8] explain that it is a theoretical ratio used in the calculus and they specify that it helps to characterise species. Using this characteristic, Eqs. (10) and (11) become 1þx for x 6 xsat ; 1 þ Crx for x > xsat .

transverse). The values obtained for xsat, Cr and qi are shown in Table 2, as well as their mean values. Introducing the mean values of xsat, Cr and qi into Eqs. (12) and (14) the wet density for the Tali species becomes

qx ¼ qi ð1 þ C r xsat Þ 

ð13Þ

qx ¼ 872.8.

qx ¼ qi ð1 þ xÞ

ð14Þ

and

1þx 1 þ 0.386x

qx ¼ 776ð1 þ xÞ

for x 6 0.32

for x > 0.32.

3.1. Thermophysical properties in term of basal density In the Eqs. (10)–(14), Cr, xsat and qi were obtained experimentally, and their mean values for the different samples of the same species are presented in the Table 1. The use of relationships (10), (11), (13) and (14) and the values in Table 1 give simple relationships between thermal physical properties and the moisture content. Taking the species Tali in example, three samples were considered. The analysis examined the problem in the three principal directions (i.e., radial, tangential and

k T ¼ 0.73qx  429.4 k T ¼ 0.69qx  388.8

ðTangentialÞ ðRadialÞ

12 % water content

k T ¼ 0.71qx  409.1.

ð19Þ

Introducing the values of qx given, respectively, by Eqs. (15) and (16) in (19) the equivalent thermal conductivity in transverse direction for Tali becomes

a: axial direction

20 % water content

Diffusitivity (10-9 m2/s)

40 % water content 60 % water content

220 180 140 100 0.3

0.4

0.5

0.6

0.7

0.8

-3 Infradensity (kg.m )

a: transverse direction

200 Thermal Diffusivity (10 -9 m2/s)

0 % water content 12 % water content 20 % water content

180

40 % water content 60 % water content

160

140

120

100 0.3

0.4

0.5

ð17Þ ð18Þ

The average of Eqs. (17) and (18) is given by

0 % water content

260

ð16Þ

In the transverse directions, the following relations were used for Tali sample to calculate the equivalent thermal conductivity in tangential and radial direction [4].

3. Results and discussion

300

ð15Þ

0.6

0.7

0.8

-3

Infradensity (kg.m )

Fig. 4. (a) and (b) Thermal diffusivity of tropical wood variation with infradensity.

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1þx  409.1 for x 6 0.32; 1 þ 0.386x k T ¼ 551.0x þ 142.0 for x > 0.32. k T ¼ 619.6

ð20Þ ð21Þ

Note that Eqs. (20) and (21) are especially for the Tali species. The process used to get Eqs. (20) and (21) for Tali species is applied for all the other species considered, both in transverse and longitudinal (i.e., axial) directions. The relationships obtained were used to plot the figures presented in the following section. 3.2. Equivalent thermal conductivity Fig. 3(a) and (b) represent the variations of the equivalent thermal conductivity of tropical woods with infradensity in axial and transverse direction, respectively. It shows that equivalent thermal conductivity increases linearly with the infradensity; this can be explained by the fact that, for a given volume, the heavier wood is constituted with matter whose particles are more favourable for conduction heat transfer. These figures also show the increase of that property with the moisture content. In fact, wood is a porous and hygroscopic material; with

the increase of water content, the air whose thermal conductivity is lower then that of water is progressively replaced by water. 3.3. Thermal diffusivity The thermal diffusivity variation in axial and transverse directions with infradensity is represented in Fig. 4(a) and (b). It can be observed that there is a decrease of thermal diffusivity when water content increases. This phenomenon can be explained by the fact that waterÕs diffusivity is lower than that of air. These figures also show that the diffusivity of tropical woods increases with the basal density. It is observed the decrease of this property with the increase of water content, but when a minimum value is reached it began to increase. In fact, as thermal diffusivity is a measure of how quickly a material can absorb energy, it cannot decrease indefinitely. 3.4. Specific heat Fig. 5(a) and (b) show the increase of the specific heat of tropical woods in axial and transverse

0 % water content 12 % water content

a: Axial direction

20 % water content

3600

40 % water content

Specific heat (J/kg.K)

60 % water content

3200 2800 2400 2000 1600 0.3

0.4

0.5

0.6

0.7

0.8

-3

Infradensity (kg.m ) 0 % water content 12 % water content 20 % water content 40 % water content

b: Transverse direction

60 % water content

Specific heat (J/kg.K)

3600 3200 2800 2400 2000 1600 0.3

0.4

0.5

935

0.6

0.7

0.8

Infradensity (kg.m-3)

Fig. 5. (a) and (b) Variation of specific heat of tropical wood with infradensity.

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directions with the moisture content; this is an obvious result since water has a greater heat storage capacity than the air. For lower moisture content (<20%) the specific heat is always a decreasing function of basal density. But when the moisture content becomes important, the decreasing process is observed only for lower basal densities (<600 kg m3), and for higher basal densities the specific heat begins to increase. This phenomenon could only be explained by the knowledge of the percentages of lignin, water and air in the wood skeleton. 3.5. Thermal effusivity The variation of thermal effusivity with water content is presented in Fig. 6(a) and (b). It shows that this property of tropical woods increases with water content. It is also observed that thermal effusivity increases linearly with the infradensity; thus low-density wood shall be softer in touch. Up to 20% of water content, tropical woods have very low effusivity (<950 J m1 K1 s1/2) compared with that of usual materials as concrete

(1420 J m1 K1 s1/2), aluminium (2157 J m1 K1 s1/2) and steel (1328 J m1 K1 s1/2) and this explain the frequent use of woods in the handles of tools. 3.6. Influence of cutting plan on thermophysical properties Fig. 7(a)–(d) represent the variation of thermal conductivity, thermal diffusivity specific heat and effusivity, respectively, with infradensity. The double thermal conductivity was observed in the axial direction than in the transverse direction. This observation is in agreement with the works of Cote and Kollman cited by Lartique [14]. For given water content and infradensity, the thermal diffusivity of tropical woods is twice as high in the axial direction as in the transverse direction. It is also observed on these figures that the cutting plan (i.e., the flux direction) has a negligible influence on the specific heat of tropical woods; this is in perfect agreement with the non-direction dependence character of this property. The higher effusivity is observed in the axial direction than in the transverse direction.

2000 0 % water content

1750

a: axial direction

12 % water content

Effusivity (J.m-2.K-1.s-1/2)

20 % water content

1500

40 % water content 60 % water content

1250 1000 750 500 250 0 0.3

0.4

0.5

0.6

0.7

0.8

-3

Infradensity (kg.m ) 0 % water content

2000

b: transverse direction

12 % water content

Effusivity (J.m-2.K-1.s-1/2)

1750

20 % water content 40 % water content

1500

60 % water content

1250 1000 750 500 250 0 0.3

0.4

0.5

0.6

0.7

0.8

-3

Infradensity (kg.m )

Fig. 6. (a) and (b) Thermal effusivity of tropical woods as function of infradensity.

Thermal Conductivity (W/m.K)

P.S. Ngohe-Ekam et al. / Construction and Building Materials 20 (2006) 929–938 0 % water content, axial direction

0.8

937

a: thermal conductivity

0 % water content, transversal direction. 40 % water content, axial direction

0.6

40 % water content, transverse direction.

0.4 0.2 0 0.3

0.4

0.5

0.6

0.7

0.8

Thermal Diffusivity (10-9) m2/s)

Infradensity (kg.m-3) 12 % water content,axial direction

b: diffusivity

300

12 % water content, transversal direction

250

60 % water content, transversal direction

60% water content, axial direction

200 150 100 0.3

0.4

0.5

0.6

0.7

0.8

Specific heat (J/kg.K)

-3 Infradensity (kg.m ) 20 % water content, Axial Direction 20 % water content, Transversal Direction 40 % water content, Axial Direction. 40 % water content, Transversal Direction

c: specific heat 3300 2800 2300 1800 0.3

0.4

0.5

0.6

0.7

0.8

-3

Infradensity (kg.m )

)

1500

-2

-1 -1/2

1800

Effusivity (J.m .K .s

2100

0 % water content, axial direction 0 % water content, transversal direction 60 % water content, axial direction 60 % water content, transversal direction

d: effusivity

1200 900 600 300 0 0.3

0.4

0.5

0.6

0.7

0.8

Infradensity (kg.m-3)

Fig. 7. (a)–(d) Influence of cutting plan on the thermophysical properties of tropical wood.

4. Conclusion An experimental study was conducted on the thermal properties of tropical woods. The influence of water content and infradensity on these properties was examined. It was shown that conductivity, diffusivity, effusivity and specific heat are strongly influenced by the infradensity. Thermal conductivity and thermal effusivity show a linear variation with infradensity. The direction dependence for thermal conductivity and thermal diffusivity was observed. Specific heat and thermal effusivity are not direc-

tion dependent. This property of thermal effusivity, can explain the tendency of tropical wood to be soft in touch in comparison with usual materials. Correlations obtained enable the prediction of the thermophysical properties of any wood species, when it infradensity is known.

Acknowledgements The corresponding author contributed to this work during his visit, as associate, at ICTP, Trieste, Italy. This

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visit is financially supported by Swedish International Development Cooperation Agency (SIDA).

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[6] NGOHE-EKAM PS. Etude expe´rimentale des proprie´te´s thermophysiques des bois tropicaux. The`se de Doctorat, UCB Lyon I, France, 1992. [7] Chesseron H. Contribution a` lÕe´tude du se´chage solaire du bois. The`se de Docteur en spe´cialite´, Universite´ de Perpignan, France, 1982. [8] GUITARD D. Me´canique du mate´riau bois et composite. Collection NABLA, CEPADUES e´ditions, 1987. [9] Degiovanni A, Moyne C. Conductivite´ thermique de mate´riaux poreux humides: e´valuation the´orique et possibilite´ de mesure. Int J Heat Mass Transfer 1987;30(11):2225–45. [10] Mourtada A. Comportement thermique des mortiers dÕisolation exte´rieure du baˆtiment. The`se de doctorat dÕinge´nieur, UCB Lyon I, France, 1982. [11] Degiovanni A. Diffusivite´ et me´thode flash. Rev Gen Ther Fr 1977:420–42. [12] Sacadura JF. Initiation aux transferts thermiques. CAST, technique et Documentation, INSA de Lyon, 3e` trimestre, 1978. [13] Ngohe-Ekam PS, Menguy G, BIindzi I. Conductivite´ thermique e´quivalente dÕun bois tropical: le sapelli. Afr J Building Mater 1999;03(1 and 2). [14] Lartique C. Me´canismes e´le´mentaires mis en jeu lors du se´chage dÕun pin maritime. The`se de Docteur de lÕUniversite´ de Bordeaux, France, 1987.