| Academic Open Internet Journal ISSN 1311-4360 |
Volume 20, 2007 |
DETERMINATION OF SOME PHYSICAL AND THERMAL CHARACTERISTICS OF MOABI
Beguide Bonoma1*, Louis Monkam2, Ernest Kaptouom3
1 Laboratoire de physique appliquée de l’Ecole Normale Supérieure B.P 47 Yaoundé, Université de Yaoundé I
2 Département d’énergétique, Institut Universitaire de Technologie B.P 8698 Douala, Université de Douala Cameroun
3 Département de génie mécanique, Ecole Nationale Supérieure Polytechnique, Université de Yaoundé I, Cameroun
* Corresponding author, Email: beguide_ed@yahoo.fr
Abstract
Experimental results of measurements carried out on wood material, in order to characterize it thermally and physically, are presented in this work. For the Moabi (Baillonella toxisperma) studied, we have presented measurements of its density and thermal conductivity as a function of its moisture content as well as the correlations resulting from these measurements. In addition, the equilibrium desorption curve is also presented; the method of least squares is used to determine the parameters of the Harkings’ function adopted for the approximation of the equilibrium points. The computation results are compared with those of experimental measurements.
Keywords : Wood, density, thermal conductivity, equilibrium desorption curve.
I. Introduction
The multiple use of wood can no longer be over emphasised, be it in furniture, lagging (insulation) or construction. Under this aspect, the study of wood material in view of characterizing it thermally and physically does not only become necessary but indispensable. A better knowledge of the particular relations between moisture content and its thermophysical properties use on the hand, and the conditions of the other (temperature and relative humidity), is necessary for many reasons ; for example the making of drying tables for lingo-cellulosic or biologic products [1,2], the modelisation and simulation of the drying procedure [3,4,5]. There exits a good number of studies on experimental and theoretical characterisations [4,5,6], but they are not sufficient for the diversity of species and their origins. It is known that diversity influences the properties of wood material. In this study, we are going to present an experiment aimed at determining the density, thermal conductivity and isothermic desorption of moabi (Baillonella toxisperma). The approximation function will also be established.
II. Presentation of the working environment
In the general study, measurement has two origins: concerning isothermic desorption, the species originates from Cameroon, the adaptability of samples and measurements were carried out thanks to the Energy and Transfer Phenomena Laboratory (LEPT-ENSAM) of Bordeaux I in France. The measurement of density and thermal conductivity were carried out in the mechanics of fluid and thermal transfer unit of the Institute of Technology of the University of Douala, Cameroon. In this unit, the desk (Deltalab) of heat transfer comprising a rectangular electric heater was used to determine thermal conductivity; an electronic scale (Mettler TE/J) at 0.005 kg, a 20 calliper rule as well as a small over having a thermostat with necessary in determining the desired parameters.
III. Operational mode
The samples with the dimension 8´6´1.6 cm, used during the test were carved from the same wood; after weighing them, the samples are placed in the oven to reduce their water content and later, the new dimension as well as mass is taken in order to determine the density. The process is repeated at different levels of temperature to obtain the low moisture level of the product. After these measurements, the density can then be calculated using the following formula:
(1)
We notice a small variation of volume during the process in relation to density. At the end of measuring, we obtain a series of data of density with respect to the humidity (3) of the wood; this is obtained using formula (2), by determining the anhydrate mass of the sample studied after subjecting it to 105°C for 24 hours. Humidity (H) is obtained using the following formula:
(2)
The obtention of absolute mass (m0) of wood also enabled us to evaluate the total volume of moabi. Here, we find about 14%, a value which is found in wood material that has not been very much deformed, and whose total volume is less than or equal to 15% [3].
For thermal conductivity, the procedure is as follows: after subjecting the wood to tension, the temperature of the electric heater is regulated at 50°C, the sample which had been earlier weighed is put in contact with the electric heater and its external surface is coated with polymethane, the temperature of the larger surface is obtained using a borer (drill) and the value pasted on the quadrant; the density of the thermal flux q emitted by the electric heater through the wood material is given by:
(W/m2) (3)
The voltage U and the current I are read respectively on the voltmeter and the incorporated milliamperemeter. The coefficient a is the relation between duration of heating on over the time of the cycle read on the display table; S is the surface of the permanent out come is got by monitoring the evolution of the sample up to stabilisation and latter, a series of 10 measurements are carried out to better judge the stability of the diet. It is only after that we deduce the value of thermal conductivity corresponding to the moisture content of the wood material, using the following formula:
(W/m.K) (4)
is the thickness of the sample, Ts
the temperature of the heater and Tex, the
temperature of the external phase at many different moisture levels.
Figure 1 : Density of Moabi
Concerning the determination of isotherms of desorption, equilibrium points are obtained using the gravimetric method. The procedure is as follows: a sample of the product is put in a container which contains saturated salt solution; the role of the solution is to maintain relative humidity Hr of constant air. All of the sample is weighed many times up to the point where the density no longer varies, equilibrium is then attained with air (Ta, Hr) by knowing the initial density, the anhydrite density can be determined and moisture content can be deduced with the aid of formula (2). In this method, equilibrium at time sis only attained after several weeks, as such, much salt as possible is used in order to place points/marks on the curve.
IV. Results and discussions
IV.1 Density
Density of wood is in general a variable property depending on the location in the trunk of the tree, or its origin [8]; Here, we are presenting in a way an average reaction of this magnitude in relation to the moisture content without restriction to a location. In effect, trials also concerned shaped samples, in the transition zone commonly referred to as normal wood. The error margin on the weight and volume shows that relative uncertainty on the determination of the density is about 9.5%. The results of measurement are presented on figure 1; although the number of points are small, we observe with all evidence, taking into consideration studies existing in the domain [6,7,8], a linear variation of the density of moabi in relation to its moisture content. The correlations deduced from these results are as follows:
(5)
This correlation shows a difference between the value of the density at the anhydrate state 586.88 kg/m3 and the value found in current literature, 780 kg/m3. This could bring about criticisms on the procedure and state of equipment used, meanwhile, emphasis should be laid on the location, origin and even the age of the sample studied.
IV.2 Thermal conductivity
The measurement of thermal conductivity, figure 2 also shows a linear variation with moisture content; which is in conformity with existing studies. These studies also show that thermal conductivity decreases slowly in weak moisture content [3], below 13%; what we cannot predict with our results, due to the fact that we did not have data from the zone. Absolute uncertainty on measurements stands at 0.017. The correlation linked to this series of measurements is as follows:
(6)
Humidity comprises between 14% and 42%. If we consider the extrapolation relations (5) and (6), we can represent on figure 3, the nature of conductivity in relation to the density of the wood material; in effect, we note with satisfaction that the more a wood material is thick, the better it is as a thermal conductor. This reaction clearly comes out in the bibliography which makes us not to attribute this result solely to the principle and equipment used, which will be better in hard wood than porous ones; Moabi, being an averagely heavy wood, thus less porous, we can accept this result which is, nevertheless, susceptible to amelioration.
Figure 2 : Thermal conductivity of Moabi
Figure 3 : Influence of density on thermal conductivity
IV.3 Equilibrium of desorption
Hydroscopic equilibrium translates the relation between wood and water, two domains of reaction are considered in relation to the moisture content of wood. The non-hydroscopic domain where the moisture content is above the level of saturation of fibre and the hydroscopic domain which is less than this point/level. The saturation level of fibre is defined as the moisture content lower than the level where, any extraction of water is accompanied by a recording of the volume. We present here, the desorption equilibrium curves, used in the drying process. The experiment carried out with the aid of LEPT-ENSAM, Bordeaux I readily correlates with this. Studies show that desorption equilibrium curves can be represented through different models (Simpson, 1973 ; Koponen, 1985 ; Avramidis, 1997 ;...), the G.A.B model (Guggenhein, Anderson, Boer), the B.E.T model (Brunauer, Emmet, Teller) just to name a few. Certain models present cases showing the influence of temperature on the isotherm studied; in other cases, the influence is not explicit and the curve can only be represented for discrete values of temperatures which have facilitated their determination. In this study, we use the Harkings’s method to correlate the experimental points; as such, for a series of data at the same temperature, the moisture content of wood material is given in relation to the activity in water:
(7)
X is the moisture content at equilibrium, Hr, relative humidity of air or activity in water.
k and n are parameters that we have determined by reducing the error margin between the value measured and that calculated. For relative humidity of air less than 33%, the values of k and n are different from values obtained for Hr > 33%. We adopted this principle because, by using all the data, the function established presented a shortage of about 35% of relative humidity. Table 1 gives the values of parameters for some temperatures.
|
T(°C) |
Hr£33% |
Hr>33% |
||
|
k |
n |
k |
N |
|
|
20 |
3.7210 |
0.0018 |
4.7047 |
0.0115 |
|
30 |
3.6383 |
0.0014 |
4.6720 |
0.0096 |
|
60 |
3.5011 |
0.0006 |
4.6213 |
0.0045 |
Table 1: Values of parameters of the Harkings’s model.
On figures 4 and 6, we present desorption curves respectively at 20°C, 30°C and 60°C; the pace conforms well to the sigmoid curves presented in other studies [2,3,4,6,7]. Absolute uncertainty between the value calculated and that measured from moisture content to equilibrium is averagely 0.005.
Figure 4 : Isotherm of desorption of Moabi at 20°C
Figure 5 : Isotherm of desorption of Moabi at 40°C
Figure 6 : Isotherm of desorption of Moabi at 60°C
Three isotherms are presented on figure 7 showing the Influence of temperature on equilibrium points/levels of wood with the area of use. In effect, when the temperature is high, the moisture content of wood is low if we remain at the same level of relative humidity, in the hygroscopic domain. At the lowest hygroscopic degree of air, the rate of humidity equilibrium is somehow equal to the temperature of the area; this reaction is observed on the curves figure 7 where isotherms are represented at 20°C, 30°C and 60°C.
Figure 7 : Isotherms of desorption of Moabi: theoretical curves
V. Conclusion
Results presented are generally satisfactory given that the behaviours observed are not different from those found in existing literature of wood drying or its characteristics; be it of experimental points remain few. The procedures could also constitute an issue of debate; these results are therefore susceptible to amelioration and constitutes for us, the first elements of a database, on the thermal and physical properties of Moabi and its reaction during drying. It is also necessary to be able to correlate isotherms by models which can enable the integration in an explicit manner, the influence of temperature on equilibrium points, in order to come up with a more complete method of representation.
Nomenclature
e: thickness of wood sample (m);
H: moisture content of wood;
Hr: relative humidity of air (%);
I: current entering the heater (mA) ;
k: parameter of the function of Harkings ;
m: mass of sample at humidity H (kg) ;
m0: anhydrate mass of sample (kg);
n : parameter of the function of Harkings ;
q : voltage of thermal flux (W/m2 .K);
S: surface of heater (m2);
U : voltage of current entering heater (V) ;
Ta : temperature of air (°C);
Ts : surface temperature of heater (°C);
Tex : temperature of external surface of sample (°C);
V : volume of sample (m3);
X : moisture content of wood at equilibrium;
r : density of wood (kg/m3);
l : thermal conductivity of wood (W/m.K);
a : transmission relation of lagging/insulation flux;
References
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[5] Jing Liu, Stravos Avramidis, Simon Ellis, Simulation of heat and moisture transfer in wood during drying under constant ambient condition. Holzforschung 48, 1994 pp. 281-293.
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[7] A. Themelin. Bois et forêt des tropiques, 256, (1998).
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