DETERMINATION OF THE DIFFUSION COEFFICIENT DURING THE SORPTION PROCESS OF MOISTURE IN WOOD

Daniela SOVA
Conf.dr. ing. - Universitatea "Transilvania" din Brasov - Facultatea de Inginerie Mecanica
Adresa: B-dul Eroilor Nr. 29, 500036 Brasov, Romania
Tel: +40 268 412921/int. 193. Fax: +40 268 474761. E-mail: sova.d@unitbv.ro

Mihaela CAMPEAN
Prof.dr. ing. - Universitatea "Transilvania" din Brasov - Facultatea de Industria Lemnului
Adresa: B-dul Eroilor Nr. 29, 500036 Brasov, Romania
Tel: +40 268 419581. Fax: +40 268 419581. E-mail: campean@unitbv.ro

Abstract: The paper presents two graphic-analytical methods for the determination of the diffusion coefficient which are based on the experimental determination of the dynamics of the moisture sorption by wood. A first method, based on Crank's integral solution, proposes the determination of the diffusion coefficient in the conditions of neglecting the convective surface resistance. The second method is based on an equation that establishes the relationship between the dimensionless potential E and the dimensionless time t for different values of the Biot number.
Within a preliminary research session, four radial pine wood specimens (Pinus nigra L.), of two different thicknesses have been submitted to experimental determinations. The specimens have been first dried to the oven-dry state and afterwards submitted to the sorption process in an environment with constant parameters: air temperature t=30ºC; relative air humidity f=56% and air velocity w=0.5 m/s.
There have resulted close values of the diffusion coefficient for the two approached methods. From the first method resulted the values: 1.619E(-11), 1.715 E(-11), 6.528 E(-12), 6.0912 E(-12), corresponding to the four specimens, and from the second method resulted only one value: 1.693 E(-11). A closer agreement of the values obtained through the two different methods could be noticed in the case of the thicker specimens. According to the first method, the diffusion coefficient is a function of the square of the specimen thickness. According to the second method, no dependence of the diffusion coefficient on the specimen thickness can be established
The two graphic-analytical methods offer suitable modalities for the determination of the diffusion coefficient for different wood species and different parameters of air during sorption, representing a characteristic of the behavior of wood as a hygroscopic material.
In order to settle more rigorously the precision level of the two methods and their restrictions, the research is to be continued on larger sets of specimens of different species and dimensions.

Key words: mass transfer, diffusion coefficient, sorption dynamics, pine wood.

REFERENCES
CRANK, J. (1975). The Mathematics of Diffusion. Oxford University Press.
MARINESCU, I. (1979). Uscarea lemnului (Wood drying). Vol. I. Editura Tehnica, Bucuresti.
MIHAILA, C., CALUIANU, V., MARINESCU, M., DANESCU, A. (1982). Procese si instalatii industriale de uscare (Industrial Drying Processes and Kilns). Editura Tehnica, Bucuresti.
SIAU, J.F. (1995). Wood: Influence of Moisture on Physical Properties. Virginia Polytechnic Institute and State University.
STEFANESCU, D., LECA, A. (1983). Transfer de caldura si masa. Teorie si aplicatii (Heat and Mass Transfer. Theory and Applications). Editura Didactica si Pedagogica, Bucuresti.