Evaluation of Diffusion Coefficient and Surface Emission Coefficient by an Optimization Technique


  • Yong Chen
  • Elvin T. Choong
  • David M. Wetzel


Diffusion coefficient, surface emission coefficient, optimization, conjugate gradient


The rate at which wood dries can be expressed by two factors:(1) the diffusion coefficient, as an indication of internal resistance to moisture removal, and (2) the surface emission coefficient, as an indication of external resistance to moisture removal. An optimization technique was applied to calculate these two coefficients. Unlike existing methods where these coefficients are evaluated at only one point, usually at one-half of the evaporable water remaining in wood, this optimization technique used the entire drying data. The technique searches for the optimum pair of coefficients based on the least squares principle. The performance of this method was demonstrated by reanalyzing published data using a digital computer. The results indicate that a more accurate prediction of the drying process can be achieved by the optimization method than by existing methods. The dependence of the diffusion coefficient on moisture content and sample thickness is discussed.


Choong, E. T. 1965. Diffusion coefficients of softwoods by steady-state and theoretical methods. Forest. Prod. J. 15(11):21-27.nChoong, E. T. and C. Skaar. 1969. Separating internal and external resistance to moisture removal in wood drying. Wood Sci. 1(4):200-202.nChoong, E. T. and C. Skaar. 1972. Diffusivity and surface emissivity in wood drying. Wood Fiber 4(2):80-86.nComstock, G. L. 1963. Moisture diffusion coefficients in wood as calculated from absorption, desorption, and steady-state data. Forest. Prod. J. 13(3):97-103.nHart, C. A. 1964. Principles of moisture movement in wood. Forest. Prod. J. 14(5):207-214.nLiu, J. Y. 1989. A new method for separating diffusion coefficient and surface emission coefficient. Wood Fiber Sci. 21(2):133-141.nNewman, A. B. 1931. The drying of porous solids:Diffusion calculations. Trans. Am. Inst. Chem. Eng. 27: 203-216, 310-330.nRosen, H. N. 1978. The influence of external resistance on moisture adsorption rates in wood. Wood Fiber 10(3): 218-228.nSkaar, C. 1954. Analysis of methods for determining the coefficient of moisture diffusion in wood. Forest. Prod. J. 4(6):403-410.nStamm, A. J. 1964. Wood and cellulose science. Ronald Press, New York, NY.n






Research Contributions