Superheated Steam Drying Of Chinese Cedar Lumber And Its Heat & Mass Transfer Modeling

The influence of superheated steam?SHS?drying on Chinese Cedar?Cryptomeria fortunei?lumber was studied in this dissertation.The drying quality,microscopic structure,physical and mechanical properties of Chinese Cedar lumber dried by SHS were investigated.Characteristic of the boundary layer generated during SHS drying process was also analyzed.Heat and mass transfer of the Chinese Cedar lumber during SHS-drying process was studied systematically.A one-dimension mathematical model describing heat and mass ransfer was estibulished,which provided theoretical basis for the SHSdrying analysis of Chinese Cedar lumber,and also provided important significance to promote the efficient processing and high value-added utilization of plantation wood.The main research conclusions are as follows:?1?Drying quality and microstructure.The drying quality of Chinese cedar lumber beyond the 2^nd drying grade is 89% for SHS drying.The extent of damaged perforated membrane observed in the radial section of Chinese Cedar lumber with SHS drying was greater than that of the lumber with conventional?CON?drying and air drying,and the permeability of wood was increased.Average drying rate of the Chinese Cedar lumber with SHS drying was 84% higher than that of Chinese Cedar lumber with CON drying.?2?Physical and mechanical properties.Equilibrated MC and swelling efficiency of Chinese cedar were significantly decreased after SHS drying.The value of moisture excluding efficiency?MEE?for CON dried spencimens were 15.49% and 11.33% under 20?/65%RH and 40?/90%RH conditions,which increased to 17.68% and 22.44% for SHS dried spencimens.The value of anti-swelling efficiency?ASE?for CON dried spencimens were 6.22% and 7.13% under two environmental conditions,which increased to 24.38% and 22.19% for SHS dried spencimens.Hygroscopicity was reduced and dimensional stability was enhanced by SHS drying.Modulus of elasticity of SHS dried spencimens was slightly higher than that of CON dried spencimens and air dried spencimens,but modulus of rupture of SHS dried spencimens was slightly lower than that of CON dried spencimens and air dried spencimens.?3?Mechanism analysis.The storage modulus of SHS dried,CON dried and air dried spencimens was decreased with temperature increasing;the decreasing extent was increased with lower stiffness and higher moisture content?MC?of spencimens.Two relaxation processes were observed for the three kinds of spencimens,the temperature of relaxation peak was drceased with increasing MC.Cellulose crystallinity and crystallite size of SHS dried spencimens was greater than that of CON dried and air dried spencimens.?4?The value of Ma was less than 0.3 for drying medium,and its density may be considered costant in the duct as steady flow.The drying medium flows under turbulent state,and turbulent boundary layer was formed.The thickness of boundary layer was increased with increasing temperature of drying medium and the distance from the entrance.?5?Heat and mass transfer during drying process was separately expressed by Fourier's law and Fick's second law of diffusion.A mathematical model descrbing heat and mass transfer during SHS drying process was established,and the definite conditions were also obtained.Partial differential equations of heat and mass transfer were converted to difference equations followed by numerical solution by using finite difference method.The results of comparsion for theoretical and measured values of MC were showed that the deviation range from 1% to 9%.The results of comparsion for theoretical and measured values of temperature were showed that the deviation range from 1% to 8%.