Study On Preheating And Rheological Behavior Of Eucalyptus Camaldulensis Lumber Drying

E. camaldulensis is one of the main fast-growing plantation species in Yunnan Province, but drying defects including severe collapse and twist limit its use widely as solid lumber. Aiming to optimize drying technology and improve drying quality, this dissertation presents studies on preheating and Theological behavior of eucalyptus lumber drying. These studies include: (1) effect of preheating on shrinkage coefficient, longitudinal gas permeability, extractives content, water diffusion coefficient, drying rate and quality; (2) development of strain during drying and elimination of it during final conditioning; (3) effect of conditioning on collapse recovery and its mechanism.Because of change of extractives content and distribution during pretreatment, shrinkage coefficient, longitudinal gas permeability and water diffusion; coefficient increase significantly after pre-steaming over 3h or hot-water pre-soaking over 60℃ . Because of the same reason, drying rate of first period increase and final collapse magnitude decrease with 4h pre-steaming or 4h 80 ℃ hot-water pre-soaking.One-dimensional stress-strain mathematical model across to the grain during drying is established and can be used to illustrated stress-induced phenomena. The model takes into account all the needed components in low- and middle temperature drying: shrinkage, elastic, visco-elastic and mechano-sorptive strains. The recovery of mechano-sorptive strain during conditioning is also taken account. Shrinkage strain is modeled by shrinkage unit. Elastic strain and visco-elastic strain are modeled by spring and Kelvin body, separately. Mechano-sorptive strain is modeled by a modified Kelvin body, in which the conventional Kelvin body is parallel with the pinned-slider and the removal or replacement of the pin represents hydrogen-bond breaking or re-making.Rheological behavior during drying and conditioning is studied by slicing method, theresults show that development of elastic strain and visco-elastic strain are both affected by instantaneous drying stress and independent of drying history, so the law of them is similar; that of mechano-sorptive strain is affected by drying stress and drying history; because of the limited moisture pick-up in surface zone, elastic strain and mechano-sorptive strain decrease significantly during final conditioning; mechano-sorptive strain is the main component of net strain during conditioning as well as the middle- and later- period of drying; visco-elastic strain is little and appears to be negligible.Effect of different conditioning at different moisture content(MC) on collapse recovery were studied with software Matlab 6.0 and digital cameral, the results show that effect of conditioning on collapse recovery is relevant to MC of lumber at conditioning significantly, the effect of conditioning at mean MC of 27% is little, 3h steaming conditioning and 3h 90℃ wet saturation air conditioning at mean MC of 18% both can increase collapse recovery significantly while the former is more significant.