Periodic Hot-platen Drying And Heat&Mass Transfer Mechanism Of Poplar Lumber

In order to reveal the moisture state and pressure variety,and its influence mechanism on heat and mass transfer of wood during hot-platen drying,the moisture pressure and temperature inside poplar(Populus tomentosa)lumber was investigated by the integrated temperature-pressure probe during periodic hot-platen drying.The moisture state variety and migration mechanism in lumber were analyzed according to the thermodynamic principle during hot-platen drying.The hot-platen drying schedule was further optimized.A mathematical model with moisture pressure term for modeling heat and mass transfer inside lumber during hot-platen drying was constructed and validated,which provided theoretical basis for precise control of wood hot-platen drying process.When the initial moisture content(MC)range of lumber is from 50%to 70%,the heating platens temperature ranging from 120℃ to 180℃,the main conclusions are as follows:(1)The moisture state variety and migration mechanism inside lumber during hot-platen drying was studied.① It was revealed that moisture pressure inside lumber with MC above fiber saturation point(FSP)was greater than the atmospheric pressure during the closing period of heating platens.Moisture state distribution characteristic was formed as superheated vapor-saturated vapor-liquid water from the surface to center of lumber.Moisture state distribution of superheated vapor under different superheat along the thickness of lumber was formed with MC lower than FSP.②Internal vapor driven by pressure difference between the lumber and atmospheric environment was discharged from the surface of lumber and resulted in the decrease of MC in the opening period of heating platens.This is the main moisture transfer style in lumber during periodic hot-platen drying.(2)The hot-platen drying schedule was optimized in accordance with the effect of internal moisture pressure on wood drying efficiency.The optimized periodic hot-platen drying schedule of 25 mm-thickness poplar lumber is as follow:In the pretreatment stage,the heating platens temperature was 140 ℃,the closing and opening time of heating platens was 14.0 min and 1.5 min respectively;As MC>30%,the heating platens temperature was 140 ℃,the closing and opening time of heating platens was 11.5 min and 2.0 min respectively;the heating platens temperature was increased to 150 0C,the closing and opening time of heating platens was 7.5 min and 2.0 min within the stage of 20%<MC<30%;while the heating platens temperature was increased 160 0C,the closing and opening time of heating platens was 8.5 min and 2.0 min within the stage of 6%<MC<20%.The drying efficiency of lumber dried by the optimized periodic hot-platen drying schedule was increased by 104.90%as compared with the lumber dried by traditional periodic hot-platen drying schedule.(3)The effects of hot-platen drying on dimensional stability of lumber was analyzed.Compared with the air-drying poplar lumber,the swelling rate in tangential and volume of hot-platen dried poplar lumber were decreased by 56.15%and 28.87%,and the corresponding shrinkage rate was decreased by 55.45%and 37.06%,respectively.The swelling and shrinkage rate in radial of hot-platen dried lumber was increased by 36.00%and 14.58%,respectively.The swelling and shrinkage ratio between tangential and radial of hot-platen dried lumber was decreased by 67.69%and 61.14%as compared with that of the air-drying lumber.The anisotropy of dimensional change in lumber was decreased after hot-platen drying.It was concluded that the dimensional stability of lumber was obvious improved after hot-platen drying.(4)A mathematical model for hot-platen dried lumber containing pressure term was constructed and validated.The internal pressure field,temperature field and MC Field of hot-platen dried lumber was investigated and analyzed experimentally.A mathematical model describing heat and mass transfer with moisture pressure term was constructed.The numerical simulation and verification of model describing heat and mass transfer during hot-platen drying was completed using finite difference methods and MATLAB programming.The results showed that:① the MC values predicted by the mathematical model was well agreed with the experimental data.②Compared with the values of temperature predicted by the traditional heat transfer model without vapor pressure term,the theoretical temperature values from heat transfer model with vapor pressure term agree with experimental data the better.