Study On Heat And Mass Transfer In Wood During Ultrasound-Vacuum Combined Drying

Wood ultrasound drying is a kind of innovative approaches. Ultrasound technology was taken into wood vacuum drying process. The characteristics of boundary layer at wood surface and the heating principle of ultrasound inner wood, the drying characters, and the heat and mass transfer principle were studied during ultrasound-vacuum combined drying process. This paper could enrich the basic theory of wood drying and provid a new way for wood drying. The main results and innovation point were as follows,(1)Wood equilibrium moisture content and the law of heat and mass transfer during vacuum drying process were studied among the temperature of30℃to90℃and the absolute pressure of0.02MPa to0.1MPa. Results showed that, wood equilibrium moisture content increased along with the decrease of temperature and increase of absolute pressure; The convection mass transfer coefficient decreased with the increase of temperature and with the decrease of absolute pressure; What’s more, the convection heat transfer coefficient decreased with the decrease of absolute pressure and increase with the increase of temperature. Also, the equilibrium moisture content model and convection heat and mass transfer coefficient model were established.(2)The characteristics of boundary layer at wood surface was studied at the temperature of35℃and50℃, the absolute pressure of0.03MPa,0.06MPa and0.1MPa, ultrasound power of60W and100W, and ultrasound frequency of20kHz and28kHz. The results indicated that, comparing with control group, ultrasound could increase the temperature of boundary layer and wood surface, the increase of which were10.7℃and5.6℃, respectively, and ultrasound power and frequency had significant impact on boundary layer temperature. Also, ultrasound could enhance the water evaporation power, which is110.1%higher than that of control specimens. What’s more, the thickness of boundary layer model and the water evaporation model at wood surface were established.(3)The heating principle of ultrasound inner wood was studied under the conditions that, the temperature were20℃,40℃and60℃, the absolute pressure were0.03MPa,0.06MPa and0.1MPa, the ultrasound power were60W and100W and the ultrasound frequency were20kHz,28kHz and40kHz. The results showed that, the temperature of control group was lower or equal to ambient temperature, while that for ultrasound treating group was higher than that of ambient temperature, the maximum value was85℃, and the temperature value is proportional to ultrasound power and ultrasound frequency, while is inverse proportional to absolute pressure.(4)Ultrasound attenuation coefficient model, ultrasound intensity attenuation law, ultrasound heating and temperature increasing models were established based on wood temperature field and ultrasound propagation law during ultrasound-vacuum combined drying process. The results indicated that ultrasound attenuation coefficient is proportional to ultrasound power, ultrasound frequency, while is inverse proportional to pressure and this model was well-matched to the actual value.(5)Ultrasound-vacuum drying, ultrasound pretreatment-vacuum drying, and ultrasound and vacuum combined pretreatment-vacuum drying were carried out at different temperature, absolute pressure, ultrasound power and ultrasound frequency conditions to study the impact of ultrasound on wood drying characteristics. The results indicated that, comparing with control group, the water diffusion coefficient was increased by23.25-40.9%for ultrasound-vacuum drying, the drying time was shorten by29.7-48.1%for ultrasound pretreatment-vacuum drying, the drying time was shorten by8-11%for ultrasound and vacuum combined pretreatment-vacuum drying. All in all, the three drying methods could be used to increase wood drying rate, shorten wood drying time and increase water diffusion coefficient.(6)Wood ultrasound-vacuum combined dying was carried out at the temperature of60℃, at the absolute pressure of0.02MPa, at the ultrasound power of100W and the frequency of20kHz. The water and heat distribution model, the relationships among wood moisture content, water diffusion coefficient and time were established based on Fick’s law of diffusion, heat and mass transfer principle and mathematical analysis. The results showed that, the water diffusion coefficient is exponential growth with the moisture content. This model was well-matched to the actual value and could be used to simulate water and heat distribution during ultrasound-vacuum combined drying process.