【作者】 何正斌；

【导师】 伊松林；

【作者基本信息】 北京林业大学 ， 木材科学与技术， 2014， 博士

【摘要】 木材超声波干燥是一种创新的干燥方法,本研究将超声波技术引入到木材真空干燥过程,并就超声波-真空协同干燥条件下的边界层特性、超声波发热机理、协同干燥特性及协同干燥过程中热质传递规律进行了较系统的研究,丰富了木材干燥基础理论,为木材干燥技术的创新和发展开辟了一条新的道路。论文主要成果与创新点如下：(1)在温度为30-90℃,绝对压强为0.02-0.1MPa条件下,对木材平衡含水率和表面热质传递规律进行研究。结果表明,木材平衡含水率随温度的升高而减小,随绝对压强的增加而增加；液体表面对流传质系数随温度的升高而减小,随绝对压强的减小而升高；液体表面对流换热系数随绝对压强的减小而减小,随温度的升高而升高；基于试验数据,建立了真空干燥过程中的木材平衡含水率模型及液体表面对流传热传质系数模型。(2)在温度为35℃和50℃,绝对压强为0.03MPa、0.06MPa和0.1MPa,超声波功率为60W和100W,超声波频率为20kHz和28kHz条件下对木材干燥过程中的边界层特性进行研究。结果表明,超声波协同处理试件的边界层底层温度和木材外层温度均高于对照组,其差值可分别达10.7℃和5.6℃,且超声波功率和频率对边界层温度影响显著；相比对照组,超声波协同处理可提高木材表面水分蒸发能力,最大可提高110.1%；建立了超声波-真空协同干燥过程中边界层厚度及木材表面水分蒸发模型。(3)在温度为20℃、40℃和60℃,绝对压强为0.03MPa、0.06MPa和0.1MPa,超声波功率为60W和100W,超声波频率为20kHz、28kHz和40kHz的条件下对超声波-真空处理过程中,木材内部发热机理进行研究。结果表明,对照组的温度均小于或等于环境温度,而超声波处理材的温度均高于环境温度,最高温度可达85℃,且木材温度与超声波功率和频率正相关,与环境压强负相关。(4)基于超声波-真空处理过程中,木材内部温度场和超声波传播规律,得到了协同作用下,超声波衰减系数模型、超声波声强衰减模型、超声波发热模型和木材内部温度升高模型。结果表明,超声波衰减系数与超声波功率、频率正相关,与环境压强负相关,且该模型得到的理论值与实测值吻合良好。(5)在不同温度、绝对压力、超声波功率和频率条件下,分别采用超声波-真空干燥,超声波预处理-真空干燥及超声波、真空预处理-真空干燥三种联合方式对木材进行干燥处理。结果表明,与对照组相比,超声波-真空干燥使得木材水分扩散系数提高23.25-40.9%；超声波预处理-真空干燥使得干燥时间缩短29.7-48.1%,超声波、真空预处理-真空干燥使得干燥时间缩短8-11%。三种协同方式均能加快木材真空干燥速率,缩短干燥时间,提高水分扩散系数。(6)在温度为60℃,绝对压强为0.02MPa,超声波功率为100W,频率为20kHz的条件下对木材进行超声波-真空协同干燥,并基于菲克扩散定律、热质传递规律及数学分析方法得到了协同干燥过程中,木材内部不同位置的水分分布和热量分布规律模型、木材含水率变化值与水分扩散系数和时间的关系模型。结果表明,水分扩散系数随含水率的增加呈指数形式增长,且模型得到的理论值与实际值相吻合,可用于模拟协同干燥过程中木材内部温度和水分的变化情况。更多还原

【Abstract】 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.更多还原