基于水势的玉米真空干燥传热传质模型及介电特性的研究
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摘要
目前粮食真空连续干燥机械装置已经研制成功,粮食低温真空连续干燥的有效性得到证实,而该技术的深入发展受到自动检测和控制技术落后的严重制约。粮食低温真空连续自动检测和控制技术的基础是真空条件下生物多孔介质介电特性和传热传质的研究,这方面的基础研究相当缺乏。而真空条件粮食介电特性的测量与粮食在常规条件下的介电特性有很大的区别。与普通热风干燥条件下的介电特性比较,在真空连续干燥条件下的介电特性存在影响因素更多、数学模型建立难度大、过程参数探测难度大、信号干扰大等特点,另外,由于真空压强的介入,粮食颗粒传热传质的分析也发生改变,因此,有必要研究真空干燥条件下粮食的介电特性和传热传质特性,进而为改进真空干燥技术和连续干燥设备,优化干噪工艺,提高粮食干燥品质和干燥效率提供依据和参考,为实现粮食真空干燥自动化控制奠定基础。
本文基于水势分析了真空干燥条件下玉米传热传质和应力的变化情况,并在设计叉指式平板电容介电参数检测元件的基础上,构建了真空状态下的介电测试系统,实验研究了玉米介电特性的变化规律,为真空干燥过程测试控制提供了基础。本文的研究工作是依托于国家自然基金项目“低温真空连续干燥条件下粮食介电特性及水分检测和控制”中的部分内容,从理论和试验两方面研究真空干燥介电特性变化、传热传质和应力变化过程。
全文的主要内容如下：
1)基于水势理论,根据热力学第一定律分析了单粒玉米真空干燥传热传质特性,建立了数学模型并利用差分解析法进行数值解析,得到真空干燥条件下传热传质的模拟系统,该系统操作性强,性能可靠,可通过输入初始物料物理参数和干燥条件,直接模拟出玉米内部温度、水分、压强及水势的分布情况和该籽粒的平均温度和平均水分值,由于对于单粒玉米来说,真空干燥室内提供的能量足够大,粮温的变化表现出的是持续增加的趋势直至达到干燥箱温度。
2)在单粒玉米真空干燥传热传质的基础上,分析了深床玉米真空干燥传热传质特性,并进行模拟分析,与单粒玉米传热传质分析相比,粮温的变化有一恒温阶段,是由于真空干燥室内提供的热量有一段时间全部用来水分蒸发,所以在研究深床真空干燥时加入了该条件的限制,该模型能较好的模拟传热传质过程。建立了真空干燥过程中质热传递的模拟系统。由于该模型综合考虑了压强、温度和水分对质热传递的影响,输入其他物料的初始参数,也可应用于其他物料的真空干燥。另外,模型经变形亦可应用于其他干燥方式,如热风干燥、微波干燥等。
3)基于水势理论对真空干燥的玉米进行应力分析和预测,真空干燥的玉米颗粒内部应力比热风干燥所受应力小得多,为了更好的避免应力裂纹的产生,对真空干燥加入缓苏阶段,并可通过模型对缓苏时间进行确定,即可保证干燥品质又可保证干燥效率。
4)以叉指式电容板作为水分传感器的主要元件,利用LCR测试仪检测玉米真空干燥条件下的介电特性,结果表明,真空度对粮食介电特性有较大的影响,为了更好的考虑综合因素对粮食介电特性的影响,在这里引入了水势的概念,得到玉米水势与介电特性的关系,并分析了粮食的介电特性在真空中和大气压下介电特性的差别。
5)根据介电特性的试验数据,利用剔除粗大误差的方法得到有效数据,再对有效数据进行神经网络数据融合,并通过试验进行了验证分析。
Present, continuous vacuum drying machine has been developed, and the effectiveness of continuous vacuum drying of grain at low temperature is confirmed. But the further development of this technology is seriously constrained by the automatic detection and behindhand control technology. Grain cryogenic vacuum continuous automatic detection and control technology is based on the vacuum dielectric properties of biological porous media and heat and mass transfer research, and the basic research in this area is scarce. There is great different between the vacuum measurement of dielectric properties and conventional dielectric measurement. Compared to the dielectric measurements under ordinary drying conditions, the dielectric measurement under the condition of continuous vacuum drying has the properties of more effect factors, more difficult mathematical modeling, more difficult to detect process Parameters and larger signal interference. In addition, because of the vacuum pressure intervention, grain Particles heat and mass transfer analysis is also changed, therefore, it is necessary to study the dielectric properties and heat and mass transfer characteristics under the conditions of vacuum drying, and thus to improve the vacuum drying and continuous drying equipment, optimize drying technology, improve grain quality and drying efficiency, and provide the basis for the realization of automatic control of grain vacuum drying.
In this Paper, the vacuum dielectric test system was constructed based on the design of comb plate caPacitor dielectric Parameters detection device. The change rule of dielectric properties was studied to provide a basis for test control during vacuum drying process. The heat and mass transfer and stress changes were analsized under vacuum drying conditions and based on water potential. This research is relying on the National Natural Science Foundation project "The dielectric properties of grain and moisture detection and control under continuous low-temperature vacuum drying conditions" and the changes of vacuum drying dielectric properties, heat and mass process and stress were investigated in both theoretical and experimental aspects.
The main contents are as follows:
1) Based on water potential theory and due to the first law of thermodynamics, vacuum drying heat and mass transfer characteristics of a single grain were analyzed, a mathematical model was built and. using analytic method for numerical solution of differential. The heat and mass transfer simulation system under vacuum dry conditions was obtained. This system operable, performance and reliable, which can input physical Parameters and drying conditions of initial material and simulate internal temperature, moisture, pressure, water potential distribution and average temperature and moisture directly. Because the vacuum drying chamber is large enough to provide energy, food temperature is shown increasing trend until to dryer temperature.
2) Based on vacuum drying heat and mass transfer in a single grain, the heat and mass transfer characteristics of the deep vacuum drying were analyzed, and did simulation analysis. ComPared to heat and mass transfer of single grain, the grain temperature presented a constant phase. Because the heat of drying chamber was all used to water evaporation, this conditional limit was added to deep vacuum drying. ComParison of simulation results with the experimental results, the model is caPable of simulating heat and mass transfer. As the model took into account the imPact of pressure, temperature and moisture on heat and mass transfer, enter the initial Parameters of other materials can also be applied to vacuum drying of other materials. In addition, the model after deformation can be applied to other drying methods, such as microwave drying.
3) Based on water potential theory, stress analysis and forecasting for vacuum drying were carried out. The inner stress of vacuum drying grain is much smaller than hot air drying. In order to avoid the stress cracks, on the stage of vacuum drying to join tempering stage and may determine tempering time through the model. It can guarantee drying quality and efficiency.
4) Take interdigital electrode as the main moisture sensor component and use of LCR, the dielectric properties was detected under the conditions of vacuum drying. The results show that the vacuum has a greater influence on dielectric properties of grain. In order to take into account for the effect of factors on dielectric properties, the concept of water potential was introduced, the relationship of water potential and dielectric properties was obtained, and the differences of dielectric properties between a vacuum and the atmospheric pressure was analyzed.
5) According to test data of the dielectric properties, the method was used to eliminate the careless error and got the effective data, did data fusion for the effective data, and the model verified by tests.
本文基于水势分析了真空干燥条件下玉米传热传质和应力的变化情况,并在设计叉指式平板电容介电参数检测元件的基础上,构建了真空状态下的介电测试系统,实验研究了玉米介电特性的变化规律,为真空干燥过程测试控制提供了基础。本文的研究工作是依托于国家自然基金项目“低温真空连续干燥条件下粮食介电特性及水分检测和控制”中的部分内容,从理论和试验两方面研究真空干燥介电特性变化、传热传质和应力变化过程。
全文的主要内容如下：
1)基于水势理论,根据热力学第一定律分析了单粒玉米真空干燥传热传质特性,建立了数学模型并利用差分解析法进行数值解析,得到真空干燥条件下传热传质的模拟系统,该系统操作性强,性能可靠,可通过输入初始物料物理参数和干燥条件,直接模拟出玉米内部温度、水分、压强及水势的分布情况和该籽粒的平均温度和平均水分值,由于对于单粒玉米来说,真空干燥室内提供的能量足够大,粮温的变化表现出的是持续增加的趋势直至达到干燥箱温度。
2)在单粒玉米真空干燥传热传质的基础上,分析了深床玉米真空干燥传热传质特性,并进行模拟分析,与单粒玉米传热传质分析相比,粮温的变化有一恒温阶段,是由于真空干燥室内提供的热量有一段时间全部用来水分蒸发,所以在研究深床真空干燥时加入了该条件的限制,该模型能较好的模拟传热传质过程。建立了真空干燥过程中质热传递的模拟系统。由于该模型综合考虑了压强、温度和水分对质热传递的影响,输入其他物料的初始参数,也可应用于其他物料的真空干燥。另外,模型经变形亦可应用于其他干燥方式,如热风干燥、微波干燥等。
3)基于水势理论对真空干燥的玉米进行应力分析和预测,真空干燥的玉米颗粒内部应力比热风干燥所受应力小得多,为了更好的避免应力裂纹的产生,对真空干燥加入缓苏阶段,并可通过模型对缓苏时间进行确定,即可保证干燥品质又可保证干燥效率。
4)以叉指式电容板作为水分传感器的主要元件,利用LCR测试仪检测玉米真空干燥条件下的介电特性,结果表明,真空度对粮食介电特性有较大的影响,为了更好的考虑综合因素对粮食介电特性的影响,在这里引入了水势的概念,得到玉米水势与介电特性的关系,并分析了粮食的介电特性在真空中和大气压下介电特性的差别。
5)根据介电特性的试验数据,利用剔除粗大误差的方法得到有效数据,再对有效数据进行神经网络数据融合,并通过试验进行了验证分析。
Present, continuous vacuum drying machine has been developed, and the effectiveness of continuous vacuum drying of grain at low temperature is confirmed. But the further development of this technology is seriously constrained by the automatic detection and behindhand control technology. Grain cryogenic vacuum continuous automatic detection and control technology is based on the vacuum dielectric properties of biological porous media and heat and mass transfer research, and the basic research in this area is scarce. There is great different between the vacuum measurement of dielectric properties and conventional dielectric measurement. Compared to the dielectric measurements under ordinary drying conditions, the dielectric measurement under the condition of continuous vacuum drying has the properties of more effect factors, more difficult mathematical modeling, more difficult to detect process Parameters and larger signal interference. In addition, because of the vacuum pressure intervention, grain Particles heat and mass transfer analysis is also changed, therefore, it is necessary to study the dielectric properties and heat and mass transfer characteristics under the conditions of vacuum drying, and thus to improve the vacuum drying and continuous drying equipment, optimize drying technology, improve grain quality and drying efficiency, and provide the basis for the realization of automatic control of grain vacuum drying.
In this Paper, the vacuum dielectric test system was constructed based on the design of comb plate caPacitor dielectric Parameters detection device. The change rule of dielectric properties was studied to provide a basis for test control during vacuum drying process. The heat and mass transfer and stress changes were analsized under vacuum drying conditions and based on water potential. This research is relying on the National Natural Science Foundation project "The dielectric properties of grain and moisture detection and control under continuous low-temperature vacuum drying conditions" and the changes of vacuum drying dielectric properties, heat and mass process and stress were investigated in both theoretical and experimental aspects.
The main contents are as follows:
1) Based on water potential theory and due to the first law of thermodynamics, vacuum drying heat and mass transfer characteristics of a single grain were analyzed, a mathematical model was built and. using analytic method for numerical solution of differential. The heat and mass transfer simulation system under vacuum dry conditions was obtained. This system operable, performance and reliable, which can input physical Parameters and drying conditions of initial material and simulate internal temperature, moisture, pressure, water potential distribution and average temperature and moisture directly. Because the vacuum drying chamber is large enough to provide energy, food temperature is shown increasing trend until to dryer temperature.
2) Based on vacuum drying heat and mass transfer in a single grain, the heat and mass transfer characteristics of the deep vacuum drying were analyzed, and did simulation analysis. ComPared to heat and mass transfer of single grain, the grain temperature presented a constant phase. Because the heat of drying chamber was all used to water evaporation, this conditional limit was added to deep vacuum drying. ComParison of simulation results with the experimental results, the model is caPable of simulating heat and mass transfer. As the model took into account the imPact of pressure, temperature and moisture on heat and mass transfer, enter the initial Parameters of other materials can also be applied to vacuum drying of other materials. In addition, the model after deformation can be applied to other drying methods, such as microwave drying.
3) Based on water potential theory, stress analysis and forecasting for vacuum drying were carried out. The inner stress of vacuum drying grain is much smaller than hot air drying. In order to avoid the stress cracks, on the stage of vacuum drying to join tempering stage and may determine tempering time through the model. It can guarantee drying quality and efficiency.
4) Take interdigital electrode as the main moisture sensor component and use of LCR, the dielectric properties was detected under the conditions of vacuum drying. The results show that the vacuum has a greater influence on dielectric properties of grain. In order to take into account for the effect of factors on dielectric properties, the concept of water potential was introduced, the relationship of water potential and dielectric properties was obtained, and the differences of dielectric properties between a vacuum and the atmospheric pressure was analyzed.
5) According to test data of the dielectric properties, the method was used to eliminate the careless error and got the effective data, did data fusion for the effective data, and the model verified by tests.
引文
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