The Research Of Heat And Mass Transfer Model And Dielectric Peoperties Of Maize Under Acuum Drying Conditions Based On Water Potential

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.