Study On The Discharge In Microwave Vacuum Freeze-Drying

Microwave vacuum freeze-drying technology (MVFD) is the combination of microwave drying technology and vacuum freeze-drying technology. It uses the microwave as heat source which can provide heat not only to the surface but also to the inner of the object simultaneously. Comparing with conventional vacuum freeze-drying technology MVFD has the advantages of fast drying, time saving, high efficiency, low energy consumption and etc. MVFD still faces technology challenges in large scale industry application although it could get good drying effect in laboratory. The biggest problem is the microwave discharge phenomenon under low pressure. In recent years, many studies have been done on microwave vacuum freeze-drying technology and heat and mass transfer model, but few literatures was on the research of electric discharge in microwave drying process.At first the mechanism of microwave discharge under low pressure condition was studied and analyzed. It was showed microwave discharge is a kind of diffusion control non-polar microwave discharge under high frequency situation. On this basis, Boltzmann equation in electromagnetic field was used to explain this phenomenon and obtained the impact factors of microwave discharge are properties of working gas, pressure P, applied electric field frequency f, applied electric field strength E and feature size of the discharge container.Secondly, the regular pattern of microwave discharge under the condition of vacuum 50-1000Pa was discussed. The influence laws on microwave discharge of the factors vacuum pressure, feature size of drying cavity, material weight and moisture content were studied as well. Under the condition of drying cavity feature size 9.63cm,3.29cm,2.12cm, the models of microwave discharge point power value were regressed respectively with the parameters vacuum pressure, material quality and moisture content with using SAS software. Using Matlab software, quadratic fit and three-dimensional imaging analysis of the power value of microwave discharge points were done under different material weight and vacuum pressure. The results showed that the influence of pressure value on microwave discharge power value was U-shape in the scope of 50-1000Pa and the minimum value was between 200-400Pa. The power value of microwave discharge points has positive correlation with material weight and moisture content. That is, the power value of microwave discharge gets increased with the increasing of material weight and as well the moisture content. For the impact degree of the parameters on power value of microwave discharge:the vacuum pressure> material weight and water content.Thirdly, based on the microwave discharge theory, experimental research experience and the actual situation of original microwave freeze-drying equipment the experimental equipment was designed and optimized. The microwave field of resonance cavity with three different sealing flange thickness 10mm, 15mm and 20mm were simulated respectively with using HFSS Electromagnetic Field simulation software. It was showed that the best thickness of sealing flange was 10mm.