| Zanthoxylum bungeanum’s industrialized drying has always been the focus of Zanthoxylum bungeanum industrial research because of the great economic and medicinal value.In the study on the drying of agricultural products,the dehydration characteristics,heat and mass transfer mechanism and dynamic law of materials are the theoretical basis for designing drying system and optimizing drying equipment and technology.At present,the traditional hot air drying technology is the most widely used drying method of agricultural products,but it takes a long time,consumes a lot of energy and has low efficiency.The characteristics of high efficiency and high energy utilization of the new microwave technology can make up for the shortcomings of dry ing process for hot air.Therefore,this paper combines the two drying technologies to investigate the heat and moisture transfer law of hot air and microwave combined drying through experiments and numerical simulation.For the two technologies of hot air and microwave series combined drying and parallel coupled drying,the research contents are as follows:In the study of series drying,firstly,the single-stage hot air drying and the single-stage microwave drying experiments were conducted to investigate the dehydration characteristics and dynamic law of Zanthoxylum bungeanum in the hot-air section and microwave section,separately,and the range of the target moisture content was determined.Then,the microwave drying technology was used for deep drying and combined with the traditional hot air drying to study the effect of hot air parameters,the target moisture content and microwave power on the drying characteristics of Zanthoxylum bungeanum.The results show that the increase in air temperature,air velocity and microwave power all result in the acceleration of moisture migration of Zanthoxylum bungeanum.The sooner the process enters the microwave drying stage,the higher the drying efficiency is.And the best target moisture content is the critical moisture content of hot air dying as 65%（w.b）.The fitting results of thin-layer drying model and experimental data indicate that the Wang and Singh model and Page model were the best models to predict the moisture variation of Zanthoxylum bungeanum in the hot air section and microwave section,respectively,which determines the kinetic model of hot air/microwave combined drying of Zanthoxylum bungeanum.The Fourier criterion number（F0）was introduced to solve Fick’s second diffusion law equation,the effective moisture diffusivities（Deff）of hot air drying section and microwave drying section were obtained and varied from 1.908 × 10-9 to 3.547 × 10-9 m2/s and from 1.883 × 10-8 to 3.321 × 10-8m2/s,separately.Based on Arrhenius function,Ea of hot air section and microwave section of Zanthoxylum bungeanum were 29.363 kJ/mol and 1.439 W/g,respectively.In the study of parallel drying,hot air coupled microwave drying experiments were performed to investigate the dehydration characteristic of Zanthoxylum bungeanum at different air temperatures and microwave powers.The obtained results show that there are an increasing rate period and a falling rate period in the hot air coupled microwave drying of Zanthoxylum bungeanum,and the increase in hot air temperature and microwave power results in the acceleration of moisture migration of Zanthoxylum bungeanum and reduction of drying time.The common drying models were fitted to the experimental data and the Logarithmic model was selected as the best one to predict the moisture variation of Zanthoxylum bungeanum.Based on Fick’s second law and Arrhenius function,the effective moisture diffusivity（Deff）and the activation energy（Ea）of Zanthoxylum bungeanum were obtained and varied from 1.65026×10-8 to 4.51093×10-8 and from 14.95 to 24.09 kJ/mol,respectively.The simulation on the heat and mass diffusion phenomenon of hot air microwave coupled drying was conducted through FLUENT.The results show that microwave is able to improve the uneven distribution of heat and moisture during drying,and the heating effect of microwave weakens the influence of hot air.It is verified by experiments that the mathematical model of drying heat and mass transfer and the UDF program compiled are reliable.The advantages and disadvantages of four drying methods（i.e.hot air drying,hot air and microwave series combined drying,hot air and microwave parallel coupled drying and microwave drying）in terms of drying characteristics and energy characteristics were compared and analyzed in this paper.The calculation results show that microwave-assisted hot air drying can significantly shorten the drying time,reduce energy consumption and improve drying energy efficiency.And it is more suitable for the drying demand of fresh Zanthoxylum bungeanum to apply the hot air and microwave series combined drying process based on the existing experimental conditions. |
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