| Berry has high nutritional value and health-promoting efficacy with increasing production and sale in China.Unfortunately,fresh berry is soft texture,high moisture content and high viscosity with containing heat sensitive components,which are prone to be perishable and mildewed in the process of storage and transportation,resulting in greater economic losses.Microwave heating has the advantages of high thermal efficiency and controllable product quality,which is widely used in the drying treatment of food and agri-products.However,the low energy efficiency and nonuniformity of microwave heating limits its high-quality application,especially for the continuous microwave dryer which is used in the industrial production.Under the premise of ensuring the uniformity of microwave heating and drying quality of materials,the main purpose of this study is to improve the energy utilization efficiency of continuous microwave drying.Raspberry,as a typical small berry,is taken as the research materials.Based on the bench test and computer simulation,the electromagnetic and mass-heat transfer model for microwave drying is established by the multiphysics field software COMSOL Mutliphysics,and the distribution of electric field strength and microwave energy absorption of berry pulp layer are characterized.Also,the change rules of energy efficiency and temperature uniformity are explored incroporating the variation characteristics of temperature and moisture.By altering mass of berry pulp processed,microwave power and energy input mode,the main studies are focused on the drying characteristics of berry pulp,absorption and conversion efficiency of microwave energy,characteristics of electric field strength,energy input mode,retention rate of ascorbic acid and anthocyanin affected by the changes of temperature and temperature uniformity,as well as the interaction between processing parameters.Firstly,the effects of opening modes of magnetrons and microwave transmition modes in the waveguide on the distribution of electric field strength inside the berry pulp layer and the relationship between energy input and demand under different drying stages are analyzed.Then,the effects of temperature uniformity on the retention of ascorbic acid and anthocyanin are investigated.Finally,the interactions among experimental factors on energy efficiency and temperature uniformity are investigated through the response surface method and the optimum parameters are also proposed,which is conductive to determine the reasonable scheme to improve the energy efficiency and final quality for berry in continuous microwave drying.The main research contents and conclusions are as follows:(1)The continuous microwave drying process of berry is numerically simulated by using the electromagnetic,mass heat transfer and energy conservation modal.The procedure code written by MATLAB is used to achieve the movement for material geometry position,the storage and call of calculation results for realizing the dynamic simulation for moving material layer of continuous microwave drying process in the drying chamber.The reliability of model is verified by comparing the variation of temperature and moisture content measured by experiment with that by simulation.This simulation method can be used to characterize the distribution of electric field strength and microwave energy absorption inside berry pulp layer in continuous microwave drying process,which can provide theoretical basis for analyzing the variation mechanism of energy efficiency and uniformity of drying process.(2)The effects of different treated mass of berry on energy efficiency and uniformity are examined under full microwave power input(21k W)of microwave dryer and different microwave intensity(2-5 W/g)conditions.Under certain microwave power,the higher microwave intensity with thinner material layer casues the less microwave energy consumption and absorption by berry pulp layer per unit time,which reduces the efficiency of microwave energy absorption.Simultaneously,the higher microwave intensity results in the greater effective diffusion coefficient of moisture,which is beneficial to accelerate the drying rate of pulp and improve the conversion efficiency of microwave energy.High microwave intensity may shorten drying duration and improve uniformity of microwave energy absorption.The uniformity of temperature is positively correlated with the uniformity of electric field and microwave energy absorption under different microwave intensities.Due to the influences of the movement of material layer and the internal heat conduction of berry pulp layer,the uniformity of temperature is higher than those of electric field and microwave energy absorption under the same microwave intensity.The energy efficiency and temperature uniformity are affected by the muli-factors,such as microwave intensity and drying characteristics of berry materials,among which the distribution of electric field strength is the key factor.(3)To analyze the effects of the distribution of electric field strength on energy efficiency and uniformity,the mode of microwave transmission in rectangular waveguide is determined based on the established model for microwave drying.The different working amouts of magnetrons at the ceiling of cavity of continuous microwave dryer are introduced to achieve the input mode of microwave powers as 12,15,18,21 k W,and the rules of microwave transmission and distribution of electric field strength for berry pulp layer are analyzed by combining the modes of microwave transmission in rectangular waveguide.The electric field uniformity,the ratio of electric field strength to power and the ratio of microwave energy absorption to mass of the whole berry pulp under different opening modes of working magnetrons are obtained,and the changes of temperature and moisture are combined during drying process to analyze the variation of energy efficiency.The results show that the amounts and positions of working magnetrons determine TE or TM plane wave,in which the incident angle of plane wave dominates the conversion capacity of microwave energy to thermal energy.The distribution of electric field strength of berry pulp layer is affected by the transmission mode of microwaves in the rectangular waveguide and the opening mode of the magnetrons.Under the transmission modes of rectangular waveguide,the distribution of electric field in berry pulp only presents the transverse component.The active mode of magnetrons is parallel arrangement,and the distance of central points of magnetrons is odd times of quarter microwave wavelength in the direction of long side of magnetron,which can improve the uniformity of electric field for the whole material layer due to the cross distribution with high and low spots in electric field.High uniformity of distribution for electric field reduces the energy loss caused by electric field superposition and the difference of temperature distribution across the material layer,improvinh the energy efficiency of the whole drying process and the temperature uniformity of postdrying.The temperature uniformity of post-drying is positively correlated with the uniformity of overall electric field and the electric field in the late drying period,which is more strongly correlated with the uniformity of electric field in the late drying period.(4)Under the same microwave input power of 12 k W and different energy input modes achieved by altering the amount and position of working magnetrons,the energy input mode with high uniformity distribution of electric field for the whole berry pulp layer can improve the efficiency of microwave energy absorption,while the energy input mode with energy demand associating the drying characteristics of berry pulp improve the conversion efficiency of microwave energy.The efficiency of microwave energy conversion has a greater influence than the efficiency of microwave energy absorption on the total energy efficiency in the drying process.The drying characteristics of berry pulp layer should be considered more for the energy input mode to improve the total energy efficiency.To improve the energy efficiency,sufficient energy input should be provided to obtain a higher level of drying rate in the early stage,and suitable energy should be provided to keep the constant-rate stage and the evaporation of partial bound water in the fallingrate drying period.Combining simulation analysis with the drying characteristics of berry pulp layer is helpful to select specific energy input mode.At the early stage of drying,temperature and temperature uniformity have little effect on the retention of ascorbic acid and anthocyanin.As the temperature rises to a certain level in the middle and late stages of drying,both of temperature and temperature uniformity affect the retention of ascorbic acid and anthocyanin of berry pulp.In order to ensure the quality of post-drying,reasonable temperature range is determined as 70?80 ? for drying berry pulp,and the temperature uniformity in the middle and late heating stages should be improved.Overall,the optimal energy input mode should pay more attention to the ‘supply on demand of energy' in the early stage,and the uniformity of electric field distribution should be improved in the middle and later stages so as to determine an appropriate opening mode of magnetrons considering both energy efficiency and final quality of berry pulp.(5)The interactions between microwave intensity,microwave power and air ventilation on moisture content,temperature,temperature uniformity and energy efficiency are studied by experimental design of center combination.Results show that proper ventilation can enhance the moisture diffusion inside berry pulp and promote the temperature tending to be the same distribution in the continuous microwave drying,which is beneficial to improve the energy efficiency and temperature uniformity of drying.The strong synergistic effects between high microwave power and high air ventilation can improve the energy efficiency by accelerating the process of the moisture diffusion from interior to surface of berry pulp layer and moisture evaporation on the surface.However,the conditions of low microwave power and certain air ventilation are conductive to control the drying temperature and temperature uniformity,and further to ensure the drying quality of berry pulp.The optimal parameters obtained from the prediction model are microwave intensity of 4 W/g,microwave power of 17 k W,air velocity of 1.3 m/s.The verification experiment is carried out under the same conditions,and the response indexes of the prediction model are in good agreement with experimental ones.The results may provide technological reference for the continuous industrial production of berries with high energy efficiency and quality. |
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