Effects Of Conveyor Movement And Mixing On Radio Frequency (RF) Heating Uniformity In Wheat

Radio frequency (RF) heating involves utilizing electromagnetic energy at a frequency range of 3 kHz to 300 MHz. Because of its rapid and volumetric heating, RF treatment has been used as alternatives to chemical fumigation for disinfestations and pasteurization in postharvest agricultural products and foods. The major obstacle for implementing RF technology in industrial applications is the non-uniform heating. The hot and cold spots may cause quality loss and microbial or insect pest survivals in RF treated products. It is very important to improve RF heating uniformity to make RF treatment be widely used in industrial scale. To understand the complex mechanisms on the heating uniformity of food products subjected to RF treatments, mathematical modeling and computer simulation are commonly used as valuable tools with time-saving and effective cost. The simulation model is based on high frequency electromagnetic field and heat transfer, which can predict the temperature distributions of RF treated products effectively and systematically and obtain deep insights of complex RF heating mechanism and detailed transient temperatures of treated products. The validated computer model can be used to study the effects of material properties on RF heating patterns and uniformity. Computer models of wheat samples treated in a 6 kW 27.12 MHz RF heating system under moving and mixing conditions were developed using commercial software COMSOL Multiphysics based on finite element method (FEM). After validations of simulation models, they were used to study effects of related parameters on RF heating uniformity and to provide optimal parameters to industrial RF treatment. The main research contents and conclusions are as follows:(1) A computer simulation model using finite element-based commercial software, COMSOL, was developed to simulate temperature distributions in wheat samples packed in a rectangular plastic container and treated in the RF system under stationary and moving conditions. Temperature distributions of three horizontal layers (top, middle and bottom) and central point of second middle layer in wheat sample were collected by thermal imaging camera and fiber optic sensors. Good agreements were obtained when comparing the simulated and experimental temperatures in three horizontal layers of wheat sample under stationary and moving conditions, and both showed higher temperatures in the middle and bottom layers compared with those of the top layer. Corners and edges were heated more than central parts in all three layers.(2) After validation of this developed model, the validated model was further used to predict the influences of conveyor movement speeds (v1=8.57 m h-1, v2=14.23 m h-1, 和v3 =17.14 m h-1), three different top electrode lengths (a1=0.5 m, a2=0.83 m, and a3=1m) and turning the container by 90° under moving conditions on RF heating uniformity. The uniformity index (λ) was used to evaluate the effects of conveyor movement, three different lengths of the top electrode, and turning the container from Position A to Position B by 90° under moving conditions on RF heating patterns using simulation. Simulated results showed that moving and turning the container by 90° could improve the RF heating uniformity. This simplified and effective simulation approach could be effectively used to understand and analyze the effects of conveyor movement on RF heating uniformity.(3) A computer simulation model was developed using finite element-based commercial software, COMSOL, to simulate temperature distributions in wheat samples packed in a rectangular plastic container and treated in the RF system with and without mixing conditions. Temperature profiles of internal six points, top surface and three points of second middle layer in wheat sample were collected by thermocouples, thermal imaging camera and fiber optic sensors so that the developed computer model was validated.(4) The uniformity index (UI) was used to evaluate effects of mixing on RF heating uniformity. Both experimental and simulated UI showed decreasing trend with the increasing mixing times. With increasing mixing times from 0 to 3 times, the temperatures of central points (2 and 5) at mid and bottom layers increased gradually, whereas the temperatures of corner and edge points (1,3,4 and 6) of these two layers decreased both in experiments and simulations. Because of the heat loss during mixing process, both the average temperatures of surface and interior wheat samples were also reduced in experiment and simulation. In practical applications using several RF systems in series with mixings in between, forced hot air should be used to minimize the amount of heat loss during the mixing process. The developed model can help to understand the RF heating patterns and effects of mixing conditions on RF heating uniformity and provide valuable strategy for developing effective industrial-scale RF treatments with mixing processes.