Influences Of Different Filling Shapes On Radio Frequency Heating Behaviors Of Millet

Millet,as an important forage and food crop,is widely cultivated in semi-arid regions of north China,including Inner Mongolia,Gansu and Shaanxi.However,millet is easy to be infected by pests in the storage process,resulting in the large quality and economy losses of millets.Radio frequency(RF)energy,as a new insecticidal method,has been widely considered by many researchers.The method can effectively avoid environmental pollution and chemical residue caused by chemical fumigation.But the RF heating non-uniformity is one of the important problems that restricts the further promotions and industrial applications.Therefore,it is important to explore a simple,efficient and suitable method to improve the RF heating uniformity in industrial processing.This study took millet as the research object and used a 6 k W,27.12 MHz free-oscillation parallel plate RF heating system to study the influence of different concave and convex degrees at the top and different step shapes at the bottom on the RF heating uniformity.According to the obtained rules of the shape change,the effects of different sizes and locating position of package samples,hot air holding time,and gap between package samples on RF heating performances of millets were further determined,providing a reliable theoretical basis in developing the RF process protocol for disinfesting millets.The major contents and results of the research are as follows:(1)The dielectric properties of millet measured by an impedance analyzer generally increased with increasing temperature and decreased with increasing frequency.Based on the thermal death temperature(54℃)of the most heating resistant pest(Rhyzopertha dominica)in millet storage process,the best RF heating parameters were determined as the electrode gap of 100 mm,the heating time of 6.5 min,and the corresponding heating rate of 4.46℃/min.These parameters ensured the faster heating rate and better heating uniformity.(2)The influence of different concave and convex shapes on the uniformity of millet rf heating was studied by using the shapes of different convex height and concave depth.The results showed that the heating rates at the center of millet samples with the same weight increased when the heights of the convex-concave degrees of sample shapes increased.But the trend of the heating rates at the corner of millet samples was opposite to that at the center.The difference of heating rates between center and corner decreased.The difference of millet heating rate at center of millet samples with the same center height between convex and concave shapes was small,but the corner heating rates increased gradually with the sample shape from the convex-concave degrees of 20 mm to-20 mm.The convex shape can significantly improve the RF heating uniformity of samples.However,the RF heating uniformity indexes were not significantly different(P>0.05)between the heights of 10 mm and 20 mm.The overheating at the corners and edges was worse with the increase of the concave degree.(3)Polypropylene(PP)blocks with different lengths and heights were added at the bottom of the rectangular samples to obtain different step shapes,and the influence of different shapes at the bottom on the RF heating uniformity of millets was explored.The results showed that different step shapes could affect the RF heating uniformity of different layers in millets,and there was an obvious improvement of the RF heating uniformity of millets with the increasing width of the PP blocks in the same height.The RF heating uniformity index values of the samples above the PP blocks decreased with increasing height,but those under the PP blocks increased with increasing height.The step shape can significantly improve the RF heating uniformity of entire samples,and the optimal shape was found to be the 30 mm(W)×20 mm(H)of the PP blocks,which had the minimum heating uniformity index values.Compared with the samples without PP blocks,the RF heating uniformity index value of the optimal shape decreased by 38.8%,which could achieve an effective improvement of the RF heating uniformity.(4)The samples were selected to be packed in packages and piled into different step shapes to study the influence of different shapes of package samples on the RF heating uniformity.The results showed that when the size of large package was 240×180×25 mm~3and the size of small package was 220×160×20 mm~3,the RF heating uniformity of package samples was the best.The locating positions of different package samples had an effect on the heating uniformity of different surfaces.Considering the simplicity of placements,small packages on the top layer and large package on the bottom layer were more appropriate.When the hot air was added to the RF heating system,increasing the holding time of hot air had a significant effect on the heating uniformity of package samples.With the increase of the gap between two package samples in a horizontal position,the RF heating uniformity indexes decreased first and then increased.The optimal gap was found as 20 mm,resulting in the minimum heating uniformity index values.To sum up,changing the shape of samples is a simple and effective method to improve the uniformity of RF heating.The method mainly reduces the energy of electromagnetic field concentrated at the corners by reducing the thickness of the corners of samples,so as to improve the overheating phenomenon of corners.Moreover,this method is also applicable to packaging materials and easy to use in industrialization.