Study On Dielectric Heating Characteristics Of LDPE/TiO₂ Composites And Its Application

Dielectric heating technology,which is a physical heating method,has the advantages of rapid heating,large penetration depth,selective heating and safety.Therefore,it has broad applications in the process of insecticide,sterilization,and drying of agricultural products.However,non-uniform heating is the main obstacle to the large-scale industrial application of dielectric heating technology.To solve this problem,LDPE/TiO₂ composites with different TiO₂ content were prepared in this study,and the dielectric heating characteristics of LDPE/TiO₂ composites were discussed.Meanwhile,it was applied to improve the dielectric heating uniformity of agricultural products.The main contents and results of this research are as follows:（1）LDPE/TiO₂ composites with dielectric constants between 2 and 8 were prepared.According to the prediction model of composite dielectric constant,LDPE/TiO₂ composites with different dielectric constants were prepared by adding different volume contents of titanium dioxide（TiO₂）into LDPE.Then,the main characteristic parameters of the composites were measured.The results showed that the dielectric constant of the composite varied from2 to 8 at 25℃and 27.12 MHz when the content of TiO₂ were 0%,10%,20%,30%and40%,respectively,which were 2.41±0.00,3.38±0.01,4.45±0.00,5.84±0.01 and 7.78±0.01.With increasing the TiO₂ content,the density and loss factor of the composite increased,while the resistance and resistivity decreased.（2）The dielectric heating properties of different LDPE/TiO₂ composites and the effects on the dielectric heating properties of agricultural products were discussed.LDPE/TiO₂composites with different TiO₂ contents were treated by dielectric heating.The results showed that LDPE/TiO₂ composites were hardly heated during the dielectric heating treatment and did not consume the power of the dielectric heating equipment.Then,the LDPE/TiO₂ composite materials with different TiO₂ content were made into containers to conduct dielectric heating treatment on different agricultural products.The results showed that there was no significant difference in the heating rate of soybeans,mung beans,rice,wheat flour and peanuts in different LDPE/TiO₂ containers.However,they obtained better dielectric heating uniformity when they were treated in LDPE/TiO₂ composite containers with 10%,10%,20%,20%and30%TiO₂ content,respectively.（3）The influence of various characteristic parameters of LDPE/TiO₂ composite in dielectric heating uniformity was analyzed.A dielectric heating model of soybeans was established by the finite element software COMSOL.The effects of the dielectric constant,thermal conductivity,density,heat capacity,loss factor and resistivity of LDPE/TiO₂composites on the dielectric heating uniformity index of soybeans were analyzed through the verified model.The results showed that the dielectric constant of LDPE/TiO₂ composite has the greatest influence on the dielectric heating uniformity of soybeans.Furthermore,the electric field distributions in soybeans would be more uniform and the dielectric heating uniformity of soybeans would be better when the soybeans were treated in the LDPE/TiO₂container when its dielectric constant is 0.75～1.22 times of that of soybeans.（4）The structure of LDPE/TiO₂ composite container was optimized to improve the electric field distribution and heating uniformity during the dielectric heating process.The effect of LDPE/TiO₂ composite container structure on the internal electric field distribution and dielectric heating uniformity of soybeans was studied through the verified dielectric heating model of soybeans.The results showed that the LDPE/TiO₂ square container with a thickness of 2 mm,a height consistent with the soybeans,a length and width of 747 mm and360 mm respectively,and a chamfer radius of 40 mm was an ideal container for dielectric heating of soybeans.