| Radio frequency(RF)heating is a new type of heating technology applied in the food processing industry in recent years because of its rapid heating rate and the overall heating.The applications of it include heating,drying,sterilization,insecticidal,thawing,baking,blunt enzyme and so on.But due to the edge heating effect,the edges and corners of the material will be overheated in the RF heating process.Though some solutions to improving the heating uniformity have worked,the heating rate has dropped as a result,which lead to incomplete sterilization and the nutrition loss.There are also some solutions that do not affect the heating rate,but they often need adjustment of the RF equipment,which is more expensive and troublesome.In addition,some materials are heated relatively slowly in the RF field,so it will be meaningful to improve the heating rate by changing the structure of the container.Inspired by the RF heating rules of porous materials,this research applied dispersion structure to liquid materials(sodium chloride solution)and solid materials(rice)to raise the RF heating rate and improve the heating uniformity.COMSOL Multiphysics software is used to build the RF heating simulation model of rice to find the effect of the dispersed structure on the distribution of electric field and the temperature.Using the validated model to explore the influence of the packaging container structure parameters on heating rate and heating uniformity,the main research results are as follows:(1)The influence of solution concentration and diameter of particle dispersion structure on heating rate and heating uniformity of sodium chloride solution was investigated,and it was found that the dielectric constant and dielectric loss coefficient of sodium chloride solution were linearly related to temperature.The dielectric constant decreased with the increase of temperature,firstly increased and then decreased with the increase of solution concentration.The dielectric loss factor increased with the temperature and solution concentration.With the increase of the concentration of sodium chloride solution,the heating rate firstly increased and then decreased.The maximum heating rate in different solutions corresponded to different particle diameters,but the dispersion structure of different particle diameters could significantly increase the heating rate,and the heating rate first increased and then decreased with the increase of particle diameters.In addition,the heating uniformity of the horizontal surface is better than that of the vertical surface.(2)The influence of hole dispersed structure on RF heating rate and heating uniformity of rice was explored,and it was found that the hole dispersed structure could improve the RF heating rate.The concrete embodiment was: the result of the vertical hole was superior to the transverse hole;Hollow was better than solid;For the same hole diameter,the RF heating rate increased with the number of holes.With the same number of holes,the RF heating rate generally increased with the hole diameter.When the number of holes is eight,in the three distribution modes of concentrated cold zone distribution,staggered distribution and equidistant distribution,equidistant distribution had faster heating rate and better heating uniformity.After the same treatment time,the death rate of insects in the sample containing porous dispersion structure was significantly higher than that in the control group.(3)The verified rice RF heating model was used to explore the influence of container fillet radius,wall thickness,material and length-to-width ratio on the material RF heating rate and heating uniformity.It was found that for polypropylene(PP),increasing the fillet radius appropriately could improve the heating uniformity and rarely affected the heating rate.The effects of increasing wall thickness on heating uniformity and heating rate of different materials are different.In polypropylene(PP),increasing the wall thickness will make the heating uniformity index decrease first and then increase,while for polycarbonate(PC)and polyester(PET),the heating uniformity index decreased with the increasing wall thickness.The wall thickness of polypropylene(PP)had a relatively limited effect on temperature,that was,it would not cause the material temperature to rise and fall significantly.However,in polycarbonate(PC)and polyester(PET),the average volume temperature of materials increased with wall thickness,and the increase was more obvious in polyester(PET).Finally,the aspect ratio had a significant effect on the RF heating of materials,and increasing the aspect ratio of the container properly could increase the average volume temperature of materials and improve the heating uniformity. |
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