Heating Performance And Uniformity Improvement Of Radio Frequency Systems

Radio frequency(RF)system is one of dielectric heating methods and widely used in food and agricultural products processing due to rapid and volumetric heating,greater penetration depth,higher energy efficiency and non-chemical pollution.However,the uncertainty of the heating performance and non-uniform heating are the main obstacles to the commercial application of the RF system.Therefore,the theoretical research and industrial application has to be perfect by clarifying the heating performance and improving the heating uniformity of RF system.In this study,the heating performance of the two common radio frequency(the free running oscillation and 50 Ω)systems were compared.Then,a screw conveyors which can be used in radio frequency is processed to improve the heating uniformity by combination of motion and agitation.At the same time,the theoretical basis for the industrial application of the mixing method was completed,and the development of RF technology was promoted.The major contents and conclusions are as follows:(1)In the performance comparison test between RF systems(the free running oscillator and 50 Ω systems),the temperature-time history and heating rate of the milled rice were analyzed under different conditions(electrode gaps,sample weights and moisture content).The results showed that heating time increased with the increasing electrode gap or the heating rate decreased with increasing electrode gap when RF heated sample to the same target temperature.The heating rate of the free-running RF system was higher than that of the 50 Ω RF system under the same electrode gap,sample weight and moisture content when RF heated 3.9 kg milled rice.(2)The energy efficiency of RF system gradually decreased with the increasing electrode gaps or decreasing the RF power when RF heated 3.9 kg milled rice with the moisture content of 12.3% wet basis(w.b.).The free running oscillation RF heating systems had high energy efficiency at the same electrode gaps,but the heating uniformity and the RF system stability of the 50 Ω RF heating system were better than those of the free running oscillation RF system.Thus,Determining the advantages and disadvantages of different RF systems is important for the development of RF Technology in the future.(3)A plastic screw conveyor used in the free running RF system to improve the RF heating uniformity was manufactured.Three kinds of granular products with different diameters(soybean,corn and peanut)were used as target samples in the experiment to study the heating rates and uniformity when RF heated samples in the screw conveyer.(4)The studies on the temperature distribution and heating uniformity of samples after RF heating showed that the variations between the highest and lowest sample temperatures in the screw conveyor were clearly reduced regardless of soybean,corn and peanut compared with the stationary conditions.The standard deviation of average temperature and heating uniformity index value for the three samples were also clearly reduced using the screw conveyor due to its stable,continuous,uninterrupted sample movement and mixing.This experimental study on the screw conveyor used in RF heating could achieve the replacement of artificial mixing and provide a new and effective method to improve RF heating uniformity,and help to promote further practical applications of RF heating in food and agricultural products processing industry.