Structure optimization and experiment of radio frequency dryer based on heating uniformity

Abstract: Drying of agricultural products immediately after harvest is critical to preserve their quality and prevent mildew deterioration. Fresh agricultural products must be dried to the required moisture content level for long-time storage in an ambient environment. Considering its great penetration depth and volumetric heating advantages, radio frequency (RF) technology can be used in agricultural products drying for its high efficiency, quick speed and high quality. However, the heating non-uniformity caused by the edge or corner effect and the thermal runaway heating restrict its industrial applications. Therefore, the structure of RF system need be optimized. In order to solve the problems of non-uniform heating during RF drying caused by internal heat and moisture accumulation due to large material thickness and caused by the edge or corner effect due to bending electric field lines, a convective hot air heating device and a charge device were designed. The convective hot air heating device was composed of heat source supplying system, hot air velocity and temperature control system and air distribution room. The charging device was composed of many trays. The automatic weighing system, which was composed of weighing sensor, weighing trays and supporting pillars, was also designed in order to realize real-time weighing during drying process. The air distribution performance, inlet diameter, size and shape of shunt cylinder were simulated using computational fluid dynamics software named Flow Simulation. When changing shape of the air distribution room to trumpet with tilt angle of 19.3° as well as length of 500 mm and adding 3 shunt cylinders with diameter of 20 mm, the air distribution uniformity could be improved, with the velocity deviation ratio decreasing from 31% to 10.5% and the velocity non-uniformity index decreasing from 19% to 7.6%. The result showed that the velocity magnitude of simulation seemed to be basically equal to the velocity magnitude of measurement. Taking corn seeds as an example, the optimized performance verification tests were carried out. Installation location of automatic weighing device would not affect the RF heating; hot air would not affect the weighing quality, and weighing relative deviation was within the deviation range. The deviation range is 1~5 g with a maximum range of 24 kg. The maximum capacity of flake or granular material treated is 18 kg. In the maximum range, if the weighing quality was larger, the weighing was more accurate. The drying conditions were electrode gap of 180 mm, hot air temperature of 40 ℃, and hot air velocity of 1.5 m/s. After RF heating for 30 min, the corn seeds were immediately removed out of the cavity. The surface temperatures of 3 layers of corn seeds were sequentially measured using an infrared camera. The result showed that thermal runaway heating phenomenon and edge or corner effect phenomenon were not observed for corn seeds during RF drying process with auxiliary convective hot air heating device and layered loading of corn seeds under the suitable matching parameters of hot air and RF. The study avoids the influence of outside temperature and humidity in the drying process and reduces the drying time by 10%. The results can provide a reference for improving the uniformity of RF heating.