# Using Modeling to Select the Type of Microwave Field Emitter for Dense-Layer Grain Dryers
## 1 Introduction  
Normally, wheat grains are subjected to pre-seeding treatment and disinfection with a moisture content of 14%. In order to ensure grain viability and technological quality, grain-drying equipment has to be applied [1]. The initial water content in grains subjected to drying may vary over a wide range, from 18% to 30%. If wet grains are not dried in a timely manner, they will promote the growth of mold and self-heating of the grain. As a result, they become unsuitable for both food and feed purposes. In terms of technological convenience and price, processing plants with thick grain layers are preferable [2], such as hopper-type units [3, 4]. Such processing plants are applicable for grain moisture content not exceeding 16%. Processing grains with higher moisture content requires the application of flow-through grain dryers. Depending on the grain crop under processing and the required production rate, various types of grain dryers can be used, such as vertical silage-type dryers [5], ray-type grain dryers [6], free drop flow grain dryers [7], and table-type grain dryers [8]. It should be noted that grain drying is a rather energy-demanding process that accounts for 80% of aggregate energy consumption. That is why various methods designed to reduce the energy consumed for drying, including the application of infrared radiation [9], ozone-air mixtures as drying agents [10], and processing plants with ultrasonic grain drying effect [11] are applied. Grain dryers with alternating blow-through directions in the grain layer [12] and those with grain recycling are also widely used [13]. One of the environmentally safe grain-drying methods is treatment in convective-microwave processingplants [14,15]. The application of microwaves for processing grains with the purpose of drying, disinfection, and pre-seeding treatment makes it possible to reduce energy consumption and enhance the productivity of the dedicated equipment [16, 17, 18]. In the course of pre-seeding treatment, there is a possibility to combine the effect of disinfection with that of improving both the viability of seeds and the yield formula [19,20]. The basic element of such processing plants is the so-called active convective-microwave treatment zone, where waveguides and air ducts are installed. Waveguides serve as microwave radiation conductors, whereas air ducts supply drying agents into the convective-microwave zone [21,22]. One of the principal disadvantages of microwave radiation application for grain processing is defined by the dependence of the penetration depth into the grain layer on the dielectric properties of the materials. The higher the moisture content of the grain, the smaller the distance of its penetration into the grain layer, and the greater the share of microwave power absorbed in the surface sublayer adjacent to the radiation source. At the same time, the maximum productivity of the convective-microwave processing plant can be attained under conditions of uniform interaction between the microwave field and grain. Previously, the authors designed a shaft-type convective-microwave dryer on the basis of a processing unit in which microwave field sources were installed on the opposite sides of the grain processing zone [23,24] (Figure 1). The unit in the processing plant had the following dimensions 0.3 \(\times\) 0.2 \(\times\) 0.2 m, and the volume of the inner space was 0.012 m\({}^{3}\). The length of the active zone, i.e., the distance between the waveguide's output gates, was 0.3 m. The wheat grain weight that could be processed in the convective-microwave zone was 8 kg and 9.1 kg with moisture content of 14% and 25%, respectively. Small-power magnetrons of 0.9 kW with a radiation frequency of 2.45 GHz are normally used in grain processing units. Radiation is supplied to the microwave-active zone with the help of horn-type antennas.