Experimental study of microwave slow wave comb and ceramic applicators for soil treatment at frequency 2.45 GHZ

[EN] In many cases in industry it is required to heat or treat surface layers of different material (soil, timber, concrete, plastics and so on) with microwaves (MW). Traditional MW irradiators (antennas) cannot provide heating only to the surface areas and energy penetrates deep into the material, where it decays exponentially due to normal attenuation. To reduce energy losses it was required to develop special MW applicators for surface treatment to increase process efficiency. To address this problem, a slow wave ("surface wave") comb and ceramic applicators were designed, built and studied. The main property of slow waves is that the energy concentration is very near impedance electrode – comb or ceramic plate surface. Comb and ceramic slab applicators for frequency 2.45 GHz operation were designed for the soil treatment and studied using soil with moisture content range 32-174% and density range 590-1070 kg/m3 . 30 kW MW plant was used for experiments. Results of the experiments showed that a ceramic applicator provides better uniformity of energy distribution across the width of the applicator. It reduces overheating of the soil surface and energy losses. The depth of energy penetration provided by ceramic applicator is lower compared with the comb applicator. It means that the ceramic applicator provides better energy localization and more energy absorption in the soil surface layers compared with the comb applicator. The ceramic applicator is more effective for MW treatment of the soil surface areas and is recommended for practical use in machines for thermal treatment and sterilization of surface layers of the soil and other materials.Brodie, G.; Torgovnikov, G. (2019). Experimental study of microwave slow wave comb and ceramic applicators for soil treatment at frequency 2.45 GHZ. En AMPERE 2019. 17th International Conference on Microwave and High Frequency Heating. Editorial Universitat Politècnica de València. 127-136. https://doi.org/10.4995/AMPERE2019.2019.9651OCS12713

Microwave Soil Heating with Evanescent Fields from Slow-Wave Comb and Ceramic Applicators

Microwave soil heating deactivates weed seeds; however, in many modern agricultural settings, weed seeds are mostly found in the top 1–2 cm of the soil profile. Until recently, microwave soil heating has been achieved using various antennas, which project the microwave energy deeply into the soil. The aim of this research was to develop new microwave applicators that provide shallow heating (less than 50 mm). This paper presents two applicator designs, one based on a comb slow-wave structure and the other on the frustrated total internal reflection (FTIR) principle, which utilise evanescent microwave fields to restrict the depth of microwave heating. The background theory to their performance is presented, followed by experimental evidence of their constrained heating performance under different soil moisture scenarios. Experimental measurements of the heating performance of these applicators, in soils of varying moisture content, demonstrate that the evanescent microwave fields restrict the depth of heating, so that most of the energy is manifested in the top 50 mm of soil. The evanescent field decay rate for the FTIR applicator changes from 44.0 ± 0.7 m−1 to 30 ± 1.2 m−1 as the soil moisture changes from 32% to 174% (dry weight basis). This is higher than the evanescent field decay rate for the comb slow-wave applicator (17.6 ± 0.7 m−1 to 19.9 ± 1.5 m−1). The FTIR applicator has a wider and shallower heating pattern than the comb slow-wave applicator. Because of the double heating lobes of the FTIR applicator, the effective half temperature heating width is approximately 150 mm. This is wider than the half temperature heating width of the comb slow-wave applicator (95 mm)