Homogeneous Dielectric Heating in Large Microwave Ovens by Excitation of Multiple Eigenmodes at Their Resonance Frequencies

Abstract

The proper use of microwave heating can significantly increase the production cycle time and energy efficiency in industrial heating processes compared to conventional heating methods. The main challenge of this technique is to improve the temperature uniformity in the product exposed to standing waves inside the microwave oven. In opposite to the magnetron, solid-state amplifiers (SSA) offer the possibility to increase the homogeneity by changing the amplitude, frequency and phase with the help of intelligent control methods [1]. In this work, the variation of the frequency and the amplitude of the SSA is considered. The multimode microwave oven used in the experiment has an industrial size of 535 mm x 510 mm x 395 mm (Figure 1). The SSA was operated in the frequency range from 2.4 GHz to 2.5 GHz. It consisted of a new 300 W solid state microwave source from HBH microwave GmbH, Germany. An antenna system was developed based on numerical simulation with CST Microwave Studio. The positions of four loop antennas were optimized to excite at least 90 % of the possible 32 eigenmodes [2] of the unloaded cavity. At the roof of the cavity, an IR camera was installed to observe the temperature distribution of the load during heating. A sheet of paper was used as the thermal load. It was placed on a PTFE plate as a sample holder. Figure 1 exemplary illustrates the comparison of the simulated power distribution with the measured temperature distribution for two representative eigenmodes. As can be expected from the figures, an optimized combination of different modes will lead to a significantly improved temperature uniformity in the material. Latest results obtained with different type of loads will be presented.

Microwave-Assisted Freeze-Drying of Foodstuffs Using a Novel Frequency-Based Control Concept

“…Both frequency sweeps exhibit a higher homogeneity factor than the single frequencies for most drying states. In the literature, in agreement with the present results, a more homogeneous power input into the product is achieved by applying multiple microwave frequencies in both simulations [13] and experiments [12,15] . The reason for the increase in heating homogeneity is the superposition of several distributions of the electromagnetic field, the so-called modes, in the cavity.…”

“…One approach to overcome the challenges of microwave-assisted processing is the use of semiconductor-based solid state generators (SSGs) instead of conventionally employed magnetrons [11] . SSGs enable a targeted modulation of power, frequency, and phase of the generated microwaves [12] . Targeted frequency modulation using SSGs has already been demonstrated to provide more uniform heating within a product and more efficient power conversion in electromagnetic simulations of microwave-assisted heating [13] .…”

“…Additionally, varying the phase shift and the fraction of power input utilizing multiple SSGs allowed targeting a specific energy efficiency or homogeneity of power input in the material in electromagnetic simulations [14] . Experimentally, a more uniform power input into the product was also demonstrated for microwave-assisted heating via frequency modulation [12,15,16] . But in the sparse literature concerning the use of SSGs in MFD [8] , the effect of modulating the frequency during the drying process has not been investigated.…”

A Uniform Heating Technique for Cavity in Volatile Organic Compound (VOC) Removal System Using Slotted Waveguide Array

“…In general, much care is needed for the design of the feeding structure of a cavity since many modes exist inside the cavity as determined by the cavity structure, and the excitation of the mode is dependent on the feeding structure. Many research studies related to this have been actively conducted [6][7][8][9][10][11][12]. The optimization process for improving the electric field's uniformity by analyzing the modes existing in the cavity and expressing the electric field inside the cavity in a linear combination of the modes has been presented [6].…”

“…Many research studies related to this have been actively conducted [6][7][8][9][10][11][12]. The optimization process for improving the electric field's uniformity by analyzing the modes existing in the cavity and expressing the electric field inside the cavity in a linear combination of the modes has been presented [6]. However, modal analysis in a cavity and finding the feeding structure are very difficult because the cavity structures have different characteristics.…”