Vadim V. Yakovlev

Examination of Contemporary Electromagnetic Software Capable of Modeling Problems of Microwave Heating

Despite all the progress in numerical mathematics and computational technologies, computer simulation of processes and systems of microwave power engineering remains a new and unexplored arena for most practitioners. Engineers dealing with microwave non-communication applications currently seem to lack not specific technical data but general information on modern computational opportunities. At the same time, a number of modeling tools do allow one to get valuable data about the characteristics of the considered system prior to constructing a physical prototype. The goal of the present paper is to update the database of the modern electromagnetic (EM) software suitable for the modeling of microwave heating and outline a few conceptual and practical issues associated with the efficient use of these simulators.

Efficiency optimization for systems and components in microwave power engineering.

The paper is concerned with one aspect of optimization of microwave thermal processing, namely, with optimization of energy coupling interpreted as a numerical characteristic of system efficiency. Since in computer-aided design coupling can be evaluated through the computed reflections, an optimization scheme is presented that is particularly suitable for minimizing the reflection coefficient in typical systems and elements of microwave power engineering. Based on response surface methodology and the sequential quadratic programming for constrained optimization, the procedure is linked with the full-wave 3-D FDTD electromagnetic simulator QuickWave-3D. Credibility and effectiveness of the method is illustrated by four examples: dimensional optimization is performed for a dry waveguide load, a waveguide T-junction with a partial-height post, a water cylinder in a cavity, and a slotted waveguide-backed radiating element.

Waveguide Microwave Imaging: Neural Network Reconstruction of Functional 2-D Permittivity Profiles

A new microwave imaging technique is proposed for reconstruction of 2-D complex permittivity profiles in dielectric samples located in a waveguide system. The spatial distributions of the dielectric constant and the loss factor are approximated by continuous functions whose functional parameters are determined using a neural network technique backed by full-wave finite-difference time-domain analysis. The profiles are reconstructed from measurements of reflection and transmission characteristics obtained with the tested sample at different locations. Operational capabilities of the technique are illustrated through a series of computational experiments for rectangular and cylindrical samples at two (original and 90deg -rotated) positions. The results demonstrate excellent agreement between the reconstructed and actual profiles approximated by linear, quadratic, and Gaussian functions: the average relative errors do not exceed 0.4%, 2.2%, and 4.8%, respectively. Finally, the assumption of functional approximation, uniqueness of the reconstruction, and prospects of practical use of the technique are thoroughly discussed.

Applicability Study of Classical and Contemporary Models for Effective Complex Permittivity of Metal Powders

Abstract Microwave thermal processing of metal powders has recently been a topic of a substantial interest; however, experimental data on the physical properties of mixtures involving metal particles are often unavailable. In this paper, we perform a systematic analysis of classical and contemporary models of complex permittivity of mixtures and discuss the use of these models for determining effective permittivity of dielectric matrices with metal inclusions. Results from various mixture and core-shell mixture models are compared to experimental data for a titanium/stearic acid mixture and a boron nitride/graphite mixture (both obtained through the original measurements), and for a tungsten/Teflon mixture (from literature). We find that for certain experiments, the average error in determining the effective complex permittivity using Lichtenecker’s, Maxwell Garnett’s, Bruggeman’s, Buchelnikov’s, and Ignatenko’s models is about 10%. This suggests that, for multiphysics computer models describing the processing of metal powder in the full temperature range, input data on effective complex permittivity obtained from direct measurement has, up to now, no substitute.

Method of control and optimization of microwave heating in waveguide systems

A deterministic process of microwave heating in waveguide systems is considered. Use of the concept of optimal material design is suggested to secure a desired distribution of heat release within processed material via supplementary dielectric focusing structures. The method is illustrated by application to a rectangular waveguide with a thick E-plane dielectric layer and by finding parameters of the focusing structure maintaining uniform electric field within this layer.

Analysis of operational regimes of a high power water load

Reflections, electric field and dissipated power of a 915 MHz water load produced by The Ferrite Company, Inc. are computed by QuickWave-3Dfor water temperatures from 5 to 60C. A simple empirical model generalizing experimental data is used for calculating the dielectric constant of fresh tap water at 915 MHz and for different temperatures. The computer model is verified by measurement of reflection’s magnitude and phase. The loads power reflection and patterns of electric field and dissipated power are demonstrated to be nearly independent of water temperature. Microwave power is dissipated primarily within the conical part of internal water cavity, and possible shortening of its cylindrical part (by up to 70 mm) is suggested for water temperatures less than 60°C The computational strategy used for modeling is assessed to be beneficia1for analyzing and designing other systems for microwave heating.

Commercial EM Codes Suitable for Modeling of Microwave Heating — a Comparative Review

Electromagnetic modeling software available in the market is studied in accordance with specific criteria to compose contemporary database of codes applicable to designing industrial systems of microwave thermal processing. Operating systems, kernel computational methods, interfaces, optimization options, possibilities of solution of associated thermal problem and financial information are among the basic characteristics addressed. It is shown that sixteen selected codes are very differently adjusted to practical needs of the field. Vendors are rated with respect to their dedication to the microwave power engineering. Issues related to further evaluation of the codes (solution of benchmark problems) are discussed.

NEURAL NETWORKS FOR FDTD-BACKED PERMITTIVITY RECONSTRUCTION

Purpose – To outline different versions of a novel method for accurate and efficient determining the dielectric properties of arbitrarily shaped materials.Design/methodology/approach – Complex permittivity is found using an artificial neural network procedure designed to control a 3D FDTD computation of S‐parameters and to process their measurements. Network architectures are based on multilayer perceptron and radial basis function nets. The one‐port solution deals with the simulated and measured frequency responses of the reflection coefficient while the two‐port approach exploits the real and imaginary parts of the reflection and transmission coefficients at the frequency of interest.Findings – High accuracy of permittivity reconstruction is demonstrated by numerical and experimental testing for dielectric samples of different configuration.Research limitations/implications – Dielectric constant and the loss factor of the studied material should be within the ranges of corresponding parameters associate...

Practical Aspects of Complex Permittivity Reconstruction with Neural-Network-Controlled FDTD Modeling of a Two-Port Fixture

The paper discusses characteristics of a new modeling-based technique for determining dielectric properties of materials. Complex permittivity is found with an optimization algorithm designed to match complex S-parameters obtained from measurements and from 3D FDTD simulation. The method is developed on a two-port (waveguide-type) fixture and deals with complex reflection and transmission characteristics at the frequency of interest. A computational part is constructed as an inverse-RBF-network-based procedure that reconstructs dielectric constant and the loss factor of the sample from the FDTD modeling data sets and the measured reflection and transmission coefficients. As such, it is applicable to samples and cavities of arbitrary configurations provided that the geometry of the experimental setup is adequately represented by the FDTD model. The practical implementation of the method considered in this paper is a section of a WR975 waveguide containing a sample of a liquid in a cylindrical cutout of a rectangular Teflon cup. The method is run in two stages and employs two databases-first, built for a sparse grid on the complex permittivity plane, in order to locate a domain with an anticipated solution and, second, made as a denser grid covering the determined domain, for finding an exact location of the complex permittivity point. Numerical tests demonstrate that the computational part of the method is highly accurate even when the modeling data is represented by relatively small data sets. When working with reflection and transmission coefficients measured in an actual experimental fixture and reconstructing a low dielectric constant and the loss factor, the technique may be less accurate. It is shown that the employed neural network is capable of finding complex permittivity of the sample when experimental data on the reflection and transmission coefficients are numerically dispersive (noise-contaminated). A special modeling test is proposed for validating the results; it confirms that the values of complex permittivity for several liquids (including salt water, acetone and three types of alcohol) at 915 MHz are reconstructed with satisfactory accuracy.

Waveguide Microwave Imaging: Spherical Inclusion in a Dielectric Sample

A new 3-D microwave imaging technique is proposed for the detection of a position and size of a spherical object in a dielectric sample located in a waveguide system. The reconstruction is based on a radial basis function neural network inversion, backed by finite difference time domain analysis and requires only elementary measurements of complex reflection and transmission coefficients. Functionality of the technique is illustrated by computational experiments in reconstructing the parameters of a glass sphere and an air bubble in a rectangular Teflon block. It is shown that, at 915 MHz, the spheres of not less than 15 mm diameter are reconstructed with the average errors of 0.9%-2.2%.