Microwave heating of electrically conductive materials (original) (raw)
International Journal of Numerical Methods for Heat & Fluid Flow, 2020
Purpose -This paper aims to investigate the thermal performance involving larger heating rate, targeted heating, heating with least non-uniformity of the spatial distribution of temperature and larger penetration of heating within samples vs shapes of samples (circle, square and triangular). Design/methodology/approach -Galerkin finite element method (GFEM) with adaptive meshing in a composite domain (free space and sample) is used in an in-house computer code. The finite element meshing is done in a composite domain involving triangle embedded within a semicircular hypothetical domain. The comparison of heating pattern is done for various shapes of samples involving identical cross-sectional area. Test cases reveal that triangular samples can induce larger penetration of heat and multiple heating fronts. A representative material (beef) with high dielectric loss corresponding to larger microwave power or heat absorption in contrast to low lossy samples is considered for the current study. The average power absorption within lossy samples has been computed using the spatial distribution and finite element basis sets. Four regimes have been selected based on various local maxima of the average power for detailed investigation. These regimes are selected based on thin, thick and intermediate limits of the sample size corresponding to the constant area of cross section, Ac involving circle or square or triangle. Findings -The thin sample limit (Regime 1) corresponds to samples with spatially invariant power absorption, whereas power absorption attenuates from exposed to unexposed faces for thick samples (Regime 4). In Regimes 2 and 3, the average power absorption non-monotonically varies with sample size or area of cross section (A c ) and a few maxima of average power occur for fixed values of A c involving various shapes. The spatial characteristics of power and temperature have been critically analyzed for all cross sections at each regime for lossy samples. Triangular samples are found to exhibit occurrence of multiple heating fronts for large samples (Regimes 3 and 4). Practical implications -Length scales of samples of various shapes (circle, square and triangle) can be represented via Regimes 1-4. Regime 1 exhibits the identical heating rate for lateral and radial irradiations for any shapes of lossy samples. Regime 2 depicts that a larger heating rate with larger temperature nonuniformity can occur for square and triangular-Type 1 lossy sample during lateral irradiation. Regime 3 depicts that the penetration of heat at the core is larger for triangular samples compared to circle or square samples for lateral or radial irradiation. Regime 4 depicts that the penetration of heat is still larger for triangular samples compared to circular or square samples. Regimes 3 and 4 depict the occurrence of multiple heating fronts in triangular samples. In general, current analysis recommends the triangular samples which is also associated with larger values of temperature variation within samples. Originality/value -GFEM with generalized mesh generation for all geometries has been implemented. The dielectric samples of any shape are surrounded by the circular shaped air medium. The unified mesh generation The author would like to thank anonymous reviewer for critical comments, which improved the quality of the manuscript.
Numerical Analysis of Heat Transfer Characteristics in Microwave Heating of Magnetic Dielectrics
Metallurgical and Materials Transactions A, 2011
A numerical simulation of heat transfer during the microwave heating process of magnetite, which is a two-dimensional (2-D) magnetic dielectric, subjected to heat conduction, convection, and radiation was performed. The heat transfer process was modeled using an explicit finitedifference approach, and the temperature profiles for different heating parameters were generated through developing a code in Mathematica 7.0 (Wolfram Research, Inc., Champaign, IL). The temperature in the sample increases rapidly in 1 minute and nonuniform temperature distribution inside the object is observed. An obvious temperature hot spot is formed in the corner of the predicted temperature profile initially, which shifts to the center of the object as heating power increases. Microwave heating at 915 MHz exhibits better heating uniformity than 2450 MHz mainly because of the larger microwave penetration depth. It is also observed that the heating homogeneity in the object can be improved by reducing the dimension of object. The effects of heating time, microwave power, microwave frequency, and object dimension need to be considered to obtain high heating performance and avoid/minimize thermal runaway resulting from temperature nonuniformity in large-scale microwave heating.
IEEE Transactions on Plasma Science, 1999
Microwave heating processes involve electromagnetic and thermal effects coupled together through the local temperature dependence of the material dielectric properties. This paper presents a one-dimensional model for the coupled electromagnetic-thermal process and demonstrates its solutions for typical problems. The local temperature dependence of the lossy dielectric medium is taken into account in two different time scales. One is the heat-generation time scale due the microwave radiation, and the other is the temperature diffusion time scale. The two timescale approach minimizes the computation time and provides an efficient simulation tool for the analysis of various phenomena. The two-scale model presented in this paper is benchmarked by a comparison of its numerical results with other models published in the literature. Several examples of microwave heating processes in various materials are simulated. Effects of heatwave propagation in matter are predicted by the model. The results show the temporal and spatial evolution of the temperature and power-dissipation profiles. Variations in the (microwave) impedance profile in the medium due to the heating are computed. A further development of this model, including more complicated geometries and various loss mechanisms, may yield useful numerical tools for the synthesis and design of microwave heaters in which the heated material acts as a nonlinear load in the microwave circuit.
Numerical Study of Microwave Heating of Micrometer Size Metal Particles
ISIJ International, 2008
Absorption of microwave energy in conductive nonmagnetic spherical particles is analyzed by means of finite element method. The frequency of the microwave is 2.45 GHz. To find out roles of the electric and magnetic fields in the heating process, conditions of the electric and magnetic anti-nodes in a standing wave are simulated. Results clearly show that single metallic particles are mostly heated by the magnetic component of the electromagnetic field. Density of the absorbed energy has maximum at some fixed particle radius, which equals to 3.3 mm for the case of copper particles. Penetration length into multi particle system is estimated.
A theoretical analysis on the effect of containers on the microwave heating of materials
International Communications in Heat and Mass Transfer, 2017
Microwave heating is generally performed by positioning the sample within a container. The container can reflect, absorb or transmit microwaves based on the dielectric properties and that can influence the microwave heating characteristics of the sample. This work is an attempt to theoretically analyze the alteration of the microwave heating characteristics of materials due to the use of either a low-lossy alumina container or a high lossy SiC container. The heating characteristics have been simulated for the high-lossy beef and low-lossy bread samples of a fixed dimension by solving the coupled energy balance equation and detailed Helmholtz wave propagation equations within the sample-container assembly. It has been shown that the microwave heating characteristics can be significantly altered in the presence of the container based on the relative dielectric properties of the materials. The alumina container has been found to be efficient to enhance the microwave heating efficiency of the high lossy material such as beef, while the rapid microwave heating of the SiC container has been found to be beneficial to enhance the heating of the low lossy material such as bread in some cases.
International Journal of Heat and Mass Transfer, 2020
The coupled electromagnetic wave equations and energy balance equations are solved via Galerkin finite element method with Crank-Nicholson time integration scheme to numerically simulate the microwave heating of model lossy samples enclosed in various low (alumina) and high (SiC) lossy containers. The primary objective of this work is to identify the alteration of heating or power absorption characteristics of the lossy sample in the presence of containers and analyze the role of container thickness. Microwave propagation is modeled via Helmholtz equations, which are the time-harmonic representation of Maxwell equations. The scattering at the air-material interface is efficiently modeled via an integro-differential radiation boundary condition. Power absorption and temperature distribution are evaluated for a range of container thicknesses and sample dimensions representing thin sample with uniform power absorption, intermediate sample with resonating power absorption and thick sample with exponentially attenuated power absorption. For each case, simulations are also carried out in the absence of the containers. It is found that the power absorption and temperature distribution are strongly influenced by both the low (alumina) and high (SiC) lossy containers. Overall, power absorption and/or heating quality can be greatly enhanced in the presence of alumina containers. SiC containers suppress power absorption within the sample. However, SiC containers lead to more controlled and uniform heating. Thickness of the container is found to be the critical factor for enhanced power absorption or heating quality of the sample.
Microwave heating: an evaluation of power formulations
Chemical Engineering Science, 1991
heating of food systems have conventionally been modeled based on a Lambert law formulation of the absorbed power. However this is strictly valid for semi-infinite samples only. The correct power dissipation must be computed from Maxwell's equations. To determine the conditions of the approximate applicability of Lambert's law for finite slabs, we have compared it with the microwave heating predicted by Maxwell's equations. We have found that the critical slab thickness L,,,, (in cm) above which the Lambert law limit is valid can be estimated from L,,. = 2.78-l -0.08, where the penetration depth, p-', is the distance in cm from the sample surface where the field reduces to l/e of its incident intensity. Temperature profiles calculated with the Lambert law limit for slabs thicker than &, are within 0.5% of those predicted with the power calculated from Maxwell's equations. Using Maxwell's equations we have developed a general formulation for power absorbed in a homogeneous, isotropic multilayered medium exposed to plane waves from both faces. We report temperature profiles obtained by solving the transient heat conduction equation with the microwave power as a source term. Thermal and dielectric properties are assumed to be temperature-independent.