Review of Numerical Simulation of Microwave Heating Process

Since the great experiments were carried out by Dr. Percy Spencer in 1946, microwave heating has been used widely in food industry, medicine, chemical engineering and so on [1][2][3][4][5], for processing many materials ranging from foodstuffs [1][6][7][8] to tobaccos [9][10][11], from wood [12][13][14] to ceramics [15][16][17][18], and from biological objects [19][20] to chemical reactions [21][22][23]. Typical applications include heating and thawing of foods, drying of wood and tobaccos, sintering of ceramics, killing of cancer cells and accelerating of chemical reactions. Microwaves are electromagnetic (EM) waves with frequencies between 300MHz and 300GHz. Under microwave radiation, dipole rotation occurs in dielectric materials containing polar molecules having an electrical dipole moment. Interactions between the dipole and the EM field result in energy in the form of EM radiation being converted to heat energy in the materials. This is the principle of microwave heating. For each material, permittivity, or dielectric constant, ε is the most essential property relative to the absorption of microwave energy. Permittivity is often treated as a complex number. The real part is a measure of the stored microwave energy within the material, and the imaginary part is a measure of the dissipation (or loss) of microwave energy within the material. The complex permittivity is usually a complicated function of microwave frequency and temperature. Compared with conventional heating, microwave heating has many advantages such as simultaneous heating of a material in its whole volume, higher temperature homogeneity, and shorter processing time. However, the nonlinear process of interaction between microwave and the heated materials has not been sufficiently and deeply understood. Some phenomena arising from this process, such as hotspots and thermal runaway, decreases the security and efficiency of microwave energy application and prevents its further development. Hotspot means that where temperature non-uniformity in heated material of local small areas having high temperature increase occurs. For example, some zones in material are overheated, and even burned, while some others have not reached the required minimum temperature. This phenomenon has become one of the major drawbacks for domestic or industrial applications. “Thermal runaway” refers to temperature varies dramatically with small changes of geometrical sizes of the heated material or microwave power. It may lead to a positive