Study of electromagnetic field uniformity in radio frequency heating applicator.

摘要

A 6kW 27MHz pilot scale radio frequency (RF) heating system was developed at Washington State University for processing 6-pound packaged foods to achieve shelf-stability. Although RF heating presents the possibility of improving food quality in a shorter processing time than the conventional retort heating method, it still confronts the heating non-uniformity caused by fringe effects. The objectives of this research were to gain a better understanding of the dielectric heating process, to study the factors that may influence the RF heating pattern and heating rate, and to investigate the possibility of achieving heating uniformity during the RF heating process.;To support this project, a dielectric properties measurement system was used to determine the complex permittivity of materials used in dielectric heating over a temperature range from 20 to 120°C at radio frequencies (27 and 40 MHz) and microwave frequencies (915 and 1800 MHz). The dielectric properties of egg products were measured with an open-end coaxial probe method. The dielectric property information was used to adjust the electric conductivity of the circulating water that immersed the packaged foods and mitigated fringing electric field and edge-heating effects during the RF heating process, and to support the implementation of the computer simulating on RF heating process.;Experiments and computer simulations using the finite element method were performed to study the influence of food dielectric properties and circulating water electric conductivity on RF heating rate and heating pattern. The results of this work indicated that with the increase of electric conductivity of circulating water or food loss factor, the heating rate decreased. Both the dielectric properties of food and electric conductivity of circulating water affected the electric field distribution inside the food.;Experiments and computer simulations were also conducted on RF heating of a heterogeneous food, meat balls in lasagna. The results demonstrated that RF heating had a potential to process pre-packaged heterogeneous food. Proper distribution and suitable size of each component, though with vastly different dielectric properties, are possible to ensure relatively uniform RF heating.