16th International Conference
on Microwave and High Frequency Heating

18-21 September 2017, Delft, The Netherlands
11:00   Modeling of microwave and RF power applications (2)
Chair: Kama HUANG
11:00
15 mins
METHOD FOR VALIDATING THE CALCULATED THERMAL PROFILES IN A 2.45 GHz MICROWAVE HEATED A CERAMIC CYLINDER
Juan A Aguilar-Garib, Junwu Tao, Beatriz C López-Walle, Tan-Hoa Vuong
Abstract: Thermal profile in a microwave heated sample depends of geometrical conditions as well as the relation between thermal and dielectric properties of the sample material. Therefore, knowing or mathematically predicting this profile, is a difficult task which accuracy is often evaluated from indirect measurements, such as a thermal evaluation at the end of microwave exposition in each process, such as cooking or curing. Continuing in this concept, a method for validating calculations of thermal profiles is proposed in this work, which consists in reproducing the cooling evolution of the heated samples considering the calculated thermal profile left by the microwave heating, as the initial condition for cooling. The applied equation with heat transfer terms is well established in software such as COMSOL, and describing cooling correctly means that the initial condition was also fine. To confirm this proposal, a hollow cylinder of a ceramic (37 mm external diameter, 15 mm internal diameter, 30 mm high) was exposed to 2.45 GHz microwaves in a cavity (35 cm x 34 cm x 24 cm). Conditions, such as the coincident values in the cooling curves of the two points (figure 1) over the sample, must be recreated with the validated thermal profile at the end of the heating process.
11:15
15 mins
IN-SITU SINGLE MODE DIELECTRIC MEASUREMENTS OF MICROWAVEABLE SNACK PELLETS
Erik Esveld, John Bows, Martijntje Vollebregt, Ruud van der Sman
Abstract: To capture the fast dynamics of microwave expansion of starch based snack pellets, a single mode shorted waveguide set-up was used. It is equipped with a six-port impedance analyser (1) which measures the complex reflection and absorbed power during heating. The pellet is suspended in the E-field maximum, with a centre inserted optic temperature sensor. Also a video of the moment/progression of snack expansion is recorded. Warm air is passed along the sample to prevent steam condensation. The dielectric properties of the pellet during heating are obtained via an inverse mapping of the recorded reflection on the expected reflection of the shorted wave guide for the whole possible range of dielectric properties. See Figure 1. These were pre-computed by finite elements simulation (2) both for unexpanded and expanded pellet geometry. The dielectric properties of the starch pellets change significantly during heating, expansion and subsequent drying. The dielectric properties increase with increasing temperature up to a maximum when the pellet expands. During expansion the power absorption shows a sudden decline. This is mainly due to the sudden change in porosity and can be reasonably predicted with the refraction mixing rule. The change of dielectric properties with temperature and different moisture content fit within the dipolar dispersion theory. Addition of salt (3%) to the starch pellet composition results in a slight decrease of the dielectric constant and loss factor, as it apparently lowers the effective mobility of the dipoles.
11:30
15 mins
MODELING THE TEMPERATURE FIELD DYNAMICS DURING THE MICROWAVES ASSISTED EXTRACTION OF ACTIVE PRINCIPLES FROM VEGETABLES
Vasile Lavric, Ioan Calinescu
Abstract: The conventional and microwave solvent extraction of active principles from vegetables is a mature experimental domain. Still, the mathematical modeling of microwaves (MWs) irradiation using either multimode or monomode MWs applicators should be improved. MWs irradiation intensifies extraction process since, due to its higher tangent loss, the vegetable gets preferentially heated. In a recent paper1, this is explained based upon the chemical potential: for only 1K positive difference in temperatures of the solid and liquid phases, the latter is responsible for osmosis of solvent from the liquid to the solid phase, thus increasing the internal pressure, which disrupts the cell’s membrane. Another recent paper2 emphasizes experimentally that the intensification of the extraction in MWs field has the same causes. Still, there is no direct proof that such a non-uniform heating appears. We present the modeling, in Comsol Multiphysics®, of a multimode MW applicator in which ethanol/water solution extracts active principles from vegetables. The solid – liquid temperature difference is the result of two dichotomic processes: in solid MWs generated heat and transferred heat to the surrounding liquid. With working temperature increase, the vegetable losses increase, while the solvent’s decrease, therefore selective heating will be enhanced at high temperatures. The authors acknowledge the financial support received from the Competitiveness Operational Programme 2014-2020, Action 1.1.4: Attracting high-level personnel from abroad in order to enhance the RD capacity, project: P_37_471, „Ultrasonic/Microwave Nonconventional Techniques as new tools for nonchemical and chemical processes”, financed by contract: 47/05.09.2016