(156e) Study of Microwave Combination Heating Using a Coupled Electromagnetics- Multiphase Porous Media Model

Combining microwaves with other heating modes such as infrared and hot air provides an excellent method to speed up cooking processes and potentially provide automated custom-cooking ability by implementing temperature and moisture profiles needed for specific processes. Combining heating modes may, however, pose technical challenges without the basic understanding of the combined processes. Through improved understanding of the combination heating process, overheating, underheating, overdrying and sogginess of food can be minimized while enhancing its quality. A physics-based computational model that captures spatial and temporal temperature and moisture patterns in the heated sample would provide valuable insight into the nature of combination heating. This work investigated combination heating in a novel microwave combination system that heats the sample through a combination of microwaves and hot air using a coupled electromagnetics-multiphase porous media model to determine the effect of combining different modes on heating patterns.

The Maxwell's equations of electromagnetics were solved in the three dimensions to obtain the electric field inside the oven cavity and the sample. Power absorbed per unit volume from the microwaves at any location of the sample was calculated from the electric field distribution inside the sample. A 3D multiphase porous media model was formulated that described the heat and mass transfer inside the sample. The three phases included in the porous media model were solid, liquid water and gas. The gas phase had two components: water vapor and air. The mass conservation equation for the different transportable phases included the effects of diffusion, capillary flow and convection. The porous media model included the change of phase between liquid water and vapor (evaporation/ condensation) throughout the domain. The energy conservation equation was solved for the mixture and the effect of microwave heating was included as a source term obtained from the electromagnetics model. The contribution due to hot air was incorporated as a boundary condition using a surface heat transfer coefficient measured experimentally. The electromagnetics and the multiphase porous media models were fully coupled as the dielectric properties of the sample, which were also measured experimentally, were a function of temperature and moisture content. All the equations were solved in the commercial solver, COMSOL Script, using the finite element method by applying appropriate boundary conditions. Temperature, moisture, pressure and evaporation rates in the sample were obtained as a function of location and time. Higher temperatures were observed at the outer walls and the top boundary due to hot air heating. Heating by the focusing effect of the microwaves was also observed near the center of the sample. Moisture content was lower on the top open surface of the sample as the moisture was carried away in form of water vapor due to high surrounding temperature. Pressure build up near the surface was observed whereas pressure (gauge) in the interior of the sample became negative as condensation of water vapor took place. Greater evaporation occurred at the top and the sides where temperatures were higher. Negative values indicated condensation and it occurred where temperatures were lower at locations of the sample that were not heated by the microwaves or the hot air. It was, therefore, concluded that the different heating modes complement each other increasing the efficiency of the combination heating process. Combination heating can also overcome the problem of surface sogginess reported in microwave only heating. The results can be used in further development of combination heating methods for different food processes.