Numerical Modeling of Continuous Flow Microwave Heating - Coupling of High Frequency Electromagnetism with Fluid Flow and Heat Transfer

Abstract

This study focused on numerical and experimental investigation of continuous flow microwave heating. A numerical model was developed to predict the temperature of a liquid product heated in a continuous-flow focused microwave system by coupling electromagnetism, fluid flow, and heat transfer in two different software packages (ANSYS Multiphysics 9.0 and COMSOL Multiphysics 3.4). The temperature and energy distributions simulated in ANSYS were verified against other numerical studies published in the literature. Comparison of the simulated temperature in ANSYS with experimental data for salt water of two different salinities showed good agreement. To simulate the experimental conditions more accurately, a comprehensive numerical model was developed in COMSOL Multiphysics by incorporating non-Newtonian flow and phase change coupling (which were not considered in the earlier ANSYS model). Comparison of the results from COMSOL model with the results from pre-developed and validated ANSYS model ensured accuracy of the COMSOL model. A comprehensive validation of the model with experimental data suggested that the simulated results were in fairly good agreement with the experimental data for saltwater and CMC solution. Rigorous experimental data was collected for freshwater, saltwater, and CMC solution flowing at three different flow rates through a 915 MHz continuous flow microwave system at 4 kW of power. A custom made temperature measurement system employing a single fiber optic probe was used to get 110 radial and longitudinal temperatures measurements. The experimental temperature values were used in determining the effect of different dielectric properties and flow rate on heating patterns of freshwater, saltwater and CMC during continuous flow microwave heating. The presented work greatly aids in understanding of the microwave heating process through accumulation and analysis of a large body of experimental data and through mathematical prediction of temperatures for a variety of operating parameters (i.e. dielectric and physical properties, and flow rates). The methodology developed can be further applied to study temperature distributions for a multitude of materials, and continuous flow microwave cavity geometries. The developed model is a vital tool for researchers and engineers in the academic and industrial settings interested in continuous microwave heating of liquids.