Temperature Profiling of Fluids in a Continuous Flow Microwave System Using Fiber-Optic Technology.

Abstract

Determining temperature distribution within materials during microwave heating (including in continuous systems) is a major challenge in establishing temperature uniformity. Numerical modeling, infrared imaging and fiber optic probes are some of the methods employed to determine the distribution, but all of them have limitations. Modeling approaches using finite element and finite difference methods lack comprehensive validation of the results as temperature measurement within the dielectric in the microwave cavity is difficult without changing electromagnetic field distribution. Infrared imaging is only able to remotely measure the surface temperature of the tube. Fiber optic temperature probes have the potential to greatly improve the ability to measure temperatures within a microwave field and have been used to measure temperatures at select locations. This study presents the design and testing of a fiber optic measurement system capable of measuring temperature at different locations in the fluid being heated in the microwave cavity. Salt water (3%) was used as a model fluid The 915 MHz continuous-flow, focused microwave system used consisted of a 5 kW microwave generator, tuning coupler, connecting waveguide, elliptical focusing cavity and a PTFE applicator tube and was operated in TE10 mode. Results showed that in saltwater at a given cross section of the tube, temperatures could vary by as much as 20 C in the same plane. Inlet and outlet temperatures were measured using standard thermocouples. Inlet to outlet temperature increases were found to be as high as 40 C, 25 C and 20 C for flow rates of 1, 1.6 and 2 L/min, respectively. The results of this study proved that fiber optic temperature probes are an effective method to determine temperature distribution within a fluid heated in a continuous flow microwave system.