Remote three-dimensional temperature sensing using planar laser induced fluorescence : development and applications to microwave heated liquids

Abstract

Microwave heating is an important technology that has been hampered in application by difficulties in measuring temperatures and temperature distributions during the microwave heating process. This thesis describes the development of a 3D imaging fluorescence thermometry system that was used to examine temporal and spatial variations in temperature within various aqueous solutions during their heating by microwave irradiation. The work provides one of the first experimentally-determined temperature maps for a system undergoing microwave heating. A 3D thermometry instrument was built based on the principles of Planar Laser Induced Fluorescence (PLIF) imaging. Temperatures were optically determined from ratiometric measurements of the fluorescence from a pair of molecular probes. A Dextran polymer labeled with Rhodamine B was used as a temperature-sensitive probe that operates between 20 and 60 ⁰C. A second temperature-insensitive probe, Rhodamine 110, was used to monitor changes in the laser emission intensity. A dual camera fluorescence detector system was employed to capture a 2D x-y plane at a specified z-axis position. A dichroic mirror and optical filters were used to separate the fluorescence signals from the two probes. The instrument was able to achieve a spatial resolution of 0.2 mm in x-y plane, a 5 mm spatial resolution in z-axis, and a temperature resolution of ±1.6 ⁰C. The 3D imaging thermometry instrument was modified for investigations into microwave heating. A microwave plasma applicator was adapted for heating experiments with water and salt solutions at concentrations ranging from 0-0.5 M.

(cont.) heating with reduced convective flows. The dynamics of microwave heating were captured in images with a 0.5 second interval. Microwave heating was observed at node positions in the microwave cavity and varied with the dielectric properties of the heated medium. The experimental results for initial heating were successfully modeled by 2D calculations of the electric field in the microwave cavity. 3D experiments were performed on both pure water sample and on a 0.1 M salt solution. Due to the rapid rate of microwave heating, the 3D experiments were conducted by repeating experiments at different positions in the microwave cavity under the same starting conditions and heating profiles. The simulations of the 2D electric fields in the microwave cavity suggest that the electric field intensity varied little across the z-axis positions. Experiments at different z-axis positions in the cavity had identical profiles within the error of the experiments.