Design of a measurement system for temperature distribution in continuous-flow microwave heating of pumpable fluids using infrared imaging and fiber optic technology

Continuous microwave heating has the potential to revolutionize processing operations in which high energy densities per unit volume are required. The adoption of microwave heating for large scale commercial application has been hampered by a lack of reliable information on the temperature distribution during heating of pumpable fluids. This study represents an initial attempt to design a system that quantifies the temperature distribution during continuous microwave heating and its relationship with processing tube surface temperature distribution. The novel measurement system designed consisted of a semi-rigid thin axial support system on which fiber optic probes were attached at different lengths in the microwave heating cavity. Inlet, outlet and in-line longitudinal temperatures were monitored during continuous flow microwave heating (1.5 kW, 2.5 kW, 3.5 kW, and 4.5 kW) of water of three different salinities (0 ppt, 15 ppt, and 30 ppt) at a flow rate of 1.6 l/min. Inlet and exit temperatures were measured using standard thermocouples; in line temperatures were determined using fiber optic temperature probes. The surface temperature distribution of the applicator tube was measured using an infrared camera. Results showed that fiber optic-based internal temperatures were 10 degrees C higher than observed external infrared temperatures for fresh water and 14.6 degrees C higher for 30 ppt salt water. The measurement system and method presented are intended to provide experimental validation of numerical models of continuous flow microwave heating, as well as to help develop empirical models of temperature distribution under varying processing parameters.