A domestic microwave oven (1000？W) was modified to permit the continuous flow of liquids run through a helical coil centrally located inside the oven cavity. Heating characteristics were evaluated by measuring inlet and outlet temperatures of coil as a function of system variables. The influence of number of turns, coil diameter, tube diameter, pitch and initial temperature were evaluated at different flow rates. The average residence time of water was computed by dividing the coil volume by the volumetric flow rate. The influence of Dean number was evaluated. Results from this study showed that (1) higher number of turns resulted in lower heating rate, lower temperature fluctuations, higher exit temperature and longer time to achieve temperature equilibrium; (2) larger tube or coil diameter gave larger coil volume causing the heating rate to decrease; (3) faster flow rates resulted in lower exit temperatures, lower temperature fluctuation, higher Dean number and slightly higher heating rate; (4) higher initial temperatures resulted in higher exit temperatures; (5) higher Dean number resulted in more uniform heating and slightly higher heating rate. Overall, the coil volume was the more dominant factor affecting heating rate as compared with flow rate and Dean number. 1. Introduction Curved or helically coiled pipes are usually used as heat exchangers in food processing and numerous engineering fields, such as chemical, food, boiler, refrigeration, air conditioning, and nuclear engineering due to the unique flow patterns resulting from tube curvature and the advantage of volume compactness. Many experimental and theoretical papers have reported on convective heat transfer and temperature profiles in helical coil tubes [2–12]. The results from the above studies generally show that the centrifugal force causes secondary flow within the tubes which increases the associated rate of heat transfer as compared with the values obtained for straight tubes. Thomson [13] was the first to observe the striking effects of curvature on open-channel flow. Eustice [14] observed the trajectories of ink injected into water flowing in tubes wound around pipes of different diameters. Dean [1] was the first one to study, using a perturbation technique, the secondary flow field as a deviation from Poiseuille flow. As illustrated in Figure 1, Dean indicated that there exists a secondary flow in the form of a pair of vortices rotating in opposite directions. According to Dean, the lines in the figure “represent what may loosely be called the projections of the paths of fluid
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