Comparison of microwave and thermal cure of epoxy–anhydride resins: Mechanical properties and dynamic characteristics

The objective of this work was to compare the mechanical properties of epoxy resins cured by thermal heating and microwave heating. Epoxy–anhydride (100:80) resins were cured in a domestic microwave oven and in a thermal oven. The hardening agents included methyl tetrahydrophthalic anhydride and methyl hexahydrophthalic anhydride. Three types of accelerators were employed. Thermal curing was performed at 150°C for 20 and 14 min for resins containing 1 and 4% accelerator, respectively. Microwave curing was carried out at a low power (207 or 276 W) for 10, 14, and 20 min. All cured resins were investigated with respect to their tensile properties, notched Izod impact resistance, and flexural properties (three‐point bending) according to ASTM standards. The tan δ and activation energy values were investigated with dynamic mechanical thermal analysis, and the extent of conversion was determined with differential scanning calorimetry. The differences in the mechanical properties of the thermally cured and microwave‐cured samples depended on the resin formulation and properties. Equivalent or better mechanical properties were obtained by microwave curing, in comparison with those obtained by thermal curing. Microwave curing also provided a shorter cure time and an equivalent degree of conversion. The glass‐transition temperatures (tan δ) of the thermally and microwave‐cured resins were comparable, and their activation energies were in the range of 327–521 kJ/mol. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1442–1461, 2005     

Experimental and numerical study on heat transfer and flow characteristics in an alternating cross‐section flattened tube

An experimental and numerical study on convection heat transfer of water flowing through an alternating cross‐section flattened (ACF) tube are investigated in this paper. The thermal‐fluid characteristics were evaluated by numerical simulation. The test run conditions covered a mass flux of 200 to 800 kg m^?2 s^?1, a heat flux of 10 kW/m², and an inlet temperature of 40°C. The results showed that the Nusselt number increased with the increase in mass flux. Moreover, the heat transfer was also affected by the flow characteristics. Vortices were formed at the curved wall, and their intensities were increased along the flow direction. It was also found that the heat transfer and pressure drop were larger than that of the circular tube. However, the thermal performance was greater than the pressure loss penalty. The comparison results showed that the ACF tube had better performance than the circular tube. Further, the details of heat transfer, flow resistance, and fluid behavior were investigated and discussed in this study.