Optimized design of shell–core structural aggregate considering improving diminishing marginal effect and heat transfer efficiency of microwave heating

The diminishing marginal effect during microwave heating and lower heat transfer efficiency in asphalt mixture limit the improvement in microwave heating property. In order to minimize the diminishing marginal effect and improve the heat transfer efficiency, a shell–core microwave-enhanced aggregate (SMEA) with different electromagnetic parameters and thermal conductivity was designed based on the principle of equivalent electromagnetic parameters. A three-phase model was established using COMSOL to investigate the influences of the core and shell radius ratio of SMEA (R1/R2) on the electromagnetic intensity and temperature distribution. The results demonstrated that SMEA improved the impedance matching between asphalt mixture and air by extending the microwave propagation path through the cavity resonance of SMEA. The heat generated by SMEA during the microwave heating was concentrated on its surface. The value of R1/R2 influenced the microwave heating efficiency by affecting the distribution of electromagnetic and temperature fields. The optimum R1/R2 was 0.5, where the electromagnetic loss of asphalt mixture containing SMEA was 7.05% higher than that of asphalt mixture with ordinary microwave-enhanced aggregate, and the heat transfer efficiency from SMEA to non-microwave enhanced structure increased by 10.03%. The laboratory experiments also verified the SMEA can improve the microwave heating efficiency.