Microwave air heater: experimental investigation of a green heat transfer approach in helical coil

Heat transfer to a helical coil induced by microwave heating was performed in this study. Microwave radiation provided indirect and volumetric heating to the activated carbon (microwave absorbent) to store and transfer heat energy to the helical coil and heat up the air indirectly. The helical coil caused centrifugal force and induced a secondary flow to significantly enhance the heat transfer. Microwave radiation provided a high heating rate of 50°C/min to heat the activated carbon and the heat energy that was retained by activated carbon showed low heat loss of 10°C/min during cooling down. Then, the convective heat transfer between helical coil and air in non-isothermal conditions was performed. The overall heat transfer coefficient of the helical coil was evaluated under the condition of curvature ratios of 0.083, 15-25 L/min airflow rate and activated carbon temperature reduced from 300°C to 100°C. Dimensionless number of Reynolds number, Nusselt number and Dean Number were then analysed to evaluate the airflow and heating performance in the helical coil. The experimental results showed good agreement with literature whereas convective heat transfer in the helical coil is more efficient at the high-temperature region as well as higher flow rate applied.