Coupled heat transfer and temperature uniformity analysis in vacuum microwave drying of pineapple

This study aims to solve the common problem of uneven heating in vacuum microwave drying. Combining numerical simulation and experimental measurement, the effects of operating parameters on the heating efficiency and temperature uniformity of pineapple slices are systematically analyzed. A three-dimensional porous medium physical coupling model is established to integrate the mechanisms of microwave electromagnetic field, heat conduction, and moisture migration, simulating the transient heat and mass transfer behavior during the drying process. The results are verified by six-point temperature and infrared thermal images. The parameters studied include input microwave power (200, 300, and 400 W), pulsing ratio (PR = 0.25 to 1), and turntable speed (n = 0 to 6 rpm). The effects of these parameters on temperature distribution and uniformity are evaluated. The results show that increasing the power can accelerate heating, but high microwave power can easily lead to local hot spot aggregation, which affects the quality. Adjusting pulsing ratio to 0.5 ∼ 0.6 can improve the thermal field stability compared with continuous heating. Increasing the turntable speed to 3 rpm can effectively break up the standing wave field, reducing the temperature coefficient of variation (COV(T) by 93.1 % and significantly improving temperature uniformity.