Flow field and drying process analysis of double-layer drying chamber in heat pump dryer

Heat pump drying is an effective method to reduce the energy consumption and environmental pollution especially for industry and agriculture. Drying process analysis is the basis of optimizing operating condition, structure design and improving product quality. In this paper, a mathematical model is developed to simulate the flow field and drying process of forage seed in a double-layer drying chamber, and the simulated results are verified by the experiment. The moisture content of seeds in the tray is approximate normal distribution in the total drying process except the lowest tray (tray-1). The lowest and highest moisture contents of seeds are 5.95% (tray-1) and 8.77% (tray-8) respectively while the maximum and minimum non-uniformities are 13.7% (tray-9) and 3.7% (tray-5) after the drying time of 3 h. All the moisture content of seeds and the surrounding air increase rapidly in the first half hour and decrease rapidly in the following half hour, and then they change slowly until the end of drying time. The moisture content of seeds is still uneven after 3 h and the range of 5.95%-8.77% is shown for different trays. The final moisture content of seeds decreases 3.00%, 4.23%, 1.67% respectively when the air temperature increases from 45 & DEG;C to 60 & DEG;C, air velocity increases from 1.5 m.s(-1) to 2.5 m.s(-1), and the thickness of seeds decreases from 6 cm to 4 cm. Although double-tray structure improves the air flow though the central seeds, it has little effect on the average drying degree.