Optimal design and experimental verification of tilted tray air-impingement dryers

Drying has been widely used to remove the water from the food for the long shelf-life of agricultural products. The dried products can greatly prevent the deterioration related to the high moisture content from the growth of microorganisms, particularly for the cost-saving packing, storage, and transportation. A hot air-impingement technology has been commonly used to consider the high heat and mass transfer rates in the field of agricultural products and food drying, where the thin boundary layer can be formed on the surface of the material by the high-velocity hot-air impinged from the nozzles. It is very necessary for the uniform drying and large load capacity in the hot air-impingement. More importantly, uniform drying can greatly contribute to the efficiency, quality, and energy consumption during drying, even the market value of the products. In view of the improved loading capacity, the drying uniformity has still remained unclear in the tilted tray air-impingement dryer. Computational fluid dynamics (CFD) as a powerful tool has also been used in the structural design of dryers, where the air can be taken as the drying medium. This study aims to enhance the uniform distribution of hot air in the drying chamber for the high efficiency and product quality of the tilted tray air-impingement dryer. A numerical simulation software (Fluent) was applied to design the structural parameters of the flow distribution plate and the air deflector, as well as the key components in the air distribution chamber, using the differential equation and the stand k-ε turbulence model. A handheld anemometer was selected to detect the outlet velocity of the chamber. A response surface method (RSM) was adopted to optimize the structural parameters, such as the inclination angle of the tray, the distance between nozzles, the arrangement of nozzles, the distance between the disc and the nozzles. Two models of arrangements were then established to reveal the effect of structural parameters on the drying uniformity in the flow field. The results showed that an optimal combination of parameters was achieved, where the crossed nozzles are arranged, the ratio of the nozzle tray height to the diameter (H/D) was 6.78, the ratio of the nozzle spacing to the diameter (S/D) was 1.25, and the inclination angle of the tray (θ) was 24o. In this case, the unevenness coefficient and deviation ratio of air velocity was reduced to less than 1%, indicating the improved uniform velocity at the outlet of the chamber. An experiment was conducted to verify the optimization, where the fresh carrot slices with a thickness of (5±0.5) mm and a diameter of (26±1.0) mm were taken as the test material. The drying rate increased under the optimal parameters, and the drying time was shortened from 420 min to 330 and 300 min with the squared array and crossed array, respectively. The shrinkage ratio in the projected area was calculated from the image parameters of dried carrots using an industrial camera. The scanning electron microscope (SEM) images also showed that the dried carrot slices under the optimal parameters were retained the original bright red color, while the shrinkage ratio was reduced from 64.2% to 50.56%, and the rehydration ratio was improved from 3.84 to 4.51 in the crossed array, indicating the better retention of microstructure. As such, Even drying and high product quality can be achieved in the titled tray air-impingement dryer. The finding can provide a strong reference for further study on high efficiency, low energy consumption, and high-quality drying processing.