Numerical simulation and optimization of air distribution in kiwifruit controlled atmosphere pallet bags

Pallet bag storage is essentially the same as controlled atmosphere storage. The pallet bag is placed in cold storage, and then to control the composition and proportion of gases in it. Fruits and vegetables can be expected to store in small batches, due to the low cost, convenient management, and excellent storage. Among them, Kiwi fruit is well-suited for storage in pallet bags. However, the current storage conditions cannot be accurately and efficiently controlled for Kiwi fruits, because the simple facilities are confined to the manual operation of the storage environment in China. In this study, the optimal parameters were determined for the pallet bag storage, in order to enhance the storage performance and quality. The natural stacking state of the kiwifruit in the box was analyzed using SolidWorks Motion. The micro-environment model was established for a single kiwifruit in pallet bag storage. An air-conditioning storage test device of pallet bags was constructed to verify the accuracy of the micro-environment CFD simulation model. The verification test was then carried out. There was better consistency between the test and CFD simulation, indicating that the CFD model accurately simulated the O₂ concentration in the pallet bag storage. After that, the single-factor experiment was carried out to analyze the influence of four key parameters (namely nitrogen purity, flow rate, pipe diameter, and outlet height) on the deoxygenation time and gas distribution in the regulating process of the storage environment. The results indicate that N₂ purity, flow rate, and outlet height significantly affected the deoxygenation time and non-uniformity coefficient. However, there was no significant effect of pipe diameter on the deoxygenation time. Furthermore, a four-factor three-level orthogonal experiment was designed using Box-Behnken. Response surface analysis was conducted to establish a regression model for the four selected factors using Design-Expert software. The simulation experiment indicated that the influencing factors on deoxygenation time were ranked in descending order of outlet height, flow rate, N₂ purity, and tube diameter. The influencing factors on the non-uniformity coefficient were ranked in descending order of the N₂ purity, outlet height, pipe diameter, and flow rate. Taking the deoxygenation time and non-uniformity coefficient as the optimization objectives, the optimal operation parameters of pallet bag storage were obtained as follows: N₂ purity 96.0%, flow rate 23.0 m³/h, pipe diameter 50 mm, and outlet height 3126 mm. The optimal deoxygenation time and non-uniformity coefficient were 989 s and 6.1%, respectively. The CFD simulation was conducted using the optimal parameters. The predicted value of the response surface regression model was compared with that of CFD simulation. The relative error between the predicted and simulated values of oxygen reduction time, and non-uniformity coefficient were 3.1%, and 8.4%, respectively. The finding is of great theoretical and practical significance to promote the controlled atmosphere pallet bag of kiwifruit.