Numerical simulation and experimental verification of forced-air precooling temperature field inside fruit packaging box with multiple-layer grids.

Forced-air precooling is more conductive to achieve a rapid cooling, and it is widely applied in the commercial processing treatment of fresh produce after harvest and recognized to be an efficient cooling method. Emphatically, the temperature plays a critical role during the whole cooling process that influencing the comfort of packed fruits and then the shelf life. In order to predict and monitor the temperature changes of produce inside the ventilated packages, a transient mathematical model was developed that considering the latent heat source due to respiration and evaporation of spherical fruits inside tray-layered packaging box during forced-air precooling process. Based on computational fluid dynamics, Fluent TM 2.3.26 along with the standard k-e model and SIMPLE algorithm for the pressure velocity coupling solution was employed to simulate the three-dimensional transient temperature field of layered fruits in forced-air precooling. Particularly, the inner heat source was added to the energy equation and loaded to the numerical solution dynamically by UDF (user defined function) for interpreting corresponding heat source files. Temperature fields of fruits stacked in three different patterns (spaced stacking pattern, paralleled stacking pattern and crossed stacking pattern) in different ventilated packages with circle and oblong side vents in the same opening area of 11.2% were analyzed. The experimental validations were performed, and the simulated data agreed well with the experimental results. Thus the mathematical model was reliable and can be used to the research of produce precooling. It is demonstrated that temperature distribution was more homogeneous for fruits in paralleled stacking pattern, while the slowest cooling rate and the highest temperature of fruit for spaced stacking pattern condition. What's more, the temperature gradient in fruit can not be ignored for the center temperature and surface temperature difference up to about 7?. Additionally, oblong vents design can balance the lateral and longitudinal diffusion of inlet airflow and ease the turbulent fluid inside packages for promoting the overall cooling rate and uniformity. Specifically for fruits in crossed stacking pattern, the cooling uniformity of oblong vents condition increased by about 10% and the slowest cooling temperature of fruit was 2? lower compared with the circle vents condition. As a result, ventilated packaging design for cooling fruits should be in tune with the internal stacking pattern as well as the trays to adjust the airflow and provide a adequate but uniform cooling environment, and for tray-layered fruits in crossed stacking pattern, oblong vents configuration with large ratio of the major axis to the minor axis can be suggested as better ventilated packaging design. [ABSTRACT FROM AUTHOR]