Convective heat transfer characteristics within a multi-package during precooling

Strawberries are packed using a multi-packaging system, with clamshells serving as the primary packaging and placed inside a tray, acting as the secondary packaging. The maintenance of uniform and desired convective heat transfer coefficients (CHTCs) at various walls of clamshells containing strawberries, as well as at different positions inside the tray, is essential during precooling. This ensures the prevention of condensation issues within the clamshells and safeguards the overall quality of the fruit. Most research studies have focused on the temperature heterogeneity of strawberries inside ventilated clamshells. However, this study aims to characterize the CHTCs on the external sides of an empty airtight clamshell (AC) placed inside an open tray. Measurements were performed on a half layer of a pallet to determine the CHTCs at various AC positions. The performance of four tray designs in terms of CHTCs was evaluated. Experimental results demonstrated different levels of CHTC heterogeneity inside the tray, both between the different sides of the AC and different positions. Upon comparing different tray designs, it was observed that adding vent holes in the trays improved the average CHTC within the trays, but increased heterogeneity between the different sides of one AC. In the other hand, increasing the air headspace height above the AC resulted in lower but more uniform CHTC values at the different positions. Dimensionless correlations related to CHTC were proposed based on the effect of mixed convection on heat transfer, using Nusselt, Reynolds and Rayleigh numbers. Additionally, a 3D CFD model was developed using Ansys-Fluent code to predict CHTCs on the different packaging sides within the trays. Good agreement was found between numerical and experimental values (mean relative error <20%).