PhD Defense by Nasser-Eddine Ahmad the 4th of october 2024

Jury :

Directed by :  Jean MOUREH, Researcher at INRAE, Supervision by Steven DURET INRAE Frise and Denis FLICK, UMR SayFood, AgroParisTech-INRAE, University Paris Saclay

Abstract : The influence of packaging on the airflow dynamics within ventilated pallets of horticultural products is crucial for maintaining recommended temperatures and ensuring their uniformity. This condition is essential to preserve the quality of the products, particularly for highly perishable products, where multi-packaging system is common. 
Multi-package includes primary packaging (for consumer sales), such as clamshells, and secondary packaging, such as corrugated trays. Non-ventilated primary packaging, like Modified atmosphere packaging (MAP), can act as a barrier between the cold air and the product, significantly affecting the cooling rate. However, no studies have examined the impact of the aeraulic design of the tray, the geometric shape of the MAPs, and their packaging mode within a tray on the uniformity of ventilation and, thus, the cooling of palletised products. 
The scientific approach of this thesis relies on the development and use of numerical and experimental methods to characterize and predict ventilation heterogeneities, heat transfer and cooling kinetics at the scale of a half layer of a pallet of strawberries conditioned in a multi-packaging system. This involves ventilated trays and airtight clamshells representing the MAPs.
The experiments include measurements of the convective heat transfer coefficient (CHTC) using flux sensors on the walls of the clamshells, monitoring the temperatures of strawberries during cooling, and velocity measurements inside the trays, conducted using Laser Doppler Velocimetry (LDV). The analysis of the aerodynamic and thermal results allows for a better understanding of the effect of the design of the secondary packaging (tray) on the airflow distribution and uniformity around the clamshells, on the convective transfers they induce, and on the characteristic cooling times within the clamshells. 
A 3D CFD (RANS) numerical model was developed in an unsteady regime using ANSYS-Fluent software. In this modeling approach, the interior of the clamshells was simulated as a solid block with thermal characteristics equivalent to a porous medium: strawberry + air. The validation of this model was carried out by comparisons with experimental data (velocity and temperature) obtained in a test cell with controlled conditions of air temperature and velocity. This model allows for examining the influence of the main geometric and aerodynamic parameters, such as the effect of adding vent holes, the thickness of the headspace above the clamshells, or the dimensions of the trapezoidal orifice, on the air circulation within the trays and on the cooling kinetics of the clamshells. Additionally, the CFD model is coupled with a quality model to evaluate the effect of cooling heterogeneity within a tray on the heterogeneity of strawberry quality. Numerical simulations revealed that a trapezoidal orifice wider than that of the current tray design yielded better results, with a slight improvement in strawberry cooling, a slight reduction in product deterioration within the package compared to the current design, and a significant decrease in pressure drop, which affects energy consumption.