Dependency of Air Curtain Performance on Discharge Air Velocity (Grille and Back Panel) in Open Refrigerated Display Cabinets

Abstract

This study performs a Computational Fluid Dynamics (CFD) modeling of air flow and heat transfer of an open refrigerated display cabinet in order to evaluate the influence of the discharge air velocity on the performance of its recirculated air curtain. The physical-mathematical model considers the flow through the internal ducts, across the fans and the evaporator, and also the thermal response of food products. The fan boundary condition is modeled in order to vary the air velocity at the discharge grille. The back panel perforation is modeled as a porous medium. The density and dimension of the back panel perforation variation is modeled by the Darcy’s law with the Forchheimer extension, varying the viscous and inertial resistance coefficients of the porous medium, based on its porosity, permeability, air velocity and pressure loss coefficient. Experimental tests were conducted to characterize the phenomena near the physical borders and to prescribe boundary conditions as well as to validate the numerical predictions on the temperature, relative humidity and velocity distributions. The numerical results show that the lowest average temperature in the conservation area of the display cabinet is achieved at a discharge air grille velocity of 1.15 ms−1. This value is lower than the experimental one, 1.51 ms−1, measured on the real equipment. The absence of a velocity component in the third dimension, which can destabilize the air curtain, is assumed to be the reason for this discrepancy. The profiles of the numerical predictions of the air curtain indicate that in the optimum case the air curtain is not so stable to bear big disturbances from outside. In order to increase the thermal performance and to reduce the energy consumption of these equipments, it’s not recommended to run the re-circulated air curtain velocity below 1.15 ms−1. For each CFD model, the values and directions of the air mass flow rate and heat transfer across the re-circulated air curtain by unit length are predicted and compared with the experimental ones in order to evaluate its thermal energy gains and losses.