Entrainment in Refrigerated Air Curtains

Abstract

Refrigerated air curtains are used in supermarket display cases to keep warm ambient air from entering the refrigerator case. Entrainment of ambient air into the curtain by shear layer mixing contributes to both the sensible and the latent heat load on the cases. In order to reduce this entrainment, a closer look at the air curtain is undertaken here. Particle Image Velocimetry was used to examine idealized air curtains of various Reynolds and Richardson numbers. The air curtain entrainment was seen to be dominated by eddies that engulfed ambient air into the air curtain. For the isothermal (Ri=0) curtains, it was observed that the air curtain dynamics and entrainment were not sensitive to Reynolds number variation between Re=3800 and Re=8500. At low air velocities (Re=1500), the curtain was found to detach, leading to extremely high entrainment rates. Variations in Richardson number of the curtain affected the flow at the curtain inlet most dramatically, where the acceleration of gravity caused the curtain to neck inwards. The overall thermal entrainment was shown to reduce with reducing Reynolds number, implying that a reduction in air curtain entrainment could be achieved by reducing the air velocity of the curtain, as long as air curtain integrity is maintained.