|Abstract:||This objective of this work was an extensive study of the parameters that affect ambient entrainment in
refrigerated air curtains used in medium temperature vertical multi-deck display cases. The purpose of this study is
to increase the understanding of the air-curtain physics and entrainment through a systematic parametric study.
Experiments were conducted using flow visualization, cinematic Particle Image Velocimetry and thermo-couple
measurements at various locations in the flow cycle. These techniques were used to investigate various effects
including variations in: air jet width, velocity profile of the jet, velocity of the jet, temperature difference between
the air curtain and to the ambient air and geometry of the display case (e.g. fully-stocked configuration with backflow
perforations vs. an idealized wall jet). Thermal entrainment and velocity distributions were obtained for
several test conditions to understand these effects.
The air curtain was found to be highly turbulent at the inlet. Hence, the observed air curtain dynamics were
found to be largely different from previous wall-jet experiments and simulations which employed laminar inflow
conditions. The results indicated that the entrained mass flow rate was found to reduce nearly linearly with
decreasing Reynolds number (jet velocity), but the effect of product heat flux would need to be taken into account
before lower velocity levels can be recommended. In addition, the air curtain entrainment was found to reduce
when decreasing the air curtain width and/or using a stepped profile instead of a uniform profile. However,
increasing the Richardson number tended to increase mixing which somewhat unexpected since negative-buoyancy
effects were thought to act as a stabilizer. As such, the increased mixing was attributed to modifications in the
downstream velocity profile shape, where decreased jet temperatures led to increasing necking effects, which
subsequently increased the velocity gradients and thus led to more mixing of the air curtain. With respect to
geometry, the fully stocked configuration with perforations open was found to give the minimum entrainment of all
configurations studied, even less than that from the solid-wall configuration.