ABSTRACT
Acknowledgment................................................................................................................ 140
Nomenclature ..................................................................................................................... 140
References .......................................................................................................................... 140
Open vertical refrigerated display cases are widely used in supermarkets and grocery stores to
maintain the food products at a prescribed temperature while allowing the customers to easily
reach for the food products. The operation and main features of all contributors to the
dynamics of an air curtain are shown in Figure 5.1. The cold air is supplied to the case
through a discharge air grille (DAG) at the top and in some cases through a perforated back
panel (BP). A fan located at the lower portion of the case pulls the air flow toward a return air
grille (RAG) that is recirculated after coming into contact with cold coils. The air curtain is
referred to the region of air flowing from the DAG to the RAG. The existence of a
momentum gradient between the air curtain and its surrounding air causes an entrainment
of the outside warm air that will partially mix with the cold air forming the air curtain.
Therefore, the temperature of the air curtain as it travels down toward the RAG increases.
Since the total air flow rate inside the display case is constant, a portion of the entrained air
will overspill from the case after mixing with the cold air curtain. The overspill air tempera-
ture is higher than the cold air temperature originating from the DAG and lower than the
room temperature. Similarly, the temperature of the air at the RAG is between the cold air
and warm air temperatures. The magnitude of the air temperature at the RAG and also for
the overspill air is a function of the amount of entrained air and turbulence-enhanced mixing
of cold and warm air along the air curtain flow. The portion of the warm air that is mixed
with the cold air and is going through the RAG is referred to as the infiltration rate.
