Influence of Supply Air Ceiling Diffusers on Surface Temperature of Display Cabinet Doors and on Customers Comfort in Food Stores

Vertical display cabinets are widespread in food stores, because they have a large display area and an ease access for customers. Recently, open cabinets has been replaced with closed ones in order to reduce energy consumptions for refrigeration. However, the presence of the glass doors can have an effect on product visibility because of mist formation on the glass, particularly for low temperature display cabinets. This effect appears when the temperature at the external surface of the glass falls below the ambient dew temperature, which is a quite common situation in humid climate in the mid-season, when neither indoor air heating nor cooling is performed. Today the mist formation is prevented increasing the surface temperature at external side of the glass doors by means of electrical resistances embedded in the doors frame. The glass door surface temperature can be raised enhancing the heat transfer coefficient driving the supply air towards the display cabinet doors. This paper investigates the behaviour of different air ceiling diffusers (vortex diffusers and linear diffusers) in order to reduce the risk of mist formation and the energy consumption of electrical resistances. Initially, 3D numerical simulations of isothermal airflow field has been performed and, later, summer and winter conditions were considered. The air flow field in the display zone, the temperature distribution on the glass surfaces and the influence of different diffusers on customers comfort condition are investigated.