Investigation of the entrainment and infiltration rates through air curtains of open low-front refrigerated display cabinets

This thesis was submitted for the degree of Doctor of Philosophy and was awarded by Brunel UniversityThe high energy demand associated with open multi-deck refrigerated display cabinets is a direct consequence of their open design. The interaction between the cold refrigerated air inside the cabinet and the relatively warm air of the supermarket takes place across the air curtain, which serves as a non-physical barrier between the customers and the products. It has been estimated that 70% to 80% of the cabinet’s cooling load is due to ambient air infiltration into the cabinet refrigeration apparatus, which was previously entrained through the descending air curtain. A new generation of display cabinets has immerged in recent years, where the display-to-floor area has increased for the sake of maximizing sales. This modification leaves the air curtain with a larger display opening to seal against. Therefore, the design of such cabinets has now become more challenging, especially when attempting to ensure product integrity and temperature homogeneity while attempting to minimize their energy consumption. In this work, advanced numerical and experimental techniques have been integrated to quantify and also minimize the entrainment rate through the air curtain and the infiltration rate into open low-front refrigerated display cabinets. Experimentally, the Particle Image Velocimetry (PIV) technique has been used to map the velocity profile along the air curtain while the Infrared (IR) Thermography technique has been used to map the temperature profile across the cabinet. The Computational Fluid Dynamics (CFD) technique has been used in both case and parametric studies after confirming its validation with experiment. CFD was found to be a valuable tool for the simulation of open low-front refrigerated display cabinets, and the credibility of the results was assured when the boundary conditions were fine-tuned by experimental data. This thesis has demonstrated a systematic procedure where the entrainment rate through the air curtain can be quantified. The effect of various Discharge Air Grille (DAG) parameters was studied, and it was found that the entrainment rate is highly sensitive to the velocity profile and magnitude at the DAG. A velocity profile with a ramp shape having the maximum velocity near the cabinet yielded the minimum entrainment rate, hence the cabinet cooling load was reduced. In addition, two techniques were introduced for the determination of the infiltration rate of the cabinet. The first utilises the tracer-gas method to determine the specific amounts of ambient dry air and water vapour entering the evaporator coil, and the second uses psychrometrics to quantify the infiltration load as well as the other cooling load components by identifying the various heat transfer processes encountered during the operation of the cabinet. The ambient air infiltrated into the cabinet, although corresponds to 31% of the total mass flow rate, was found to be responsible for at least 85% of the total cooling load of the cabinet. This indicates that low-front cabinet suffer more from infiltration. The contribution of this work is by providing a better understanding towards the entrainment and infiltration processes related to open refrigerated display cabinets. The new techniques introduced in this work can help designers to better assess the impact of different design parameters and quantify the amounts of the entrainment and infiltration rates associated with open low-front refrigerated display cabinets