Summary

This study presents a numerical and experimental analysis of the thermal-hydraulic performance of tube-fin evaporators for light commercial refrigeration systems under frosting conditions. Since such a kind of appliance is generally designed with air curtains or subjected to frequent door opening, a certain amount of the ambient air infiltrates into the refrigerated space. This promotes the growth of a frost layer on the evaporator surface, which may completely block the gap between the fins if a proper defrost operation does not take place. The frost layer reduces the system cooling capacity by adding an extra thermal resistance to the evaporator tubes and also by reducing the fan-supplied air flow rate. In this study, a closed-loop wind tunnel was used to measure the thermal performance of tube-fin evaporators with different geometries. The air temperature and humidity at the inlet of the evaporator were controlled by the wind tunnel system, while a secondary refrigeration loop was used to set the evaporator surface temperature. The air flow rate was controlled by a variable-speed fan which emulates the real fan behavior. Moreover, frost thickness measurements were also carried out by using a digital camera equipped with zoom lens. The numerical results were compared with the experimental accumulated mass of frost, air pressure drop, air flow rate and cooling capacity, with all the predictions falling within the experimental band of uncertainties. Based on this comparison, a new correlation for the frost density was developed to be specifically applied to light commercial refrigeration products. The results show evaporator cooling capacity reductions up to 40% due to frost formation process. It was observed that the effect of the reduction of the air flow rate on the evaporator cooling capacity surpasses that of the extra thermal insulation of the frost layer. Furthermore, it was observed that, after a reasonable amount of frost has been grown, the louvered-fin evaporator showed a 10% lower cooling capacity when compared to the wavy-fin evaporator with the same fin pitch. These results indicate that under frosting conditions the pair fan-evaporator must be designed as a coupled system, in order to maintain the original refrigeration capacity for longer periods, increasing the time between successive defrosting processes and, therefore, improving the thermal performance of the entire system.

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