Summary

A theoretical and experimental analysis of tube-fin evaporators utilized in household top-mount #no-frost# refrigerators is carried out in the present dissertation. The performance of an alternative evaporator concept, the so-called Accelerated Flow Evaporator (AFE) is systematically investigated. In this evaporator, the air-side cross sectional area decreases with the distance from the air flow inlet causing the air flow to accelerate and promote an enhancement of the air-side heat transfer coefficient. This heat transfer enhancement allows a reduction of the heat exchanger volume and hence its material cost. An optimization analysis is also carried out to identify an optimized configuration of the AFE utilized in household refrigerators. An experimental apparatus was constructed to measure the air-side pressure drop and the heat transfer in 15 evaporators prototypes as a function of the air flow rate, the ratio of the outlet and inlet cross-section area and the number of fins. The test facility consists of an open wind tunnel connected to a water loop. A mathematical model which simulates the air-side thermal-hydraulic behavior of the evaporator was also developed. The modeling approach consists of dividing the evaporator into n control volumes in the direction of the air flow. The tube pitches, fin density and inlet and outlet cross-sectional areas can be set independently for each control volume. A mathematical model for the refrigeration system was also developed to take into account the overall impact on the system performance due to a change in the configuration of the evaporator. The agreement between the models and the experimental results is within ±15%. The optimization results have shown that the AFE concept presents a significant potential for household refrigeration applications, since all configurations obtained from the optimization runs contained evaporators with some acceleration. When the AFE concept was employed, the mass of the evaporator could be reduced by as much as 40%, with an associated COP reduction of only 1%.

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