Effects of Air Curtains on Momentum, Heat and Mass Transfer in an Open Vertical Refrigerated Display Cabinet- Numerical Simulation

• Main Authors: Chun-Ta Lin, 林君達
• Other Authors: Tsing-Fa Lin
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/21163068015486074943

碩士 === 國立交通大學 === 機械工程系所 === 94 === A steady two-dimensional numerical simulation is conducted in the present study to investigate the momentum, heat and mass transfer resulting from cold air curtain discharge over the open surface of a vertical open cavity, simulating that in a vertical refrigerated display cabinet. The commercial computational fluid dynamics software PHOENICS [25] is employed to solve the problem. Attention is focused on how the parameters associated with the air curtain affect the characteristics of the flow in the cabinet. Computations are performed for the jet speed at the air discharge grille varying from 0.25 to 3.0 m/s and injection slot width ranging from 0.05 to 0.12 m for the air discharge-to-return grille separation distance fixed at 1.20 m and for the cabinet depth of 0.57 m. The temperature difference between the air discharge and ambient is 20 ℃, corresponding to Tj = 5 ℃ and Tamb = 25 ℃. The relative humidities at the air discharge and ambient are respectively fixed at 90% and 60%. Effects of various parameters on the flow, thermal and solutal characteristics in the cabinet are examined in detail. The results indicate that for the limiting case in the absence of the buoyancy force the cabinet is dominated by a single large flow recirculation. But when the Richardson numbers (buoyancy-to -inertia ratios) exceed certain levels, the bending of the air curtain toward the cabinet core and the intrusion of the warm moist air from the ambient into the cabinet become significant. At high Rit and Rim the air curtain bending can be large enough to induce two counter-rotating flow recirculations in the cabinet. Besides, a reduction in the injection slot width is found to result in a milder air curtain bending and warm air intrusion. Similar effects can be obtained by inclining the air curtain at the discharge grille slightly toward the ambient and by injecting the air flow into the cabinet from the back panel perforations. In a two air-curtain design, the air curtain bending and warm air intrusion can be reduced by a larger inner jet width and a smaller outer jet width at the discharge grille for a fixed total width of the two jets. Besides, the relative magnitudes of the inner and outer jet Reynolds numbers are noted to produce nonmonotonic influence on the performance of the cabinet. Moreover, it is found that inclining the outer air jet toward the ambient with the inner jet still in the vertically downward direction can improve the cabinet performance.