數值模擬冷藏垂直開放櫃之氣簾對其動量及熱質傳特性影響研究

Abstract

本篇論文利用二維穩態數值模擬方法探討一之氣簾穿過一垂直開放展示櫃之溫度與溼度濃度特性影響研究；以商用套裝計算流體力學軟體PHOENICS對流體的統馭方程式進行求解，論文著重確認由相對溼度對流場的溫溼度濃度影響；針對不同氣簾出口寬度5.0， 7.0， 10.0 及12.0 cm，及氣簾長度1.20 m與展示櫃深度0.57m，及出口流速0.25至3.0 m/s，之冷卻氣簾包含兩種不同渦流結構之穩態開放櫃流場研究。其中氣簾出口與環境溫度分別為5.0及25.0℃，溫度差為20℃。氣簾出口及環境的相對溼度分別為90%與60%。結果指出在沒有加入溫度差及濃度差的效應下，展示櫃內為單一的flow recirculation。當加入溫度及濃度的浮力效應時，氣簾將會改變方向，向展示櫃內bending，且環境的濕暖空氣將滲入展示櫃內。當熱傳與質傳的浮慣比大到一個程度時，氣簾的bending將會碰到展示櫃內壁面，形成兩個不同方向旋轉的flow recirculations。除此之外縮小氣簾出口寬度將會減緩氣簾的bending。而進一步將氣簾出口環境方向偏移一個角度以及展示櫃內的垂直壁面perforation出風，也更有效的減緩氣簾的bending現象。 在雙氣簾的設計中，氣簾的bending現象及環境的暖濕空氣可藉由內外氣簾分別使用較小及較大的出口寬度。除此之外內外氣簾的相對出口雷諾數再展示櫃的效應有非單一的影響。並且在外氣簾偏移一個角度後也將有效的改善展示櫃的整體效能。

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.