數值模擬冷藏垂直開放櫃之層流氣簾之動量及熱質傳特性研究

Abstract

利用二維穩態數值模擬方法針對垂直開放櫃層流氣簾的熱質傳及動量特性影響探討；以商業套裝計算軟體PHOENICS進行求解，主要焦點著重於相對濕度對流場的溫濕度影響；針對六個不同雷諾數100.5至717.8及在相同雷諾數之下，不同氣簾寬度0.016m、0.02m及0.024m及氣簾長度0.2m與展示櫃深度0.1m，及出口速度0.0583m至0.62m冷卻氣簾包含兩相反方向渦流結構之穩態開放櫃流場研究。其中氣簾出口處及環境溫度溫度分別為5 ℃ 及25 ℃，溫度差為20 ℃。而氣簾出口處和環境的相對溼度分別為90% 及 60%。結果指出相同Grt下，未加入濃度差及溫度差之效應時，在慣性力作用下展示櫃內形成單一渦流；然而當熱傳及質傳的浮慣比大到一個相當程度時，氣簾產生bending現象且形成兩個反方向的旋轉渦流。而在相同雷諾數之下，縮小氣簾出口寬度將可減緩氣簾的bending現象。除此之外，將氣簾出口傾斜一個角度後，等溫線及等濃度線的扭曲現象也隨之減緩。然而安置第二氣簾後的結果指出，在較低的V2j下，氣簾出口寬度及安置位置的影響較小。然而在較高速的第二氣簾效應作用之下，氣簾的bending現象有明顯的減緩，且在兩者相對比較下，原先氣簾速度對氣簾bending現象影響較小。

A steady two-dimensional numerical simulation is conducted to investigate the momentum, heat and mass transfer in a vertical open cavity resulting from a laminar cold air curtain flowing over the open surface of the cavity, simulating that in a vertical refrigerated display cabinet. The commercial computational fluid dynamics software PHOENICS is adopted to solve the problem. Particular 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 air curtain Reynolds number vanying from 100.5 to 717.8 and the injection slot width ranging from 0.016 to 0.024 m for the air discharge-to-return grille separation distance fixed at 0.2 m and for the cabinet depth of 0.1 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. Furthermore, we proceed to investigate the possibility of retarding the air curtain bending by inclining the jet at the air discharge grille toward the ambient. Moreover, how the installation of a secondary air curtain affects the flow in the display cabinet is examined. For the limiting case in the absence of the thermal and solutal buoyancy forces, the results indicate that a single large recirculating vortex is induced by the inertia force of the air curtain for all Reb. However, when the Richardson numbers (buoyancy to inertia ratio) increases to certain level, the air curtain bends toward the cavity and the intrusion of the warm and humid air from the ambient into the cavity becomes relatively prominent. At the same Grt, when the Reb is lowered to some degree, the air curtain bending is so large and two counter-rotating flow recirculations are induced in the cabinet. However, at the same Reb, milder air curtain bending and warm air intrusion are noted at reducing the injection width. Besides, inclining the jet toward the ambient can significantly delay the air curtain bending and the distortions in the isotherms and iso-concentration lines. The results from installing a secondary air curtain indicate that at this low V2j the effects of the jet width and location on the flow are rather slight. But at higher V2j, installing the jet some distance away from the cabinet causes a significant bending of the original air curtain. However, installing a high speed secondary air curtain near the cabinet can substantially reduce the bending of the original air curtain and hence improves the cabinet performance.