凸形百葉窗型與波浪型鰭片冷凝器的性能測試比較與設計

Abstract

本論文選擇七個凸形百業窗型及七個波浪型鰭片凝器分別進行， 熱傳與壓降分析。冷凝器是在相同的管徑與鰭片厚度及縱、橫向管距， 不同的管排數與鰭片節距下進行測試分析。測試的結果以執傳ｊ因子及 磨擦因子ｆ相對雷諾數Redc的變化來表示，雷諾數中的特性長度是取 管外徑加上兩部鰭片厚度做為計算的基礎，雷諾數範圍則取300至8,000 之間。實驗的結果顯示，在雷諾數大於1,000以上時，管排數對f值 影響非常微小；在不同的管排數下，鰭片節距為2.54mm時對凸形百 葉窗型與浪型鰭片的j因子影響很小；但鰭片節距為1.21mm，當雷諾數 小於2,000時，j因子隨著管排數的增加而減小，而在雷諾數大於2,000以 上時，鰭片節距的大小對j因子的影響則逐漸減少，這是因為管排數的效應 已經開始對能過冷凝器的氣流產生影響，導致氣流之漩渦消散，因而 使得鰭片節距的效應對j因子影響減少。 另外本文除了直接以j與f因子比較鰭片的性能外，亦使用多種以 冷凝器面積或體積的觀點，來做鰭片間的性能比較。比較的結果以凸 形百葉窗型鰭片的性能比波浪型鰭片高，唯一例外的是在低雷諾數、 管排數4排、鰭片節距為1.21mm的凸形百葉窗型鰭片，造成其熱傳性 能降低的主要原因是低雷諾數時，鰭片波形角掩蔽區域內的氣流無法 排放至主流，導致熱邊界層受到抑制，無法充分發展。 根據本實驗所得鰭片空氣側之j、f因子，經多重線性迴歸方法分 別求得凸形百葉型與波浪型鰭片的j與f因子的經驗式，可做為小型空 調機冷凝器尺度率輔助設計程式中所需的熱傳與壓降係數。藉由本文 所發展的電腦輔助設計程式，於程式執行時鍵入冷凝器鰭管型式與參 數、額定熱傳量，然後設定空氣側是壓力降範圍，藉由輔助設計程式 的演算，可求得其所需的熱傳面積及最佳的冷凝器尺寸大小。

In this research report, seven convex-louver fin condensers and seven wavy fin condensers with indentical tube diameter (Do), longitudinal and transverse tube pitch (Pl, Pt) but differential numbe rof tube rows (N) and fin pitch (Fp) were tested and analyzed for the heat transfer and pressure drop characteresties. Results are presented with the Colburn j factor and Fanning factor f against Reynolds number (ReDc) based on the tube collar diamater (Dc) in the range of 300 to 8,000. Experimental datas show that the effect of the number of tube rows does not influence the f factor for both convex-louver and wavy fin geometry when ReDc＞1,000 and the effect of Fp＝2.54mm on j factor variation is also very small. For the case of Pf＝1.21mm, the j factors decrease with the increasing number of tube rows when ReDc＜2,000 but the effect of Fp is negeligible when ReDc＞2,000 for the reason that the effect of tube rows starts to influence the air passing the condenser. The vortices are weakened so that the fin pitch might not disturb the j factors anymore. This study compare not only the performances of the j and f factors but also the performances of area and volume. The results show that the performace of convex-louver fin is higher than that of wavy fin with the expection that for the case at lower Reynolds number with 4-row coils with Fp＝1.21mm for convex-louver fin. The reason of j factor drop off is that a recirculation region will be established in the "sheltered" area formed by the apex angle of the chevron, and that the trapped fluid in the "sheltered" area often can not eject to the main stream at low Reynolds number so that the thermal boundary layer will not fully develope. The correclations of j and f factor for convex-louver and wavy fin geometries are abtained for condenser sizing problems with the pattern and parameters of the heat exchanger and the rate of heat transfer and air side pressure drops, the program can give the heat transfer area and the size of the condenser.