四、 結果與討論
4.2 濕盤管
實驗結果依照管排數為：2 排、4 排、8 排和 12 排，分別探討其熱傳係數以及壓降，為
了比較乾濕條件的影響，亦會將先前所做的結果[66]標上，如圖 4-2-1 所示。如預期般 的，熱傳係數以及壓降都會隨著面速的上升而增加。在除濕的條件下，當管排數為 2 時，熱傳係數相對地與鳍片截距較無關係；隨著管排數上升至 4，可以發現：當鰭片截 距較小時，熱傳係數會有明顯地上升；但若是管排數上升至 8 或 12，又會呈現相反的 走向，也就是隨著鰭片截距的減小，熱傳係數反而會隨之下降。若依乾溼條件不同，則 在管排數為 2，熱傳係數整體而言差異性並不大；但在管排數大於 2，濕式的熱傳係數，
很明顯地較乾式的來得大，尤其是當鰭片截距減小時，很更加地顯著。另一個值得注意 的點是：波浪型鰭片的波浪數，是與管排數的多寡成正比的；也就是說，管排數的增加，
會造成鰭片波浪數的上升，提高波浪形狀對於流場的影響程度。事實上，若是將流體通 過一個波浪型通道，對於沿著流道的熱傳係數而言，影響的變數相當多。像在波谷以及 吸力側(suction side)，可能就會有較小的熱傳係數。而這個理論，對於一個典型的波浪 型鰭片，在完全乾燥的情況下，也是適用的。關於這部分，McNab et al.[67]已經以數值 模擬的方式得到一些結論。他們在報告中指出：當流體通過波浪狀通道的波谷時，會有 不少流體從主流方向分離，進而形成二次流，如圖 4-2-2(a)所示。Hwang et al.[68]也發 表過類似的結論，是同步以實驗和數值方法，模擬在波浪型通道內的流體的流動現象，
以及熱傳分佈。當管截距減小時，反而會增加流道的幾何形狀對於流場的影響，流體在 通道內的流動示意圖，如圖 4-2-2(a)以及圖 4-2-2(b)所示。從上述的兩張圖中，很明顯地 可以看到：在流經通道的轉彎處，由於流體受到阻礙，會產生分離現象、迴流區以及二 次流區。這個圖也充分說明了：為什麼在波浪型峰谷附近，會產生較低的熱傳係數。總 而言之，在乾燥的條件下，對於管排數較大而言(N=8 或 12)，熱傳係數會隨著管截距的 減小而下降。
另外，對於在除濕條下的波浪型通道而言，由於鰭片表面的溫度低於露點溫度，所 以在表面上會有凝結水的產生。Lin et al.[69]就從致力於這方面的研究，並將其結果予 以可視化，實驗圖如圖 4-2-2(c)所示。圖中顯示出：在波浪型鰭片的峰谷區，以及吸力 側附近的表面，會遍佈水滴；也就是說，會有充分的水蒸氣在這附近凝結。從這點來看，
水蒸氣在表面凝結成水滴後，可能會造成氣流向這一帶流動，因此，分離現象或二次流
所造成的影響會被降低。此結果會使得，與乾燥的氣流比較起來，較佳的熱傳現象會發 生在除濕的條件下。因此，在管排數較大(8 排或 12 排)的情況下，管截距對於濕式的熱 傳係數影響，會比乾式的要來得小。
為了瞭解形狀的影響程度，參照先前平板型鰭片的結論[70，71]，該鰭片與本研究 所用鰭片的幾何尺寸相同(如：管徑，以及橫、縱向管間距等)，並繪於同一張圖上，如 圖 4-2-3 所示。從圖中可以發現：對於平板型鰭片而言，在濕式的條件下，顯熱熱傳系 數與乾式的相比，呈現差不多或是較小的趨勢。而這個現象，在擁有較小的管徑，以及 橫、縱向管截距的平板鰭片上(P
t
= 25.4 mm, Pl
= 22 mm, Dc
= 10.3 mm, [72,73])，也可以 觀察得到。這是因為：平板型並不像波浪型一樣，會在通道的轉彎處形成二次流。所以，即便是表面上有水滴的形成，也無法引導氣流改善熱傳。不僅僅是如此，表面上液滴的 形成，反而會造成氣流與鰭片間的阻抗增加，因此，平板型在濕式的條件下，熱傳係數 反而會有下降的趨勢。
除了上述的部分，另一個值得注意的就是壓損的部分。如圖 4-2-3 所示，在進口條 件同為濕度 85%的情況下，平板型鰭片在濕式的狀況，壓損普遍高於乾式約 80-90%；
然而，若是對於波浪型鰭片而言，這個數值頂多為 50%或是在更低。這是與波浪型的幾 何種類有關係。關於波浪型鰭片，在實際應用上可分成兩種，一種為”魚脊型”(herringbone wavy fin)；而另一種為”平滑型”(smooth wavy fin)。Sparrow and Hossfeld [72]就曾經針對 波浪型鰭片尖端突起的部分做研究，發現若是改用圓形的突起，會明顯地降低這一帶的 壓損。最近也有 Islamoglu [74]得到相同的結論：若是將波浪型鰭片的峰谷尖端部分，
做成較為平滑的，則可以大幅地降低壓損。從圖 4-2-2(c)中，可以很清楚地看到：本研 究所使用的為”魚脊型”鰭片，且由於在除濕條件下，鰭片表面佈滿液滴，故可以推測液 滴也是影響壓損的重要原因之一。凝結的水滴在魚脊型的鰭片上，會使得峰谷尖端的部 分較為平滑，進而降低壓損。雖然水在表面上會使得粗造度上升，使得摩擦壓損上升；
但此現象會使得通道較為平順，降低壓損。這兩個效應結合的結果，就是會使得波浪型 鰭片在除濕的條件下，相較於完全乾燥的條件，壓損仍會有限地增加；若與平板型相比，
其值會小得許多。
五、 結論
本研究以實驗方法進行大管徑波浪型鰭片，分別在乾、濕兩種條件下，探討其熱 傳表現，以及可能影響的原因；並試著建立相關資料庫。主要結果如下：
5.1 乾盤管
(1)管排數對於熱傳性能的影響是相當明顯的。當管排數上升時，熱傳性能隨之下 降，由其是在低雷諾數時，這現象更加明顯。事實上，在管截距約為 1.7 ㎜，當管排數 由 1 上升至 16，熱傳性能大約會下降 85%左右。
(2)至於管排數對於壓損的影響，則會遇到出乎預料的狀況。
(3)在管排數為 1 或 2 時，鰭片截距對於熱傳性能的影響相對地小。然而，當管排 數增加時，熱傳性能會很明顯地下降；若是在鰭片截距較大的情形下，則熱傳性能與壓 損普遍也較大。
5.2 濕盤管
(1)在除濕的條件下，管排數為 2 時的熱傳係數，相對地與管截距較無關係係。然 而，可以發現到：在管排數為 4、較小的管截距時，熱傳係數會稍微地上升。但在管排 數為 8 或 12 時，這趨勢是恰恰相反的。
(2)若根據鰭片表面乾濕條件的影響，熱傳係數在濕式時，通常會較乾式佳；尤其 是在管排數增加或是管截距減小的情況下，會更加地明顯。相較之下，對於平板型鰭片 而言，濕式的熱傳係數會稍微較乾式來得低。
(3)濕式的壓降很明顯地較乾式大得許多。然而，若觀察壓降在乾濕條件下的差值，
則可以發現：波浪型鰭片會較平板型來得小許多。
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