光滑與鰭片平板之滴淋蒸發實驗

本研究探討冷媒R134a與R-141b在光滑表面與兩種鰭片表面於平板上的薄膜蒸發熱傳性能。探討的主要參數範圍為平板的傾斜角度（10o~40o）和熱通量（17.9~45.7kW/m2），R-134a液態冷媒滴淋溫度至測試室的溫度約在10.5+-0.5oC之間，而R-141b的測試溫度約在15.5+-0.5oC。本研究的數據發現，傾斜平板的角度，對R-134a能有效的增強熱傳效果，傾斜角度在20o~30o左右的平均蒸發熱傳係數最高，而R-141b傾斜角度則沒有影響。本研究的鰭片表面之鰭片間距為0.6mm，鰭片厚度為0.3mm，鰭片高度分別為0.3mm與0.5mm。R-134a實驗中，鰭片高度0.5mm的平均蒸發熱傳係數是光滑表面的2~4倍，而鳍片高度0.3mm的平均蒸發熱傳係數是光滑表面的2.5~4.8倍。R-141b的實驗中，鰭片高度0.3mm的平均蒸發熱傳係數是光滑表面的1.5~1.8倍。本研究並以高速攝影機進行滴淋蒸發的流場觀測，拍攝時每秒400張，快門速度為1/400秒，發現熱通量的增加，使平板表面產生沸騰的情形漸趨頻繁，使得熱傳機制隨著熱通量的改變，而由純粹的蒸發熱傳變為蒸發熱傳與核沸騰兩種效應並行的狀態，影響熱傳效率。而實驗中還發現，氣泡的方向是隨著冷媒向下流動，與滿液式蒸發器有很大的不同。本論文將文獻中的經驗公式與實驗值比較，發現文獻中大部份的經驗公式皆未考慮到核沸騰的影響而低估熱傳係數，而考慮到核沸騰效應的經驗公式卻高估了其結果，因此本研究以實驗值利用回歸分析提出經驗公式，除了部分數據因實驗時滴淋條件不佳以外，預測平板薄膜蒸發的其他數據，誤差小於20%。

關鍵字

滴淋薄膜蒸發；傾斜角度；核沸騰

並列摘要

This thesis discusses the effect of surface geometries and inclination angle on the falling film evaporation performance. Falling film evaporation experiments were conducted on a smooth plate and two finned plates using refrigerant R-134a at 10.5+-0.5 oC system temperatures, and R-141b at 15.5+-0.5 oC. The fin pitch = 0.6 mm and the fin thickness = 0.3 mm for both finned surfaces. The fin height is 0.5 mm for Fin-A, and 0.3 mm for Fin-B. All plates are tested with an inclination angle between 10o and 40o, and the heat fluxes are varied between 17.9 and 45.7 kW/m2. The local heat transfer coefficients are measured at four different heights, and the falling film flow distribution is observed through a sight glass. The test results showed that the falling film evaporation heat transfer coefficient increases as the heat flux increases. For the finned surfaces, the heat transfer coefficient increases as the inclination angle increases. However, the inclination angle showed negligible effect on the smooth surfaces for both R-134a and R141b. The finned surface yields better falling film evaporation heat transfer performance than the smooth surface for R-134a an R141b. For the R-134a, the Fin-A surface, having 0.5 mm fin height, yields 2~4 times enhancement of the heat transfer coefficient over the smooth surface, and the Fin-B surface enhanced the heat transfer by 2.5~4.8 folds. For the R-141b, the Fin-B surface yields 1.5~1.8 times heat transfer coefficients of the smooth surface. The visualization observation showed that bubble nucleation is more pronounced on the finned surface than the plain surface. The enhancement of the falling film evaporation is mainly attributed to the bubble nucleation effects on the finned surfaces.

並列關鍵字

Falling film evaporation ； inclination angle ； boiling

參考文獻

[15] 簡良翰與鄭全宏，「噴淋式蒸發器之熱傳性能預測新模型」，冷凍與空調期刊，33期，頁75-84，民94。

[1] Chun, K. R. and Seban, R. A., “Heat Transfer to Evaporating Liquid Films,” Journal of Heat Transfer,93,pp.391-396, 1971.

[3] Alhusseini, A. A., Tuzla, K.; Chen, J. C., “Falling Film Evaporation of Single Component Liquids,” Int. J. of Heat and Mass Transfer, Vol. 41, n 12, Jun, pp 1623-1632, 1998.

[4] Chyu, M. C., and Bergles, A. E, “An Analytical and Experimental Study of Falling-Film Evaporation on a Horizontal Tube” ASME J. of Heat Transfer, Vol. 109, pp. 983-990, 1987.

[5] Owens, W. L., “Corrlation of Thin Film Evaporation Heat Transfer Cofficients for Horizontal Tubes,” ASME Paper No.78-WA/HT-67, 1978.

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光滑與鰭片平板之滴淋蒸發實驗

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