沸騰具有絕佳的熱傳能力，在對流熱傳機制中，沸騰亦表現出極高的熱傳係數。然而，向下加熱式表面之沸騰熱傳，由於熱浮力效應受到表面之限制，其熱傳能力遠不及一般向上加熱式之情況。本研究設計並建置了一小尺寸實驗模組，搭配一套實驗系統與環路，旨在探討水平向下加熱式表面之沸騰熱傳現象，並深入研究表面附近冷卻水的對流熱傳機制，以及次冷態水流造成之影響。
本研究記錄了各組實驗條件下之加熱表面溫度及熱通率數值，並將實驗過程中表面之情況以高解析度攝影機全程錄影，以對向下加熱式沸騰熱傳機制做進一步的分析。再者，本論文亦呈現出不同實驗參數影響下，熱傳能力探討及汽泡動態分析之結果，並展示了數張具有代表性的加熱表面汽泡及冷卻水之照片，以佐證其沸騰熱傳現象。
本研究發現向下加熱式沸騰熱傳表面產生之汽泡會停滯於表面無法脫離，此乃造成其熱傳能力遠低於向上加熱式沸騰之主因。然而次冷態水流之沖擊，會使得加熱表面附近冷卻水之溫度梯度提升，有助於提高其熱通率，且次冷態水流之流量越大，對其熱通率提升之效果越顯著。此外，在核沸騰(nucleate boiling)區域，汽泡之快速生成、冷凝及移動等作用，會使冷卻水產生紊流而擾動周圍之流場及溫度分布，亦有助於提升向下加熱式表面之沸騰熱傳能力。

Boiling heat transfer has a high heat removal capability in convective cooling. However, the heat removal capability of downward-facing boiling is significantly worse than that of upward-facing cases due to the confined buoyancy effect. In the present study, a small-scale test facility had been established to investigate the local phenomena of boiling heat transfer under a downward-facing horizontal heated surface with impinging coolant flow.
In this study, the surface temperature, heat flux information and several specific snapshots of bubbles are taken down throughout the boiling processes for detailed investigation. Furthermore, the heat transfer rate evaluation, bubble dynamics analysis and pictures regarding the heat transfer mechanisms are performed under the present condition.
It is observed that bubbles are confined under the downward-facing heated surface, which causes a worse heat transfer capability than upward-facing boiling. Nevertheless, the impinging coolant flow is found to raise the temperature gradient formed by the heated surface, hence the heat transfer rate increases with an increase of coolant flow rate. In addition, during nucleate boiling, it is discovered that the formation, condensation and motion of bubbles induce turbulent wakes and therefore enhance the heat transfer rate.

1. 潘欽，沸騰熱傳與雙相流，國立編譯館，台北市，2001。
2. G. H. Su, Y. W. Wu and K. Sugiyama, “Natural Convection Heat Transfer of Water in a Horizontal Gap with Downward-Facing Circular Heated Surface”, Appl. Thermal Eng., vol. 28, pp. 1405-1416, 2008.
3. F. B. Cheung and K. H. Haddad, “A Hydrodynamic Critical Heat Flux Model for Saturated Pool Boiling on a Downward Facing Curved Heating Surface”, Int. J. Heat Mass Transfer, vol. 40, pp. 1291-1302, 1997.
4. A. H. Howard and I. Mudawar, “Photographic Study of Pool Boiling CHF from a Downward-Facing Convex Surface”, Int. Comm. Heat Mass Transfer, vol. 35, pp. 793-799, 2008.
5. G. H. Su, Y. W. Wu and K. Sugiyama, “Subcooled Pool Boiling of Water on a Downward-Facing Stainless Steel Disk in a Gap”, Int. J. Multiphase Flow, vol. 34, pp. 1058-1066, 2008.
6. D. Qiu and V. K. Dhir, “Experimental Study of Flow Pattern and Heat Transfer Associated With a Bubble Sliding an Downward Facing Inclined Surfaces”, Exp. Therm. Fluid Sci., vol. 26, pp. 605-616, 2002.
7. J. Yang, F. B. Cheung, J. L. Rempe, K. Y. Suh and S. B. Kim, “Critical Heat Flux for Downward-Facing Boiling on a Coated Hemispherical Vessel Surrounded by an Insulation Structure”, Proc. Int. Congress on Advances in Nuclear Power Plant (ICAPP ’05), Seoul, Korea, 2005, pp. 139-146.
8. G. Dewitt, T. Mckrell, J. Buongiorno, L. W. Hu and R. J. Park, “Experimental Study of Critical Heat Flux with Alumina-Water Nanofluids in Downward-Facing Channels for In-Vessel Retention Applications”, Nucl. Eng. Technol., vol. 45, pp. 335-346, 2013.