下加熱面之臨界熱通率與其熱傳現象為近年熱流研究探討之重點之一。其應用範圍遍及多種工業，如核能、電子設備散熱冷卻、燃料電池、煉鋼及化學反應槽作動。許多研究發現下加熱式之臨界熱通率小於上加熱式之臨界熱通率，肇因於氣泡因浮力與力平衡之影響，容易累績於下加熱面上，造成臨界熱通率(CHF)提早發生。下加熱面沸騰熱傳是一種特殊的熱傳現象，其特徵與上加熱式熱傳差異極大，起因是核沸騰熱傳與臨界熱通率發生之物理機制造成之差異。沸騰過程中，下加熱傳面所產生之氣泡因為浮力與力平衡因素導致氣泡不斷在熱傳表面累積，且表面過熱度也不斷增大，最終於表面形成氣膜並觸發臨界熱通率現象。許多工業生產、製造與應用皆會產生此種下加熱面現象；如核能工業可能之應用時機，像西屋公司設計之進步型輕水壓水式核能機組AP1000之熔融物保封暨反應爐壓力槽外壁移熱防熔穿系統In Vessel Retention - External Reactor Vessel Cooling (IVR-ERVC)之相關應用。此時反應爐壓力槽底部外壁之冷卻水沸騰熱傳移熱能力變得相當重要，其能順利移除壓力槽底部內壁承受之熔融爐心衰變熱，以保障壓力槽底部不被熔穿之嚴重情況發生。IVR-ERVC所用之不同注水距離亦會影響熱傳表面之臨界熱通率限值。本研究主要探討IVR-ERVC設計中不同冷卻水進口與加熱表面距離與冷卻水不同流率下，壓力槽底部外表面發生臨界熱通率限值之比較；同時也針對不同冷卻水除氣條件對於下加熱面臨界熱通率值影響之比較。由實驗結果中得知，越短的冷卻水進口距離或冷卻水流率越大時，臨界熱通率值越高，其移熱效果越佳，反之亦然。除氣主要為防止水中含有之空氣於次冷態沸騰中提早因表面過熱度效應，所濾出之空氣於下加熱表面產生氣泡，此現象會引起臨界熱通率之提早發生；實驗結果也顯示除氣時，加熱溫度越高，除氣效果越好，其臨界熱通率值亦隨著除氣溫度增加而上升。最終，本研究也探討下加熱面不同傾斜角度對於熱傳表面之臨界熱通率值之影響，並推導出一套準確且實用之依傾斜角度為函數之臨界熱通率預估經驗公式。

The critical heat flux (CHF) of downward facing heating process and its heat transfer has been a hot study topic heat flow in recent years. The application has been widely applied into a variety of industries, including nuclear, cooling of electronic devices, fuel cells, steel-making and chemical reactors. Many studies have found that the CHF of downward facing heating is lower than that of upward facing heating for the reason that of bubbles are easily accumulated at the heating surface due to buoyancy and gravity, causing premature CHF. Downward facing heating is a special heat transfer phenomenon; the feature is very different from traditional upward facing heating, especially nuclear boiling heat transfer and the process of CHF. The bubbles are forming constantly at the heating surface during downward facing heating because of buoyancy and gravity, and the surface is superheated, leading to a layer of gas film and CHF. This phenomenon may exist during many industrial production, manufacturing and application, such as reactor core boiling and severe accidents related to heat transfer of coolant and its mitigation system - In Vessel Retention External Vessel Cooling (IVR-ERVC) in nuclear industry. Under this condition, coolant and its heat transfer capability become very critical. In addition, different fluid properties and water inlet distance will also affect the CHF at the heat transfer surface. This study aims to investigate the CHF at different distances between coolant inlet and heating surfaces, different coolant inlet flow rates, and how different degas coolant conditions affect the CHF. The result shows that shorter coolant inlet distance or larger inlet flow rate leads to better heat transfer effect, and vice versa. The purpose of degas is to prevent air in the water from sub-cooled boiling and premature purge boiling bubbles on the surface, causing premature CHF. The result also indicates that the higher the temperature for degas is, the better the degas effect is, and the CHF also increases positively. Finally, the heating surface with inclining effects are also discussed and a new CHF model with downward facing inclined angles has been derived of this study.

[1] Westinghouse,“AP-1000 Design Control Document, Revision 17”, 2008.
[2] Westinghouse main website : http://www.westinghousenuclear.com/
[3] Kutateladze, S.S., "On the Transition to Film Boiling under Natural Convection," Kotloturbostroenie, No. 3, pp. 10-12, 1948.
[4] Katto, Y., Kosho, Y., "Critical Heat flux Of Saturated Natural Convection Boiling In A Space Bounded By Two Horizontal Co-Axial Disks And Heated From Below", Int. J. Multiphase Flow, 219, 1979.
[5] Theofanous, Syri, "The Coolability Limits Of A Reactor Pressure Vessel Lower Head", Proceedings of NURETH-7, Saratoga Springs, NY, NUREG/CP-0142, Vol 1, 627-647, 1995.
[6] Theofanous, S, "The Coolability Limits Of A Reactor Pressure Vessel Lower Head", Nucl. Eng. Des., 169, 59-76, 1997.
[7] Theofanous, "In-Vessel Coolability And Retention Of A Core Melt", Nucl. Eng. Des. 169, 1-48, 1997.
[8] S. Rouge, “SULTAN Test Facility for Large-Scale Vessel Coolability in Natural Convection at Low Pressure”, Nuclear Engineering and Design, 169, 185, 1997.
[9] S. Rouge, I. Dor, and G. Geffraye, "Reactor Vessel External Cooling for Corium Retention SULTAN Experimental Program and Modeling with CATHARE Code", Proc. OECD/CSNI Workshop In-Vessel Core Debris Retention and Coolability, NEA/CSNI/R (98) 18, Garching, Germany, March 3– 6, 1998.
[10] F. B. Cheung, K. Haddad, and Y. C. Liu, "Critical Heat Flux（CHF）Phenomena on a Downward Facing Curved Surface", NUREG/CR-6507, The Pennsylvania State University, 1997.
[11] F. B. Cheung and Y. C. Liu, "Critical Heat Flux (CHF) Phenomenon on a Downward Facing Curved Surface: Effects of Thermal Insulation", NUREG/CR-5534, The Pennsylvania State University, 1998.
[12] F. B. Cheung, J. Yang, M. B. Dizon, J. L. Rempe, K. Y. Suh, and S. B. Kim, "Scaling of Downward Facing Boiling and Steam Venting in a Hemispherical Heated Channel" International Journal of Transport Phenomena , 6, 81, 2004.
[13] X.F Peng, Y.J Huang, D.J Lee, " Transport Phenomenon Of A Vapour Bubble Attached To A Downward Surface", International Journal of Thermal Sciences, Volume 40, Issue 9, October, Pages 797–803, 2001.
[14] Masanori Monde *, Yuhichi Mitsutake，"Critical heat flux of natural circulation boiling in a vertical tube Effect of oscillation and circulation on CHF"，International Journal of Heat and Mass Transfer, 45, 4133–4139, 2002.
[15] Yong Hoon Kim, Seong Joong Kim, Sang Woo Noh, Kune Y. Suh，"Critical Heat Flux in Narrow Gap in Two-dimensional Slices under Uniform Heating Condition", Transactions of the 17th International Conference on Structural Mechanics in Reactor Technology (SMiRT 17)Prague, Czech Republic, August 17 –22, 2003.
[16] J.J. Kim, Y.H. Kim, et al. " Boiling Visualization And Critical Heat Flux Phenomena In Narrow Rectangular Gap", NTHAS4, Sapporo, Japan, November 28 - December 1, 2004.
[17] Shigefumi, Nishio, Hiroaki Tanaka, "Visualization Of Boiling Structures In High Heat–Flux Pool-Boiling", International Journal of Heat and Mass Transfer 47, 4559–4568, 2004.
[18] Alicia H. Howard, Issam Mudawar, "Photographic study of pool boiling CHF from a downward-facing convex surface", International Communications in Heat and Mass Transfer, 35, 793–799, 2008.
[19] Hongxing Yu, Yapei Zhang, Guanghui Su, Suizheng Qiu, Wenxi Tian ,"A theoretical CHF model for downward facing surfaces and gaps undersaturated boiling", International Journal of Multiphase Flow, 45, 30–3, 2012.
[20] Q.T. Pham, T.I. Kim, S.S. Lee, S.H. Chang, " Enhancement of critical heat flux using Nano-fluids for In vessel Retention External Vessel Cooling", Applied Thermal Engineering, 35, 157-165, 2012.
[21] ASME PTC 19.1-2005, "Test Uncertainty", The American Society Of Mechanical Engineers, 2005.
[22] ASME V&V 20-2009, "Standard for Verification and Validation in Computational Fluid Dynamics and Heat Transfer", The American Society Of Mechanical Engineers, 2009.