隨著科技的進步，高功率LED使用的範圍越來越廣，在不同工作環境下所會遇到的散熱問題也更需要去各別討論。汽車LED頭燈使用的LED主要以高功率為主，且因使用環境為封閉空間且位於汽車引擎旁，故對於高功率LED來說散熱模組更是不可或缺。由於LED散熱模組在封閉環境下工作會產生溫度分層之散熱問題，而一般LED散熱模組設計之方向並無法改善封閉空間之散熱問題，故研究利用煙囪來改善此散熱問題。
本研究設計下煙囪與LED散熱模組進行結合，利用下煙囪之幾何來改善封閉空間之散熱問題，並以擋板設計、增加下煙囪出口寬度來提升下煙囪之效果，但由模擬分析的結果知道下煙囪之幾何設計並不能產生明顯之煙囪效應，故再由煙囪效應文獻得知利用延伸下煙囪出口高度可產生煙囪效應，使得引進更多的空氣進入煙囪裡達到提升散熱效果的目的，讓LED散熱模組在與煙囪結合後能夠使LED溫度下降近4.6℃。最後將煙囪散熱模組運用於H牌汽車近光燈上，利用模擬分析來與原近光燈散熱模組做比較，可以發現煙囪散熱模組確實改善了原近光燈的散熱模組不能解決的溫度分層問題，並且LED溫度還比原散熱模組的LED溫度低3.8~4.6℃，且煙囪散熱模組之散熱鰭片的散熱面積比原近光燈的散熱模組還少20％，故本研究所建立的LED煙囪散熱模組設計，於封閉環境下能有更好的散熱效果及實用性。

LED vehicle headlamps consist largely of several high-power LEDs with the thermal module inside an enclosed space, and locate near the car engine which represents a high-temperature environment. Certainly, the enclosed space and engine compartment make the thermal management of LED headlamp becoming an indispensable and challenging task. Obviously, the temperature stratification inside this bounded region is an unfavorable phenomenon for the natural-convection thermal module and becomes the topic of this thesis. This study utilizes the chimney effect to achieve a more uniform temperature distribution and thus enhance the heat dissipation ability. First of all, an enclosed volume with a similar size like vehicle headlamp is selected for executing the parametric study on the chimney geometry. Several important results are obtained and verified via both numerical and experimental means on the chimney design. Among these alternatives, enlarging the chimney exit area, extending the length of chimney, and adding guide plate between sink and chimney are the most effective strategies to eliminate the undesirable temperature stratification and induce more airflow to the fin passages. Also, a significant 4.6℃ decrease on LED junction temperature is found on the best design of chimney-enhanced heat sink.
Thereafter, these approved guidelines are applied to design the chimney-enhanced thermal module for a commercial LED headlamp. Besides, the numerical flow visualization is performed and observed carefully to check its influence on the thermal dissipating ability. As a result, the temperature-stratification phenomenon is largely improved and a range of 3.8~4.6℃ reduction on LED temperatures are identified for this innovative chimney thermal module, which has only 80% cooling area compared with the original thermal design. In conclusions, this work not only successfully demonstrates the cooling enhancement of chimney design, bust also provides a systematic scheme and guidelines to design an appropriate chimney-enhanced thermal module for the enclosed-space applications.

[1] 黃國長, "高亮度 LED 之熱模擬分析探討", 碩士學位論文, 國立清華大學工程與系統科學系, 2007年。
[2] Veeco Investor Presetation ,
http://www.sec.gov/Archives/edgar/data/103145/000110465907033846/a07-11059_2ex99d2.htm
[3] Philips Lumileds Lighting, U.S. LLC., "Power Light Source LUXEON III Emitter", Technical Datasheet DS45, www.lumileds.com.
[4] 陳智禮，揚鈞杰，"高功率LED的散熱處理"，工業材料雜誌，231期，139-145頁，2006年。
[5] 王崇岳，“桌上型電腦散熱模組之參數分析”, 碩士學位論文, 中原大學機械工程系, 2005。
[6] Elenbaas, W., "Heat Dissipation of Parallel Plates by Free Convection", Physica, Vol. 9, pp. 1-28, 1942.
[7] Kraus, A.D., "Analysis and Evaluation of Extended Surface Thermal System", Hemisphere Published, 1982.
[8] Morrison, A. T., "Optimization of Heat Sink Fin Geometries for Heat Sinks in Natural Convection", InterSociety Conference on Thermal Phenomena, pp. 145-148, 1992.
[9] Morega, A. M. and Bejan, A., "Plate Fins with Variable Thickness and Height for Air-Cooled Electronic Modules", Int. J. Heat Mass Transfer, Vol. 37, pp. 433-445, 1994.
[10] Bejan, A., Tsatsaronis, G., and Moran, M., "Thermal Design and Optimization", Wiley, pp. 241-256, 1996.
[11] Bar-Cohen, A., Iyengar, M., and Kraus, A. D., "Design of Optimum Plate-Fin Natural Convective Heat Sinks", Journal of Electronic Packaging, Vol. 125, pp. 208-216, June 2003.
[12] Rahnama, Mohammad and Farhadi, Mousa, "Effect of Radial Fins on Two-Dimensional Turbulent Natural Convection in a Horizontal Annulus", International Journal of Thermal Sciences, Vol. 43, pp. 255-264, 2004.
[13] Haaland, S. E., and Sparrow, E. M., "Sloutions for the Channel Plume and the Parallel-Walled Chimney , "Numerical Heat Transfer, Vol. 6, pp. 155-172, 1983.
[14] Asako, Y., Nakamura, H. and Fagfri, M., "Natural Convection in a Vertical Heated Tube Attached to a Thermally Insulated Chimney of a Different Diameter, "Journal of Heat Transfer, Vol. 112, pp. 790-793, 1990.
[15] Straatman, A. G., Tarasuk, J. D., and Floryan, J. M., "Heat Transfer Enhancement from a Vertical Isothermal Channel Generated by the Chimney Effect , "ASME J. Heat Transfer, Vol. 115, No. 2, pp. 395-402, 1993.
[16] Fisher, T. S., Torrance, K. E. and Sikka, K. K., "Analysis and Optimization of a Natural Draft Heat Sink System, ”IEEE Transaction on Components, Packaging, and Manufacturing Technology-Part A, Vol. 20, No. 2, pp. 111-119, 1997.
[17] Fisher, T. S. and Torrance, K. E., "Free Convection Limits for Pin-Fin Cooling, " Journal of Heat Transfer, Vol. 120, pp. 633-640, 1998.
[18] Fisher, T. S. and Torrance, K. E., "Experiments on Chimney-Enhanced Free Convection, " Journal of Heat Transfer, Vol. 121, pp. 603-609, 1999.
[19] Fisher, T. S., Torrance, K. E. and Thrasher, W. W., "Experiments on Chimney-Enhanced Free Convection from Pin-Fin Heat Sink, " Journal of Electronic Packaging Transactions of ASME, Vol. 122, No. 4, pp. 350-355, 2000.
[20] Churchill, S. W. and Chu, H. H. S., "Correlating Equations for Laminar and Turbulent Free Convection from a Vertical Plate", Int. J. Heat Mass Transfer, Vol. 18, pp. 1323-1329, 1975.
[21] Patankar, S. V. and Spalding, D. B., "A Calculation Procedure for Heat Mass and Momentum Transfer in Three-Dimensional Parabolic Flows", International Journal of Heat Mass Transfer, Vol. 15, pp. 1787-1806, 1972.