本論文研究主要改進本實驗室先前研發的微通道自然循環迴路，冷凝段部份以致冷晶片結合冷凝銅流道替代一公升的冷凝水槽，以減少大幅體積，且只需要通電即可達到冷卻的效果。為達實際應用於電腦中央處理器(CPU)的散熱，本研究根據現行CPU尺寸修正微流道蒸發測試段之加熱底面積，從先前研究的10.5x10.5mm2更改為31x31mm2。本研究在加熱基底上製作共78條微流道，每一通道蝕刻深度為300μm，並採用本實驗團隊的研究成果，以漸擴微流道設計來有效抑制環路內雙相沸騰時所產生的不穩定性，同時考量此多重平行微流道的總加熱表面積(每一流道包含一底部面積加上兩個側面面積)必須大於加熱底面積，因此漸擴流道的設計只存在於微流道的前半段，亦即微流道入口端寬度150μm，漸擴至離入口16mm處、寬度為300μm；之後為均勻截面至出口端、寬度維持在300μm。爰此，平行微流道的總表面積與基底面積的比值為2.1。
　　本研究以99.8%的乙醇作為實驗工作流體，以上述的改良迴路進行自然循環實驗。實驗結果顯示利用致冷晶片搭配冷凝銅流道將增加自然循環的流阻，造成移熱能力下降。在填滿工作流體(填充比100%)的情況下，實驗觀察到蒸發測試段出口的溫度高於飽和溫度，推測此時的自然循環內部壓力可能高於一大氣壓，這可能肇因於迴路填滿工作流體，沸騰時因液體變成氣體造成體積大幅膨脹，但無足夠的成長空間促使流體壓力上升，進而抑制自然循環使移熱能力下降。因此，本研究再以不同的填充比進行自然循環實驗，觀察其升流段流譜、各溫度變化、自然循環的不穩定性等，發現不同填充比對本自然循環迴路特性有顯著影響，最佳的填充比為90%。
　　為提升自然循環迴路的移熱能力，本研究將升流段與降流段由原本的4mm擴大孔徑至8mm，以進行自然循環實驗。實驗結果顯示擴大孔徑，將使升流段的液膜太厚，冷凝銅流道的流阻又使液體不易流通，造成液體回流的現象，讓整個自然循環移熱能力下降。
　　綜合以上實驗結果，致冷晶片結合冷凝銅流道的設計可能不適於此微通道自然循環迴路，必須進一步改良以達應用於CPU散熱的目標。

　　In order to reduce the large space of 1L condensing water tank in our previous microchannel natural circulation loop (NCL), this study employs thermoelectric cooler incorporating with cooling copper channels instead as the condensing section in this NCL to develop the cooling methodology for the electronics, such as Central Processing Unit (CPU). According to the real size of current CPU, the base area of microchannel evaporator in the present NCL is modified from 10.5x10.5mm2 to 31x31 mm2. Our previous studies had recognized that the divergent microchannels can significantly stabilize the two-phase microchannel NCL. In addition to increase the wall-to-base area ratio, the divergent design is only applied to the front section of all the 78 parallel microchannels with uniform depth of 300 μm. Accordingly, the width of each microchannel is diverging from 150 μm at the inlet to 300 μm at the location of 16mm from the inlet, and then with an uniform cross-section with a width of 300μm until the outlet. Thus, it will result in a wall-to-base area ratio of 2.1.
　　The 99.8% ethanol is adopted as the working fluid in the NCL. Its boiling temperature is about 78.4 ℃ at 1 atm. With a filling ratio of 100%, the experimental results show that thermoelectric cooler together with cooling copper channels may increase loop flow resistance and reduce heat removal capability. The temperature at the evaporator outlet is higher than the saturated temperature at 1 atm. It implies that the fluid pressure inside the loop is possibly higher than 1 atm. The higher pressure may be caused by the confined space for bubble growth after the boiling inception and thus affect the heat removal capability of NCL. Therefore, this study also investigates different filling ratios on the performance of the present NCL. The experimental results reveal that the filling ratio has a significant effect on the two-phase flow characteristics of this NCL. The optimum filling ratio is supposed to be about 90%.
　　In order to further improve heat transfer capacity of the NCL, this study enlarges the diameter of the riser and downcomer from 4mm to 8mm. However, the results show the counter-current flow appears in the riser due to the thicker liquid film deposited in the riser and the larger flow resistance existing in the cooling cooper channels. This will reduce the heat removal capability of this NCL.
　　Based on the above results, the thermoelectric cooler incorporating with cooling copper channel may not be suitable for this microchannel NCL. It needs a further improvement to increase the heat removal capability and meets the cooling requirement of CPU.

Laszlo B. Kish, “End of Moore’s law: thermal (noise) death of integration in micro and nano electronics”, ELSEVIER Physics Letters A, vol.305, pp.144-149, 2002
http://www.intel.com.tw/content/www/tw/zh/homepage.html (intel)
J.F. Tullius, R. Vajtai and Y. Bayazitoglu, A Review of Cooling in Microchannels, Heat Transfer Engineering, p.1-1, 2010
黃俊霖﹐漸擴微流道自然循環迴路之移熱能力提昇研究﹐碩士論文﹐國立清華大學﹐民國103年
C.T. Lu and C. Pan, “Stabilization of flow boiling in microchannel heat sinks with a diverging cross-section design,” Journal of Micromechanics and Microengineering, vol.18, doi:10.1088/0960-1317/18/7/075035, 2008
Z.J. Edel and A. Mukherjee, “Experimental investigation of vapor bubble growrh during flow boiling in a microchannel,” International Journal of Multiphase Flow, vol.37, pp.1257-1265, 2011
H.J. Lee, D.Y. Liu and S.C. Yao, “Flow instability of evaporative micro-channels”, International Journal of Heat and Mass transfer, vol.53, pp.1740-1749, 2010
W. Qu and I. Mudawar, “Measurement and prediction of pressure drop in two-phase micro-channel heat sinks,” International Journal of Heat and Mass Transfer, vol.46, pp.2737-2753, 2003
G. Wang, P. Cheng and H. Wu, “Unstable and stable flow boiling in parallel microchannels and in a single microchannel,” International Journal of Heat and Mass Transfer, vol.50, pp.4297-4310, 2007
D. Bogojevic, K. Sefiane, A.J. Walton, H. Lin, G. Cummins, “Two-phase flow instabilities in a silicon microchannels heat sinks, International Journal of Heat and Fluid Flow, vol.30, pp.854-867, 2009
C.J. Ho, Y.N. Chung and C.M. Lai, “Thermal performance of Al2O3/water nanofluid in a natural circulation loop with mini-channel heat sink and heat source,” Energy Conversion and Management, vol.87, pp.848-858, 2014
C.J. Ho, Y.N. Chen, F.J. Tu, and C.M. Lai, “Thermal performance of water-based suspensions of phase change nanocapsules in natural circulation loop with a mini-channel heat sink and heat source,” Applied Thermal Engineering, vol.64, pp.376-384, 2014
K.K. Kumar and M.R. Gopal, “Experimental studies on CO2 based single and two-phase natural circulation loops,” Applied Thermal Engineering, vol. 64, pp. 3437-3443, 2011
K. Fukuda and T. Kobori, “Classification of two-phase instability by density wave oscillation model,” Journal of Nuclear Science and Technology, vol.16, pp.95-108, 1979
L. Chen, B.L. Deng and X.R.Zhang, “Experimental investigation of CO2 thermosyphon flow and heat transfer in the supercritical region,” International Journal of Heat and Mass Transfer, vol.67, pp.202-211, 2013
L. Chen, B.L. Deng and X.R.Zhang, “Experimental study of trans-cirtical and supercritical CO2 natural circulation flow in a closed loop,” Applied Thermal Engineering, vol.59, pp.1-13, 2013
L. Chen, X.R. Zhang, B.L. Deng and B. Jiang, “Effects of inclination angle and operation parameters on supercritical CO2 natural circulation loop,” Nuclear Engineering and Design, vol.265, pp.895-908, 2013
V. Jain, A.K. Nayak, P.K. Vijayan, D. Saha and R.K. Sinha, “Experimental investigation on the flow instability behavior of a multi-channel boiling natural circulation loop at low-pressures,” Experimental Thermal and Fluid Science, vol.34, pp.776-787, 2010
S. Mukherjee, I. Mudawar, “Smart pumpless loop for micro-channel electronic cooling using flat and enhanced surfaces,” IEEE Transctions on Components and Packaging Technologies, vol.26, No.1, 2003
S. Mukherjee, I. Mudawar, “Pumpless loop for narrow channel and micro-channel boiling,” Journal of Electronic Packaging, vol.125, pp.431-441, 2003
C.T. Lee, “The effect of heater-to-riser configuration on the stability of a two-phase natural circulation system,” Journal of C.C.I.T, vol.41, No.2, 2012
M. Misale, P. Garibaldi, J.C. Passos and G.G.D. Bitencourt, “Experiments in a single-phase natural circulation mini-loop,” Experimental Thermal and Fluid Science, vol.31, pp.1111-1120, 2007
A.K. Nayak, M.R. Gartia and P.K. Vijayan, “An experimental investigation of single-phase natural circulation behavior in rectangular loop with Al2O3 nanofluids,” Experimental Thermal and Fluid Sience, vol.33, pp.184-189, 2008
D. Enescu, E.O. Virjoghe, “A review on thermoelectric cooling parameters and performance,” Renewable and Sustainable Energy Reviwes, vol.38, pp.903-916, 2014
A.K. Nayak, M.R. Gartia and P.K. Vijayan, “Thermal-hydraulic characteristics of a single-phase natural circulation loop with water and Al2O3 nanofluids,” Nuclear Engineering and Design, vol.239, pp.536-540, 2009
A.K. Nayak, P.P. Kulkarni and P.K. Vijayan, “Study on the transient and stability behavior of a boiling two-phase natural circulation loop with Al_2 O_3 nanofluids,” Applied Thermal Engineering, vol.31, pp.1673-1681, 2011
H.Y. Zhang, Y.C. Mui and M. Tarin, “Analysis of thermoelectric cooler performance for high power electronic packages,” Applied Thermal Engineering, vol.30, pp.561-568, 2010
R. Chein and G. Huang, “Thermoelectric cooler application in electronic cooling,” Applied Thermal Engineering, vol.24, pp.2207-2217, 2004
P. Naphon and S. Wiriyasart, “Liquid cooling in the mini-rectangular fin sink with and without thermoelectric for CPU,” International Communications in Heat and Mass Transfer, vol.36, pp.166-171, 2009
A.Y. Faraji, H.J. Goldsmid and A. Akbarzadeh, “Experimental study of a thermoelectrically-driven liquid chiller in terms of COP and cooling down period,” Energy Conversion and Management, vol.77, pp.340-348, 2014
Madou, M., Fundametals of microfabrication: the science of miniaturization. CRC, 2002
吳梃睿﹐微流道雙相自然循環迴路研究﹐碩士論文﹐國立清華大學﹐民國102年
B.E. Poling, J.M. Prausnitz, J.P. O’Connell, (Eds), The properties of gases and liquid, McGraw-Hill, New York, 2001
B.R. Fu, M.S. Tsou and C. Pan, “Boiling heat transfer and critical heat flux of ethanol-water mixtures flowing through a diverging microchannel with artificial cavity,” International Journal of Heat and Mass Transfer, vol.55, pp1807-1814, 2012
潘欽﹐沸騰熱傳與雙相流﹐1ed. 台北市:俊傑書局股份有限公司, 2001