- 學位論文
微流道蒸發測試段搭配致冷晶片冷凝段之雙相自然循環迴路研究
Investigation of Two-Phase Natural Circulation Loop with Microchannel Evaporator and Thermoelectric Cooler
摘要
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。
並列摘要
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.