二氧化碳超臨界流體在板式熱交換器的數值分析

有鑑於合成冷媒對環境的污染，本研究考慮將板式熱交換器中的冷媒改為二氧化碳，可是二氧化碳在壓力與溫度分別為7.8Mpa、330K時，屬於超臨界狀態，因此在板式熱交換器內部，二氧化碳因為溫度與壓力的關係將會在臨界點以上操作。但是二氧化碳在臨界點附近之物理性質差異非常大，有別於一般傳統冷媒，因而不可用一般傳統熱交換器的數值模擬方式分析，因此，本研究藉由數值模擬的方式，考慮二氧化碳在U型平板式熱交換器之內部，將推導出方程式聯立，再利用程式模擬出二氧化碳在超臨界狀態之下的流速、壓力與溫度等分佈，並探討其影響。而模擬結果顯示，當入口速度為0.1m/s時，壓力與速度呈現越往下游越大的傾向；而當入口速度增加，速度越往下游會越大的趨勢也會更加明顯。另外，水的流道出口溫度，會呈現越往下游溫度越高，而流道入口溫度則變化不大。二氧化碳則是在流道出口溫度呈現越往下游溫度下降越多的情況，與文獻結果趨勢一致。最後確定，本程式可以正確的模擬出，板式熱交換器中，二氧化碳在超臨界狀態下，熱交換器內部的流速、壓力與溫度分佈，可作為後續進一步的分析之用。

關鍵字

二氧化碳；超臨界狀態；板式熱交換器

並列摘要

The study attempts to use carbon dioxide at supercritical states to replace traditional refrigerants in plate heat exchanger. The critical states of carbon dioxide is at 7.8Mpa and 330 K, and therefore the plate heat exchanger is operated over the super-critical statet. Conventional numerical simulation methods for plate heat exchangers are not available, because the physical properties of carbon dioxide vary drastically around the critical point. In the study, a numerical code has been developed to simulate the distributions of pressure, temperature, and flow velocity of both carbon dioxide and water. The simulation results show that, when inlet velocity is at 0.1m/s, the corresponding pressure and velocity become higher while approaching the outlet; the increase of inlet velocity makes the phenomenon more obvious. Besides, the outlet temperature of water channel becomes higher while approaching the outlet; on the contrary, the outlet temperature of carbon dioxide becomes lower while approaching the outlet. Comparing to the literature, we found that the simulation results obtained by the developed code are correct and the code can thus be further applied to other cases.

並列關鍵字

carbon dioxide ； super-critical state ； plate heat exchanger.

參考文獻

[1] M. K. Bassiouny and H. Martin, “Flow distribution and pressure drop in plate heat exchangers-I”, Chemical Engineering Science Vol. 39, pp. 693-700, 1984.

[3] F. W. Dittus, L. M. K. Boelter, “Heat Transfer in Automobile Radiators of Tubular Type”, 1930.

[5] S. M. Liao, T. S. Zhao, “Measurement of Heat Transfer Coefficient from Supercritical Carbon Dioxide Flowing in Horizontal Mini/micro Channels”, 2002, Volume 124, Issue 3, 413.

[6] Discussion: “Flow Distribution Manifolds”, Bajura, R. A., and Jones, Jr., E. H., 1976, ASME J. Fluids Eng., Vol. 98, pp. 654–665.

[8] M. H. Kim, J. Pettersen, C. W. Bullardj, “Fundamental Process and System Design Issues in CO2 Vapor Compression Systems”, 2004.

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二氧化碳超臨界流體在板式熱交換器的數值分析

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