本研究是封閉腔體內之高溫差自然對流。在左右腔壁溫差的方形腔體，以Boussinesq流體與理想氣體流體作模擬，當使用Boussinesq流體模擬時，等溫線與流線呈左右反對稱，在冷熱壁面的溫度梯度相等。當使用理想氣體流時，於低溫差腔體，冷熱壁面的溫度梯度相等，於高溫差腔體，冷壁面的溫度梯度大於熱壁面的溫度梯度，且高溫差的封閉腔體，壓力隨著腔體內氣體能量增加而升高。在研究中比較相同Ra時，高溫差在冷壁面的溫度梯度大於低溫差在冷壁面的溫線梯度，在熱壁面則相反。高溫差的流線較低溫差的流線偏右下方。

　　在Rayleigh-Benard問題，長寬比為25，壓力隨著腔體內的能量增加，直到近似穩態，壓力呈些微的振盪。Ra大於14590時，流場隨著時間改變的暫態現象，其暫態現象是無週期性，旋轉數隨著Ra增加而減少。在Ra=14590時，旋轉數約為12~16個。在Ra=136700時，旋轉數約為8~10個。

　　The objective of this thesis is to study natural convection in enclosure with large temperature difference. Comparisons between results with ideal gas and Boussinesq fluid have been made. In the large temperature difference, the isotherms near the cold wall is denser than that in the realm of hot wall. This study focus on comparing the streamline of diverse temperature difference in the same Rayleigh number. The streamline moves toward the direction of cold wall, and the rolls change the form when increasing the temperature difference of left-right wall.

　　The other topic is Rayleigh-Benard problem. The pressure increases with the energy of the air of cavity, until quasi-steady the amplitude of the pressure with time is small. In this research aspect ratio is 25, the stream of cavity which has no period, changes with time. The numbers of rolls decrease with increasing Rayleigh number. When the temperature difference is 1000K, the difference of Nusselt number in the realm of cold wall is twice of that in the realm of hot wall, that stand for the different temperature gradient in the realm of wall.

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