It is widely believed that the behavior of vapor bubble/blanket over heating surface plays a critical role in determining the critical heat flux (CHF) in the subcooled flow boiling. Various CHF models are based on phenomenon observations of vapor bubble/blanket in the flow channel and use vapor bubble/blanket physical parameters to determine CHF values. In this study, subcooled flow boiling tests were conducted on the experiment facility “Test of External Vessel Surface with Enhanced Cooling” (TESEC). A series of natural circulation subcooled flow boiling CHF experiments is performed in a 30 mm by 61 mm rectangular flow channel with a 200 mm long heated surface along the flow direction at various inclination angles of the test section. With the assistance of high speed video technology, the process of flow boiling in the experiments was recorded and analyzed. A novel image processing method based on a MATLAB code is used to analyze high speed images at 999 frames/second and is able to provide detailed statistical information of vapor behavior on the heating surface. By this process, the static and dynamic information of vapor blanket is obtained at the pre-CHF conditions at various inclination conditions of flow channels (30 to 90 degrees). In addition, the Fast Fourier Transform (FFT) algorithm is used to further analyze the dynamic behavior of the vapor blanket.

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