## Abstract

For an oscillating heat pipe (OHP), the hydraulic diameter must be sufficiently small so that vapor plug and liquid slug can be formed by the capillary action. Therefore, the hydraulic diameter should not exceed a critical value named maximum hydraulic diameter (MHD). In the current research, a glass OHP with a hydraulic diameter of 6 mm was fabricated, and ethanol was used as the working fluid (Figs. 1 and 2). With a hydraulic diameter far exceeding the maximum hydraulic diameter (MHD) defined by $dh,max≤{2σBo[(ρl−ρv)g]}1/2$, the OHP can function. But the flow pattern is very different from that with a diameter smaller than the MHD, and depends on the filling ratio. When the OHP is charged with a higher filling ratio, the flow pattern is from the bubbly flow to the slug flow as shown in Fig, 3. When the charging ratio is low, the dispersed bubbly flow with many small bubbles generated on the surface was observed, and the dispersed bubbly flow was directly transformed into the annular flow as shown in Fig. 4. [This research work was supported by National Natural Science Foundation of China under Grant No. 51876019, the Preresearch of General Armament Department (6140922011310), LiaoNing Revitalization Talents Program (XLYC1807117), and the Fundamental Research Funds for the Central Universities of China under Grant No.3132019331.]

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