The phenomena of two different types of unidirectional circulating flow pattern in a copper oscillating heat pipe (OHP) were firstly discovered and investigated. The OHP has six turns and three sections: evaporator, condenser and adiabatic sections with lengths of 40 mm, 64 mm, and 51 mm, respectively. R152a was used as the working fluid, the effects of the tilt angle from 0° to 90° and the heat input on the flow and heat transfer of the working fluid in OHP was studied. The experimental results showed that (1) the OHP charged with R152a can form a unidirectional circulating flow at any tilt angle under certain heat input, and the unidirectional circulating flow become steady as the heat input increases; (2) another type of circulating flow was found in the same OHP as the heat input increased to a relative high level, the difference between the two types of circulating flow is that the liquid slugs move forward with or without back forward oscillating movement; (3) the unidirectional circulating flow of the working fluid without back forward oscillating movement in the OHP significantly enhance the heat transfer of OHP.

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