This research was to study the effect of inner diameter and inclination angles on operation limit of a closed loop oscillating heat pipes with check valves (CLOHP/CV). A set of CLOHP/CV was made of copper tubes in combination of following dimension: 1.77 and 2.03 mm inside diameter: 10 turn, with R123 was used as the working fluid. The working fluid was filled in the tube at the filling ratio of 50%. The inclination angles were 0, 20, 40, 60, 80 and 90° with 5 equal lengths for evaporator, adiabatic and condenser sections. The evaporator section was given heat by heater while the condenser section was cooled by volume water in a cold bath. The adiabatic section was properly insulated. In the test operation, it could be concluded as follows. It indicated that when the inner diameter changed from 1.77-2.03 mm the critical temperature increased. And when increase the inclination angles from 0 until to 90° the critical temperature increased.

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