Operational limit of closed loop pulsating heat pipes

This paper presents an experimental study on the operational limitation of closed loop pulsating heat pipes (CLPHPs), which consist of a total of 40 copper tubes with 1 mm and 2 mm inner diameter, respectively. R123 was employed as the working fluid with filling ratios of 30%, 50% and 70%, respectively. Three operational orientations were investigated, viz. vertical bottom heated, horizontal heated and vertical top heated orientations. The effects of inner diameter, operational orientation, filling ratio and heat input flux on thermal performance and performance limitation were investigated. The results show that for the CLPHP with 2 mm ID tubes the best performance existed in the vertical orientation with heating at the bottom, while for the CLPHP with 1 mm ID tubes, orientation played almost no role. A filling ratio of 50% was optimum for both CLPHPs to obtain best performances in all orientations. The CLPHPs were operated till a performance limit characterized by serious evaporator overheating (dry-out) occurred. Rather high heat loads could be accommodated. Dry-out heat fluxes in the vertical bottom heat mode were about 1242 W/cm2 (1 mm ID) and 430 W/cm2 (2 mm ID) for axial heat transport, and about 32 W/cm2 (1 mm ID) and 24 W/cm2 (2 mm ID) for radial heat input, always with respect to the inner tube diameter.