The start-up and heat transfer performance of a micro-grooved tube oscillating heat pipe (OHP) and three other smooth tube OHPs were experimentally investigated and compared both at vertical and horizontal orientations. Deionized water was used as the working fluid with a volumetric filling ratio of 50%. The internal diameters (IDs) of three smooth tube OHPs were 3.4 mm (1#)

4.0 mm (2#) and 4.8 mm (3#)

respectively

and the internal hydraulic diameter of the micro-grooved OHP was about 2.82 mm. The results showed that at the vertical bottom heat mode

the micro-grooved OHP had lower heating power input and average evaporator temperature as compared to other smooth tube OHPs. At a heating power input of about 305 W

the reductions in the thermal resistance of the micro-grooved tube OHP were about 41.7%

35.6% and 30.9% as compared to that of the 1#

2# and 3# OHPs

and the corresponding reductions in the evaporator temperature were about 12.1℃

11.8℃ and 7.6℃

respectively. At the horizontal orientation

only the micro-grooved OHP could start up favorably at the relatively low heating power inputs within all these four tested OHPs and indicated better heat transfer performance. According to a qualitative analysis

the heat transfer enhancement of the micro-grooved OHP was mainly attributed to the sensible/latent heat transfer intensification as well as enhanced liquid backflow to the evaporator due to the microgroove-induced capillary action.