## 4 Conclusions  
By combining the high-speed visualization method and infrared thermal imaging technology, the vapor-liquid two-phase fluid flow in the OHPs with different inner diameters are observed and presented. The motions of working fluid, distributions of vapor and liquid phases, as well as the temperature distribution in the condenser under different diameters, are compared and analyzed to elucidate the fundamental effects of the inner diameter on the fluid flow as well as heat and mass transfer in the OHPs. In addition, the relationship between the motions of working fluid and heat transfer characteristics in the OHPs is discussed and clarified. Accordingly, several conclusions are drawn as follows:  
(1) The major fluid flow motions inside OHP under quasi-steady operation condition are small oscillation, bulk oscillation and circulation, which are closely correlated with the heat and mass transfer performance in the OHPs, which can be reflected by the characteristics of infrared thermal images of the condenser. These three fluid flow motions coexist or correspondingly exist alone with the increasing heating load under different inner diameters.  
(2) The general flow patterns in OHP include bubbly flow, slug flow and annular flow. The occurrence and transition of the flow patterns are mainly dependent on the phase-change phenomena in the evaporator and condenser of the OHPs. In particular, nucleate boiling in the evaporator is restricted by decreasing the inner diameter of the OHP, which leads to the smallest proportion of bubbly flow in the 1 mm OHP with respect to the other two. In addition, the decrease in inner diameter of the OHP can enhance the effects of surface tension and thus enlarge capillary instability on the vapor-liquid interface, which produces more collapses on the vapor core and long vapor slugs, forming more short slugs in the OHP with \(D_{\rm i}=1\) mm than that under \(D_{\rm i}=2\) and 3 mm.  
(3) Due to the increasing driving pressure difference in the OHPs with growing heating load, the major fluid flow motions are changed from small oscillation to bulk ones and finally reach the circulation, which triggers a decrease in the overall thermal resistance of the OHPs. Particularly,  
Figure 9: Variation of thermal resistance and fluid flow motions versus increasing heating load under different inner diameters of the OHPs.  
the occurrence of bulk oscillation of working fluid causes the most apparent decrease in the thermal resistance versus the increasing heating load, whereas the thermal resistance of the OHPs drops to become a steady value when the working fluid circulation appears. Owing to the large frictional flow resistance and small amount of filled working fluid under the small diameter, the OHP with an inner diameter of 1 mm possesses the largest thermal resistance and undergoes the dry-out of working fluid most easily. When compared with the 2 mm OHP, the increasing role of gravity with growing inner diameter induces the thermosyphon effect and weakens the 'bubble pumping' action, which results in a little smaller and bigger thermal resistances of 3 mm OHP under small and bulk oscillation of working fluid, respectively.  
The authors gratefully acknowledge the support provided by the National Natural Science Foundation of China (51406175) and the Natural Science Foundation of Jiangsu Province (BK20140488).  
[PERSON] provided the guidance and supervision. [PERSON] implemented the main research, checked and discussed the results, and wrote the paper. [PERSON] and [PERSON] performed the experiment, collected the experimental data, and checked the paper. All authors read and approved the final manuscript.  
The authors declare no conflict of interest.