## 3 Results and Discussions  
### Fluid Flow Motions inside OHP  
As mentioned above, a deep insight into the vapor-liquid two-phase flow motions in the OHP during its quasi-steady operation is helpful for understanding the oscillating operation characteristics and heat transfer mechanisms of the OHP. In this work, after the start-up process [45], a quasi-steady station is achieved in the OHPs. Based on the summary of our visualization results under the quasi-steady operation of the OHPs, it can be seen from Figure 2 that the fluid flow inside the OHPs with three inner diameters under quasi-steady operation condition mainly exhibits three motions: small oscillation (S-O), bulk oscillation (B-O) and circulation (C). Since the evaporator and adiabatic section are embedded into the thermal insulation box in the current experiment, in order to show these three motions clearly, Figure 2b selects the corresponding fluid flow motions in either U-turn at the condenser of the OHP as the representative for analysis. As shown, when the small oscillation appears in the OHP, the vapor-liquid two-phase fluid only exhibits small, local oscillations in the tube, with limited mass exchange between the evaporator and condenser in each single vertical tube. Correspondingly, when the bulk oscillation occurs, the working fluid begins to oscillate with large amplitude in the OHP, resulting in bulk mass exchange between the evaporator and condenser and even among multiple U-turns. Differing from the oscillation motion, the circulation of working fluid is characterized by the circulation of working fluid in the whole OHP along the fixed direction. However, due to the non-uniformity driving pressure distributions in the OHP, the circulation speed of working fluid is changed. In order to reflect the heat transfer performance of the OHP produced by these three motions, Figure 2c gives the typical infrared thermal images of the condenser under different motions. In addition, corresponding to the infrared thermal images in Figure 2c, Figure 3 further quantitatively represents the vertical temperature distribution in two adjacent tubes in a typical U-turn of the condenser under different quasi-steady operation states. In this figure, each curve is plotted by the  
Figure 1: Experimental apparatus: (**a**) schematic of experimental setup; (**b**) schematic of experimental oscillating heat pipe; and (**c**) cross section geometry of A–A in inset (**b**). average temperature data of every seven horizontal pixel points along each vertical tube in the typical U-turn. As indicated in Figures 2c and 3a, under small oscillation motion, the heat transfer of the condenser mainly relied on its own heat conduction, except for the bottom regime, which is dependent on the sensible and latent heat transfer of the locally oscillating working fluid. Therefore, with the exception of high temperature at the bottom of the condenser, the temperature at the other part of the condenser nearly decreases linearly along the vertical direction to the top. As the bulk oscillation comes out, working fluid at the hot and cool ends of the OHP are able to exchange with each other, which obviously enhances the heat and mass transfer from the evaporator to the condenser. As a result, the area of high-temperature region at the bottom of the condenser is expanded, leading to an increase in the temperature level of the condenser (see Figures 2c and 3b). Note that, under the bulk oscillation motion, due to the non-uniform heat and mass transfer strength caused by the uneven oscillation of working fluid among the OHP, every parallel tube at the condenser has different temperature distributions along the vertical direction. When the circulation of working fluid appears in the OHP, the adjacent parallel vertical tubes become 'upheaders' and 'downcomers' alternatively with corresponding hot and cold fluid flowing inside, resulting in alternatively high and low temperature on them (see Figure 3c).