## 1 Introduction
Accordingly, several researchers [12; 30] suggested the inner diameter of the OHP should be satisfied as  
\[D_{\rm i}\leq 2\sqrt{\frac{\sigma}{(\rho_{\rm I}-\rho_{\rm v})g}}. \tag{2}\]  
Meanwhile, it should be noted that the frictional flow resistance increases considerably with the decreasing inner diameter of the OHP, which impedes the normal oscillatory motions of working fluid in the OHP. Therefore, the optimal range of \(D_{\rm i}\) is suggested within the following range [31]:  
\[0.7\sqrt{\frac{\sigma}{(\rho_{\rm I}-\rho_{\rm v})\,g}}\leq D_{\rm i}\leq 1.8 \sqrt{\frac{\sigma}{(\rho_{\rm I}-\rho_{\rm v})\,g}}. \tag{3}\]  
Thus, it can be concluded that the inner diameter has great influence on the fluid flow and heat transfer in the OHP, which has been further demonstrated by several available studies. [PERSON] et al. [32] experimentally investigated the thermal performance of the horizontal OHPs with \(D_{\rm i}\) = 1.0 and 2.0 mm, which indicated the better thermal performance of the OHP with bigger \(D_{\rm i}\). In addition, via the comparison between the thermal resistance of two OHPs with different inner diameters, [PERSON] et al. [33; 34] found that the thermal performance of the OHP with \(D_{\rm i}\) = 1.0 mm was decreased by about 10% relative to that with \(D_{\rm i}\) = 2.0 mm. In addition, the shape of the cross section of the OHP also affects its thermal performance under the same hydraulic diameter. The effects of inner diameter on the thermal performance of OHP might be opposite for different working fluid, which is indicated by the experimental results by [PERSON] et al. [35; 36]. Particularly, based on an experimental test on the open-loop OHPs, [PERSON] et al. [37] found that when compared with the OHPwith \(D_{\rm i}\) = 0.9 mm, the OHP with \(D_{\rm i}\) = 1.5 mm demonstrated worse thermal performance, which is different from the conclusions by [PERSON] et al. [(32)] and [PERSON] et al. [(33; 34)]. In summary, although available research has demonstrated significant influence of the inner diameter on the thermal performance of the OHP, the real causes of this influence have not been fully revealed due to the limited insight into the fluid flow in the OHP during its operation. It is worth noting that several researchers (e.g., [PERSON] et al. [(38)], [PERSON] et al. [(39)], [PERSON] et al. [(40; 41; 42)], and [PERSON] et al. [(43; 44)] have proven that high-speed visualization of the operating OHP is an effective way for understanding the fluid flow in it. In addition, this previous research also pointed out that the detailed fluid flow behaviors inside are closely correlated with the thermal performance of the OHP, which are critical for clarifying the heat transfer mechanisms of the OHP. However, the available high-speed visualization experiments are mainly carried out on the OHPs with single inner diameters, with limited focus on the effects of inner diameter on the fluid flow. As a result (from the scenario described), the effects of inner diameters on the thermal performance of the OHP have not been deeply understood. Therefore, in this work, based on the high-speed visualization method and infrared thermal imaging technology, the vapor-liquid two-phase fluid flow in the OHPs with \(D_{\rm i}\) = 1.0, 2.0, 3.0 mm is investigated and compared to reveal the fundamental effects of the inner diameter. In addition, the motions of working fluid, distributions of vapor and liquid phases as well as the temperature distribution in the condenser are clarified and analyzed, in an effort to elucidate the relationship between the motions of working fluid and the heat transfer characteristics in the OHPs.