## 1 Introduction  
With rapid increase in energy consumption, in order to realize the sustainable development of energy and environment, it is necessary to improve the efficiency of energy transfer, especially that of thermal energy transfer [1; 2; 3; 4; 5]. Note that, as one of several types of high efficiency heat transfer elements, the heat pipe has been widely used for improving the efficiency of thermal energy transfer and reducing the environmental impact during the heat exchanging process in real applications [6; 7; 8; 9]. The oscillating heat pipe (OHP) is a new member in the family of heat pipes, which was proposed by [PERSON] [10; 11] in 1990. Generally, OHP is fabricated by a sealed wickless capillary partially filled with the working fluid, which is arranged in an interconnected meandering manner. Compared with the conventional wick heat pipe, OHP has its own special operation mechanism due to its unique wickless structure [12]. In the OHP, owing to the sufficient small scale of the meandering capillary, working fluid can be distributed naturally into a series of vapor-liquid slugs by surface tension. When heated in one section (i.e., evaporator) and cooled in another (i.e., condenser), OHP establishes the temperature difference inside. As a result of this temperature difference and the uneven vapor-liquid distribution, a saturation pressure difference is established between the evaporator and condenser, coupled with non-uniform pressure oscillation produced in the OHP. These make the working fluids undergo complex oscillatory motions in the OHP, which achieves the heat transfer between the hot and cold sections. Both the special configuration and operation mechanism produce several additional advantages for the OHP in contrast to the conventional heat pipe [13; 14]: simple construction and low cost, operational flexibility, capability for efficient heat transfer among multiple heat sources and sinks, etc. Therefore, the OHP possesses great application prospects in the areas of waste heat recovery [15; 16], solar energy utilization [17; 18], thermal management of hybrid vehicle [19; 20], etc. In this context, considerable efforts have been devoted to investigating the fluid flow and heat transfer in the OHP, so as to deeply understand its heat transfer mechanisms which are helpful for the design and optimization of the OHP in practical applications. Several theoretical models were proposed to investigate the oscillating motion and heat transfer of the vapor-liquid slugs in the OHP [21; 22]. In addition, some researchers made efforts to enhance the thermal performance of the OHP by using the nanofluids [23; 24] and mixed working fluid [25]. The optimal design of capillary channel geometry has also been conducted to successfully improve the operation stability and heat transfer of the flat-plate OHP [26; 27; 28]. Note that the previous studies indicated that the thermal performance of OHP is affected by many parameters such as geometric parameters, working fluid properties, heating load and heating modes, inclination angle, filling ratio, number of turns, and gravity load, etc. [29]. In particular, the inner geometry scale of the OHP must be small enough to form the necessary uneven distribution of vapor-liquid slugs inside, which is essential for successful operation of the OHP. Note that this uneven distribution of vapor-liquid slugs mainly depends on the ratio between the surface tension and gravity, as characterized by the Bond number:  
\[Bo=\sqrt{\frac{g(\rho_{\rm I}-\rho_{\rm v})}{\sigma}}D_{\rm i}, \tag{1}\]  
where \(D_{\rm i}\) is the inner diameter of the OHP, \(\rho_{\rm I}\) and \(\rho_{\rm v}\) are the corresponding densities of liquid and vapor, \(\sigma\) is the surface tension coefficient, and \(g\) is the gravity acceleration.