## 1 Introduction  
The thermosiphon is a natural circulation loop. With proper arrangement of the heating and cooling sections, the resulting thermal buoyancy can drive the working fluid in the loop to achieve thermal-energy transportation. The cooling and heating sections are usually positioned at the left and right or upper and lower sides of the loop, respectively. The increase in working-fluid temperature in the heating section decreases fluid density. The working fluid flows upwards by thermal buoyancy, which dissipates heat in the cooling section. Gravity opposes the upward flow of working-fluid flows, and promotes the downward flow of the working fluid. The absence of external driving forces results in considerable operational reliability for the thermosiphon. The self-adjusting mechanism and stability of the thermosiphon have resulted in wide application, such as in solar heating or cooling [1; 2], nuclear thermal coolers [3; 4], geothermal systems [5], waste-heat recovery [6], and electronic cooling devices [7]. There are many considerations in the design of thermally efficient thermosiphons, such as the selection of the working fluid, pipe-wall material, the locations of the heating and cooling sections, and geometric loop parameters [1; 8; 9; 10]. Phase-change material (PCM) can effectively store and release a large amount of latent heat during phase transition; therefore, is a promising material choice for applications of energy storage, heat-transfer enhancement [11; 12], thermal-environment control [13], boiling-heat-transfer enhancement with nanofluids [14], etc. Comprehensive literature studies on thermophysical properties, impregnation methods, thermal-performance analyses, numerical simulations, and other applications can be found in the review articles [15; 16]. [PERSON] et al. [17] reviewed the techniques and challenges of engineering surfaces for phase-change heat transfer. Fine solid-liquid PCM particles can be dispersed in a carrier or a suspending working fluid to form PCM slurries or suspensions. The absorption and release of latent heat of the PCM at designated locations of the loop decreases the maximal temperature and increases heat transfer. Methods for manufacturing PCM particles can be categorized into microencapsulation [18; 19; 20; 21] and emulsification [22; 23]. In microencapsulation, PCMs are encapsulated by polymer materials, separating the disperse phase (PCM) from the dispersion media (such as water and glycerol) and suspending in the dispersion media. The advantages of microencapsulation are that the size and shape of the particles are fixed, and the physical properties of the particles are stable. The disadvantages of this method are a complex manufacturing process, high cost, and that the polymer layer on the PCM particle surface can reduce the heat-transfer efficiency, and may expand and break after repeated heating-cooling. In emulsification, the surface tension between a dispersed phase and a dispersion medium is reduced via the addition of an appropriate emulsifier. PCM particles can be homogeneously suspended in a dispersion medium as a stable emulsion. The heat-transfer enhancement obtained by using PCM suspensions as a working fluid was widely demonstrated for forced convection [24; 25]. By contrast, limited studies have been carried out on the heat-transfer characteristics for PCM suspensions in a thermosiphon. [PERSON] et al. [26] numerically simulated the heat transfer of a rectangular thermosiphon containing PCM suspensions. A flow regime was identified where the latent heat absorption/release of the PCM suspension effectively enhanced heat transfer. [PERSON] et al. [27] performed more extensive parametric simulations to investigate the effects of heating-section ratio, cooling-section height, and loop size on the heat-transfer characteristics of a rectangular thermosiphon containing PCM suspensions.