## 2 Design, Working Fluid, and Experimental Apparatus of the MF-AHPA
### Working Fluids Selection and Evaluation  
#### 2.2.1 Working Fluids Selection and Charging Processes  
Working fluids would be a key factor in the performance of the present design. According to the compatible principle, ammonia [36], Freon, cyclopentane [37], acetone [38], and n-hexane are used as working fluids for aluminium heat pipes. However, ammonia and Freon are only applicable for low temperature heat pipes. In this study, cyclopentane, acetone, and n-hexane are selected for the MF-AHPA working at regular temperature. The properties of the selected working fluids are listed in Table 2.  
Acetone has potential to show good thermal performance due to its high thermal conductivity and large latent heat of vaporisation. Since the boiling point is as low as 49 \({}^{\circ}\)C at 1 atm, cyclopentane tends to phase change and works at a lower temperature. N-hexane, with its relatively large density of vapour, has the potential to reduce the vapour velocity and the liquid-vapour shearing force. \begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Value (mm)** \\ \hline Thickness of MF-AHPA (b) & 3 \\ Width of MF-AHPA (f) & 26 \\ Total length of MF-AHPA & 970 \\ Wall thickness (a) & 0.45 \\ Width of microfin (c) & 0.35 \\ Height of microfin (h) & 0.32 \\ Width of microgroove (e) & 0.25 \\ \hline \end{tabular}
\end{table} 
Table 1: Key dimensions of MF-AHPA. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Working Fluid** & \(\sigma\) **(mN/m)** & \(\mathbf{h_{fg}}\) **(K)/kg** & \(\mathbf{\mu_{i}}\) **(mN-S/m\({}^{2}\))** & \(\mathbf{\lambda_{i}}\) **(W/m-K)** & \(\mathbf{C_{p}}\) **(K)/kg-K** & \(\mathbf{\rho_{i}}\) **(kg/m\({}^{3}\))** & \(\mathbf{\rho_{v}}\) **(kg/m\({}^{3}\))** \\ \hline acetone & 19.13 & 517 & 0.226 & 0.143 & 2.36 & 744.7 & 2.12 \\ cyclopentane & 17.2 & 390 & 0.292 & 0.113 & 2.24 & 823 & 2.56 \\ n-hexane & 14.0 & 343 & 0.377 & 0.108 & 2.46 & 620 & 3.14 \\ \hline \end{tabular}
\end{table} 
Table 2: The thermal–hydraulic properties of three selected working fluids (60 \({}^{\circ}\)C). Figure 2: Schematic of heat transfer path and liquid film distribution inside a channel while heated from the upper surface. (**a**) A view of the whole channel cross-section; (**b**) upper groove liquid distribution; and (**c**) bottom groove liquid distribution. The filling rate of working fluid can be defined as [39]:  
\[\eta=\
u_{\rm wf}/\
u_{\rm g}\times 100\% \tag{1}\]  
where \(\
u_{\rm wf}\) is the volume of the working fluid and \(\
u_{\rm g}\) is the volume of the grooves. In this study, the filling rates of the three working fluids were all 100% (6.4 mL), meaning that the grooves are full of working fluid. The aluminium grooved tube was ultrasonically cleaned by ethanol, acetone, sulfuric acid, and deionised water for 10 min sequentially. After air-drying at room temperature, the cleaning process was finished. The purity of working fluids used in this study was more than 99.5% and a 10-min degassing process was conducted. Before charging, pipes were vacuumed to be lower than 10 Pa in the first vacuum process. The second vacuum process [39; 40] was adopted to minimise non-condensable gas. A non-condensable gas gathering section with a length of 80 mm was sealed off after heating the evaporation section in 100 \({}^{\circ}\)C for 60 s.  
The heat pipe performance is affected by many other factors, including non-condensable gas (NCG) [41], charging ratio [42], and groove wick structure [33]. Heat pipe performance is very sensitive to the amount of NCG. Only 5% NCG could induce a reduction of 50% in condensation heat transfer coefficient [43]; an excessive charging ratio leads to flooding in the condenser, whereas an insufficient working fluid induces dry-out in the evaporator; a groove with a high depth-width ratio has a higher critical heat flux, but a lower heat transfer coefficient. Although heat pipe thermal performance is affected by other factors, the variable factor in this comparative experiment is inclination angle or working fluid.