## 5 Conclusions  
In this study, the effects of working fluid and inclination angle on the thermal performance of the MF-AHPA have been firstly systematically investigated. The heat pipe start-up performance, isothermal performance, and thermal resistance were investigated experimentally. Some calculated parameters of working fluids were evaluated and linked to the final heat pipe performance. The liquid film distribution was theoretically analysed to assist explaining the inclination angle effect. The liquid film mechanism that determines the optimal inclination angle has been firstly discovered and verified. A criterion was originally proposed to quantitatively predict the optimal inclination angle for the MF-AHPA. The optimal volumetric fraction in this study was further compared with those of other heat pipes from the literature. The main conclusions are drawn as below:  
1. The acetone MF-AHPA outperforms the other two heat pipes in terms of start-up time, temperature uniformity, and thermal resistance for its superior fluid properties. 2. The underlying mechanisms of the acetone heat pipe are the small viscous friction of flowing liquid in grooves and a small shearing force of liquid-vapour enabled by low viscosity, high thermal conductivity, a high latent heat of vaporisation, and a high La number of acetone. Figure 13: The optimal volumetric fraction of heat pipes from literature as a function of the volume of the wick. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{1}{c}{**Volume of Wick (mL/m\({}^{2}\))**} & \multicolumn{1}{c}{**Input Power (W)**} & \multicolumn{1}{c}{**Working Fluid**} & \multicolumn{1}{c}{**Optimal**} & \multicolumn{1}{c}{**Volumetric**} & \multicolumn{1}{c}{**Resource**} \\ \hline
111.1 & 90–160 & water & 75” & 51.4–65\% & Z-H. [PERSON] et al. 2010 [23] \\
124 & 90–230 & acetone & 25”-75” & 70–81.3\% & Present experiment \\
124 & 90– & n-hexane and cyclopentane & 75” & 66.5–69\% & Present experiment \\
144.1 & 1185 & water & 30” & 93.6\% & K. [PERSON] et al. 2005 [55] \\
144.1 & 1161 & water & 40” & 84.4\% & K. [PERSON] et al. 2005 [17] \\
176.6 & 45–60 & water & 60” & 19.6–22\% & [PERSON] dry et al. 2017 \\
199.0 & 120 & water & 90” & 28.8\% & [PERSON]. [PERSON] et al.2016 [25] \\
336.3 & 70–150 & water & 90” & 13.6–17.68 & [PERSON] et al. 2011 [24] \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The optimal volumetric fraction of specific groove heat pipes from the literature. 3. The acetone MF-AHPA can effectively prevent unwanted local dry-out due to a high La number, with liquid quickly spreading. 4. The optimal inclination angles of heat pipes are not constant; it is affected by the working fluid and heat load. However, the liquid film distributions are unified when the heat pipes are working at their optimal inclination angle. 5. In the present study, a volumetric fraction of 74% \(\pm\) 7% has been shown to well predict an optimal inclination angle for MF-AHPAs with various working fluids and heat loads. 6. The optimal volumetric fractions vary for grooves with different structures or dimensions, and decrease as the volume of the wick increases. It is significant to select the optimal working fluid (acetone) and make the heat pipe work on the optimal liquid distribution by adjusting the inclination angle. The thermal resistance could have a reduction of more than 34% and 24% by selection of optimal working fluid and optimal inclination angle, respectively. Note that the optimal volumetric fraction value of 74% \(\pm\) 7% is only applicable to the MF-AHPA in this study. Future work will further build the predicted model of the optimal volumetric fraction for different wick structures. The Original data of the Figures 6, 7 and 9-13 are available online at [[http://www.mdpi.com/2076-3417/8/7/1052/s1](http://www.mdpi.com/2076-3417/8/7/1052/s1)]([http://www.mdpi.com/2076-3417/8/7/1052/s1](http://www.mdpi.com/2076-3417/8/7/1052/s1)). Conceptualisation, G.H. and Y.T. ; Formal analysis, G.H. ; Funding acquisition, Y.T. ; Investigation, G.H. ; Methodology, G.H.