The evaporator heat transfer coefficient ratios between NF and water for increasing and decreasing steps are shown in Fig. 8. As can be seen, using 5 wt.% of Al2O3 NF increased the heat transfer coefficient of the heat pipe for both increasing and decreasing steps. In contrast, the evaporator heat transfer ratios for the heat pipe with 10 wt.% Al2O3 NF for increasing and decreasing steps drop below unity which shows that at this Cyclopamine concentration utilizing Al2O3 NF is not beneficial for the heat pipe performance. The reasons of heat pipe performance deterioration with 10 wt.% of Al2O3 NF may be due to the evaporator wick surface structure and also two times higher viscosity increase at this concentration compared with Al2O3 NF at mass concentration of 5%. At higher nanoparticle concentration, partial detachment of the nanoparticle layer is imminent and unavoidable because of not very firm adhesion of the nanoparticles to the wick surface . So, a significant reduction of evaporator heat transfer coefficient is detected after detachment.