The heat transfer characteristic of horizontal screen heat pipe using nanofluid was studied experimentally.The nanofluid consists of water and CuO nanoparticles having a mean diameter of 50 nm

the mass concentration of nanofluid ranges from 0.5% to 2.0%

and the operational pressures of the heat pipe varies from 7.45 kPa to 19.97 kPa.The experimental results show that the effect of the particle concentration on both evaporating and condensing heat transfer coefficients and the maximum heat flux is very significant.At the pressure of 19.97 kPa

the evaporating heat transfer coefficient rises from 170% to 410%.The operational pressure yields very significant effect on the heat transfer enhancement for the heat pipe.The lower operational pressure indicates more significant thermal performance enhancement of the heat pipe.The heat transfer characteristic improves with the rise of mass concentration

and then drops after reaching a certain value

while the optimal mass concentration is 1%.