How to improve the thermal performance of pulsating heat pipes: A review on working fluid

Pulsating Heat Pipes (PHPs) are cooling devices that are compact in size and have an ability to transfer heat in low temperature differences. Working fluids strongly affect the thermal performance of PHPs. In this paper, effects of some thermophysical parameters relating to working fluids, such as boiling point, latent heat of vaporization, surface tension, thermal conductivity and dynamic viscosity, are presented based on experimental and numerical studies done in recent years. Addition of nanoparticles to fluids, or making nanofuild, is a new method of improving thermophysical properties of fluids. Recently, many studies are carried out on thermophysical properties of nano-fuild. Results indicate that using nanofuild could improve thermal performance of heat pips. Finally, in this review, flow regimes of some working fluids are represented under different conditions to obtain a better insight into the effect of input heat on working fluid flow pattern. It is concluded that lower dynamic viscosity and surface tension and higher thermal conductivity improve thermal performance of PHP. For lower heat inputs, lower boiling point of working fluid is more favorable due to faster start-up onset; however, at higher heat loads it causes some problems, such as dry-out.