S. Zeinali Heris, M. Nasr Esfahany11 investigated the effect of Al2O3 nanoparticles suspended in water in circular tube. They concluded that as the concentration of Al2O3 nanoparticles increases in the fluid the heat transfer coefficient is much higher as compared to conventional working fluids. It was also concluded in their research that enhancement of thermal conductivity was not the sole reason for heat transfer enhancement but dispersion and chaotic movement of nanoparticles, Brownian motion and particle migration are also the reason for this enhancement.
The effect of CuO nanofluids suspended in heat pipe was examined by Xue Fei Yang12. They used an experimental setup with a micro grooved heat pipe with CuO nanoparticles induced in water. Heat transfer coeffecients of evaporator and condenser section increases with increasing particle concentration in the fluid, but it reaches a critical point at 2% mass concentration beyond which heat transfer coefficient start to decrease. They also stated that at 1% mass concentration optimum heat transfer is achieved.
Maryam – Vincenzo 15 used different nanoparticles like CuO, Al2O3, and TiO2 in flat shaped heat pipes and analysed the difference in thermal conductivity between heat pipes with nanoparticles and heat pipes with distilled water. They concluded that the presence of nanoparticles in the working fluid result in reduced speed of the liquid, lesser temperature difference at the ends of the heat pipe. So in effect it reduces the thermal resistance of the heat pipe and provides for a smoother heat dissipation to external environment.
As the driving potential of the working fluid is the capillary pressure i.e. the pressure difference between the evaporator and the condenser section, the sum of all the resisting pressure should be less than the capillary pressure for the liquid to be able to flow. The different pressure drops that take place across the heat pipe are liquid pressure drop, vapour pressure drop and inter phase pressure drop.
Since liquid pressure in heat pipe wick is generally low, the dynamic pressure can be neglected. Assuming uniform heat addition & removal at the evaporator & at the condenser, the liquid pressure drop through porous wick can be expressed as 13
The dynamics of the vapour flow 25 is more complex. The vapour flowing the evaporator & condenser of a heat pipe is dynamically identical to pipe flow with injection of suction through a porous wall (Cotter 1965). The vapour pressure drop in the adiabatic section can be estimated using Poiseuille’s Law 14:
Paulis & Lang (1976) have considered inter-phase transfer pressure drop in addition to liquid & vapour pressure drops, inter-phase pressure drop is due to non – equilibrium pressure difference between the true pressure in the vapour, Pv & the pressure the vapour would have if it were in equilibrium with the liquid. They have derived an expression for the pressure drop using the Kinetic Theory of Gases.
If the total pressure drop exceeds the capillary pressure, then there won’t be a force sufficient enough to force the fluid into liquid section. There won’t be any more liquid left in the evaporator section to absorb heat and transport it to condenser section and eventually the heat pipe will dry out. The capillary limitation of a heat pipe can be found from the following expression: