In this paper, the effect of several different parameters on the thermal resistance of a Closed Loop Pulsating Heat Pipe (CLPHP) has been investigated. These parameters include the working fluid, the inclination angle, the filling ratio and the heat influx. Also, the impact of using nanofluids with different nano-particle concentrations has been analyzed. It was observed that a CLPHP can increase the heat transfer up to 11.5 times compared to an empty pipe. Optimum performance for a system with the water-silver nanofluid was achieved at conditions of 50% filling ratio and 0.9 K/W of thermal resistance, and for the water-titanium oxide system, these optimal conditions were found to be 40% filling ratio and 0.8 K/W of thermal resistance. In addition, the optimum performance for pure water occurs at a filling ratio of 40% and a thermal resistance of 1.15 K/W. The nanofluid reduces the thermal resistance by 30%. With a decrease in the concentration of nano-particles in the base fluid, the performance of the system decreases as well and the total thermal resistance increases. In low powers (under 20 W), the two-phase flow pattern inside the pipes was slug-plug, but in higher powers (over 30 W), this changed to an annular flow. The performance of the system was better in the annular mode, but there was a probability of dry out and sudden increase of condenser temperature.

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