Experimental Investigations On Heat Transfer Performance Of A Closed Loop Pulsating Heat Pipe

Heat transfer and operating principles of pulsating heat pipes are quite complex. In the recent two decades, many people make great effort on experimental investigations of pulsating heat pipes. However, some problems still not make a progress. In this paper, a closed loop pulsating heat pipe (CLPHP) was taken as the object of the study. A visualized experimental table was built to observe the main characteristics and phenomenon of the start-up and steady operation of the CLPHP. Effects of the filling rate, working fluid and inclination angle to heat transfer performance of the CLPHP were also investigated.The heat transfer performance of the CLPHP was mainly evaluated by calculating the heat transfer rate, effective conductivity and thermal resistance. The last two were related to temperature distribution of both evaporator and condenser sections of the CLPHP, also to the heat transfer between these two sections.In the research of the filling rate, the CLPHP separately within distilled water, ethanol and 0.7% water-TiO2 nano-fluids under 40%, 57%, 70% filling rate was investigated. The research indicated that the effect of filling ratio on heat transfer performance of the CLPHP within distilled water and ethanol is obvious, and the best filling ratio was approximately at 57%. About the 0.7% nano-fluids, the influence of the filling ratio is worse than the characteristics of the nano-fluids. There was a best value of the heat transfer performance under every filling ratio. And over this value the performance get worse because of the sedimentation of nano-particles.In the research of the working fluids, the CLPHP separately within distilled water, 0.2%, 0.7%, 1.5% water-TiO2 nano-fluids was investigated. The research indicated that heat transfer performance of nano-fluids was better than distilled water under low filled ratio. However, nano-particles adding did not necessarily increase the base water's heat transfer performance under high filled ratio. The start-up characteristics of the CLPHP within nano-fluids was worse than water and the whole operation phenomenon of the CLPHP within 0.2% nano-fluids was the worst.In the research of the inclination angle, it was adjusted to 30°,45°,60°when the CLPHP operated stably. The research indicated that the operation phenomenon of the CLPHP was the best under the vertical bottom heating pattern, as long as the CLPHP was tilted, the heat transfer rate and effective conductivity got low. The heat transfer performance was better when the inclination angle was at 45°, then got worse over 60°. The CLPHP's working condition would be even worse and more difficult to operate on the level direction.