The flow of looped pulsating heat pipe was studied by a visualizing experiment, and the PHP is made of high quality glass capillary tube. Under different fill ratio, heat transfer rate and many other influence factors, the flow patterns were observed in the start-up, transition and stable stage. The experimental results indicate that bulk flow, transition flow and annular flow are the major flow patterns in PHP. Under different fill ratios and heat transfer rate, the flow pattern in PHP is transferred form bulk flow to semi-annual flow and annual flow, and the performance of heat transfer is improved. In the experiment, nuclear boiling, the convergence and break up of liquid-plug and vapor-slug were observed. The influence characterization has been done for the variation of fill ration, heat transfer rate, non-condensable gas and inclination angle. The experimental results indicate that the total heat resistant of PHP is increased with fill ratio, and heat transfer rate achieves optimum at filling rate 50%; the heat resistance is decreased with heat transfer rate, and non-condensable gas also has significant influence on it. The temperature of tubes in heating, condensing and observe sections were recorded, The fluctuation of heat pipe wall temperature was analysed, and the phenomena of suddently increase or decrease of temperature, the unregular fluctuation of temperature were analyzed. Otherwise the response time under different conditions was also analyzed.

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