Study on the operation limit of micro pulsating heat pipes

Abstract

In this thesis, the effects of the channel geometry and the number of turns on the operation limit of micro pulsating heat pipes (MPHPs) are studied. In the first technical chapter, the operation limit of MPHPs is investigated with different channel geometries: cross-sectional shapes and hydraulic diameters. Five-turn closed-loop MPHPs, which have either the circular or square channel with hydraulic diameters of 390, 480, and 570 μm, are fabricated onto a silicon wafer using MEMS techniques. FC-72 is used as the working fluid with the filling ratio of 50%. Experimental results show that the operation limit is observed to increase with the hydraulic diameter in both cross-sectional shapes. At the same hydraulic diameter, the square-channel MPHP can handle approximately 70% higher maximum allowable heat flux than the circular-channel MPHP. Based on the results of flow visualization, a falling-film model for the operation limit of the MPHPs with 5 turns in a vertical orientation is proposed: the MPHPs reach the operation limit when the falling film flow rate is smaller than the evaporation rate. Using a scale analysis and experimental data, a correlation for predicting the maximum allowable heat flux of the MPHPs with 5 turns in a vertical orientation is developed. According to the model, the maximum allowable heat flux is found to be proportional to the third power of the hydraulic diameter regardless of the cross-sectional shape. In the second technical chapter, the effects of the number of turns and the inclination angle on the operation limit of MPHPs are investigated through a series of experiments. The MPHPs with 5, 10, 15, and 20 turns are fabricated: square channels with a hydraulic diameter of 480 μm are engraved onto a silicon wafer. Experimental results show that the dependence of the maximum allowable heat flux on the inclination angle becomes smaller as the number of turns increases, and the MPHP with 20 turns has almost the same maximum allowable heat flux regardless of the inclination angle. For this orientation-independent case with a large number of turns (20 turns or more), a film-formation model of the operation limit is proposed: the MPHPs reach the operation limit when the liquid-film formation rate is smaller than the evaporation rate. This postulate is experimentally confirmed through comparison between the two rates. By combining the film-formation model with the falling-film model, a correlation for predicting the maximum allowable heat flux in terms of the number of turns and the inclination angle is developed. According to the correlation, the number of turns has to be larger than 17 in order for the MPHP to have orientation-independent maximum allowable heat flux.