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ISSN Online: 2377-424X

ISBN Print: 978-1-56700-421-2

International Heat Transfer Conference 15
August, 10-15, 2014, Kyoto, Japan

Effects of Filling Ratio and Input Heat Flux on the Thermal Performance and Flow Pattern of a Pulsating Heat Pipe

Get access (open in a dialog) DOI: 10.1615/IHTC15.tpa.009899
pages 8269-8277

Abstract

The pulsating heat pipe(PHP) which has advantage of its structural simplicity has been the focus of next generation cooling device, because the trend of electronic devices is toward reduced size and thickness. The filling ratio which is defined as the ratio of charged liquid volume to total channel volume is major design variable that influences the thermal performance. The goal of this research is to investigate the thermal performance and flow pattern of a flat-plate PHP depending on input heat flux and filling ratio. The thermal performance of a copper based flat-plate PHP was experimentally evaluated and its internal flow pattern discussed. 20 parallel interconnected square channels which has a width and height of 1 mm were engraved on copper plate of 2 mm thickness forming a meandering closed loop. The thermal performance of the PHP charged with ethanol as a working fluid was tested in a vertical orientation and evaluated as thermal resistance which is defined the ratio of average temperature difference between evaporator and condense section to input heat flux. The thermal performance and observed flow pattern show the 2 regimes depending on input heat flux and filling ratio. When filling ratio is lower than about 50 %, the thermal resistance is shown to decrease with increasing input heat flux level and reaches an optimum input heat flux that means maximum thermal performance. Thereafter, the thermal resistance increases with increasing input heat flux because dry-out area of evaporator becomes broader actually and fluid flow becomes slower. From the flow pattern perspective, a PHP operates as Mode 1 which has low amplitude of meniscus. Heat transfer mechanism of Mode 1 seems to consist of phase change and convection in liquid slug. When the filling ratio is higher than 50 %, on the other hand, the mode change is generated from Mode 1 to Mode 2 in condition of high input heat flux. Significant transition is observed not only the thermal resistance trend also flow pattern. Mode 2 has large amplitude of meniscus pulsating between from evaporation and condenser section. The sensible heat transfer is maximized between liquid slug and wall. When the filling ratio is about 57 % and input heat flux is more than about 6 W/cm2, active pulsating motion is observed and maximum thermal performance is achieved to 0.52 K/W. Finally, thermal performance region map was developed.