ISSN Online: 2377-424X
ISBN Print: 978-1-56700-421-2
International Heat Transfer Conference 15
Effects of Filling Ratio and Input Heat Flux on the Thermal Performance and Flow Pattern of a Pulsating Heat Pipe
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.