Characteristics Of Liquid-vapor Phase Change And Transport In Nanofluid Oscillating Heat Pipes

As electronic devices continuously decrease in size, traditional cooling methodsand technologies are facing great challenges and an urgent demand appears for higherefficiency cooling methods. Oscillating heat pipes, for its high heat-removal efficiency,attract many researchers' attention. It is found that capabilities of oscillating heat pipecould be improved by adopting nanoparticle suspension and/or nanofluids of diluteparticle volume concentration as the working fluid. The nanofluid oscillating heat pipeoffers a new future to electronics cooling. To provide a knowlegement base fordeveloping higher heat-efficiency cooling technologies, basic phenomena and transportcharacteristics of the nanofluid oscillating heat pipe need to be explored to understandthe mechanism of the improvement of heat efficiency by nanoparticles addition.A series of experimental observations were conducted on oscillating heat pipeswith nanfluids and DI water for comparing their operating performance and internalflow phenomena. In a vertical oscillating heat pipe system, internal flow oscillationswere studied. it was found that when the oscillating heat pipe operated normally, theinternal flow was alternately small-amplitude slow oscillation and large-amplitude fastoscillation. The transition depended on drastic nucleation and quick phase change insidethe liquid slug in the evaporator. At the same heat load, nanoparticles addition made thedrastic nucleation and quick phase change to happen at relatively low temperatures. Inanother horizontal oscillating heat pipe system, the internal flow patterns were the focus.In the DI water heat pipe, simple column flow was mainly observed and the evaporatordried out at a relatively high heat load. In the nanofluid heat pipe, simple column flowwas also mainly observed at relatively low heat load but the heat transfer performancewas improved compared with the DI water; however at relatively high heat loads,several interesting flow patterns were observed: bubbly flow, slug flow and annularflow. The flow-pattern changes ensured oscillatory flow inside the oscillating heat pipeand thereby increased operation the limit of operation heat flux and improved the heattransfer performance of the oscillating heat pipe.Based on the classical nucleation theory and the thermodynamic theory of solution,a new model of nanofluid nucleation was built up. It was found that nanoparticles accumulation at the vapor-liquid interface decreased the embryo bubble radius and thepotential barrier of nucleation, and facilitated liquid nucleation. Additionally,nanoparticles accumulation in the three-phase contact region and deposition duringnucleate boiling on the wall both improved wettability of nanofluid on the wall, whichimproved liquid evaporation. An experiment about liquid-vapor phase change inside acapillary tube was conducted to further validate the nucleation and wettingcharacteristics of nanofluid: nucleation was easier and growth rates ofbubbles/vapor-plugs were faster in nanofluids than in pure liquids.Based on pure fluid oscillating heat pipe computation models, a simplified modelfor nanofluid oscillating heat pipe was built up to model heat and mass transfer innanofluid heat pipes. Results showed that the easy liquid-vapor nucleation andphasechange was the key factor for the improvement of heat transfer of the nanofluidheat pipe. This model can provide essential information for further investigations onnanofluid oscillating heat pipes.