Study On The Start-up And Themo-hydrodynamic Characteristics Of Micro/mini Oscillating Heat Pipes

As new heat transfer devices,micro/mini oscillating heat pipes(OHPs)have drawn much attention in recent years due to their high heat transfer performance and good integration fabrication potential.However,the two-phase system which embodies multidisciplinary physics is so complicated that the investigation on OHPs is still at a primary stage,and many fundamental problems are still unclear and need to be clarified.In this dissertation,the study on micro/mini oscillating heat pipes(OHPs)were performed to investigate the start-up,heat transfer and flow characteristics.To aim at a fundamental understanding of the operation mechanism and heat transfer enhancement,the thermohydrodynamic behaviours in different-sized mini/micro OHPs as well as enhanced heat transfer mechnisms with different structures were studied.Firstly,the start-up and heat transfer of silicon-based micro-OHPs were investigated,and various effects on the thermal performance of micro-OHPs were analyzed.To enhance the heat transfer and start-up performance,channels with non-uniform structures or artificial nucleation cavities were introduced to the micro-OHPs.Secondly,a copper-foam wicks was firstly sintered on the channel bottom of an FPOHP,and the start-up characteristics and heat transfer performance of a porous-based FPOHP were investigated and compared with a conventional FPOHP.Finally,considering the contact angle hysteresis,a theoretical model was developed to investigate the oscillating motion characteristics of working fluid in OHPs.The main contents and results are summarized as follows:(1)The start-up and heat transfer characteristics of a micro-OHP,having trapezoidal channels with a hydraulic diameter of 357μm,had been investigated using a high-speed CCD camera in conjunction with the temperature measurement.Two different start-up behaviors,so-called start-up with and without bubble nucleation(STWBN/STWOBN),subject to different heat input levels were firstly observed,and the underlying mechanism was elaborately analyzed.The STWBN mode is sometimes indispensable to activate the two-phase oscillation in micro-OHPs,leading to much higher evaporator temperatures even greater than 140 ℃ to initiate the bubble generation.Additionally,the gravity action on the micro-OHP performance still functioned and can’t be neglected,and the thermal performance of micro-OHP at the vertical orientation is much better than that at the horizontal orientation.An optimal filling ratio of about 40% was observed for the microOHP with respect to the lowest thermal resistance.(2)Vapor plugs/liquid slugs in the channels with non-uniform structures can create an additional capillary driving force and facilitate the movement of plugs/slugs from the narrow channel region to the wide channel region.Therefore,this structure is introduced to the micro-OHP to enhance the heat transfer.Compared with the micro-OHP with straight channels,the micro-OHP with expanding channels has better heat transfer and start-up performance at the vertical orientation.With the increase of filling ratio,the advantages become more obvious.Moreover,the micro-OHP with expanding channels can shorten the time duration from start-up to the steady state oscillation and reduce the frequency of stopover.(3)Artificial nucleation cavities were fabricated on the bottom surface of microchannels inside the micro-OHP.The cavities could not only significantly improve the start-up performance of micro devices,but also promote the transition of working fluid from the start-up stage into the steady state.What’s more,the start-up characteristics of the nonuniform micro-OHP with artificial nucleation cavities were influenced by the filling ratio.When the filling ratio was small,the influence of non-uniform channel structure on the startup could be ignored due to the existence of artificial nucleation cavities.With the increase of filling ratio,the combination of artificial nucleation cavities and non-uniform channel structure could enhance the start-up performance further owing to the increase of freedom for the oscillating motions of working fluid inside the non-uniform channels.(4)The two-phase flow behavior inside the micro-OHPs with different structures have been investigated,and different flow patterns,including bubbly flow,slug flow and annular/semi-annular flow,were observed.Noted that the injection flow appeared when HFR-7100 was used as the working fluid,while this flow behavior did not occur in the micro device using R113 or R141 b.Additionally,the occurrence region and possibility of the bubbly flow and annular/semi-annular flow could be enlarged because the robust nucleation boiling induced by artificial nucleation cavities or non-uniform channel structures.The heat transfer and start-up performance were improved.(5)A flat-plate OHP(FPOHP)with its channel bottom covered by sintered copperfoam wicks was firstly fabricated.The start-up and heat transfer performance of porousbased FPOHP were experimentally investigated and compared with a conventional smoothchannel FPOHP.The existence of cavities in porous metal-foam wicks enabled a profound improvement in the FPOHP performance and temperature uniformity,reducing the start-up temperature significantly.However,local dry-out appeared earlier in the porous-based FPOHP when the power input was large enough because the robust nucleation boiling induced vapor layer covered the wicks,causing the heat transport limitation.Thus the redesign and re-fabrication of novel wicking structures are required to promote the heat transfer performance and delay the dry-out simultaneously.(6)A mathematical model to investigate the oscillating motion characteristics of liquid slugs and vapor plugs/bubbles in OHPs was developed considering the contact angle hysteresis(CAH)and interconnected-tube induced pressure fluctuations.The influence of the natural angular frequency on the oscillation amplitudes and frequencies for both of slug/bubble displacement and velocity were studied.The slug/bubble displacement and velocity display quasi-sine oscillating waves with small pressure fluctuations induced by the interconnected-tube.However,small oscillation waves are superimposed on a main quasi-sine oscillation wave and cause a chaotic oscillating behavior of slug/bubble inside the OHP if the induced pressure fluctuation is large enough.Besides,the effects of filling ratio,tube length,inner diameter,temperature difference between the evaporator and condenser sections,and working fluid on the oscillating motion were numerically analyzed and discussed.