Study On Heat Transfer Characteristics And Application In Oscillating Heat Pipe With Wick

Oscillating heat pipe(OHP)is a new type of heat pipe that relies on both sensible heat and latent heat of working fluid to transfer heat.It has the advantages of good heat transfer performance,low cost and strong adaptability,so it has a broad application prospect in the fields of battery thermal management,solar thermal utilization and heat dissipation with high heat flux in confined space.However,the traditional OHP is still a gravity-assisted heat pipe,its heat transfer performance is more sensitive to the change of gravity,and the heat transfer performance needs to be further improved.In order to strengthen the traditional heat transfer performance of OHP and reduce its dependence on gravity,this paper introduces the capillary wick into the OHP,designs and manufactures the oscillating heat pipe with wick(OHPWW),and builds the heat transfer performance test and visualization experimental platform of the OHPWW.The operation mode,starting performance and quasi-steady state heat transfer performance of the OHPWW under different working conditions was studied.The gas-liquid two-phase flow pattern and behavior in the OHPWW were investigated,and the heat and mass transfer mechanism and thermal resistance composition of the OHPWW were analyzed.Finally,the OHPWW and phase change material(PCM)are coupled to improve the thermal conductivity of PCM,the advantages and disadvantages of OHPWW/PCM in waste heat recovery are discussed,and the heat transfer characteristics of the coupling module are explored.The main work and conclusions of this paper are as follows:(1)An experimental platform for the performance testing of tubular OHPWWs was built,and tubular capillary OHPWWs with inner diameters of 2.5mm and 3.5mm and tubular OHPWNWs with inner diameters of 2mm,3mm and 4mm were designed and manufactured.The working fluid is acetone,and the filling rate is 55%.The heat transfer characteristics of OHPs under different heat loads and inclination angles were tested.The experimental result showed that with the increase of heat load,the OHP firstly experienced a static state and intermittent oscillation.Then the OHP experienced an oscillation state and oscillation accompanied by circulation,or experienced an oscillation accompanied by static and circulation state.At last,part of the OHP entered into cycle state.The inner diameter of the OHP had an important influence on the evolution of the operation state,the OHP with a small inner diameter was easier to enter a cycle state,while the large inner diameter OHPWNW with an inner diameter of 3mm and 4mm cannot enter the pure cycle state.As the inclination angle decreases,the minimum heat load corresponding to each operation state of the OHPWNW increases,and the heat load range covered by the stagnation accompanying the oscillating operation state increased until the inclination angle is 0°.The thermal performance of OHPWNW also gradually deteriorated,showing a strong gravity dependence;While the OHPWW entered each operating state as the inclination angle decreases,the minimum thermal load corresponding to each operating state decreased,and could operate normally in the horizontal direction.The heat transfer performance did not change much,especially the OHPWW with a large inner diameter has better heat transfer performance in the horizontal direction.The heat transfer performance of the OHP was closely related to the operating state of the working fluid,but it is also affected by the absolute liquid filling amount.The OHP achieved the best heat transfer performance during the cyclic operation state.(2)The flat plate OHPWW visual observation platform was built,and a flat plate OHP with sintered capillary wick on the inner wall of the channel was designed and manufactured.Meanwhile,a flat plate OHPWNW was designed for comparison.The hydraulic diameter of the channel was about 3mm.The working fluid is FC-72.The flow of two phases in OHPs under different filling rates and different heat loads was observed.The visualized experimental result showed that the basis of the two-phase flow behavior in the OHP are formation,growth,merging,shrinking,fracture and breakage of bubbles/gas plugs.The various flow patterns were derived,including bubble flow,bubble-plug transition flow,plug flow and annular flow.It should be pointed out that these operation patterns often did not appear alone,but existed simultaneously and transform into each other,which leads to the complexity and diversity of two-phase flow behavior in OHPs.In the quasi-steady state operation,the working fluid operation mode of the flat-plate OHP could be divided into stagnation,one-way upward flow of vapor,small oscillation,large oscillation and local drying operation.In addition,the OHPWW had a unique local directional circulation of vapor at 40% filling rate.Reducing the filling rate can reduce the minimum heat load corresponding to entering each operating pattern.The operation pattern of the two-phase flow was directly related to the heat transfer performance of the OHP.In the case that the circulation operation could not be achieved,the greater the amplitude and the faster the frequency of the oscillating motion of the two-phase flow,the better the heat transfer performance of the OHP.Dued to the intense evaporation/boiling of liquid working fluid in evaporator,the OHPWW had stronger driving force better heat transfer performance than the OHPWNW in most working conditions.However,it also leads to local drying phenomenon in evaporator of the OHPWW,and the heat transfer mechanism during drying was similar to that of the traditional capillary core heat pipe.(3)The experimental platform for waste heat recovery of PCM based on tubular OHPWW was built.Paraffin wax with a melting point of 67.3℃ was used in PCM.The experimental result showd that under the condition of no external cooling source,the drying of the three OHPs is easy to occur when PCM is used for heat absorption.The decrease of inclination angle makes the drying advance,and the overall drying was easy to occur under high power input.Dued to its unique capillary core structure,the condensate could return to the evaporator without being affected by the direction of steam flow,which alleviated the drying phenomenon in the evaporator to a certain extent.Therefore,the heat transfer performance of the OHPWW was better than that of the OHPWNW when drying occured,OHPWW could take the lead in heating paraffin to the specified temperature.In the heat release stage,the OHPWW showed better heat transfer performance before the paraffin was solidified,and the paraffin decreased faster.However,after the paraffin begins to solidify,the OHPWW had little advantage.On the contrary,the heat release of paraffin cooled by OHPWNW with a large diameter was faster.In general,OHPWWs were more suitable for the PCM endothermic stage in short distance waste heat recovery applications.