| Flat plate heat pipe is a kind of high efficiency heat transfer element with nearly two-dimensional plane structure.It has the characteristics of strong heat transfer capacity,simple and compact structure,miniaturization,low cost,large contact area with heat source,close fit,easy to change structure and so on.It is suitable for the thermal management of electronic equipment with different needs.Aiming at the three pain points of high electrical safety requirement,high heat flux and complex and compact heat dissipation space in electronic device cooling,the applicability design of plate heat pipe was proposed and systematic experimental study was carried out.In order to meet the electrical safety requirements,a new type of electronic coolant with good insulation of fluoether was selected as the working medium,and the operation characteristics of a variety of fluoether working medium in plate heat pipe were studied.Combined with 522 groups of experimental data of the research team,an experimental correlation formula for plate heat pipe suitable for fluoether working medium was proposed.Aiming at the requirement of high heat flux,multiple plate heat pipes were superimposed to form a superimposed plate heat pipe,which shared the heat load and effectively improved the heat transfer capacity.The heat transfer operation characteristics were experimentally studied.According to the complex space heat dissipation requirements,the flat-plate heat pipe was bent.The operation characteristics of the flat-plate heat pipe at different bending angles and different spatial positions were studied experimentally,and the heat transfer mechanism was analyzed,which provided the basis for its application.The main conclusions are as follows:(1)The ultimate heat transfer,thermal resistance and evaporation temperature of HFE-7100,HFE-7200 and HFE-7300 flat heat pipes in most working conditions are related to the boiling point of the working medium,and all increase with the increase of the boiling point.HFE-7100 is more suitable for low heating power,HFE-7200 and HFE-7300 are more suitable for high heating power;As the cooling temperature increases,the superheat of the working medium in the evaporation section of the flat heat pipe with the same heating power increases,resulting in the flow boiling phenomenon is strengthened,the heat transfer performance is improved,and the thermal resistance is reduced.Both the ultimate heat transfer and the optimal heating power decrease compared with the low cooling temperature.The evaporation temperature increases with heating power and cooling temperature,and is negatively correlated with thermal resistance.Both HFE-7200 and HFE-7300 flat heat pipes have a heating power range,which can adapt well to different cooling temperatures and keep the temperature stable in the evaporation section.(2)Based on 522 groups of experimental data,the experimental correlation formula for the thermal performance prediction of flat plate heat pipes was fitted,which widened the range of existing experimental correlation formula Ku value to 0.084,and filled the gap of experimental correlation formula for flat plate heat pipes in working medium of fluorine ether,which can provide reference for related research.(3)With the increase of the number of tubes,the overall heat transfer increases significantly,and the maximum heat transfer increases from 60 W of a single tube to 180 W of5 tubes;The heat transfer resistance decreased significantly,and the lowest heat transfer resistance decreased from 1.09℃/W of single sheet to 0.24℃/W of 5 sheets.As the number of sheets increases,the heat transfer resistance between heat pipes begins to affect the overall operation of superimposed flat heat pipes: the heat transfer of the outer heat pipe is higher than that of the inner heat pipe,and the thermal resistance is lower than that of the inner heat pipe.When the number of sheets is 4 or more,the proportion of heat transfer of the inner heat pipe becomes less and less,and the overall performance improvement of the heat pipe becomes less and less obvious.The ideal number of superimposed heat pipe is 3～4 pieces.(4)For the bent flat heat pipe,because the diameter of the flow channel is slightly larger than the critical diameter of the working medium,the flat heat pipe is in the category between the pulsating heat pipe and the gravity heat pipe.The flat heat pipe with more characteristics of gravity heat pipe has better performance,while the pulsating heat pipe with more characteristics has worse performance.(5)Under the horizontal bending condition of evaporation section,the thermal resistance of flat heat pipe decreases with the increase of bending Angle,and the ultimate heat transfer increases with the increase of bending Angle.Under the vertical and inclined bending conditions of the evaporation section,the flow mode of the working medium begins to change from slug to gas-liquid stratification with the increase of bending Angle.The two influences of the flow mode change caused by bending and the difficulty of the working medium reflux are mutually restricted,and the temperature shows that the temperature of the evaporation section and the condensing section both increase.The performance of the flat heat pipe is improved or decreased depending on the relative magnitude of their effects.However,the overall thermal resistance changes little,and the heat transfer performance can be enhanced even in some working conditions.The ultimate heat transfer of plate heat pipe decreases with the increase of bending Angle under two bending conditions.(6)Under the same longitudinal dip Angle and different bending angles,the increase of bending Angle will also change the flow mode from slug flow to gas-liquid layered circulation.The performance of flat heat pipe also depends on the relative influence of working medium reflux and flow mode transformation,and the overall thermal resistance changes little.However,when the longitudinal dip Angle is 90°,the thermal resistance of the plate heat pipe decreases with the increase of the bending Angle.In this case,the operation mode of the plate heat pipe changes from disordered slug flow to ordered unidirectional circulation of the working medium,and the operation state is improved. |
