Theoretical And Experimental Study On The Effect Of Surface Tension On The Heat Transfer Characteristics Of The Pulsating Heat Pipe

With the rapid development of electronic industry,the electronic products have been widely used in many aspects of the national economy.At present,the miniatrization and high integration of the electronic products has become obvious development trends of this industry,especially after the proposal of "Big Data" and "Cloud Calculation" concepts in recent years.The heat generated by the electronic products must be dissipated,or it may cause the deterioration of the performance and even burnout.In some high-tech areas,the heat fluxes of the electronics are very high,which are greatly beyond the heat transfer ability of the traditional heat dissipation method.As a result,the thermal management of the electronics hasbeen a bottleneck and a new high-efficiency heat transfer device is critically needed.Pulsating heat pipe(PHP),also known as self-excited heat pipe or oscillating heat pipe,is considered as one of the most promising heat transfer devices to face the challenge of high heat flux since it was proposed.The PHP has many distinct advantages:1)Simple structure and low cost:the structure of the PHP is very simple and there is no wick structure in the pipe,which significantly simplifies the manufacturing procedure.The small diameter is also helpful for cost-saving.2)Excellent heat transfer ability:during the operation process of the PHP,the latent heat of the working fluid can be utilized to dissipate the heat.As a result,the PHP has very excellent heat transfer ability.3)High flexibility:The channel of PHP can be arranged to arbitrary configuration according to the situation of application.The investigation of the PHP has become a hot research topic in the area of thermal management of the electronics.The heat transfer performance of PHP is influenced by many parameters,and these parameters can be classified into geometric parameters,working fluid’s property parameters and working condition parameters.It is a research hot topic to improve the heat transfer performance of the PHP,and the enhancement of the performance of the PHP through optimizing the working fluid’s properties parameters is a very direct and efficient method.Among the working fluid’s properties,the surface tension has a significant influence on the flow of the working fluid as well as the heat transfer performance of the PHP.However,neither the systematical experimental investigations nor the theoretical models are presented in the public literatures.In order to better understand the influence of the surface tension and lay a solid foundation for the enhancement techniques of the PHP through optimizing the working fluid’s properties.The disseration conducted this study,which mainly include the following contents:1)The state-of-art of the current researches on experimental investigations,theoretical analyses and potential application areas of the PHP were reviewed systemically in the dissertation.For the experimental investigation aspect,the influences of the geometric parameters,the properties parameters of working fluid and the operating parameters on the heat transfer performcne of the PHP were systematically summarized.For the theoretical analysis aspect,the conclusions on the characteristics of the oscillation motions,the heat transfer performance and other figures of the PHP were presented.For the potential application area,the utilization prospects of the PHP in heat dissipation,solar heater and waste heat recovery field were commented.2)Considering both the oscillation process of the liquid slugs and vapor plugs and the evaporation process of the thin film in meniscus area,a new heat and mass transfer modelwas proposed.Based on this model,the effect of the gravity,capillary resistance and flowresistance were analyzed.The results indicated that the capillary force was an important part of the total resistance force.When the liquid slug’s velocities were between(1～5)m/s,the liquid slug’s lengths were(5～10)times of the inner diameter,the ratios of the capillary resistance to the total resistance force could be(15.38～48.45)%.At the same time,when the surface tension of the working fluid decreased,the heat transfer processes of liquid slugs were enhanced.3)The dry-out characteristics of PHP were theoretically analyzed.The dry-out mechanisms were classified into three categories.A theoretical model was proposed to analyze the second kind dry-out phenomena.Furthermore,the correlation was analyzed to calculate the critical heat flux of the pulsating heat pipe based on the dimensional analysis.The results indicated that the dry-out heat flux could be increased when the velocities of the liquid slugs increased.That is to say,decreasing the resistance force would increase the dry-out heat flux of the PHP.The conclusions are of great importance to figure out the safe application range of the PHP.4)An experimental setup was designed and built to investigate the heat transfer performance of the PHP.It mainly consisted of four modules:the PHP module,the heating module,the cooling module and the data acquisition module.The uncertainties of the temperature,charge ratio were 0.1 K and 1.27%,respectively.The relative error of the thermal resistance was below 3.75%.5)The experiment was conducted to investigate the startup,heat transfer and dry-out performance of the PHP when the working fluids were deionized water and the surfactant solution,respectively.Sodium stearate was used as the surfactant,and the concentrations of the surfactant solution studied in this dissertation were 0 ppm,lOppm,20ppm and 40 ppm.The studied charging ratio were 18.9%,30.9%,43.8%,56.7%and 67.9%,respectively.The experimental results indicated that the PHP could start up at a lower evaporation section temperature when the working fluid was surfactant solution.Furthermore,when the surfactant solution was 20 ppm,the best heat transfer performance of the PHP was achieved.The PHP could have a greater dry-out heating power with the surfactant solution as the working fluid than the deionized water.This part can be a solid data support to enhance the performance of the PHP by optimizing the surface tention of the working fluid.