Experimental Study On Pool Boiling Enhancment Using Liquid-vapor Separation Guidance Structure

Pool boiling is one of the most promising solutions to address the need for high heat flux cooling for electronic device in the form of immersing cooling,two-phase closed thermosyphon and heat pipe loop.Pool boiling enhancement is critical to improve heat transfer performance and capability of heat-dissipation devices.In this study,a novel method to separate liquid-vapor pathways is proposed to enhance pool boiling heat transfer.Different from popular methods through surface modifications including geometry or chemical variation,a guidance structure is mounted above a heating surface in a boiling pool.The basic idea is to form a rising vapor slug-liquid plug two-phase flow and a replenishing liquid impingement onto the heating surface simultaneously through a specially designed bubble-pump,which is driven by the superheated surface.In addition,the upward two-phase flow and downward single-phase liquid flow in this configuration is separated by the wall of guidance structure so that they will not interfere each other.Experiments using plain copper heating surface at different saturation temperatures are conducted.The main results are as followed:(1)The unidirectional fluid circulation with upward slug flow inside the guidance tube and downward liquid flow in the annulus between the container and the guidance can be realized.With increasing heat flux,the fluid circulation becomes steadier,and the bubble pumping velocity and liquid impingement velocity increases.(2)By facilitating the bubble departure and strengthening the replenishing liquid impingement onto the heating surface,both guidance tubes can enhance CHF and HTC drastically.(3)The trumpet guidance tube is easier to achieve unidirectional fluid circulation than straight guidance tube due to the bubble collection effect of the trumpet.It also has larger liquid impingement velocity because of its narrower annulus between the guidance tube and the container.(4)At low saturation temperature,bubble pumping velocity is larger and the enhancement is more remarkable.At saturation temperature of 50?,the CHF and maximum HTC in a trumpet guidance tube are 109.0 W/cm~2 and38.8 kW/m~2?,respectively,which are 4.1 and 2.8 folds higher than the test sections without a guidance tube.The influence of water level and guidance tube length are investigated as well above plain copper heating surface.The results show that:(1)The unidirectional fluid circulation is most robust when water level matches guidance tube length,where higher water level obtains a better performance.(2)Both the CHF and maximum HTC for 150 mm water level is 1.3 times of that for 130 mm water level at 50?saturation temperature.The liquid-vapor separation guidance structure can be performed on sintered copper powder heating surface.Experiments at different saturation temperatures and with different size sintered area indicate that:(1)The boiling performance of combined test section with straight guidance tube and copper powder sintered surface is superior to that of baseline test sections.(2)At saturation temperature of 70?,larger sintered area results higher heat transfer enhancement.Compared to plain surface,the CHF of sintered area diameter of 5 mm and 9 mm is 15%and 100%higher,respectively,while the maximum HTC is 1.4 and 2.8 folds higher,respectively.(3)A special phenomenon called boiling inversion is founded with 9 mm diameter sintered area at saturation temperature of 50?,where heating surface superheat decreases 3.8?with the heat flux increase of 4.5W/cm~2.The possible reason is that the nucleation of outside annular sintered area is restrained by the liquid impingement before boiling reversion.An even higher heat flux will initiate nucleation of that impeded area,so the superheat drops.When the cooling water temperatures are controlled constantly,the system becomes a two-phase closed thermosyphon,whose performance is measured by thermal resistance.(1)The system pressure arises with the input heat at each cooling water temperature.The test section with trumpet guidance tube has the lowest thermal resistance since it is easiest to form the unidirectional fluid circulation.(2)When non-condensable gas is added,the test sections with straight guidance tube performs better than the test section without guidance tube with a lower thermal resistance and lower system pressure owing to the ability to assist vapor contact condensing surface.