Experimental Investigation Of The Effect Of Wettability On Microchannel Pool Boiling Heat Transfer

With the development of science and technology,both energy conservation in high energy-consuming industry and heat dissipation of high heat flux electronic devices in emerging high-tech fields require much higher heat transfer enhancement technologies.The effects of surface morphology and surface wettability on nucleated heat transfer have been paid more attention by scientists in recent years.The appearance of electro-brush plating method and heterogeneous wettability provide important approach to improve nucleated heat transfer and critical heat flux.They have great potential in future industrial applications,so it is necessary to investigate heat transfer enhancement mechanism of surface wettability and morphology.This thesis study the influence of surface structure,wettability and heterogeneous wetting nano-scale morphology on pool boiling heat transfer of deionized water in experimental way,by integrating surface structure and wettability,under atmosphere condition.The major process and conclusion are following:1.Hydrophilic micro-structure surface(contact angle=13.4°)and superhydrophilic micro/nano compound surface(contact angle=6.1°)were processed by electro-brush plating system and nano-Ag regent.Similarly,superhydrophilic nano-surface(contact angle=8.6°)was prepared by high temperature oxidation.After that,hydrophobic and superhydrophobic surface were made by modification technology,which was used to modify hydrophilic and superhydrophilic surfaces.2.Ni spheres with the scale of 10 mm were observed in micro-structure surface by scanning electron microscope.Surface roughness of micro-struture is 1.46?m after the test of surface profile.Nano-Ag particles adhered to micro-Ni balls in micro/nano compound structure surface,which roughness was 1.35?m.There were nano-Cu particles(100~150nm)on nano-structure surface,which roughness was 1.15?m.3.Heterogeneous wetting surfaces,including hydrophilic/superhydrophilic and superhydrophilic/ superhydrophobic wettability,were prepared by electro-brush plating method and surface modification technology.Visualization research of the movement process in heterogeneous wetting line of water droplet found that droplet moving speed was faster in superhydrophilic/ superhydrophobic surface.Small droplet reminders leaved the original droplet and stayed in the hydrophilic surface during the movement of water droplet in hydrophilic/ superhydrophilic wetting line,because of Rayleigh Instability.4.Pooling boiling heat transfer system was designed and set up.Rohsenow empirical formula was used to identify the rationality of boiling system an d experimental data.5.Microchannels structures on cooper surface were prepared by Electrical Discharge Machining.Superhydrophilic micro/nano compound structure and nano-structure were made on the microchannels structures.These two superhydrophilic surfaces were tested for saturated pool boiling heat transfer,indicating micro/nano compound structure and nano-structure could increase critical heat flux by 110% and 120%,respectively,than copper flat surface.Heat flux coefficient of these increased three and twenty-nine times separately than copper flat surface.The onset of nucleated boiling of nanostructure decreased 6.6 ? than the copper surface.6.Superhydrophobic micro/nano structure and nanostructure surface were prepared on the the superhydrophilic surface after low surface energy medication.These two superhydrophobic surfaces were tested for saturated pool boiling heat transfer,indicating micro/nano compound structure could increase critical heat flux by 60% than copper flat surface.Whereas the superhydrophobic nanostructure decreased critical heat flux by 8% than coopper.Maxmium heat flux coefficient of superhydrophobic micro/nano compound structure and nano-structure grow 45% and 115% respectively.The onset of nucleated boiling of micro/nano structure was lower than 3.5?,and the onset of nanostructure was lower than 2.0?.7.Heterogeneous microchannels were made on the surface of superhydrophilic nano-structure(contact angle=8.6),flat copper(contact angle=88.6°)and superhydrophobic nano-sturcture(148.6°).The interior of microchannels were cauterized pore structure with scales of 8mm,which contact angle were 113.2°.The critical heat flux of microstructures with superhydrophilic top was the highest,which increased by 61% than that of copper surface,and nucleated heat transfer coefficient was greater by 232% than that of copper surface.The nucleated heat flux coefficient of microstructures with superhydrophobic top was the highest,which increased by 606% than that of copper surface,and cr itical heat flux was greater by 35% than that of copper surface.