Research On Hydrophilic/hydrophobic High Performance Heat Pipe Based On Nanofluid

The progress of information technology leads to the development of electronic equipment towards the trend of thin and light.The dense arrangement of electronic components causes a significant increase in heat flux.If the heat can not be dissipated in time,the electronic equipment is easy to face the risk of failure.Compared with the traditional sintered heat pipe,the grooved heat pipe has the advantages of small mass,high permeability and simple process.However,due to the lack of porous structure,the capillary pressure of grooved heat pipe is low.It is an effective way to improve the capillary pressure and overall heat transfer performance of grooved heat pipe by constructing micro-nano structure on the groove surface or using nanofluid as working medium.Based on the accommodation theory,a theoretical model for predicting the wetting length of nanofluids in superhydrophilic microgrooves was developed,and the flow characteristics of nanofluids in pristine/superhydrophilic rectangular microgrooved heat pipes were compared.In this paper,the hydrophilic/hydrophobic micro-nano structure was constructed on the inner surface of the microgrooved heat pipe by chemical method,and the capillary properties of the hydrophilic micromicrogrooves were analyzed.At the same time,carbon nanotube nanofluid was used as working medium and the influence of nanofluid on the heat transfer performance of hydrophilic/hydrophobic microgrooved heat pipe was investigated.This project is of great significance to enhance the heat transfer of microgrooved heat pipe.The wetting characteristics of nanofluids in pristine/superhydrophilic rectangular microgrooved heat pipe were analyzed theoretically.The results show that the surface properties of microgroove had a significant effect on the wetting length of nanofluids.In pristine microgrooves,the wetting length Lc of SiO2 nanofluid in the corner flow region could be neglected,and the accommodation wetting length La was almost equal to the total wetting length Lt.In contrast,the Lt,La and Lc in superhydrophilic microgrooves were much higher than those in pristine microgrooves.For both microgrooves,the wetting length decreased with the increase of inclination angle.In different microgrooves,the impacts of nanoparticle size and volume fraction on wetting length were different.In superhydrophilic microgroove,the wetting length was mainly affected by the surface tension,so the accommodation wetting length La and the corner flow wetting length Lc declined with the decrease of the nanoparticle size and the increase of the volume fraction.In pristine microgroove,the effect of contact angle on La was greater than that of surface tension,so La increased with the decrease of nanoparticle size and the increase of nanoparticle volume fraction.The viscous friction caused by the evaporation of the liquid was the reason of the negative effect of the heat load on the liquid flow.When the microgroove depth was less than 0.6 mm,increasing the microgroove depth could significantly improve the total wetting length Lt in both microgrooves.Subsequently,the influence of microgroove depth on the total wetting length Lt was significantly reduced.The variation of microgroove depth had no effect on the corner flow wetting length Lc.When the microgroove width was less than 0.3 mm,increasing the microgroove width had different effects on La under different heat flux;when the microgroove width was greater than 0.3 mm,increasing the microgroove width caused the accommodation wetting length La to decrease.In the corner flow region,Lc firstly increased and then decreased with the increase of microgroove width.The optimum ratio of depth to width was about 2:1.The accommodation wetting length La decreased with the increase of microgroove pitch,while the wetting length Lc was not affected by microgroove pitch.In this paper,the hydrophilic and hydrophobic micro-nano structure surfaces were constructed and their stabilities were analyzed.The results show that the contact angle,morphology and chemical composition of hydrophilic micro-nano structure surface changed with the treatment time of sodium hydroxide and ammonium persulfate mixed solution.After 6 min of chemical reaction,slender Cu(OH)2 nanorods appeared on the surface of copper,and the contact angle decreased from 76.5° to 47.2°.When the reaction time increased to 90 min,the nanorods began to buckle,forming a network like structure.At this time,the chemical composition of micro-nano structure was no longer single,including Cu(OH)2 and CuO.And the contact angle continued to decrease to 33.6°.When the reaction time reached 720 min,the surface network structure transformed into flower cluster composite structure,and the contact angle changed to 4.7°,which meant that the surface was superhydrophilic.And the chemical composition of the micro-nano structure was mainly CuO.The contact angle of superhydrophilic micro-nano structure surface was still less than 10° after heating for 6 h.After heating for 8 h,the contact angle slowly increased and then gradually stabilized.After heating for 24 h,the contact angle was still less than 30°,which indicated that the superhydrophilic structure had good stability.Four kinds of chemical reagents were used to decorate the original superhydrophilic surface.It was found that the surface modified by 1H,1H,2H,2H-perfluorodecyltriethoxysilane had the largest contact angle,which was 160.0°.The contact angles of the surfaces modified by triethoxy1H,1H,2H,2H-tridecafluoro-n-octylsilane and lauric acid were 154.3° and 153.9°respectively,and the surface modified by n-octadecyl mercaptan had the smallest contact angle of 117.5°.The modification time had little effect on the contact angle,and the maximum difference between the contact angles for 6 h and 12 h was only 7.3°.Fourier infrared spectroscopy tests show that the characteristic peaks of the four modifications had been loaded on the copper surface.The surface modified by triethoxy-1H,1H,2H,2H-tridecafluoro-n-octylsilane had the best stability.A capillary climb experiment was performed on hydrophilic microgrooves.And the results show that compared with pristine copper microgroove,hydrophilic microgrooves had higher wetting length and capillary climb speed.The capillary pressure of the superhydrophilic microgroove reached 620.6 Pa,which was 3.9 times that of the pristine copper microgroove.The gravity had an important influence on the capillary climbing process,and gravity could not be ignored when calculating capillary performance parameters.The capillary performance parameter K·ΔPcap of the superhydrophilic microgroove was 1.357×10-6 N,which was 1505%higher than that of the pristine copper microgroove.The carbon nanotube nanofluid was prepared and analyzed for stability.The results show that the absorbance of the carbon nanotube nanofluid firstly increased and then decreased with the increase of the ultrasonic dispersion time.The absorbance of carbon nanotube nanofluid was the highest when the ultrasonic dispersion time was 60 min.The cationic surfactant cetyl trimethyl ammonium bromide had a better dispersion effect on carbon nanotubes than the anionic surfactant sodium dodecyl benzene sulfonate and the nonionic surfactant gum arabic.The three surfactants all had the best mass fractions.The best mass fractions of cetyl trimethyl ammonium bromide and gum arabic were both 0.25 wt%,and the best mass fraction of sodium dodecyl benzene sulfonate was 0.15 wt%.After standing for 40 days,the carbon nanotube nanofluid added with the cationic surfactant cetyl trimethyl ammonium bromide had the highest absorbance and the best dispersion effect,but white flocs appeared in the solution.The heat transfer performance of microgrooved heat pipe was tested.The results show that using deionized water as the working medium,the thermal resistance of evaporation section of the heat pipe was only 9.23%of that of the original heat pipe after superhydrophilic surface treatment,while the thermal resistance of condensation section increased to 6.83 times after superhydrophobic surface treatment.Adding different surfactants to carbon nanotube nanofluids had different effects on the performance of hydrophobic and hydrophilic heat pipes.When sodium dodecyl benzene sulfonate was added into the carbon nanotube nanofluid as dispersant,the average total thermal resistance of the hydrophilic/hydrophobic heat pipe was only 31.5%of that when deionized water was used as working fluid;the total thermal resistance of the heat pipe with cetyl trimethyl ammonium bromide as dispersant decreased slightly,the average total thermal resistance was 90.8%of the original;after using gum arabic as a dispersant,the total thermal resistance of the heat pipe had increased significantly,which was 299.5%of the original heat pipe.When the filling rate of the microgroove heat pipe was 100%and the working inclination was 90°,the heat transfer performance of the heat pipe was the best.