| The study of two-phase flow boiling heat transfer in microchannels plays an important role in improving the safety of microelectronic components.The experiment of this paper is based on two kinds of microchannel surfaces which are coated with copper nanowires and polished with 3000#sandpaper respectively.The experimental working medium is High-purity deionized water,and the critical heat flux rate density,heat transfer coefficient,and pressure drop characteristics during forced convection heat transfer in both microchannels were studied.The flow characteristics of the working fluid in the microchannel with the increase of the heat flux rate density was studied,the mechanism by which the copper nanowires enhance the two-phase boiling heat transfer in the microchannel is deeply investigated coupling with the visual data.Based on the electrochemical deposition method,a dense,orderly and highly controllable layer of copper nanowires was deposited on the bottom surface of microchannel using a porous anodic aluminum Oxide thin film.Three-electrode system were adopted,and the copper nanowires were scanned under the scanning electron microscope(SEM)to obtain the of the top and side characteristics.Experimental study based on smooth surfaces and surface coated with copper nanowires was carried out.Compared with smooth surface,the microchannel with Copper nanowires reach the maximum heat transfer coefficient of reinforcement under the mass flow rate of 100 kg/m~2s,and the enhancement rate is 26.18%,what’s more,the degree of super heat is promoted about 5℃when dry-out happens.The maximum effective heat flux rate enhancement is obtained under the mass flow rate of 100 kg/m~2s,and the enhancement rate is 22.38%.The reason is that copper nanowires coated microchannel have stronger capillary force comparing smooth microchannel,which can pull liquid into the micro/nano structure to wet the wall and reduce the wall temperature fluctuation.Copper nanowires can change the flow characteristics of working medium with the increase of input effective heat flux rate.The flow regimes in microchannel coated with copper nanowires are different from those in smooth channel.Under the low heat flux rate,the movement cycle of working medium in copper nanowires microchannel contains bubble generation,small bubbles merge into vapor slug,side length of vapor slug spread to imports and eventually occupy the whole microchannel.The working medium flow reversely at the time,and there is liquid thin film on the side wall surface to wet the bottom wall by capillary force.Under the mass flow rate of 66.67 kg/m~2s,flow instability phenomenon lasted until 30.78 W/cm~2.the bigger the input effective heat flux rate is,the thicker the liquid film on the side wall is,aiming to wetting the wall.The copper nanowire structure provides the capillary force to pull the liquid phase,plus the bubble size is small,so that the interface between the liquid phase and the vapor phase will not be destroyed by the resulting disturbance.For smooth channel,when the input heat flux rate density is low,flow instability happens.The period contains bubbles generation,bubbles’aggregation into a slug,and the occupation of whole channel by vapor slug,export liquid’s return to microchannel until the next cycle.With the increase of input effective heat flux rate,flow instability no longer happens in microchannel,and the heat flux is much less than that in microchannel coated with copper nanowires when flow instability stops,which is 23.24 W/cm~2 under the mass flux rate of 66.67 kg/m~2s.When flow instability stops,the vapor-liquid interface is broken due to the bubble growth,polymerization and the opposite force supported by the vapor phase and liquid phase.Liquid is entrained by vapor phase to wet the heat surface. |
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