Visualization Study And Analysis Of Enhanced Boiling Heat Transfer Mechanism On Micro/Nano Hybrid Surfaces

Future technological products are increasingly becoming integrated in compact spaces with higher thermal loads,which has led to the search for better heat management solutions to ensure stable operation of equipment.Conventional heat transfer is achieved by natural or forced convection of a single-phase working fluid,however,the cooling efficiency is directly dependent on the sensible heat of the coolant in the single-phase case,which requires increasing pumping power and volume flow of the coolant,etc.Thereby,the cooling effect under the high heat flow is restricted by limited conditions.Alternatively,a large number of studies show that two-phase approaches,particularly boiling heat transfer(BHT)is an effective heat transfer approach possessing the attractive property of having a large latent heat of evaporation,so that a large amount of heat can be released at very low temperature gradients.Among the many ways used to enhance the performance of BHT,changing the wettability and microstructure of the boiling surface stands out because of its superior characteristic such as no need for external power.In this study,by combining the visual pool boiling experiment with numerical simulation,the bubble dynamics parameters related to the formation,growth and departure of the bubbles,including bubble departure diameter(D_b),bubble departure frequency(f)and bubble active nucleation sites density(N),together with the temperature and velocity distribution near the moving bubbles,were collected and calculated.The simulation results were compared with the experimental results of bubble behavior mechanism and morphological characteristics.In addition,the BHT performance reflected by boiling curve and Heat Transfer Coefficient(HTC)of DI water on the smooth flat copper(SFC)surface;CNTs-deposited Copper(CDC)surface,that is,nano-scale surface;Micropillar Arrayed(MPA)surface,namely micron-scale surface and Micropillar with Deposited CNTs(MPDC)surface,namely micro/nano hybrid surface are studied under the atmospheric pressure.The results showed that on the surfaces without CNTs(SFC and MPA surfaces),the D_b increases with the increase of wall superheat,while the f shows an opposite trend.Therefore,the above two boiling surfaces grow fully before separating from the boiling surface and stay on the heating surface for a long time.Moreover,the larger heat transfer area and capillary force brought by the micropillar structure on the MPA surface can enhance the performance of BHT,and the average HTC increment on the MPA surface is 144.55%of that on the SFC surface.However,with the increase of wall superheat,the temperature gradient on the surface with sprayed CNTs(CDC and MPDC surfaces)is bigger,leading to a greater inertia force,which is more advantageous to bubble departure,so the D_b of the above two surfaces is inversely proportional to the degree of superheat,and f is inversely proportional to the wall superheat.N is also significantly increased due to the presence of a large number of microscale defects acting as nucleation sites in the deposited coating.The average HTC increment on the CDC surface is 116.33%of that on the SFC surface.Under the medium and high wall superheat,the bubbles on the MPA surface have grown to a large contact area with the micropillar,and the larger surface tension inhibits the departure of the bubbles.While the bubbles on the MPDC surface are small enough(～0.5 mm)to quickly leave the boiling surface and combine into larger air mass with other bubbles in the water again to further attract the bubbles below.Such bubble movement behavior can fully drive the flow of liquid around the moving bubble,and the velocity of the mainstream area of liquid is close to the bubble rising velocity(0.3 m/s-0.4 m/s),leading to stronger micro-convection effect.In conclusion,the three modified surfaces could all improve the NBHT performance of deionized water.The enhancement heat transfer effect of MPDC surface was the best,and its HTC was proved to be increased to 230.12%of that of SFC surface,followed by MPA and CDC surfaces.Additionally,a heat distribution model suitable for micro/nano hybrid surface was established considering the coupling contributions of heat flux among microlayer evaporation,transient conduction and micro-convection.The model can predict the experimental boiling curves of non-smooth surfaces well,which can provide some reference for the mechanical study of performance of BHT.