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Study Of Droplet Dynamics And Dropwise Condensation Heat Transfer Enhancement On The Micro/nano Structured Surfaces

Posted on:2023-10-04Degree:DoctorType:Dissertation
Country:ChinaCandidate:X WangFull Text:PDF
GTID:1522307061952689Subject:Engineering Thermal Physics
Abstract/Summary:PDF Full Text Request
Droplet dynamics and condensation heat transfer widely exist in nature and industrial applications,such as energy utilization,seawater desalination,power generation,petrochemicals,aerospace thermal management systems,pharmaceuticals and various other industries.In recent years,with the rapid development of micro-and nano-fabrication technologies and the intersection of fluid mechanics,interfacial mechanics,heat and mass transfer,interfacial materials science and other disciplines,the preparation and design of various advanced functional surfaces have contributed to the local modulation of microdroplets(including droplet growth,jumping and shedding,etc.)and the rapid development of dropwise condensation heat transfer,which deepened the understanding of solid-liquid interaction mechanisms and condensation enhancement strategies.The local modulation of microdroplets by means of complex micro/nano structures and hydrophobic/hydrophilic properties,which can be derived to condensation heat transfer enhancement,is one of the current hot topics in heat and mass transfer science.Therefore,it is of great scientific significance and engineering value to investigate the droplet dynamics and dropwise condensation heat transfer on the micro/nano structured surfaces.It is well known that dropwise condensation is a phase change heat transfer problem involving complex multiscale droplet dynamic evolution.Although a large number of researchers have extensively studied the dropwise condensation heat transfer and enhancement since 21st century,there are still many difficult problems such as gas-liquid interfacial evolution,multiscale dynamic characteristics,heat and mass transfer across the interface and complex solid-liquid interaction mechanisms.In this work,the spontaneous movement of droplets and condensation heat transfer are comprehensively investigated by numerical simulation,theoretical analysis and experimental exploration.The lattice Boltzmann method is used to emphasize the directional migration of droplets,coalescence-induced jumping of droplets and condensation heat transfer.Variation of the surface energy,energy coversion efficiency and heat flux are quantitatively analyzed.Nanostructured and micro/nano structured surfaces are fabricated to enhance condensation heat transfer.The influence of various factors on the multiscale evolution of droplets and heat transfer performance is explored,and the mechanism of interfacial structure to enhance dropwise condensation is revealed.The detailed research contents and conclusions of this work are as follows:(1)An improved three-dimensional lattice Boltzmann pseudopotential model is applied to numerically study the spontaneous movement of droplet on the composite wedge shaped surface and microstructured multi-wetting gradient surface.The influence of structure parameters,wettability and gravity on the deformation,velocity and surface energy of moving droplets is analyzed.As well,the physical mechanism of capillary force induced self-propelled movement of microdroplets is investigated.The velocity of droplet movement is improved by optimizing the surface structure parameters.The results demonstrate that the spontaneous movement of droplet on the wedge track is driven by the net surface tension.Under the combined effect of net surface tension and solid-liquid adhesion force,the droplet velocity increases and then decreases with time.Although the increase of vertex angle accelerates the droplet movement,it reduces the transport distance of droplet.In comparison with the simulation results,the vertex angle of 30°is an optimal solution to ensure the droplet velocity and transport distance.For the microstructured multi-wetting gradient surface,the larger wetting gradient and solid fraction improve the droplet movement and surface free energy.Although two-layer wetting gradient surface prevents the droplet from penetrating into the micropillars,it has a negative effect on droplet velocity.For the Bo number ranging from 0.0126 to 0.063,the climbing-upward movement of microdroplet on the inclined surface should take the gravity into account.For Bo≤0.0126,the inclined angle has almost no effect on the climbing-upward motion of droplet.(2)The self-propelled jumping behaviors induced by coalescence of condensate droplets on the microstructured surfaces are studied numerically using the two-dimensional lattice Boltzmann method.The effects of wettability,surface morphology,gravity,inclined angle and radius ratio on the jumping velocity and energy conversion efficiency of droplet are analyzed.The results show that the width of liquid bridge and jumping velocity simulated by LBM agree well with the published experimental and numerical results,which verifies the applicability and accuracy of the model for simulating the droplet jumping dynamics.The width and spacing of micropillar arrays have a significant effect on the jumping ability of coalesced droplets,while its height has little impact on it.Comparing the simulation results,a*=0.467,w*=0.067 and h*=0.5 are the optimal structures for the spontaneous jumping of merging droplets(r0=48.6μm).The jumping droplets on the inclined surface can successfully jump off without returning to the original spot when driven by tangential gravity.The spontaneous jumping induced by the coalescence of two mismatched droplets generates a tangential velocity that drives them to detach from the surface or merge with other droplets.For Bo>(0.00764/r)0.5,the effect of gravity on the jumping behaviors of merging droplets should be considered.The jumping velocity and energy conversion efficiency decreases with the decreasing radius ratio and increasing gravitational coefficient.In addition,as the energy conversion efficiency is less than 2.0%,the droplets fail to jump off the surface after coalescence.(3)A microscopic model of vapor condensation heat transfer is developed using an improved hybrid thermal lattice Boltzmann method.Single droplet evaporation and single droplet condensation on the hydrophobic surface are modeled and compared with the theoretical and experimental results to verify the applicability and accuracy of the model for simulating condensation heat transfer.The results show that for homogeneous wettable surfaces,the initial nucleation time of condensate droplets is prolonged with increasing contact angle,while the shedding time of condensation time is significantly reduced.Due to the release of latent heat during condensation,the temperature near the gas-liquid interface,especially near the three-phase contact line region,is much higher than elsewhere.The merging and shedding behaviors of condensate droplets cause local fluctuations in the average heat flux with time.For the hybrid wettability surface,there exists a hydrophobic-hydrophilic ratio to optimize the heat transfer performance.As well,the optimizal hydrophobic-hydrophilic ratio decreases as the wetting difference decreases.For the wetting gradient surface,directional migration of droplet caused by the capillary force facilitates the removal of condensate droplets,which in turn improves the condensation heat transfer performance.In addition,a larger wetting gradient is beneficial to further enhance the heat transfer performance.For the micropillared surface,condensate droplets initially form between the micropillars.The condensate mass and condensation rate of superhydrophobic surfaces eventually exceed those of hydrophobic and hydrophilic surfaces.In addition,the heat flux ofθa=150.7osurface is slightly higher than that ofθa=164.5osurface.The smaller spacing facilitates increasing the nucleation sites,while the larger spacing increases the jumping height of coalesced droplets.The triangular microstructure enhances the jumping ability of merging droplets,while the average heat flux is significantly lower than that of the square and semicircular microstructured surfaces.As the surface subcooling decreases,the nucleation time of droplet is prolonged.Smaller surface subcooling not only increases the maximum jumping height but also decreases the jumping radius of coalesced droplets.For the vertical condensing surface,the shedding mode of droplets changes from spontaneous jumping to gravity-driven rolling as the surface wettability strengthens.Compared with the microgrooved structure,microspined structure improves the jumping ability of condensate droplets and enhances condensation heat transfer.The evolution of heat flux with time from droplet nucleation to jumping off is divided into three stages:gradual increase after droplet nucleation,slight decrease during merging and sharp decrease after jumping off.Smaller width and height of microgroove contribute to improving the jumping velocity as well as reducing the critical departure size.(4)Nanostructured copper superhydrophobic surface,micro/nano structured silicon surface and hierarchical microporous and nanostructured copper superhydrophobic surface are fabricated to conduct the pure vapor condensation heat transfer experiments.The effects of different factors on the droplet dynamics of condensate droplets and heat transfer performance are investigated to reveal the mechanism of enhanced heat transfer by micro/nano structures.The results indicate that the transition of droplet wetting state from suspended Cassie state to impaled Wenzel state caused by the increase of surface subcooling prevents the jumping of merging droplets.As the subcooling degree is increased from 0.5 K to 3.5 K,the jumping frequency of droplet is reduced significantly from 173 cm-2·s-1 to 36 cm-2·s-1 and the average droplet diameter rises by~300%.Also,the jumping diameter of droplet ranges from 20μm to280μm,with a~60%reduction in the maximum jumping height.As the inclined angle increases from 0°to 90°,the average and maximum droplet diameters atΔT=2.0 K decline by about 63%and 33%,respectively.As the surface subcooling increases,a stable and high jumping frequency can only be maintained at the vertical orientation.Furthermore,the critical sliding diameter at a 30°inclination is 1.5,1.4 and 1.5 times higher than on the vertical substrate when the subcoolings are 5.0 K,6.5 K and 8.0 K,respectively.Compared to the horizontal surface,condensation heat transfer is enhanced by 21.1%atα=30°,49.2%atα=60°,and 72.4%atα=90°atΔT=2.0 K.The larger micropillar spacing can worsen the heat transfer efficiency.At large subcoolings(~25 K),although the condensate film covers the micropillar arrays,the thickness of condensate layer is the same order of magnitude as the height of micropillars.The heat transfer coefficient of S10R30 surface is enhanced by 26.4%compared to the nanostructured surface.Compared to the nanostructured surface,the jumping frequency of droplet is greater and the maximum as well as average diameter is smaller on the hierarchical microporous and nanostructured surface.At larger subcoolings(ΔT≤16 K),the critical shedding diameter of droplet on the hierarchical microporous and nanostructured surface is one order of magnitude lower than that on the nanostructured surface,and the heat transfer coefficient is increased by at least 26.4%.This work systematically investigates the droplet dynamic characteristics and condensation heat transfer performance on the micro/nano structured surfaces.It not only develops new ideas and research interests for the simulations of droplet dynamics,but also explores the methods to enhance condensation heat transfer via theoretical analysis,experiments and simulations.As well,it provides theoretical and technical support to further advance the potential applications of droplet dynamics and condensation heat transfer in various fields.
Keywords/Search Tags:Directional migration of droplet, droplet jumping, dropwise condensation, micro/nano structured surface, lattice Boltzmann method
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