Lattice Boltzmann Simulation,Theory Revision And Electric Field Enhancement Of Condensation

Mechanisms and enhancements of condensation heat transfer have attracted widespread attention in the world.It has been demonstrated in the literature that condensation is a complex multiscale heat transfer problem,especially for dropwise condensation,which involves six orders of magnitude at length scale,ranging from the nanometer level about droplet nucleation radius to millimeter level about droplet detachment radius.Although the study of condensation problems have been for nearly one hundred years,there still exist many unsolved problems about its heat transfer mechanism.In this thesis,a profound study of condensation will be done by combining three kinds of research approach: numerical simulation,theoretical analysis and experimental exploring.The numerical work is based on a newly developed mesoscopic lattice Boltzmann method(LBM),which not only simulates filmwise condensation and dropwise condensation on a vertical cold wall,but also investigates self-propelled jumping of condensate droplets after their coalescence on a superhydrophobic surface.Moreover,modification of dropwise condensation heat transfer theory is done at four aspects: droplet nucleation radius,heat transfer rate at droplet nucleation,droplet nucleation density and heat flux of condensation.Besides,visualization of electric field enhanced moisture condensation has been carried on,and the enhancement mechanism is theoretically analyzed.The detailed research contents are described below:1.Lattice Boltzmann simulation of both filmwise condensation and dropwise condensation.Firstly,filmwise condensation on a hydrophilic subcooled vertical flat plate is simulated using a newly developed phase change LBM.The accuracy and applicability of this phases change model is evaluated by comparing the simulation results with the classical Nusselt model and existing boundary layer solutions,and some assumptions in previous analytical solution is verified.LBM can take account of interface shear stress easily without specially treatment,which has been neglected at the classical Nusselt model;on the other hand,it is difficult for the boundary layer solutions to solve the equation set when considering the effects of vapor on the condensate film.Moreover,this phase change LBM is applied to simulate droplet nucleation,growth up,deformation and detachment during dropwise condensation on a vertical cold surface.Effects of contact angle on droplets dynamic behaviors are investigated,and heat flux variance at the process is studied.It is found that the heat flux at droplet nucleation increases very fast,which means that droplet nucleation is also a heat transfer process.2.Lattice Boltzmann simulations for self-propelled jumping of droplets after coalescence on a superhydrophobic surface.Coalescence induced condensate droplets self-propelled jumping on textured superhydrophobic surfaces(SHS)is numerically simulated using three dimensional(3D)multiple-relaxation-time(MRT)LBM.The energy conversion is analyzed,and the interaction between the droplet and the texture is investigated.Simulation results shows that the downward velocity of the droplet is reverted to upward direction due to the counter action of the surface,and leading to droplets jumping away.Droplets jumping velocity by LBM simulation is compared with the existing experimental data in literature.And morphological effects of texture on condensate droplets coalescence as well as jumping are discussed for the first time to explain the large discrepancy in jumping velocity reported in different experiments.It is shown that for a texture with small spacings,the adhesion force is large,which results in a decrease of its jumping velocity.On the other hand,for a texture with large spacings,the lower contour of the droplet falls into the texture,which also decreases droplet jumping velocity.3.Development of dropwise condensation theory.Heat transfer model of dropwise condensation mainly consists of two parts: single droplet heat transfer and drop size distribution.In this thesis,an improved thermodynamic model is developed for droplet nucleation radius at condensation based on the availability(or exergy)criterion,which considers thermal resistances of the coating layer,droplets heat conduction,the liquidvapor interface,and curvature depression.Then the concept of heat transfer rate at droplet nucleation process is proposed for the first time,which is always equal to zero in previous models,thus not taken into consideration at all by them.Further more,in our LBM simulation of dropwise condensation,it is found that the heat flux at droplet nucleation increases very fast,which also validate the concept of heat transfer rate at droplet nucleation.Meanwhile,relationship between droplets nucleation density and nucleation radius is established based on an early prediction expression in the literature.Moreover,by applying the newly determined droplets nucleation radius and nucleation density to heat transfer theory of dropwise condensation,heat flux is obtained and then compared with existing experimental data with good agreement.Effects of subcooled degrees,contact angle,thickness and thermal conductivity of the coating layer as well as saturated pressure on droplet nucleation radius,heat transfer rate at nucleation,nucleation density and condensation heat flux are studied.4.Visualization of electric field enhanced moisture condensation and its theoretical analysis.The process of moisture condensation under an external electric field is experimental studied.Droplets nucleation,growth,coalescence and renucleation are visualized and investigated.The results show that high electric field can decrease the energy barrier of condensation,thus increase the droplets nucleation density,promote droplets growth,accelerate condensation cycles,aggrandize condensate coverage,increase the possibility of droplet renucleation and more small droplets(r<10?m)exist on the subcooled surface.Further more,theoretical analysis of droplet nucleation during moisture condensation under an external uniform electric field is implemented.It is shown that high external electric field will reduce energy barrier of droplet nucleation process,and decrease its nucleation radius.This thesis not only offers new ideas for the numerical simulation of condensation,but also modifies classical dropwise condensation heat transfer theory.Moreover it experimentally explores new method to enhance condensation.