Investigation On Droplet Dynamic Behaviors And Heat Transfer Enhancement During Condensation On Groove Surface

Condensation heat transfer widely exists in energy,petrochemical,aerospace and other engineering fields.It is a key thermophysical process in industrial device systems such as power generation,desalination,refrigeration and air conditioning,and thermal management of electronic devices.However,the commonly used metal heat exchange materials have hydrophilic properties,which makes the condensate easily coalesce to form a high thermal resistance liquid film.How to quickly remove the condensate has become one of the key issues in the design of heat exchange surfaces.The groove structure has attracted attention because it can coordinate the gravity and the condensate surface tension to construct the pressure gradient for fast condensate transport.Moreover,with the progress of surface treatment and micro-nano processing technology in recent years,the modified groove surface can realize the synergistic effect of various dynamic behaviors of rapid condensate removal,including coalescence-induced jumping,self-propelled dewetting,droplet suction by liquid film,and the directional transport of the droplets,etc.,to maintain stable dropwise condensation,and greatly enhance the condensation heat transfer.Therefore,it is necessary to fully understand the mechanism of the condensate dynamic behavior on the groove surface,and develop the condensation heat transfer enhancement method for the groove surface.This exploration will be of significant scientific value in the theory development for microscale phase change heat transfer and interface dynamics.It will also provide important technical supports for the optimization design of advanced functional condensation surface and high-efficiency heat exchange devices.In this context,the study on droplet dynamic behaviors and heat transfer enhancement during condensation on the groove surface is carried out based on numerical simulation and experimental methods.A phase field based lattice Boltzmann model is improved for simulation of condensate dynamic behavior.The dynamic characteristics of droplet suction by liquid film is numerically investigated and the internal mechanism of droplet dewetting from microgroove is explored.A new type of functional surface with gradient groove structure is proposed to promote the condensate rapid removal and enhance condensation heat transfer.The main research content and conclusions are summarized as follows:(1)Lattice Boltzmann model for interface capturing of multiphase flows based on the Allen-Cahn equation A phase-field-based lattice Boltzmann model is proposed for interface capturing of multiphase flows based on the conservative Allen-Cahn equation.By adopting improved forms of a relaxation matrix and an equilibrium distribution function,the time derivative?_t(?u)induced by the recovery of the diffusion term in ACE is eliminated.The conducted Chapman-Enskog analysis demonstrates that the correct conservative ACE is recovered.To validate the improved model,a Zalesak’s disk rotation and a vortex droplet simulation are firstly carried out to test its capability of capturing the interface.The proposed MRT model shows its robustness and accuracy for various mobility.Then,the proposed model coupled with a flow field lattice Boltzmann solver is tested by droplet impact on the thin liquid film.Both qualitative and quantitative analyses have proved that the proposed model is capable of dealing with complex interface deformation in a large-density-ratio multiphase system.Finally,numerical simulations were carried out for the two dimensional Rayleigh-Taylor instability developed from single mode initial perturbations.Under the condition of large Reynolds number,the four stages,including exponential growth,terminal velocity,reacceleration and chaotic development stage,are reproduced.The numerical results of terminal velocity stage shows a good agreement with the theoretical solution.In addition,the position evolution of the spike and bubble is similar to the results of previous studies,which also proves the capacity of the new model to deal with complex topological changes in the flow.(2)Study on the dynamic behavior of droplet suction by liquid film on grooved surface Based on the improved phase-field-based lattice Boltzmann model,the dynamic characteristics of the droplet suction by liquid film is analyzed.The effects of droplet size,contact angle,viscosity and liquid film thickness on the suction speed and suction time is analyzed.The simulation results show a two-stage acceleration of droplet front point during the suction process.The first acceleration is driven by the large-curvature liquid bridge generated when the liquid film merges with the droplet.The second acceleration occurs due to the squeeze of droplet top when the interface on the advancing side deflects.The suction processes of droplets with different sizes show a self-similar characteristic,which have a same collapsed displacement-time curve and dimensionless spreading length.The decrease in wall hydrophilicity and droplet viscosity accelerates the movement of the front point,increases the speed growth during the second acceleration,and prolongs the spreading length.There is a critical liquid film thickness related to viscosity,less than which suction speed increases with the increase of the liquid film thickness.(3)Dewetting regimes of condensation droplets in a microgroove The groove-embedded droplet dewetting process of spontaneous transition from Wenzel state to Cassie state is numerically investigated using the lattice Boltzmann method.The key behaviors of extension,squeeze,rupture and ejection were reproduced,and the effects of groove geometry,wettability and droplet surface tension on the dewetting process were analyzed.Three dewetting regimes,namely retention,partial dewetting,and complete dewetting,are clearly identified for a condensation droplet growing in a groove.The droplet in the retention mode continues to grow along the groove.The droplet in partial dewetting mode generates a neck,which breaks latterly and results in partial liquid remaining in the groove.The droplet in the complete dewetting mode squeezes out the groove as a whole.As the groove aspect ratio and hydrophilicity decrease,the dewetting regime moves from retention to partial dewetting,and then to complete dewetting.The droplet requires to grow to a critical volume to attain a sufficient pressure difference driving the dewetting process.The critical dewetting volume decreases with the decrease of groove width or the increase of bottom and side wall contact angle.When other parameters are determined,there is an optimal groove height that minimizes the critical dewetting volume.(4)Study on condensation heat transfer characteristics of superhydrophobic gradient groove surface A superhydrophobic gradient grooved surface is proposed inspired by the water collection process on the cactus spines.The condensate dynamic behavior and the condensation heat transfer performance under different subcooling degrees on superhydrophobic plain surface,superhydrophobic square groove surface and superhydrophobic gradient groove surface are comparatively studied.The research results show that the key to enhancing the condensation heat transfer on the surface of superhydrophobic gradient grooves lies in the regulation of the nucleation position and size of the condensate.In terms of nucleation regulation,the micron groove structure leads to a higher vapor density on the ridge than in the groove.Besides,the processing defects at the groove edge induce lower nucleation barrier and strengthen the condensate mobility.In terms of condensate size regulation,the gradient groove structure can stimulate the efficient droplet removal behavior,including self-propelled dewetting and domino coalescence-induced jumping.Thus,the surface refresh frequency is improved and the embedded droplet size is reduced.With the increase of subcooling,the condensate morphology on the superhydrophobic plate surface and superhydrophobic square groove surface changes from the suspended Cassie state to the embedded Wenzel state.Therefore,the condensate removal mechanism changing from coalescence-induced jumping to gravity-driven sweeping.The maximum droplet detachment diameter increases rapidly,as well as the thermal resistance.On the superhydrophobic gradient groove surface,the self-propelled dewetting and domino coalescence-induced jumping behaviors greatly resist the retention of large droplets,which makes the surface have the best condensation heat transfer performance at large undercooling degree.