Study Of Flow And Heat Transfer In The Process Such As Evaporation Of Nanofluid Sessile Droplet With LBM

The heat transfer performance of the fluid is the main factor that hinders the heat transfer efficiency,high compactness and effectiveness of heat exchangers.One effective method to increase the thermal conductivities of fluids is to add solid metallic,non-metallic and polymeric particles into the conventional fluids.However,the millimeter or micro-sized particles,which are added into the fluid,may cause severe problems in heat transfer equipment such as clogging when they pass through micro channels and erosion of components and pipe lines.All the problems limit the application of such suspentions.The fast development of nano material science brings opportunity to enhancement of heat transfer. A innovative concept of nanofluid is put forward,which refers to a two-phase mixture with its continuous phase being generally a liquid and the dispersed phase constituted of the 'nanoparticles' i.e.extremely fine metallic or nonmetallic particles of nanometer dimensions. The nanofluid forms a new type of coolant.As an innovative heat transfer substance,nanofluid can effectively enhance the heat transfer performance of the device and make the device more compact with high efficiency. The evaporation of nanofluid droplet will be widely applied in the field of vehicle,spaceflight, voyage,shipping and electron.The hydrokinetics,the theory of heat and mass transfer,the material science and the behavior dynamics of free interface are involved in the process of the nanofluid droplet evaporation.It is academically significant and has invaluable application to master the heat and mass transfer mechanism in this process.In this paper,the profile evolvement of nanofluid droplet during the process of evaporation on heated fiat substrate is simulated with Lattice Boltzmann moment model.The effect of nanoparticle size on the evolvement is analyzed.The changing characteristics of base diameter and contact angle for ethanol-aluminum nanofluid droplet are experimentally compared with that for pure ethanol.As a mesoscopic method between the macroscopic fluid dynamics and the molecule dynamics,Lattice Boltzmann method has many unique advantages compared to the traditional computational hydrodynamics methods,such as high efficiency,easily handle boundary condition and absolutely parallel computation.It is easy to understand and to write program. An important advantage of the Lattice Boltzmann method is that microscopic physical interactions between different phase particles can be conveniently incorporated into the numerical model,which is lacked in the conventional methods,so that it could supply an effective way to investigate multiphase flow and complex boundary condition problems. Lattice Boltzmann moment model supplies several controllable parameters as multi relaxation parameters,which can satisfy the physical requests and keep stability for different hydrokinetic systems.In addition,the extemal forces and thermal action can be facilitated treated in such moment model.The development,the characteristics and the main idea of Lattice Boltzmann method are introduced in this paper.In view of the evaporation process of nanofluid droplet being complex,where multi-component and multi-phase are coexist,the simulation begins at the simple single component single phase(SCSP) model,then it proceeds systematically to complex multi-component and multi-phase(MCMP) model.To begin with,SCSP model is built to simulate the mixed convection in lid driven rectangular when there is temperature difference between the two side walls.The couple of velocity and temperature is accomplished.And the simulation of effects of forces such as gravity and buoyancy on the velocity and temperature distribution is methodological realized.Next,based on the single component multi-phase(SCMP) model,the dynamic behavior of droplet impinging on substrate is simulated at cylindrical coordinate,and the moving interface between the liquid and the gas is successfully simulated as well.In the end,the evaporation process of nanofluid droplet on heated substrate is simulated with MCMP model and the evolution of nanofluid droplet is obtained.The results show that during the evaporation the droplet experiences the pinning stage,where the base diameter keeps constant and the height decreases,and the depinning stage,where the droplet recoils.A ring-shaped nanoparticle stain is formed and it is affected by the size of nanoparticle.The mechanism of nanoparticle acting on the evaporation is discussed.The evaporation process of nanofluid droplet on heated substrate is experimentally investigated,and the results are compared with the simulation results.By analyzing the shapes,the evaporation process of ethanol-aluminum nanofluid droplet is compared with that of ethanol droplet.Same as what concluded from simulation results,the experimental results show that the evaporation experiences pinning and depinning stage.In the pinning stage,the evaporation speed of nanofluid droplet is lower than that of ethanol droplet due to its larger viscosity.The deposition of nanoparticles at the rim changes the interface tension between the solid sustrate and the liquid droplet,which leads to a larger contact angle.As the result,the initial contact angle of nanofluid droplet is larger than that of ethanol droplet.Meanwhile,the critical angle when the droplet begins to recoil is smaller for nanofuild than for ethanol,so the nanofluid droplet pinning stage is longer than ethanol pinning stage.That is,the ethanol depinning stage is longer,when the evaporation speed is sharply decreased.Therefore,for the same droplet volume,the whole evaporation speed of nanofluid is faster than that of ethanol. The simulation is proved be valid by comparison the simulation results and experimental results.