Numerical Simulation Of Dynamic And Heat Transfer Characteristics Of Droplet Impacting On Porous Media

The phenomenon of droplet impinging on porous media exists widely in nature and plays an important role in many engineering applications.It has attracted more and more attention in recent years.Compared with that on impermeable surfaces,droplet impact on porous surfaces results in droplet penetration into the porous interior due to the capillary force,and this makes the two processes of spreading and penetration occur simultaneously and compete with each other,especially for hot porous surfaces.So far,the coupling characteristics of spreading,penetration and evaporation are still not clear.In this paper,the dynamics of single/double droplet impact on a normal temperature porous surface and the dynamics and evaporation of droplet impact on a hot porous surface are investigated numerically using a level set method to trace the droplet free surface.Firstly,the effects of droplet impact characteristics(droplet size and impact velocity),porous substrate properties(porosity and particle diameter)and the fluid properties(surface tension and dynamic viscosity)on droplet impact dynamics were investigated.The results show that when the inertial force of droplets is not sufficient to overcome the resistance of porous substrate,part of the liquid will remain on the substrate surface and will not penetrate;conversely,droplets will continue to penetrate;in addition,it was found that the impact and fluid properties have weaker effects on the droplet spreading and penetration behavior than the substrate properties,mainly because both porosity and particle diameter directly affect the resistance inside the porous media.The smaller the resistance,the easier the droplet penetration;then,a prediction model for the penetration depth of droplet impact on the porous surface was established by analyzing each factor.Secondly,the spreading infiltration process of double droplet impinging on porous media was analyzed in detail,and the effects of the horizontal diatance of two droplets,droplet size,impact velocity,particle diameter and dynamic viscosity on the flow at the phase interface were discussed.The results show that: the horizontal distance has little effect on the penetration depth of droplets in the substrate,and the correlation between the maximum height of the jet column and the horizontal distance is not monotonic;when at a fixed horizontal distance,the droplet size and impact velocity are positively related to the initial kinetic energy of the droplet,and the larger the initial kinetic energy is,the larger the maximum jet height and spreading factor that the droplet can reach,and the central jet column will produce secondary droplets;increasing the particle diameter of porous media will promote the penetration process of droplets,resulting in a decrease in the height of the jet column after the merging of two droplets;in addition,it was found that both the jet column and the spreading factor show non-monotonic changes with the increase of dynamic viscosity.Finally,the effects of droplet working medium,surface temperature,droplet size,impact velocity and porosity on the evolution of droplet morphology during evaporation were studied,and the changes of velocity,pressure and temperature fields during droplet evaporation were analyzed.The results show that the droplet size and impact velocity both show a positive relationship with the spreading factor and a non-monotonic variation with the penetration depth;the spreading factor decreases and the penetration depth increases with the increase of porosity;with the increase of temperature,the spreading factor decreases,the time for the droplet to reach the maximum spreading factor is shortened and the penetration depth decreases significantly;during the evaporation process,the droplet starts to contract from its sides and bottom,and the evaporation rate of fluid outside the porous medium is significantly higher than that inside the porous medium;increasing the surface temperature,droplet size,impact velocity and reducing the porosity,the droplet evaporation rate increases.