Study On The Characteristics Of Flow And Heat Transfer By Droplet Flow On The Surface Of Micro-Nano Structure

Droplet flow has the advantages of thin film thickness,low heat transfer resistance,high film fluctuation and high heat and mass transfer efficiency.However,the surface wettability of droplet flow is not ideal.The application of the micro-nano structure can improve the surface wettability and effectively suppress the completeness of thin liquid film,which can reduce the appearance of dry area,strengthen the droplet flow of thin liquid film layer flow and heat transfer.By employing a numerical simulation of droplet flow and heat transfer process,in this paper the change of spatial topological structure,dynamic spreading characteristics and evaporation phase transition process are fully coupled.In this paper,Ansys Fluent 18.0 software was used to conduct numerical simulation of the droplet flow heat transfer process,a two-dimensional numerical model is established.Re is 54-108,which belongs to laminar flow,and the two-phase interface was captured by VOF model.The Ψ is 3°-60°,the surface contact angle is combined with the dynamic contact angle model to characterize different tube surface structures and verified the model by quantitative analysis.In addition,the Sharp-interface model is used in the simulation to consider the evaporation phase transition phenomenon and to improve the accuracy of the calculation.The effects of different Re and Ψ on the flow and heat transfer characteristics of the droplet flow are studied.The variation rules of the film coverage,spreading velocity,film thickness and local heat transfer coefficient of the droplet flow are also analyzed in detail.The results show that the droplet has undergone a series of complex process,such as droplet growth-air stretching-fracture-bridge shrinkage-impact-film spreading-bottom convergence-regeneration.On the surface of larger Ψ,when Re is low,the thinnest part of the liquid film is easy to fracture and shrink,which makes the liquid film coverage fluctuate periodically,and the spreading velocity of the liquid film is restrained.In the stable state,the circumferential film thickness of the droplet flow is higher at the top of the tube,and then gradually decreases to the minimum value near 120°,at the bottom of the tube,the film thickness increases due to the formation of droplets.And in the stable state,the film thickness is less affected by Ψ.The local heat transfer coefficient in the liquid film reaches the maximum at the top of the tube and decreases slowly along the circumference of the tube,and decreases rapidly at the bottom of the tube.The average heat transfer coefficient on the surface with Ψ of 3° is 1.8 times of that on the surface with Ψ of 60°.In this study,theconvective heat transfer intensity at the top of the tube is higher and it increases with the increase of Re,and increases with the decrease of Ψ.