3D Numerical Simulations On Flow And Heat Transfer In Multi-Droplet Impact On Liquid Film

The Flow phenomenon,heat and mass transfer during multi-droplet impact process have always been a hot topic in the academic community.In depth exploration of the special phenomena and mechanism of impact process can improve the level of relevant industrial applications,and provide an important theoretical basis for industrial upgrading.Many scholars focus on the complex interaction between droplets,droplets and liquid film during the multidroplet impact process.Comprehensive study has been done for the multi-droplet impact on liquid film,while research focused on the flow and heat transfer by 3D numerical simulation method.Aiming at the process of multi-droplet impacting liquid film with different impact modes,including simultaneous impact,successive impact,non-simultaneous impact.Founding a threedimensional model with random disturbance subjected to Gaussian distribution,which is added in form of momentum source terms.The two phases interface captured by couple Level-set and volume of fluid method.The reliability of interface capture method and the disturbance model was verified by comparison with experimental results.The mechanism of the formation and subsequent cracking of central liquid sheet during multi-droplet simultaneous impact had been explained.The effects of Weber number,liquid film thickness and droplets horizontal interval on the height of central liquid sheet had been discussed.Results show that splashing of secondary crown is not induced with the same impact kinetic energy of successive droplets according to the theory of kinematic discontinuity.It observed that bubble entrainment occurred during the crown-crown interaction.Quantitative analysis of the size variation of primary crown had been carried out.The effects of Weber number,liquid film thickness and droplet spacing were discussed,and compared with the crown size variation of single drop impact on the liquid film.In the research of multi-droplet non-simultaneous impact liquid film,the droplet arrangement had been classified,including isosceles triangle distribution,right triangle distribution,obtuse triangle distribution.The mechanism of the liquid sheet breakup was analyzed.The single-phase heat transfer problem in the multi-droplet impact process was studied.It can concluded that the splash threshold was lower due to cracking of central liquid sheet,and successive impact results in higher splashing threshold of the trailing droplet than leading droplet due to radial flow in the residual film.It is found that the local single-phase heat transfer coefficient in the impact region is significantly higher than that in the undisturbed region,indicating that the droplet impact enhances the convective heat transfer.A heat accumulation point situated at the bottom of the crown and central liquid sheet,and identified as a heat transfer blind spot.By analyzing the cross-section temperature distribution during the successive impact of multi-droplet,it revealed that the interaction between the flow field and the temperature field of residual liquid film is enhanced,so that the convective heat transfer intensity is improved.Finally,the effects of heat flux,droplet spacing and Weber number on local heat transfer coefficient and local wall temperature under two kinds of impact were discussed.The problem of two-phase heat transfer during the multi-droplet impact on a thin liquid film covering a heated wall was studied.Many nucleation bubbles were arranged at the bottom of the liquid film at the saturation temperature,and it was found that the bubble experienced a series of movement and deformation during the simultaneous impact process.It is concluded that the configuration of vapor bubbles has little effect on the interface evolution and heat transfer coefficient in the impact process,but it has a remarkable effect of the peak value of the heat transfer coefficient in the three-phase contact lines near the bubble.It had been found that reducing the contact angle helps the vapor bubbles to quickly detach from the heated surface,thereby increase the convective heat transfer strength.The influence of droplet spacing on the local heat transfer during successive impact was discussed.The variation of local heat transfer in the impact region during successive impact was summarized.