Study On Hydrodynamic Characteristics And Heat Transfer Mechanism During Diesel Droplet Impingement

Droplet impacting on a surface widely exists in industrial applications.Research of dense droplet impacting on inclined dry and wetted surfaces and hollow droplet impacting on horizontal dry and wetted surfaces were focused on in this article.The hydrodynamics and heat transfer mechanism during the impingement process were revealed by adopting a combined analysis based on experiments,numerical simulations and theories.A better understanding of the process was achieved,which not only enriches the theory of gas-liquid flow with free interface,but also improves the spray control technology.Based on high-speed camera,an observing system for the experiment of droplet impacting on inclined surface was developed.The hydrodynamics of the droplet were experimentally studied.It was shown that surface wave is observed at a low impact velocity.The droplet is ladder-like spread after impingement.With the increase of impact velocity,the droplet successively presents spread,fingering edge and splash,where splash appears only at the front spreading edge.Comparing with the droplet impacting on a dry surface,droplet impacting on a wetted surface presents splash at both front and back spreading edges.In addition,the spreading factor is larger but with lower maximun spreading velocity.The critical criteria for droplet impacting on inclined dry and wetted surfaces were developed based on experimental results.A numerical model was developed to investigate a droplet impacting on inclined surface by using coupled Level set and Volume of Fluid method,in which surface tension,heat transfer and contacting resistance were taken into account.This model was validated with the results from experiment.The droplet spread mechanism impacting on both dry and wetted surfaces were obtained.The numerical results show an asymmetric spread of droplets,which is significantly affected by the pressure gradient inside the droplet.Inertia energy affects the formation of front jet and the liquid motion disconnection leads to the formation of back jet.The change of surface tension resulting from Marangoni effects gives rise to the formation of splash with heat transfer considered in the impact.Based on energy balance equation,a theoretical correlation for the maximum spreading factor was developed in terms of impact angle.Air entrainment and heat transfer in the droplet impingement process were studied.The difference of air entrainment was observed between droplet impingements on dry and wetted surfaces.The entrained air has a great impact on the surface heat flux distribution for dry surface impingement and the impact becomes less significant for wetted surface impingement.When a droplet impacts on a dry surface,the dynamic feature tends to lag behind that of heat transfer,its average wall heat flux is closely related to impact velocity.Comparing with dry surface impact,the liquid-to-solid heat transfer characteristics is less for wetted surface impact.Hydrodynamics and heat transfer of a hollow droplet impacting on a horizontal surface were numerically investigated,and the central jet formation and heat transfer were studied.The results show that hollow droplet impacting on a horizontal dry surface presents spread,central jet and liquid shell rupture,its dynamic feature tends to lag behind that of heat transfer.The numerical maximum spreading factor and average velocity of central jet are close to theoretical analysis,increasing as Reynolds number increases.The extended feature before impact is positively related to the cavitated gas pressure.The higher cavitated gas pressure is,the more obviously extended feature and central jet surface waving feature are.As the pressure of cavitated gas increases,the spreading feature and average wall heat flux decrease,while the dimensionless central jet height and velocity increase.When a hollow droplet impacts on a wetted surface,it presents spread,splash and central jet.Compared to dry surface impact,the jet at the spreading edge and impacting center are more obviously,the maximum spreading and central jetting velocities and average wall heat flux are less.In addition,the dynamic and heat transfer feature tend to be synchronism.Also,compared to dense droplet impact,the spreading liquid film is more smoothly and the height of jet at the spreading edge is larger for hollow droplet impact.Cavitated gas contributes to the development of jets at the spreading edge and impacting center,but diminishes spread and heat transfer features during impact.