Direct Numerical Simulation Of Liquid Swirling Atomization

Liquid atomization is a multi-phase,multi-scale process,involving complex multi-physical problems such as turbulence,interface movement and their interactions.In the present thesis,based on the theory of high fidelity direct numerical simulation(DNS),a massively parallel computing method and platform have been developed for the simulation of gas-liquid flows.In order to reveal the underlying mechanism of liquid atomization and the interaction between turbulence and interface,problems concerning droplet drag coefficient,droplet breakup in turbulence,swirling liquid atomization and atomization model have been analyzed.A mass conserving interface capturing method has been first developed to direct numerical simulation of liquid atomization.The simulation of gas-liquid flows occurs many difficulties,such as length and time scales vary over several orders of magnitude,the interface constitutes a discontinuity in material properties,surface tension forces are singular forces active only at the phase interface,turbulence and the turbulent primary atomization process are inherently three dimensional,topology changes of the phase interface occur frequently.In order to solve these problems,based on the original computing platform,a mass conserving Level Set(LS)method has been developed to simulate gas-liquid flows.Classical LS method has the drawback of mass loss due to the discretization of LS transport equation and re-initialization process.We propose a mass remedy procedure based on the local interface curvature to solve this problem.The mass remedy procedure include three steps:Calculation of volume fraction from the Level Set function,Volume fraction remedy based on the local interface curvature,Reverse calculation of the Level Set function.Three benchmark cases,including Zalesak's disk,a drop deforming in a vortex field,and the binary drop head-on collision,are simulated to validate the present method,and the excellent agreement with exact solutions or experimental results is achieved.The present method is then applied to study more complex flows,such as a drop impacting on a liquid film and the swirling liquid sheet atomization,which again,demonstrates the advantages of mass conservation and the capability to represent the interface accurately.DNS of deformable droplet is performed to investigate the droplet drag coefficient.The motivation for the parameter range focused is mainly based on spray combustion applications,such as Diesel engine,aero engine.The result shows that the whole droplet moves like a jellyfish.Since ambient gas flows around the drop,a symmetric vortex pair appears behind the drop.But at longer times,the vortices behind the drop are no longer symmetric and display the so-called Karman vortex street.The drag coefficient can be represented as the function of Reynolds number and unsteady parameter(including density ratio).We consider the droplet deformation and circulation inside the droplet,hence the drag coefficient can be represented as the function of Reynolds number,Weber number,viscosity ratio and unsteady parameter,where Weber number represents the degree of deformation and viscosity ratio represents the degree of circulation inside the droplet.The simulation results indicate that the unsteady drag coefficient is always larger than the steady standard drag coefficient for the present decelerating relative flow.The effects of Weber number,density ratio and viscosity ratio on the unsteady drag coefficient of drop deformation are studied.It is found that the unsteady term(including density ratio)has the most effect on the unsteady drag coefficient and Weber number secondly.The viscosity ratio has little effect on the unsteady drag coefficient due to low Ohnesorge number.The numerical results confirm that the difference between drag coefficient and standard steady drag coefficient has an approximately linear curve fit with the unsteady parameter for low Weber number.But for large Weber number that doesn't exceed the breakup limit,the linear correlation is not valid.The droplet breakup in homogeneous isotropic turbulence has been simulated to investigate the interaction between turbulence and interface,and to evaluate and improve the ELSA model.The external force is added in Navier-Stokes euqations to obtain statistical steady turbulence.The effect of the Weber number is investigated through the droplet breakup at three Weber numbers.With increasing the Weber number,the liquid structure tends to break down into small droplets whose DSD satisfies the log-normal distribution,and the turbulent dissipation at intermediate and small scales tend to be suppressed.Compared with the turbulent local topology in the gas-phase HIT in the Q-R analysis,it is shown that the bi-axial strain is reduced at the final state in the liquid phase.In the interaction between vortical structures and the interface,the vorticity tends to be perpendicular to the normal of the large-scale interface for early times,but the normal alignment is mitigated through the breakdown into small droplets for late times.The dispersed droplets have relatively strong surface tension resisting the deformation induced by local turbulent straining motion.The investigation of the averaged normal alignment between vorticity and the interface normal reveals that it decays exponentially at early times,reaches the strongest normal alignment at around one large-eddy turnover time,and then recover and converge to a steady state with a relatively low level of the normal alignment,particularly for the cases with large Weber numbers and small local effective Weber number for dispersed small droplets in the final stage.Based on the DNS database,we also evaluate and improve the ELSA model.In this model,we improve the definition of equilibrium Weber number and give evidence to the turbulence source term.We report detailed numerical simulations of swirling liquid atomization to investigate the formation of liquid sheet,ligament,droplet and the effect of turbulence on the atomization.The swirl and atomization characteristics of two-phase annular swirling jets with the influence of turbulent inflow are investigated.The result shows that Rayleigh-Taylor instability is dominate for the swirling liquid atomization.Through comparing the sheet thickness,the breakup length and the cone angle,the numerical convergence of the global characteristics of the swirling two phase flow has been obtained.The numerical results show that turbulent inflow can induce liquid sheet breakup near the nozzle exit,reduce the stiffness of the liquid sheet,and lead to the statistically homogeneous distribution of small-scale liquid structures in the radial direction.Compared with the single-phase jet,the two-phase jet exhibits the chaotic velocity filed downstream that can enhance the mixing of droplets and ambient gas,and the precessing vortex core(PVC)is not observed in the center of the two-phase jet.In addition,the recirculation zone is smaller and farther from the nozzle exit for the turbulent inflow case than that from the laminar inflow case,and the preferential alignment of vorticity with the intermediate strain rate indicates that the fluctuating velocity in the recirculation zone is statistically similar to isotropic turbulence.The interaction of the liquid-gas interface and vortices shows the preferential normal alignment of the vorticity and the normal of the interface,and the liquid sheet can generate high shear layers to produce anisotropic small-scale fluctuations.The results show that surface tension term,pressure difference term and swirling term contribute to the sheet formation.The sheet rim and hole extension are the main reason for the formation of ligament formation.The droplet formation is attributed to the ligament breakup and stretch of center sheet.