Zhi Ning

On the spatial stability of a liquid jet in the presence of vapor cavities

A dispersion equation describing the effect of temperature differences on the stability of three-dimensional cylindrical liquid jets in the presence of vapor cavities is presented by the use of linear stability analysis. The mathematical model and its solving method are verified by comparing them with the data in the literature, and then the effect of temperature differences between jet and surrounding gas on the spatial stability of liquid jet is investigated. Some conclusions can be drawn from the results of this investigation: (1) the temperature difference destabilizes the liquid jet when the jet liquid is cooler than the surrounding gas, (2) the smallest atomized droplet without taking into account the effect of temperature differences is significantly larger than that when the effect of temperature differences is taken into account, (3) the effect of temperature differences on the stability of liquid jet has little relationship with azimuthal wave modes, (4) cavitation destabilizes the liquid jet wh...

Spatial instability in annular swirling viscous liquid sheet

The spatial linear instability of an annular swirling viscous liquid sheet moving in an inviscid gas medium was investigated. With both para-sinuous and para-varicose modes analyzed, results show that under certain conditions the mode difference for the maximum disturbance growth rate is small. However, the para-sinuous mode is always the dominant mode for an annular swirling viscous liquid sheet in the parameter range studied here. Liquid swirls have a stabilizing effect at low swirling strengths. Two regions were identified: the initial region, where the maximum disturbance growth rate decreases with increasing dimensionless liquid swirling strength, and a region of monotonic increase of maximum disturbance growth rate with increasing liquid swirling strength. The non-axisymmetric mode is the dominant mode when swirling strength is large in an annular liquid sheet. The drop in axial size of non-axisymmetric mode is larger than that for the axisymmetric mode. Liquid viscosity suppresses increases in the ...

Breakup of cavitation bubbles within the diesel droplet

Supercavitation in the diesel nozzle increases the instability of droplets in part due to the two-phase mixture, while the effect of cavitation bubbles on the instability of drops is still unclear. In order to investigate the breakup of cavitation bubbles within the diesel droplet, a new mathematical model describing the disturbance growth rate of the diesel bubble instability is developed. The new mathematical model is applied to predict the effects of fluids viscosity on the stability of cavitation bubbles. The predicted values reveal that the comprehensive effect of fluids viscosity makes cavitation bubbles more stable. Compared with the viscosities of air and cavitation bubble, the diesel droplet’s viscosity plays a dominant role on the stability of cavitation bubbles. Furthermore, based on the modified bubble breakup criterion, the effects of bubble growth speed, sound speed, droplet viscosity, droplet density, and bubble-droplet radius ratio on the breakup time and the breakup radius of cavitation bubbles are studied respectively. It is found that a bubble with large bubble-droplet radius ratio has the initial condition for breaking easily. For a given bubble-droplet radius ratio (0.2), as the bubble growth speed increases (from 2 m/s to 60 m/s), the bubble breakup time decreases(from 3.59 μs to 0.17 μs) rapidly. Both the greater diesel droplet viscosity and the greater diesel droplet density result in the increase of the breakup time. With increasing initial bubble-droplet radius ratio (from 0.2 to 0.8), the bubble breakup radius decreases (from 8.86 μm to 6.23 μm). There is a limited breakup radius for a bubble with a certain initial bubble-droplet radius ratio. The mathematical model and the modified bubble breakup criterion are helpful to improve the study on the breakup mechanism of the secondary diesel droplet under the condition of supercavitation.

Study on Droplet Size and Velocity Distributions of a Pressure Swirl Atomizer Based on the Maximum Entropy Formalism

A predictive model for droplet size and velocity distributions of a pressure swirl atomizer has been proposed based on the maximum entropy formalism (MEF). The constraint conditions of the MEF model include the conservation laws of mass, momentum, and energy. The effects of liquid swirling strength, Weber number, gas-to-liquid axial velocity ratio and gas-to-liquid density ratio on the droplet size and velocity distributions of a pressure swirl atomizer are investigated. Results show that model based on maximum entropy formalism works well to predict droplet size and velocity distributions under different spray conditions. Liquid swirling strength, Weber number, gas-to-liquid axial velocity ratio and gas-to-liquid density ratio have different effects on droplet size and velocity distributions of a pressure swirl atomizer.

The Instability of Vapor Bubble Growth within the Diesel Droplet under the Condition of Supercavitation

A new analytical form of dispersion equation which can be used to describe the disturbance growth rate of the diesel bubble growth instability is derived. The instability analysis of vapor bubble growth within the diesel droplet is carried out in this paper. Analysis results show: the disturbance growth rate is majorly influenced by six dimensionless variables. The disturbance growth rate initially decreases gradually then increases rapidly with increasing bubble volume fraction. The disturbance growth rate increases with increasing Weber number of vapor bubble growth, increasing Mach number and increasing diesel droplet Reynolds number. Both vapor bubble Reynolds number and ambient air Reynolds number have slightly influence on disturbance growth rate.

Study of effervescent jet breakup under gas expansion disturbance

Effervescent jet breakup-based gas expansion disturbance was studied by the combination of experiment and numerical methods. A transparent outside-in-type atomizer was used to observe both internal and external gas–liquid flow behavior. Effects of internal flow patterns and flow rates on gas expansion bulge were experimentally studied. Further analysis on the disturbance of gas expansion on jet breakup was conducted through the numerical method. The present work showed the results of gas expansion disturbance exiting under various internal flow regimes. Increasing gas–liquid flow rates enlarges spray angle and gas expansion bulge, decreases adjacent gas bulges’ distance, and leads to a more stable spray. Detailed numerical results demonstrated that the gas expansion advancing jet breakup was enhanced by enlarging instantaneous gas volume and internal pressure. For efficient utilization of gas expansion potential energy, small bubble formation is suggested.Graphical Abstract

The Temporal Evolution of Particle Population Involving Simultaneous Coagulation, Diffusion Deposition, Gravity Sedimentation and Ventilation in the Fixed Space

The dynamic behavior of small particles in fixed spaces is of interest in various fields of science and engineering. In the paper, a population balance equation for particles in fixed space is established to describe the dynamic behavior of system involving simultaneous coagulation, diffusion deposition, gravity sedimentation and ventilation. Related Parameters are decided by experiments for the convenience of achieving numerical solution of the equation. Finally, the temporal evolution of particles population in fixed space is simulated and discussed under different conditions.

Linear Instability Analysis of an Annular Swirling Viscous Liquid Jet

A three-dimensional linear instability analysis was carried out for an annular swirling viscous liquid jet with solid vortex swirl velocity profile. An analytical form of dispersion relation was derived and then solved by a direct numerical procedure. A parametric study was performed to explore the instability mechanisms that affect the maximum spatial growth rate. It is observed that the liquid swirl enhances the breakup of liquid sheet. The surface tension stabilizes the jet in the low velocity regime. The aerodynamic force intensifies the developing of disturbance and makes the jet unstable. Liquid viscous force holds back the growing of disturbance and the makes the jet stable, especially in high liquid velocity regime.

Study on Spreading of Liquid Droplet Impacting on a Solid Dry Surface

A numerical computation and theoretical model are presented on spreading of a single droplet impacting on a solid surface at low Weber number. The numerical simulation uses combined Level Set-VOF method and a precise wetting model. The singularity at the moving contact line was analyzed and removed by present wetting model. A theoretical model based on the energy balance was developed to predict the maximum spreading ratio, accounting for wetting effect by a correction factor. The droplets shapes, the dimensionless spreading radius and the dimensionless height of droplet calculated by present numerical method were compared with the experimental data. The theoretical model is used to predict the maximum spreading ratio. The numerical and theoretical results agree well with the experimental data. Present theoretical model indicates that capillary effects may be neglected if 6We(We/3+4)>>Re.