STUDY ON FLUID MECHANICS OF HYPERVELOCITY LIQUID JETS

The fluid mechanics in the generation of hyper veloeity water jets, light oiljets and glycerin jets was studied. Framing high-speed photography and single-shot photography wereused to observe the jets directly. The purposes of this study is to investigate the disintegrationand atomization processes at the velocity of 2km/s-3km/s as well as the auto-ignition andself-combustion of the light oil jets. Therefore, in the jet velocity measurement in addition to thehigh-speed photography, the results by other methods such as the laser beams cutting method and theshock wave detection using pressure transducers were also given. In the observation of the jetsevents, the illumination phenomenon was found, which may be regarded as the result of theauto-ignition and combustion of the light oil jets. Finally, the Munroe jet was studied.

Formation of radially expanding liquid sheet by impinging two round jets

A thin circular liquid sheet can be formed by impinging two identical round jets against each other. The liquid sheet expands to a certain critical radial distance and breaks. The unsteady process of the formation and breakup of the liquid sheet in the ambient gas is simulated numerically. Both liquid and gas are treated as incompressible Newtonian fluids. The flow considered is axisymmetric. The liquid-gas interface is modeled with a level set function. A finite difference scheme is used to solve the governing Navier-Stokes equations with physical boundary conditions. The numerical results show how a thin circular sheet can be formed and break at its circular edge in slow motion. The sheet continues to thin as it expands radially. Hence, the Weber number decreases radially. The Weber number is defined as ρu 2 h/σ, where ρ and σ are, respectively, the liquid density and the surface tension, and u and h are, respectively, the average velocity and the half sheet thickness at a local radial location in the liquid sheet. The numerical results show that the sheet indeed terminates at a radial location, where the Weber number reaches one as observed in experiments. The spatio-temporal linear theory predicts that the breakup is initiated by the sinuous mode at the critical Weber number We c =1, below which the absolute instability occurs. The other independent mode called the varicose mode grows more slowly than the sinuous mode according to the linear theory. However, our numerical results show that the varicose mode actually overtakes the sinuous mode during the nonlinear evolution, and is responsible for the final breakup. The linear theory predicts the nature of disturbance waves correctly only at the onset of the instability, but cannot predict the exact consequence of the instability.

EVALUATION OF THE CUMULATIVE FORMATION OF HIGH-SPEED LIQUID JETS

This paper describes the generation of pulsed, high-speed liquid jets usingthe cumulation method. This work mainly includes (1) the design of the nozzle assembly, (2) themeasurement of the jet velocity and (3) flow visualization of the injection sequences. Thecumulation method can be briefly described as the liquid being accelerated first by the impact of amoving projectile and then further after it enters a converging section. The experimental resultsshow that the cumulation method is useful in obtaining a liquid jet with high velocity. The flowvisulization shows the roles of the Rayleigh-Tay-lor and Kelvin-Helmholtz instabilities in thebreakup of the liquid depend on the jet diameter and the downstream distance. When the liquid jetfront is far downstream from the nozzle exit, the jet is decelerated by air drag. Meanwhile, largecoherent vortex structures are formed surrounding the jet. The liquid will break up totally by theaction of these vortices. Experimental results showing the effect of the liquid volume on the jetvelocity are also included in this paper. Finally, a method for measuring the jet velocity bycutting two carbon rods is examined.

PARALLEL ADAPTIVE SIMULATION OF A PLUNGING LIQUID JET

This paper is concerned with three-dimensional numerical simulation of a plunging liquid jet. The transient processes of forming an air cavity around the jet, capturing an initially large air bubble, and the break-up of this large toroidal-shaped bubble into smaller bubbles were analyzed. A stabilized finite element method （FEM） was employed under parallel numerical simulations based on adaptive, unstructured grid and coupled with a level-set method to track the interface between air and liquid. These simulations show that the inertia of the liquid jet initially depresses the pool＇s surface, forming an annular air cavity which surrounds the liquid jet. A toroidal liquid eddy which is subse- quently formed in the liquid pool results in air cavity collapse, and in turn entrains air into the liquid pool from the unstable annular air gap region around the liquid jet.

Spray and mixing characteristics of liquid jet in a tubular gas-liquid atomization mixer

For the design and optimization of a tubular gas-liquid atomization mixer,the atomization and mixing characteristics of liquid jet breakup in the limited tube space is a key problem.In this study,the primary breakup process of liquid jet columnwas analyzed by high-speed camera,then the droplet size and velocity distribution of atomized droplets were measured by Phase-Doppler anemometry(PDA).The hydrodynamic characteristics of gas flow in tubular gas-liquid atomization mixer were analyzed by computational fluid dynamics(CFD)numerical simulation.The results indicate that the liquid flow rate has little effect on the atomization droplet size and atomization pressure drop,and the gas flowrate is themain influence parameter.Under all experimental gas flowconditions,the liquid jet column undergoes a primary breakup process,forming larger liquid blocks and droplets.When the gas flow rate(Qg)is less than 127 m^(3)·h^(−1),the secondary breakup of large liquid blocks and droplets does not occur in venturi throat region.The Sauter mean diameter(SMD)of droplets measured at the outlet is more than 140μm,and the distribution is uneven.When Qg>127 m^(3)·h^(−1),the large liquid blocks and droplets have secondary breakup process at the throat region.The SMD of droplets measured at the outlet is less than 140μm,and the distribution is uniform.When 127<Qg<162m^(3)·h^(−1),the secondary breakup mode of droplets is bag breakup or pouch breakup.When 181<Qg<216m^(3)·h^(−1),the secondary breakup mode of droplets is shear breakup or catastrophic breakup.In order to ensure efficient atomization and mixing,the throat gas velocity of the tubular atomization mixer should be designed to be about 51 m·s^(−1) under the lowest operating flow rate.The pressure drop of the tubular atomization mixer increases linearly with the square of gas velocity,and the resistance coefficient is about 2.55 in single-phase flow condition and 2.73 in gas-liquid atomization condition.

Turned Trochoidal Disturbance on a Liquid Jet Surface

This paper shows that a turned trochoidal function disturbance may lead to peripheral drops production. The resulting model is used to describe that a turned trochoidal disturbance leads to peripheral drops production on the liquid jet surface without the necessity for superimposed disturbances. The trochoid is a non-unique parametric function. Only non-unique parametric functions disturbances may lead to peripheral drops production. The trochoidal function disturbance is decomposed to Fourier series. Every Fourier element receives an amplification factor in accordance to the Rayleigh inviscid jet model. Peripheral drops are received on the jet surface. The paper shows that all trochoidal disturbance functions, prolate cycloid, cycloid and curtate cycloid have a capability of peripheral drops producing. A limited capability of peripheral drops production is introduced for the trochoidal curtate cycloid. Produced drops size are reduced for increasing the jet velocity and wave number. Smaller drops are also received by transition from the prolate cycloid to curtate cycloid disturbance.

Skin friction and heat transfer of liquid jet over a continuous moving horizontal hot plate

The skin friction and heat transfer occurring in the laminar boundary layerwhich caused by a vertical liquid jet impinging on a continuously moving horizontal plate werestudied. Similarity solutions for shear stress and heat distribution were obtained by using thehooting technique. The results show that the skin friction decreases with an increase of velocityparameter, the evolving of thermal boundary decrease with increasing in Prandtl number, but increasewith increasing of velocity parameter.

Numerical Study on the Pulsation Characteristics of An Attached Air Bubble Under A Nearby Oscillating Bubble

Numerical simulations of the non-spherical evolution of a pulsating bubble interacting with a stationary air bubble attached to a fixed structure were performed using a three-dimensional boundary integral method by implementing the mirror image method to simplify the processing of the numerical model. Code validation was accomplished by comparing the numerical results with the laboratory experimental data obtained in our previous study. Complex phenomena were observed, including three types of bubble jet forms, which depended strongly on the distance parameter with respect to the initial location of the bubble from the plate, the bubble strength parameter and the initial air bubble radius parameters. The results of the simulations provide detail insight into interesting bubble jetting phenomena,such as bubble splitting, jets moving away from the plate, and bubble shedding. The dimensionless distance parameter and the initial air bubble radius parameter play a major role in determining the shapes of two bubbles and the jetting direction. The air bubble strength parameter did not change the bubble jet direction but influenced the bubble shape.The detachment of the attached air bubble under oscillating bubble suction was easily observed for a small initial air bubble. These results showed that bubble jetting toward the plate was manipulated through the effect of attached air bubble, and that cavitation-based applications and underwater explosion bubble may benefit from this erosion mitigation approach.

Experimental and Numerical Study on the Bubble Dynamics near Two-Connected Walls with An Obtuse Angle

This study is concerned with the dynamic characteristics of bubbles near two connected walls(one horizontal and the other inclined with an obtuse angle from the horizontal one). In this study, we set up an experiment system to conduct typical cases, and the boundary element method is employed to explain the bubble behavior and study the effect of relative parameters. Comparing the two mutually perpendicular walls, we find that the liquid jet deviates from the horizontal direction within a much shorter range. On the intersection of the two walls, the motions of bubbles have similar trends. The relatively low pressure between the bubble and walls causes the transition of the bubble, and a local high-pressure zone leads to the formation of a liquid jet. Moreover, there are moments when the two walls are evenly stressed in the first bubble cycle. Through parameter analysis, we find that the jet directions of bubbles perform interesting discrepancies for different buoyancy and distances to walls. Some instructive conclusions are given to serve practical applications.

INSTABILITY OF GAS/LIQUID COAXIAL JET

In this article the emphasis was given to the discussion of the effects of diameter ratio and swirling on instability character for the gas/liquid coaxial jet used by Liao, et al.[1], The results indicate that the finite diameter ratio markedly increases the maximum growth rate, the most unstable wavenumber, as well as the cutoff wavenumber. It implies that the finite diameter ratio will lead to the liquid jet breakup length shorter and the liquid drop size smaller. The effect of the swirling jets is much more complex： for the axisymmetric perturbation mode, the swirling enhances the flow stability, for helical perturbation, the dominant instability mode occurs at n〈0. And it is found that in long wave region there exists a new kind of instability modes at n=l that was not mentioned in Liao et al.＇s article. For this new mode, there appears a dominated swirling ratio at which the flow has the maximum growth rate.

Three-dimensional simulation of liquid injection,film formation and transport,in fluidized beds

Liquid injection, and film formation and transport in dense-phase gas-solids fluidized beds are numerically simulated in three dimensions using a collisional exchange model that is based on the mechanism that collisions cause transfer of liquid mass, momentum, and energy between particles. In the model, each of the particles is represented by a solid core and a liquid film surrounding the core. The model is incorporated in the framework of the commercial code Barracuda developed by CPFD Software. The commercial software is an advanced CFD-based computational tool where the particles are treated as discrete entities, calculated by the MP-PIC method, and tracked using the Lagrangian method. Details of the collisional liquid transfer model have been previously presented in O＇Rourke, Zhao, and Snider （2009）; this paper presents new capabilities and proof-testing of the collision model and a new method to better quantify the penetration length. Example calculations of a fluidized bed without liquid injection show the expected effect of collisions on the reduction of granular temperature （fluctuational kinetic energy） of the bed. When applied to liquid injection into a dense-phase fluidized bed under different conditions, the model predicts liquid penetration lengths comparable to the experiments. In addition, the simulation reveals for the first time the dynamic mixing of the liquid droplets with the bed particles and the transient distribution of the droplets inside the bed.

NUMERICAL ANALYSIS OF THE 3-D FLOW FIELD OF PRESSURE ATOMIZERS WITH V-SHAPED CUT AT ORIFICE

Axisymmetric liquid jets have been studied extensively for more than one century, while non-axisymmetric jets are also very common in engineering applications but attract less concern. Based on Eulerian fluid-fluid model in Fluent software, this article analysizes the 3-D flow fields of pressure atomizers with V-shaped cut at orifice, which will result in a non-axisymmetric liquid jet. Flow rate analysis and jet structure analysis are carried out, the results show that the flow rate can be formulated by adding a correction coefficient to the formula of inviscid axisymmetric jets in atomization regime, when the Weber number is low enough to make the flow fall out of atomization regime, and the jet structure together with the flow rate formula will change. Analysis shows that the evolution of the spray and therefore the structure of the liquid jet are affected much by relative velocity and the local volume fraction of liquid phase.

EXPERIMENTAL AND THEORETICAL STUDY OF CAVITATION-INDUCED MECHANICAL EFFECT ON A SOLID BOUNDARY

The mechanical effect induced by the cavitati- on bubble collapse in the neighborhood of a solid boundary was investigated by focusing a Q-switched laser pulse on a metal target in water. By means of a fiber-coupling optical beam deflection technique, the displacement generated by liquid jet impact at the final stage of the bubble collapse was detected at the epicenter of the rear metal surface. Furthermore, by combining a widely used laser ablation model with the detection principles of this detector, the transient impact force exerting on the target material could be easily estimated. Besides, according to experimental results and the modified Ray leigh theory, the maximum bubble radius and the liquid-jet pressure were also obtained, which are in good agreement with previous results,