Particle modulations to turbulence in two-phase round jets

The particle modulations to turbulence in round jets were experimentally studied by means of two-phase velocity measurements with Phase Doppler Anemometer （PDA）. Laden with very large particles, no significant attenuations of turbulence intensities were measured in the farfields, due to small two-phase slip velocities and particle Reynolds number. The gas-phase turbulence is enhanced by particles in the near-fields, but it is significantly attenuated by the small particles in the far-fields. The smaller particles have a more profound effect on the attenuation of turbulence intensities. The enhancements or attenuations of turbulence intensities in the far-fields depends on the energy production, transport and dissipation mechanisms between the two phases, which are determined by the particle prop- erties and two-phase velocity slips. The non-dimensional parameter CTI is introduced to represent the change of turbulence intensity.

NUMERICAL MODELING OF TURBULENTEVAPORATING GAS-DROPLET TWO-PHASE FLOWS IN AN AFTERBURNER DIFFUSOR OF TURBO-FAN JET ENGINES

The two-dimensional turbulent evaporating gas-droplet two-phase flows in an afterburner diffusor of turbo-fan jet engines are simulated by the k-ε turbulence model and the particle trajectory model. Comparison of predicted gas velocity and temperature distributions with experimental results for the cases without liquid spray shows pretty good agreement. Gas-droplet two-phase flow predictions give plausible droplet trajectories, fuel-vapor concentration distribution, gas-phase velocity and temperature field in presence of liquid droplets. One run of computation with this method is made for a particular afterburner. The results indicate that the location of the atomizers is not favorable to flame stabilization and combustion efficiency. The proposed numerical modeling can also be adopted for optimization design and performance evaluation of afterburner combustors of turbo-fan jet engines.

Deduction and Validation of an Eulerian-Eulerian Model for Turbulent Dilute Two-Phase Flows by Means of the Phase Indicator Function-Disperse Elements Probability Density Function

A statistical formalism overcoming some conceptual and practical difficulties arising in existing two-phase flow (2PHF) mathematical modelling has been applied to propose a model for dilute 2PHF turbulent Hows. Phase interaction terms with a clear physical meaning enter the equations and the formalism provides some guidelines for the avoidance of closure assumptions or the rational approximation of these terms. Continuous phase averaged continuity, momentum, turbulent kinetic energy and turbulence dissipation rate equations have been rigorously and systematically obtained in a single step. These equations display a structure similar to that for single-phase flows. It is also assumed that dispersed phase dynamics is well described by a probability density function (pdf) equation and Eulerian continuity, momentum and fluctuating kinetic energy equations for the dispersed phase are deduced. An extension of the standard k-e turbulence model for the continuous phase is used. A gradient transport model is adopted for the dispersed phase fluctuating fluxes of momentum and kinetic energy at the non-colliding, large inertia limit. This model is then used to predict the behaviour of three axisymmetric turbulent jets of air laden with solid particles varying in size and concentration. Qualitative and quantitative numerical predictions compare reasonably well with the three different sets of experimental results, studying the influence of particle size, loading ratio and flow confinement velocity.

Effects of the nozzle structure and fluidized gas composition on the gas-particle two-phase jet characteristic in a powder fuel scramjet

The interaction between nozzle design and fluidization gas composition significantly influences the dynamics within a powder fuel scramjet's combustion chamber.To investigate this relationship,an experimental study utilized high-speed shadow imaging technology to explore the macroscopic aspects of powder fuel injection.The investigation examined various convergence angles,nozzle throat lengths,and fluidized gas compositions.Key findings include:During jet development,powder fuel initially concentrates near the axis,with non-convergence angle nozzles exhibiting longer concentrated distribution periods than convergence angle conditions.Decreasing nozzle convergence angles lead to increased penetration distance,frontal velocity,and radial diffusion distance during the initial stages of jet development.Additionally,stable jet shapes show larger divergence angles as nozzle convergence angle decreases,with the largest divergence angle observed atα=60°.In the initial 0-7 ms of jet development,the powder fuel jet demonstrates greater penetration distance and frontal velocity under certain conditions.Moreover,penetration distance and frontal velocity increase with throat length from 7 to 20 ms,accompanied by changes in divergence angles.Specifically,at a throat length(l)of 2 mm,the near-field divergence angle measures 46.50°,and the far-field divergence angle is 22.25°.Conversely,at l=8mm,the near-field divergence angle is 33.49°,and the far-field divergence angle is 23.21°.The fluidization gas composition minimally affects jet penetration distance and frontal velocity during the initial 0-3 ms.However,due to hydrogen's low density,hydrogen/powder fuel jets exhibit shorter distances and velocities compared to nitrogen/powder fuel jets.Hydrogen fluidization also results in larger divergence angles,particularly in the near field.These findings underscore the importance of nozzle design and fluidization gas composition in optimizing scramjet performance and efficiency.

Large-eddy Simulation of Bubble-Liquid Confined Jets

The Large-eddy simulation (LES) with two-way coupling is used to study bubble-liquid two-phase confined multiple jets discharged into a 2D channel.The LES results reveal the large-eddy vortex structures of both liquid flow and bubble motion,the shear-generated and bubble-induced liquid turbulence,and indicate much stronger bubble fluctuation than that of the liquid,the enhancement of liquid turbulence by bubbles.Both shear and bubble-liquid interaction are important for the liquid turbulence generation in the case studied.

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.

Numerical simulation of slurry jets using mixture model

Slurry jets in a static uniform environment were simulated with a two-phase mixture model in which flow-particle interactions were considered. A standard k-e turbulence model was chosen to close the governing equations. The computational results were in agreement with previous laboratory measurements. The characteristics of the two-phase flow field and the influences of hydraulic and geometric parameters on the distribution of the slurry jets were analyzed on the basis of the computational results. The calculated results reveal that if the initial velocity of the slurry jet is high, the jet spreads less in the radial direction. When the slurry jet is less influenced by the ambient fluid （when the Stokes number St is relatively large）, the turbulent kinetic energy k and turbulent dissipation rate e, which are relatively concentrated around the jet axis, decrease more rapidly after the slurry jet passes through the nozzle. For different values of St, the radial distributions of streamwise velocity and particle volume fraction are both self-similar and fit a Gaussian profile after the slurry jet fully develops. The decay rate of the particle velocity is lower than that of water velocity along the jet axis, and the axial distributions of the centerline particle streamwise velocity are self-similar along the jet axis. The pattern of particle dispersion depends on the Stokes number St. When St = 0.39, the panicle dispersion along the radial direction is considerable, and the relative velocity is very low due to the low dynamic response time. When St = 3.08, the dispersion of particles along the radial direction is very little, and most of the particles have high relative velocities along the streamwise direction.

Large eddy simulation of a particle—laden turbulent plane jet

Gas solid two-phase turbulent plane jet is applied to many natural s it uations and in engineering systems. To predict the particle dispersion in the ga s jet is of great importance in industrial applications and in the designing of engineering systems. A large eddy simulation of the two-phase plane jet was con d ucted to investigate the particle dispersion patterns. The particles with Stokes numbers equal to 0 0028, 0 3, 2 5, 28 (corresponding to particle diameter 1 μm , 10 μm, 30 μm, 100 μm, respectively) in \%Re\%=11 300 gas flow were studied. The simulation results of gas phase motion agreed well with previous experimental re sults. And the simulation results of the solid particles motion showed that part icles with different Stokes number have different spatial dispersion; and that p articles with intermediate Stokes number have the largest dispersion ratio.

PDA MEASUREMENTS OF TWO-PHASE FLOW STRUCTURE AND PARTICLE DISPERSION FOR A PARTICLE-LADEN JET IN CROSSFLOW

The two-phase flow structure and particle dispersion for a dilute particle-laden jet in crossflow （JICF） were experimentally investigated by means of Phase Doppler Anemometry （PDA） measurement. The two-phase flow experiments were conducted for different flow conditions and solid particle parameters, including the ratio of the jet velocity to crossflow velocity, the particle size and mass loading. The experimental results indicate that the fine particles with the size of 70 micron and the mass loading of 0.05% have a minor influence on the mean and fluctuation velocity fields of the two-phase JICF. However, the fine particle transport by the two-phase JICF is dominantly and preferentially affected by the shear layer vortices and exhibits a somewhat enhanced dispersion as compared to the fluid. For the coarse particles with the particle size ranging from 300 micron to 700 micron and the mass loading less than 0.16%, the effect of the particle parameters on the fluid phase is associated with both the anisotropic properties of the flow field and the trajectory deviation of the settling particles from the fluid. Compared to the single-phase JICF, the two-phase JICF laden with the coarse particles is recognized to possess more pronounced mean velocity alteration and turbulence modulation of the fluid phase in the presence of the particles with the larger particle size and higher mass loading.

Mixing effects of high-speed jets in gas-solid riser and downer reactors

For the high-temperature and short-contact time gas-solid reaction process,riser and downer are considered appropriate reactors.To realize an intensive and complete mixing of reactants with catalysts,the feed raw is always introduced in the form of high-speed jets.In this study,in order to investigate the mixing effects of different types of high-speed jets in riser and downer,traceable ozone is injected with the high-speed feed jets to react with catalyst particles.By detecting the decomposition of ozone,the gas-solid mixing and reaction in riser and downer under the influence of both co-current and counter-current injections are analyzed.The relative ozone concentration is used to reflect the location reaction extent and its radial nonuniformity index is proposed to compare the results in riser and downer.It is found that the jet influence zone in downer provides a relatively better environment for the mixing of feed jets with catalysts.In the riser,introduction of counter-current injections could improve the uniformity of gas-solid mixing in the initial contact region of feed with catalysts.

Numerical analysis of cavitation cloud shedding in a submerged water jet

Focused on the unsteady behavior of high-speed water jets with intensive cavitation a numerical analysis is performed by applying a practical compressible mixture flow bubble cavitation model with a simplified estimation of bubble radius. The mean flow of two-phase mixture is calculated by unsteady Reynolds averaged Navier-Stokes （URANS） for compressible flow and the intensity of cavitation in a local field is evaluated by the volume fraction of gas bubbles whose radius is estimated with a simplified Rayleigh- Plesset equation according to pressure variation of the mean flow field. High-speed submerged water jet issuing from a sheathed sharp-edge orifice nozzle is treated. The periodically shedding of cavitation clouds is captured in a certain reliability compared to experiment data of visualization observation and the capability to capture the unsteadily shedding of cavitation clouds is demonstrated. The results demonstrate that cavitation takes place near the entrance of nozzle throat and cavitation cloud expands consequentially while flowing downstream. Developed bubble clouds break up near the nozzle exit and shed downstream periodically along the shear layer. Under the effect of cavitation bubbles the decay of core velocity is delayed compared to the case of no-cavitation jet.

Numerical modeling and spatial stability analysis of the wall jet flow of nanofluids with thermophoresis and brownian effects

The present work describes similarity solution to a general scheme for the wall jet flow of nanofluids,accounting both the similarity branches(say upper and lower),allowed with respect to the suction and moving wall conditions in the context of Glauert type e-jets.Before proceeding with this,a spatial stability analysis is performed to check the stability of the similarity modes.Results indicated that the upper similarity branch is possibly stable;whilst,the lower branch is not likely to reside in actual physics.The governing transport equations of mass and energy subject to a general two-phase modeling framework were transformed into similarity equations.The involved equations were then solved numerically employing the standard 4th order Runge-Kutta together with shooting technique.The influence of the involved parameters is shown graphically and in a detailed manner.In the last section,it is presented closed-form algebraic solution to the energy equation for the base fluids with a general convective boundary condition.