Large-eddy Simulation of Near-field Dynamics in a Particle-laden Round Turbulent Jet

AbstractThis article investigates the near-field dynamics in a particle-laden round turbulent jet in a large-eddy simulation (LES). A point-force two-way coupling model is adopted in the simulation to reveal the particle modulation of turbulence. The particles mainly excite the initial instability of the jet and bring about the earlier breakup of vortex rings in the near-field. The flow fluctuating intensity either in the axial or in the radial directions is hence increased by particles. The article also describes the mean velocity modulated by particles. The changing statistical velocity induced by particle modulation implies the effects of modulation of the local flow structures. This study is expected to be useful to the control of two-phase turbulent jets

Particle dispersion models and drag coefficients for particles in turbulent flows

Some of the concepts underlying particle dispersion due to turbulence are reviewed. The traditional approaches to particle dispersion in homogeneous, stationary turbulent fields are addressed, and recent work on particle dispersion in large scale turbulent structures is reviewed. The state of knowledge of particle drag coefficients in turbulent gas-particle flows is also reviewed

CHARACTERIZATION OF FIBER SUSPENSIONS IN ARCHETYPAL FLOWS BY MEANS OF STANDARD AND HIGH SPEED PARTICLE IMAGE VELOCIMETRY

Multiphase flows, i.e. flows in which one or more phases are dispersed within a carrier phase, are often encountered in environmental and engineering applications. The inherent di ffculty in studying these flows, due to the phases interactions, is further complicated when the carrier flow is turbulent. A speci c category of two-phase flows relevant for industrial applications is represented by flows where the shape of the dispersed phase is best approximated by rods or fibers, rather than spheres. In this thesis dilute fi ber suspensions in a turbulent pipe jet and in a channel with backward-facing step are characterized experimentally by means of standard and high-speed Particle Image Velocimetry. A full characterization of the near fi eld region of the single-phase, unladen jet is provided with a focus on entrainment rate. To this end, a simple model is presented to predict entrainment rate and tested against experimental data in the Reynolds number range [3200-28000]. The fiber-laden case is obtained by adding Nylon fi bers featuring an aspect ratio of 13.3 to the pipe jet at two diff erent concentrations at a Reynolds number equal to 10000. A phase discrimination technique is presented and validated to obtain simultaneous carrier flow and dispersed phase velocity data. Jet mean and RMS of velocity measurements, velocity correlations and spectral data are discussed with a focus on turbulence modulation induced by the dispersed bers. High spatial resolution measurements of fiber suspensions in a channel with a backward- facing step are presented and discussed. The high spatial resolution and the use of an object-fi tting technique allow the identi fication and measurement of single fibers orientation within the flow. Fibers orientation and concentration data are compared to carrier flow velocity statistics. The results hint at an important role played by bers orientation and orientation anisotropy in turbulent modulation on the carrier phase.Co supervisore: Giovanni Paolo Romano - Struttura di aggregazione: Dipartimento di Meccanica e Aeronautica dell'Università di Roma La SapienzaopenDottorato di ricerca in Tecnologie chimiche ed energeticheopenCapone, Alessandr

Heavy particles in the near field of a turbulent jet

The behaviour of aluminium spherical particles of density ρp=2700 Kg/m3 dispersed in a turbulent round water jet, at Reynolds number Re≈15000 and exit Stokes number St0=0.013 is investigated via Particle Image Velocimetry up to x/D=16 from the nozzle exit. Average and fluctuating velocity fields are analysed and compared to the unladen jet condition. Generally, particles are reported not to behave like tracers, in particular outside the region of the jet potential core. Velocity fluctuations are reported to differ from the fluid fluctuations in particular at the jet centreline and shear layers. In particular, axial velocity fluctuations appear to be dominated by the fluid phase whereas a tendency towards an increase of radial fluctuations downstream of the jet exit is reported for particles. The complexity of this scenario suggests a selective particle response with respect to the flow dynamics

Mixing and Demixing Processes in Multiphase Flows With Application to Propulsion Systems

A workshop on transport processes in multiphase flow was held at the Marshall Space Flight Center on February 25 and 26, 1988. The program, abstracts and text of the presentations at this workshop are presented. The objective of the workshop was to enhance our understanding of mass, momentum, and energy transport processes in laminar and turbulent multiphase shear flows in combustion and propulsion environments

Large-Eddy Simulation of Axially-Rotating, Turbulent Pipe and Particle-laden Swirling Jet Flows

The flows of fully-developed turbulent rotating pipe and particle-laden swirling jet emitted from the pipe into open quiescent atmosphere are investigated numerically using Large-Eddy Simulation (LES). Simulations are performed at various rotation rates and Reynolds numbers, based on bulk velocity and pipe diameter, of 5.3x103, 12x103, and 24x103, respectively. Time-averaged LES results are compared with experimental and simulation data from previous studies. Pipe flow results confirm observations in previous studies, such as the deformation of the turbulent mean axial velocity profile towards the laminar Poiseuille-profile, with increased rotation. The Reynolds stress anisotropy tensor shows a redistribution due to pipe rotation. The axial component near the wall is suppressed, whereas the tangential component is amplified, as rotation is increased. The anisotropy invariant map also shows a movement away from the one-component limit in the viscous sublayer, with increased rotation. Exit conditions for the pipe flow simulation are utilized as inlet conditions for the jet flow simulation. Jet flow without swirl and at a swirl rate of S=0.5 is investigated. Swirl is observed to change the characteristics of the jet flow field, leading to an increase in jet spread and velocity decay and a corresponding decrease in the jet potential core. Lagrangian tracking with one way coupling is used to analyze particle dispersion in the jet flow. Three particle diameter sizes are investigated: 10, 100, and 500μm, which correspond to Stokes numbers of 0.06, 6, and 150, respectively. Particles are injected with an initial velocity set equal to the instantaneous fluid phase flow velocities at the jet inlet. The results show that, in the absence of swirl, particle dispersion is inversely proportional to particle size. With the addition of swirl, particle evolution is much more complicated. Largely unaffected by turbulent structures, the largest particles maintain their initial radial trajectory and disperse radially outward significantly more with the addition of swirl. The smaller particles, much more susceptible to turbulent structures, are shown to quickly diffuse within the jet, and their dispersion is unaffected by swirl. With the addition of swirl, dispersion of the midsize particles is shown to increase initially from the jet inlet up to a distance of approximately three diameter lengths downstream. Particle tracking and particle concentration analysis shows that the increase in particle dispersion of the midsize particles upstream is due to an initial outward migration of particles that are injected near the edge of the jet inlet

Two-phase velocity measurement in a particle-laden jet

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Experimental investigation of inertial particle transport in a turbulent boundary layer

PhDThe first major part of the work was to commission and test the newly built 3 meter openchannel experimental rig. Various development stages have been carried to improve the design specifications to meet experimental requirements. The original 2 meter open-channel working section was replaced with a new 3 meter channel working section enabling measurements to be taken further downstream allowing the boundary layer to develop. The original bell mouth inlet was replaced with a hyperbolic tangent profile 3:1 contraction with a honeycomb, coarse and fine gauzes fitted upstream. 25% porous perforated plates were installed at the channel exit and also within the inlet plenum tank to reduce the turbulence level. LDV measurement in the freestream revealed that the turbulence level is below 1% and the boundary layer profile collapses well with DNS data of Schlatter (2010). A dip in the outer wake region of the velocity profile can be observed throughout the measurements and is attributed to the aspect ratio of the channel which is 1.7 at Fr = 0.33. Nevertheless, boundary layer profile and turbulence intensity profile collapse well with published DNS data. Good agreement was obtained between measurements carried out using the available LDV and time-resolved PIV systems. Time-resolved PIV measurements were performed in a dilute particle-laden flow, tracking nearly neutrally buoyant polymer microspheres within the measured velocity field of a near wall turbulent boundary layer. Data were taken 2100mm downstream of the inlet, in a vertical light-sheet aligned in the streamwise direction on the centerline of the horizontal, open-channel channel facility. High frame-rate measurements were taken to temporally and spatially track particle motion and instantaneous visualization clearly reveal a link between particle movement and near-wall coherent structures. Structures having 2D vorticity signatures of near-wall hairpin vortices and hairpin packets, directly affect particle motion. Statistical and instantaneous results agree well with published experimental and numerical work. Conditional statistics were investigated for the particles using the Quadrant method. Particles moving outwards from the channel floor are influenced by the Quadrant 2 ejection events and those that moves inwards towards the wall are influenced by the Quadrant 4 sweeps events. v Particle-fluid velocity correlations, rpf were calculated for each particle trajectory and averaged of all the particle-fluid velocity correlations, R, were determined for the whole dataset for Re=1000. This value is estimated to be 0.0261 and 0.000643 respectively for the particle-fluid streamwise and wall-normal velocity correlation