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Velocity Measurements in a Turbulent, Dilute, Two-Phase Jet
Abstract
The results of an experimental investigation into the behavior of unconfined, steady, fully turbulent, two-phase jets are described. A round jet of 25.4 mm in diameter, exit velocity of 20 m/s and containing 80 m beads with a mass density of loading of 1.5% was examined. Mean velocity profiles at several stations, as well as the rms values and velocity cross-correlations for both phases were measured by laser-Doppler velocimetry. It was found that the particles lagged the fluid by 8% near the exit, but later, at 9 jet diameters downstream led it by about 7%. Also, the velocity profiles of the particles were flatter than those of the fluid.
... These were obtained by comparing Uz(r,z), eq. (4.12), to a few data points from Zoltani and Bicen (1990a); we also chose, arbitrarily, (The experimental circumstances there were not exactly identical. In particular, the Reynolds numbers differ by approximately 30% at the jet exit. ...
... In the figures we use conventional notation. The correspondence between the notation used in this paper (better suited for theoretical calculations) and the conventional one, as used in Zoltani and Bicen (1990a), (1990b) is the following. (All arguments are suppressed, so, for instance, G 11 stands for GII(x,x). ...
A new approach to the calculation of turbulent flows is presented. Variational principles are introduced into the statistical theory of turbulence enabling the determination of correlation functions by extremizing certain functionals. To demonstrate the efficiency of the technique a sample calculation of a mean flow profile and certain two point correlation functions of a cylindrically symmetric free jet were carried out using the Rayleigh-Ritz method. A judicious choice of the trial functions leads to the determination of the correlation functions with a modest amount of computation. The results are in reasonable agreement with experimental data.
... The particle-fluid interactions can be characterised by the dimensionless Stokes number, defined as St = τ p /τ f where τ f and τ p are the fluid characteristic time scale and the response time of physical dispersed elements to the carrier phase, respectively (Balachandar and Eaton, 2010). It is now well established that the Stokes number has a significant influence on the instantaneous particle volume fraction and velocity distributions within the flow (Aísa et al., 2002;Gillandt et al., 2001;Hardalupas, 1989;Lau and Nathan, 2014;Lau and Nathan, 2016;Prevost et al., 1996;Zoltani and Bicen, 1990). In particular, in turbulent particle-laden jets two-way coupling regime, the Stokes number has been shown to impact the time-averaged particle distributions both at the pipe exit, and throughout the evolution of the jet, such that the particle volume fraction departs significantly from that of a passive scalar field (Lau and Nathan, 2016;Picano et al., 2010). ...
This study presents experimental measurements and computational modelling of particle-laden jets for a wide range of Stokes numbers to analyse the effect of polydispersity on particle volume fraction and velocity. The polydisperse two-phase turbulent jet issues from a pipe into a low velocity coflow, resulting in inlet Stokes numbers, Stin, ranging from 0.003 to 25. The particle velocity and volume fraction are experimentally measured using digital particle image velocimetry and planar nephelometry simultaneously. The simulations are conducted using a newly developed model based on the probability density function of the population balance equation (PDF-PBE) in a large eddy simulation framework. A stochastic Monte Carlo method is adopted to solve the PDF-PBE on an ensemble of notional Lagrangian particles, while the method of Stokes binning is employed to explicitly treat inertial effects in a computationally efficient way. There is a satisfactory agreement between the measurements and simulations. A series of monodisperse simulations were also conducted to compare with the polydisperse flows in a two-way coupled flow. The results confirm that in the monodisperse jet, small particles (with Stin≤0.3) closely follow the carrier phase velocity whereas in the polydisperse jet they are affected by the presence of larger particles to reduce their axial velocity decay rate. The opposite trend is observed for large particles (Stin≥5.5), confirming that in the polydisperse jet, the presence of small particles increases the large particles’ radial dispersion that lowers their volume fraction along the centreline compared to the monodisperse jet.
... Zoltani and Bicen [5] investigated experimentally the behaviour of a confined turbulent two phase jet. A jet of diameter 25.4 mm and exit velocity of 20 m/s. ...
The flow structure of a single-and two-phase jet from a 10 mm nozzle was investigated experimentally and numerically. The measurements were conducted for Reynolds number range of 10007-31561. The velocity was measured at different radial and axial locations from the exit of the nozzle. The distances of the axial locations are 0, 2, 4, 6, 8 cm. The two-phase flow was created by introducing sand particles as a solid phase. The influence of particles size and mass loading ratio are investigated for particles diameter 220,350,550 µm and loading ratio in range 0.18-1.38. The results revealed that the introducing solid particles into the jet leads values of velocity due to the transfer of momentum to the particles. It has been observed that the velocity decrease with the higher loading ratio and smaller particles size. The numerical simulation was performed for single-and two-phase jet flow. The flow of air was simulated by implementation RNG K-ε turbulence model and the discrete phase model to simulate the sand particles. The results form numerical simulation showed a good agreement with experimental results.
... Many of the experimental studies have been undertaken in the absence of the complexities introduced by droplet evaporation, and for simple flow configurations, which reproduce the main features found in practical systems. They have selected two main fundamental cases: the particle-laden plane shear layer 7-9 and the particle-laden jet [10][11][12][13][14][15][16][17][18] . Most of the above mentioned works focused on the effects of the dispersed phase on the gas-flow properties 19 and very little information regarding particle quantities is reported. ...
Low emissions gas turbine combustors will demand fuel injection systems that are efficient in the accomplishment of the mixing of fuel and air in the shortest possible residence time. This paper describes a computational study of a polydisperse two-phase turbulent jet, which is the basic configuration to analyze how the various parameters influence the size distribution and the dispersion of the droplets in the injection process. An Eulerian frame for the gas phase was used in conjunction with a Lagrangian approach to describe both interphase slip and turbulence on particle motion. Good agreement of the computations with the experimental data is obtained for both the gas and the particles. Turbulence modulation and anisotropy effects were introduced and were found to be very important to the successful performance of the computational method. The exchange of momentum and turbulence energy between the two phases was shown to play a decisive role on the flow development. The predictions confirmed the measurements and showed that the particles do not follow the turbulent gas flow, but they affect it significantly. Considerably high values of the radial velocity were detected around the centerline far from the jet exit where the velocity of the particles lags that of the fluid, that seems to be associated with the effect of shear induced lift forces.
... The experimental setup is described in detail in Zoltani and Bicen (1990b). In short, it comprised a jet issuing vertically downward from a pipe of 12.7 mm and laden with 80 pm glass particles at a mass loading of around 5%. ...
The results of a flow visualization study of the behavior of an unconfined, steady, fully turbulent, two phase jet of diameter 12.7 mm at the exist with velocities up to 25 m/s and containing particles with a mass density of loading up to 5% are described. The significant result is the demonstration of fan spreading, whereby some of the particulates carried by the jet take paths outside of the expected envelope of an equivalent single-phase jet. Measurements of the path angles and a new theory to explain the reason for fan spreading is reported.
... Many of the experimental studies have been undertaken in the absence of the complexities introduced by droplet evaporation, and for simple flow configurations, which reproduce the main features found in practical systems. They have selected two main fundamental cases: the particle-laden plane shear layer 7-9 and the particle-laden jet [10][11][12][13][14][15][16][17][18] . Most of the above mentioned works focused on the effects of the dispersed phase on the gas-flow properties 19 and very little information regarding particle quantities is reported. ...
... Recent studies conducted with particle tracking velocimetry (PTV) and particle image velocimetry (PIV) resurfaced its significance (Muste et al. 2005). Although various techniques have been used to improve the effectiveness of phase discrimination in LDA (Qiu et al. 1991, Muste et al. 1996, 1998 and PIV/PTV (Kiger and Pan 2000, Poelma et al. 2006, Zhang et al. 2008, the crosstalk effect cannot be completely eliminated in cases where the density of the solid phase is relatively high (Modarress et al. 1984) or the tracer and the dispersed phase are not well separated in size distribution (Zoltanic andBicen 1990, Qiu et al. 1991). ...
Cross-talk was identified as an important elemental uncertainty source in dispersed two-phase flow measurement with laser-Doppler anemometry or particle tracking velocimetry. Analytical formulations are presented to quantify the cross-talk bias errors introduced to the ensemble-averaged mean and root-mean-square (rms) velocities. Experimental data available in the literature were used to demonstrate the possible unintended consequences of these errors. It was found that these cross-talk errors are closely related to the slip velocity between the two phases. The error in the mean velocity is proportional to the cross-talk ratio times the slip velocity. The error in the rms velocity depends on the rms velocities of each phase, the cross-talk ratio, and the slip velocity. In practice, the presence of cross-talk may lead to either under- or overestimation of the true velocities.
... For higher Stokes numbers Prévost et al. [3,4] measured decreased turbulence intensities in the two-phase jet compared to the single-phase jet. Fleckhaus et al. [5], Zoltani and Bicen [6], Fan et al. [7] and Modarress et al. [8] studied the effect of laden solid particles on the turbulent flow structure. Maeda et al. [9] studied the effect of particle size in a turbulent gas-particle upward pipe flow with glass beads ranging in size from 45 to 137 lm. ...
The effect of solid particles on the flow characteristics of axisymmetric turbulent coaxial jets for two flow conditions was studied. Simultaneous measurements of size and velocity distributions of continuous and dispersed phases in a two-phase flow are presented using a Phase Doppler Anemometry (PDA) technique. Spherical glass particles with a particle diameter range from 102 to 212 μm were used in this two-phase flow, the experimental results indicate a significant influence of the solid particles and the Re on the flow characteristics. The data show that the gas phase has lower mean velocity in the near-injector region and a higher mean velocity at the developed region. Near the injector at low Reynolds number (Re = 2839) the presence of the particles dampens the gas-phase turbulence, while at higher Reynolds number (Re = 11 893) the gas-phase turbulence and the velocity fluctuation of particle-laden jets are increased. The particle velocity at higher Reynolds number (Re = 11 893) and is lower at lower Reynolds number (Re = 2839). The slip velocity between particles and gas phase existed over the flow domain was examined. More importantly, the present experiment results suggest that, consideration of the gas characteristic length scales is insufficient to predict gas-phase turbulence modulation in gas-particle flows.
The behavior of dispersed particles in a turbulent round jet is experimentally investigated. The role of particle-to-fluid density ratio [Formula: see text] is analyzed by inspecting particle velocity fields and preferential concentration at four different ratios, from 0.7 to 19.3. The jet near-field region, i.e., up to [Formula: see text], is analyzed and compared to the unladen case. Particle-to-fluid density ratio is reported to have a strong impact on particle velocity field structure, in terms of jet transition and self-similar region as well as on turbulent fluctuations. Concentrations of particles show that increasing particle density corresponds to larger departure from uniformity. This occurrence is limited to the region [Formula: see text], where also the largest differences of average and fluctuating velocities with respect to the unladen case are measured.
The pulsating high-temperature particles generated in the industrial production process are common pollutants in industrial buildings. Accurately grasping the transport characteristics of pulsating high-temperature particles is an important task to achieve high-efficiency industrial ventilation system design. In this study, large eddy simulation was used to study the transport characteristics of pulsating high-temperature particles with 5 μm ≤ dp ≤ 70 μm. The aspects discussed mainly include the temperature variations of pulsating high-temperature particles, the interaction between airflow vortex structure and particles, the influence of initial temperature (303 K ≤ T0 ≤ 1273 K) on particle transport range, and the proportion variations of different particle diameters in the work area. The results show that particles with different diameters present similar heating effects on airflow. The vorticity of vortex structure is attenuated by being transferred to both sides during transport. The vorticity attenuation causes the particle transport to occur horizontally from the inside of the vortex structure to the outside. The variations in the particle concentration caused by horizontal transport will affect the shape of the small-scale vortex structure at the edge. When 0.297 ≤ St0 ≤ 0.783, the increase of the initial temperature promotes the vortex structure to carry more particles from airflow to surroundings. When T0 = 1273 K, particles with 0.003 ≤ St0 ≤ 0.107 are transported to the upper space from work area, while particles with 0.107 < St0 ≤ 0.582 are suspended in work area. The results help guide the refined design of local exhaust ventilation.
The behaviour of aluminum 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 analyzed 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 centerline 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.
This chapter describes the investigations conducted to improve the understanding of the transport processes related to dry flue gas desulfurization (FGD). It is important to improve the understanding of the various processes involved in FGD; e.g, dry sorbent injection and dispersion, sorbent and flue gas mixing, sor-bent humidification by water injection, so that the sorbent utilization can be enhanced. The objectives of this study were to experimentally obtain a basic understanding of turbulent flow structure of the mixing zone and its influence on particle dispersion; the effect of particle loading on turbulent properties and mixing; the effect of jet entrainment, water spray sorbent cocurrent flow interactions, sorbent wetting and mixing; and to investigate the flow field where certain ratios of jet velocity to flue gas velocity result in regions of negative flow.
In this work, a turbulent backward-facing step flow at Reynolds number ReH = 17 520, laden with rod-like particles, is investigated by means of particle image
velocimetry. Particle loadings tested are within 0.02% and 0.05% volume fractions; thus, the suspensions may be considered as dilute. The high spatial resolution allows the identification of fiber orientation within the flow, so that mutual interactions between phases can be investigated by considering flow modifications and phenomena related to preferential concentration and orientation of fibers. Fibers’ concentration data show a moderate trend to accumulate at the channel centreline, whereas orientation data display a strong preferential orientation of fibers. Specifically, the local fibers’ orientation is correlated to the flow field in terms of mean flow direction and plane of maximum shear. The occurrence of a symmetrical orientation profile with respect to the channel centre-line is reported as well as the tendency to recover such profile downstream of the step.
The near field of a turbulent circular pipe jet laden with rigid rod-like particles is investigated experimentally by means of particle image velocimetry. Two mass fraction loadings are examined at a Reynolds number equal to 9,000. A simple and robust phase discrimination scheme based on image intensity threshold is presented and validated. Simultaneous flow and dispersed phase velocities data are discussed and compared to literature data for spherical and elongated particles providing insight on phase interactions. Being the Stokes number around unity, both inertial and dynamical effects have high relevance, the former giving rise to velocity lag among particles and fluid and the latter to turbulence modulation in the carrier flow induced by the dispersed phase.
This paper describes the application of a computational method to the problem of a polydisperse two-phase turbulent jet. An Eulerian frame for the gas phase was used in conjunction with a Lagrangian approach to describe both interphase slip and turbulence on particle motion. Turbulence modulation and anisotropy effects were introduced and were found to be very important to the successful performance of the computational method. Good agreement of the computations with the experimental data is obtained for both the gas and the particles.
This paper describes the application of a computational method to the problem of a polydisperse two-phase turbulent jet. An Eulerian frame for the gas phase was used in conjunction with a Lagrangian approach to describe both interphase slip and turbulence on particle motion. Turbulence modulation and anisotropy effects were introduced and were found to be very important to the successful performance of the computational method. Good agreement of the computations with the experimental data is obtained for both the gas and the particles.
The effect of solid particles on the flow characteristics of axisymmetric turbulent coaxial jets has been studied. A laser Doppler anemometer was used to measure the mean and fluctuating velocities of the two phases. Both 50 and 200 μm silica particles were used. Experimental results are presented which indicate a significant influence of the solid particles on the flow characteristics.
A computational fluid dynamics analysis has been conducted for the steady state, turbulent, solid-liquid how through nozzles used in premixed abrasive water jet cutting systems. The development of a theoretical approach to the evaluation of turbulent flow and particle dynamic properties in the nozzles is attractive because of the difficulties associated with direct measurements in nozzles of high flow speed and small dimension. Axisymmetric simulations have been performed with the commercial code FIDAP, using the standard k-epsilon turbulence model. One-way coupling was considered in the simulations, which means that the effect of the presence of the dispersed solid phase on the dynamics of the liquid phase was neglected. The velocities and trajectories of solid particles were predicted. The effects of nozzle geometry on particle dynamic properties were studied. The predictions have been compared with available experimental and theoretical results published by other investigators. This modelling technique will assist in the nozzle design of premixed abrasive water jet systems and the prediction of jet cutting performance.
Flow visualizations obtained in a two-phase jet flow with 80 μm particles at a mass loading of 5% revealed the following.1.
Particles exited the jet tube in straight trajectories with “fan-spreading” angles of up to ±30°. The velocities of those particles having large angles were considerably lower (40% or more) than the mean particle velocity of the bulk flow.
2.
Reducing the mean particle velocity at the exit from 25 m/s to 7 m/s reduced the “fan-spreading” effect with a maximum angle of around 10°.
Variational principles are introduced In the statistical theory of turbulent two-phase flows. With the help of these, correlation functions can be calculated by means of extremizing certain functionals. We carry out a sample calculation of the mean flow profile and correlation functions of a cylindrically symmetric two-phase jet. A Rayleigh-Ritz method is used for the determination of the correlation functions. There is a reasonable agreement between the theoretical calculation and experimental data even with a rather simple choice of the trial functions.
A computational fluid dynamics method has been used to predict the likely concentration distribution at the exit of a direct injected abrasive waterjet nozzle. Particle trajectories were simulated and the concentration at the nozzle exit is obtained by simulating a statistically significant large number of particles. The study takes into consideration the effect of the tapered nozzle inlet angle and the randomness in flow fluctuation and particle size distribution. No published experimental data appear to be available. Validation of predictions is by comparison with experimental data for free air jets.
The particle mass loading effect on the flow structure of a two-phase turbulent jet flow was studied. A particle mass loading ratio ranging from 0 to 3.6 was used as the control parameter. The polystyrene solid particles used had nominal diameters of 210 and 780 μm. The flow Reynolds number, which was based on the pipe nozzle diameter and the fluid-phase centerline mean velocity, was 2 × 104 in the current test. A two-color laser Doppler anemometer (LDA), combined with the amplitude discrimination method and the velocity filter method, was employed to measure the mean velocity distributions for the particle and fluid phases, and the turbulent intensities and Reynolds stresses of the flow. The two-phase jet flow field was measured from the initial pipe exit to 90 D downstream. Another one-component HeNe laser LDA system was also applied to obtain the energy spectra and temporal correlations of the two-phase jet flow.
The effect of solid particles on the flow structure of axisymmetric turbulent coaxial jets has been studied. A laser-Doppler anemometer was used to measure the mean and fluctuation velocities of both phases, and a Malvern laser diffraction instrument was applied to measure particle size and concentration. A series of velocity ratios and particle loading ratios were investigated, and the results were analysed for the effects of these ratios on the mixing characteristic and the similarity behavior of the jet. The effects of particle diameter and its distribution were also studied as well as their influence on the coaxial jet behavior.
Today Particle Image Velocimetry, a technique that enables the quantitative visualization of instantaneous flow fields, has become widely spread and differentiated into many distinct applications ranging from micro flows to combustion and supersonic flows, both for research and industrial needs. This was made possible mainly by technological progress in video techniques, lasers and the development of sophisticated evaluation algorithms, having been started by a few researchers in the 1980s. At that time only printed versions of research papers were available. In 1990 Ronald J Adrian started to collect a list of references on PIV, an activity which he continued until 1995, when the number of accessible publications of PIV approached the amount of 200 per
year.
Particle
The effect of particle size on two-phase turbulent jet flow structure is studied in the present experimental investigation. Polystyrene solid particles of 210, 460, and 780 μm were used. The particles' mass loading ratios ranged from 0 to 3.6. The flow Reynolds number was 2 ‘ 104, which was based on the pipe nozzle diameter and the fluid-phase centerline velocity at the nozzle exit. A two-color laser-Doppler anemometer (LDA), combined with the amplitude discrimination method and the velocity filter method, was employed for measurement. The measurement range of the jet flow was from the initial pipe exit to 90D downstream. Results are presented for the mean velocities of particle and fluid phases, the flow's turbulent intensities and the flow's Reynolds stresses. The energy spectra and the correlation functions of the two-phase jet flow were also obtained by using another one-component He-Ne LDA system.
The velocity and mass flux characteristics in the non-self-similar region of a round turbulent bead-laden jet for three bead diameters (200, 80, and 40 microns), at two mass loadings, are described. The mean velocity of the 200-micron beads is virtually constant, with little influence of mass loading on the mean motion of the beads because the mean Stokes number is small. The centerline velocity of the 80-micron beads decays but not as a function of mass loading, at least up to 86 percent (mass/mass). For the 40-micron beads, the rate of decay of the centerline bead velocity decreases with increasing mass loading. The rate of spreading of 40-micron beads decreases with increasing loading, but the opposite is true for the 80-micron beads. The 'fan-spreading' model for the motion of the beads is considered, which refers to quasi-unidirectional trajectories of the beads from the exit of the jet up to about 20 diameters downstream for the 40-micron diameter beads, and farther for the larger beads.
The results of an experimental investigation of two-phase turbulent flow of air and solid particles in a round jet are presented. A method of laser Doppler anemometer (LDA) signal discrimination for two-phase flow measurement is proposed. It is shown that with a minimal modification of an existing two-component LDA system, the 'cross talk' error caused by the signals from both phases can be eliminated. Data are presented in support of the effectiveness of this method. Extensive data for the two-phase turbulent round jet of air and 50- mu m glass beads are also presented. The results indicate that the presence of the particles reduces the gas velocity fluctuations and the Reynolds shear stress. This reduction is proportional to the particle mass loading.
Measurements of particle mean and r.m.s. velocity were obtained by laser-Doppler anemometry in a descending solid-liquid turbulent flow in a vertical pipe with volumetric concentrations of suspended spherical particles of 270 μm mean diameter in the range 0.1–14%. Similar measurements were obtained in the flow downstream of an axisymmetric baffle of 50% area blockage placed in the pipe with volumetric concentrations of 310 μm particles up to 8% and of 665 μm particles up to 2%. In order to enable measurements in high particle concentrations without blockage of the laser beams the refractive index of the particles was matched to that of the carrier fluid.The results show that the particle mean velocity profiles become more uniform and the particle r.m.s. velocity decreases with increasing concentration in both flow cases. The particle mean velocity in the pipe flow also decreases with concentration and the relative velocity, the difference between the particle velocity and the fluid velocity in single-phase flow, decreases with increasing Reynolds number. The length of the recirculation region downstream of the baffle was shorter than in single-phase flow by 11 and 24% for particle concentrations of 4 and 8%, respectively. The particle mean velocities were hardly affected by size for concentrations up fo 2%, but the r.m.s. velocities were lower with the larger particles.
This book deals with the measurement of instantaneous velocity by laser
Doppler methods. A historical review of laser Doppler anemometry is
given, its relationship to other optical techniques is examined, and
principles of geometrical, physical, and quantum optics are outlined.
Light scattering is analyzed, the optical measurement of particle
velocity is explained physically, and principles of laser Doppler
anemometry are introduced. Optical arrangements for velocity measurement
are described, the transform equation for a laser Doppler anemometer is
derived, and principles underlying the definition of the measuring
control volume are set forth. Practical criteria for specifying optical
components are provided, and techniques are discussed for changing the
frequency of light beams as well as for removing the pedestal signal.
Electronic arrangements for processing the Doppler signal are examined
along with various signal-processing techniques. Frequency analysis,
frequency-tracking demodulation, and counting procedures are considered
in detail together with the specification, generation, and measurement
of scattering particles. Designs of laser Doppler anemometers for
specific requirements are presented, and previous investigations with
such anemometers are appraised.
Existing knowledge on particle deposition rates on walls from turbulent pipe and channel flows is summarized and it is shown that discrepancies exist between experimental and theoretical findings. To contribute to the existing experimental information, laser Doppler measurements are reported of the flow field of a glass particle-air two-phase flow. The results reveal certain seemingly peculiar behaviors of the particles which obviously defy the predictions of the conventional analyses of turbulent two-phase suspension flows.In an accompanying approximate, yet pragmatic theoretical approach, an attempt is made to find a rational basis for the explanation of these experimentally observed particle behaviors. It is shown for the particles in the present study, there exists a limiting size above which their response to the agitation of the fluctuating motion of the surrounding fluid could be treated as if the flow were laminar. On this rational basis, these experimentally observed particle behaviors can then be qualitatively explained by the existing theory of particle excursion in a laminar shear flow field.Reported also is a suggestion to extend the present analysis to a dispersion of particles of multiple sizes.
An optical technique was used to study the dispersion of 50 m, 90 m and 150 m droplets downstream of a source located in the center of a vertical pipe through which turbulent air is flowing. A turbulent dispersion coefficient,
P, and a mean-square of the fluctuations in the turbulent velocity, v
p
2
, are determined from the change of the measured mean-square displacement of the droplets over the pipe cross section with time. The interesting aspect of the experiments is that they explored conditions where the inertia of the particles is believed to be a much more important effect than that of the crossing of trajectories associated with the inequality of the average velocities of the particles and the fluid.
The mixing of particles or droplets in free shear layers is encountered in a variety of combustion systems and industrial applications. Free shear layers are characterized by large scale vortical structures which evolve and interact with time. These vortex structures can play a major role in particle or droplet dispersion. It has recently been postulated that the organized rotating motion of the large-scale structures can enhance the dispersion of intermediate size particles. This paper first reviews the currently-accepted mechanisms and models for particle dispersion in homogenoue, isotropic turbulence and addresses the difference between such flows and free shear layers. The essential features of free shear flows are then described and experiments on particle dispersion in free jets and mixing layers are reviewed. Numerical models which have been developed for particle dispersion in free shear layers, such as plane mixing layers, jets and wakes, are outlined and the results are interpreted in light of the postulated physical model. Both experimental results and numerical simulations strongly imply that particle dispersion in free shear layers is controlled by the motion of large scale vortex structures.
Axisymmetric particle-laden jets were measured using three different devices: a Pitot tube, an LDV (laser Doppler velocimeter) and a specially designed optical fiber probe. Particle sizes in the present experiment ranged from 170 to 1400 μm, which is larger than in most previous works. Results are presented for particle and air velocities, particle concentration and air turbulence. Particle diffusion was unexpectedly large. A delay in the decrease of the centerline air velocity and a reduction in the jet width were observed, as in jets laden with fine particles, but the magnitude of such particle effects on the air flow is less with large particles than with small particles for the same loading ratio.
Mean and fluctuating velocities of both phases, particle mass fluxes, particle size distributions in turbulent particle-laden jets were measured. The following models are considered: (1) a locally homogeneous flow (LHF) model, where slip between the phases was neglected; (2) a deterministic separated flow (DSF) model, where slip was considered but effects of particle dispersion by turbulence were ignored; and (3) a stochastic separated flow (SSF) model. The SSF model performed reasonably well with no modifications in the prescriptions for eddy properties from its original calibration. A modified k- model, incorporating direct contributions of interphase transport on turbulence properties (turbulence modulation), was developed within the framework of the SSF model.
The state-of-the-art in analytical methods for sprays and drop/turbulence interactions in combustion processes is presented with reference to dilute sprays, and dilute dispersed flows, that contain well-defined dispersed-phase elements in volume fractions lower than 1 percent. Attention is given to the near-injector dense spray region. The methods tested are those of locally homogeneous flow, deterministic separated flow, and stochastic separated flow; the first of these was ineffective due to its ignoring finite interphase transport rates, and the second failed by ignoring turbulent dispersion. Stochastic separated flow methodology yielded encouraging results through its treatment of both finite interphase transport rates and dispersed-phase/turbulence interactions.
Yule et al. (1982) have conducted a study of vaporizing sprays with the aid of laser techniques. The present investigation has the objective to supplement the measurements performed by Yule et al., by considering the limiting case of a spray in a stagnant environment. Mean and fluctuating velocities of the continuous phase are measured by means of laser Doppler anemometry (LDA) techniques, while Fraunhofer diffraction and slide impaction methods are employed to determine drop sizes. Liquid fluxes in the spray are found by making use of an isokinetic sampling probe. The obtained data are used as a basis for the evaluation of three models of the process, including a locally homogeneous flow (LHF) model, a deterministic separated flow (DSF) model, and a stochastic separated flow (SSF) model. It is found that the LHF and DSF models do not provide very satisfactory predictions for the test sprays, while the SSF model does provide reasonably good predictions of the observed structure.
Principles and practice of laser-Doppler anemometry Academic Press Faeth, G. M. 1987: Mixing, transport and combustion of sprays Velocity and particle-flux characteristics of particle-laden jets
- E Durst
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Durst, E; Melling, A.; Whitelaw, J. H. 1981 : Principles and practice of laser-Doppler anemometry. 2nd edn. New York: Academic Press Faeth, G. M. 1987: Mixing, transport and combustion of sprays. Prog. Energy Combust. Sci. 13, 293-345 Experiments in Fluids 9 (1990) Hardalupas, Y.; Taylor, A. M. K. P.; Whitelaw, J. H. 1989: Velocity and particle-flux characteristics of particle-laden jets. Proc. R. Soc. London Ser. A 426, 31-78
LDV measurements of gas-particle confined jet flow and digital data processing
- T Boerner
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Scattering of an inert admixture of different grain size in a two-phase axisymmetric jet
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