Gas turbulence modulation in a two-fluid model for gas-solid flows

Recent rapid progress in the theoretical and experimental study of turbulence modulation has led to greater understanding of the physics of particle-gas turbulence interactions. A new two-fluid model incorporating these advances for relatively dilute gas-solid flows containing high-inertia particles is established. The effect of aerodynamic forces upon the particulate stresses is considered in this kinetic theory-based model, and the influence of the particles on the turbulent gas is addressed: the work associated with drag forces contributes to the gas turbulent energy, and the space occupied by particles restricts the turbulent length scale. The interparticle length scale, which is usually ignored, has been incorporated into a new model for determining the turbulent length scale. This model also considers the transport effect on the turbulent length scale. Simulation results for fully developed steady flows in vertical pipes are compared with a wide range of published experimental data and, generally, good agreement is shown. This comprehensive and validated model accounts for many of the interphase interactions that have been shown to be important

Turbulence modulation effects caused by small droplets using one-dimensional-turbulence

This thesis presents a stochastic model to study turbulence modulation effects on gas phases caused by small droplets or more generally speaking particles to achieve a better understanding about the physics and with the aim to provide data for a subgrid-scale (SGS) model for Large-eddy-simulations. The one-dimensional-turbulence (ODT) model addresses one of the major issues for multiphase flow simulations, namely computational costs. It is a dimension-reduced model resolving all turbulent time and length scales and reaching parameter ranges, which are inaccessible for Direct-Numerical-Simulations (DNS). ODT is a stochastic model simulating turbulent flow evolution along a notional one-dimensional line of sight by applying instantaneous maps which represent the effect of individual turbulent eddies on property fields.For an efficient investigation of turbulence modulation effects, ODT has been extended in this thesis in many ways. First, the Lagrangian particle tracking method developed by Schmidt et al. was modified for spatial, cylindrical flow simulations. Therefore, the two-way coupling mechanism was extended as well. This case serves to study the overall influence of particles on a jet configuration, which conforms with the droplet-laden flow in the dilute region of a spray. Here, the most significant effects are expected. Secondly, a concept for developing a SGS model is presented. Based on this concept, ODT was modified to capture two canonical test cases of stationary, forced isotropic turbulence (HIT) and homogeneous shear turbulence (HST). For this purpose, a forcing scheme that maintains statistical stationarity and a new energy redistribution mechanism during\ua0 the eddy events are introduced. The latter enables ODT to predict anisotropic turbulent structures. ODT is validated against several data sets of DNS studies and showed its capability to access parameter ranges beyond previous limits. It turned out to have a lot of potential to contribute to a SGS closure of LES for turbulence modulation caused by small droplets

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

Particle concentration and stokes number effects in multi-phase turbulent channel flows

This investigation examines the effect that particle concentration has on the dynamics of two-phase turbulent channel flows at low and high density ratios. In the literature, little explanation is offered for the existence of high particle turbulence intensities in the buffer layer and viscous sublayer for particles with high Stokes number. The present study aims to explore particle dynamics in those regions. The spectral element method DNS solver, Nek5000, is used to model the fluid phase at a shear Reynolds number Re=, Particles are tracked using a Lagrangian approach with inter-phase momentum exchange (two-way coupling). Mean fluid and particle velocity statistics are gathered and analysed to determine the effect of increasing both Stokes number and concentration. Results indicate that the system with the greater Stokes number (air) has a much larger impact on the mean streamwise velocity and turbulence intensity profiles. As the concentration is increased, the mean flow velocity and turbulence intensity are reduced in the bulk and increased very close to the wall. For the low Stokes system, there is negligible effect on the flow statistics at low concentration. One-way coupled solid-phase statistics indicate that particles in water follow the flow very closely. At the higher densityratio, particles lag behind the flow in the bulk, but overtake the flow in the near-wall region, where the existence of increased streamwise turbulence intensities is also observed. To elucidate the dynamics, concentrations and fluxes are analysed. Particles are observed to be distributed more densely close to the wall in air, compared to a reasonably uniform distribution in water. Finally, contour plots indicate that particles in air tend to congregate in regions of low streamwise fluid velocity, and the extent to which this differs between the two systems is then quantitatively measured

Micro- and macromixing studies in two- and three-phase (gas-solid-liquid) stirred chemical reactors

The iodide/iodate reaction scheme was used to study the effect of gas sparging and/or solid particles on micromixing in a stirred vessel. A literature review illustrated the need for focused work on this matter and gave valuable ideas for the experiments. For this, the experimental method was first validated for using the added gaseous and solid phases air and glass beads, respectively. The experiments covered a range of conditions for micromixing in single-phase, for validation, in gas-liquid, solid-liquid and gas-solid-liquid systems: power inputs up to 1.94 W/kg, gas sparge rates up to 1.5 vvm and up to 11.63 wt.% solids with diameters from 150 to 1125 μm. For comparison, the power inputs from the impeller were kept constant when affected by the added phase(s). In order to allow better quantification of the experimental data, variations of the Incorporation model were evaluated for taking recent suggestions for the reaction scheme into consideration. The second dissociation of sulphuric acid was included in the model and different kinetic rate laws from the literature on the Dushman reaction were implemented. These variations allowed order-of-magnitude estimates and further comparisons of local specific energy dissipation rates

Transport in complex flows: reactive, multiphase and supercritical jets

The present work deals with the dynamics of turbulent jet in different configurations and geometries. In particular two aspect, important both in the engineering applications and in the scientific research, are stressed. The first one deals with the mixing in the turbulent jets at near-critical thermodynamic conditions. The second addresses the dynamics of inertial particle in turbulent premixed Bunsen flames. In order to perform a Direct Numerical Simulation (DNS) of a turbulent jet at supercritical conditions a suitable method was developed to mimic the gas thermodynamic behavior. The Van der Waals equation of state has been chosen and an Low Mach number expansion of Navier Stokes equations has been performed. This approach is completely original in the context of real gas equation, and it is considered as useful as the whole Navier Stokes system in the fully compressible formulation especially at very Low Mach number. The new equations are implemented in a numerical code in order to perform the first, in our knowledge, DNS of a fully turbulent coaxial jet in supercritical thermodynamic conditions. The configuration adopted is similar to the coaxial injectors of the liquid rocket engines and consists in an inner jet with liquid-like density and low velocity and in an outer jet characterized by a gas-like density and high velocity. Aim of the simulation is to observe high-density finger-like structures observed in previous experimental visualizations, the so-called “ligaments”, and to understand the mechanism of their formation. In particular these finger-like structure formation is ascribed to the joint effects of the jet dynamics and the thermodynamics condition. In fact the Kelvin-Helmholtz structures, which generates by the peculiar jet configuration, contributes to the “ligaments” formation while the thermodynamic conditions allow these high-density structures to persist in low-density field. In the real gas jet the interface between the high and low density fluid is observed to be thinner than the perfect gas jet, hence the diffusion occurs at smaller and smaller scales. The obtained data are considered useful for the study of the mixing and combustion processes in the near critical conditions. In the LES/RANS context, in addition, DNS data are necessary for the evaluation of sub-grid terms and for development of new models. The dynamics of the inertial particles in turbulent premixed flame is addressed with the same code. The DNS of a reactive Bunsen jet laden with inertial particle is performed. The simulation reproduces a lean Methane/Air premixed flame in the ”flamelet“ regime. The flow is seeded with four particle population of different inertia with the mass density much larger than the fluid one and diameter much smaller than the Kolmogorov length scale. In these conditions, the particle dynamic equation is forced by only the Stokes drag and the one-way coupling regime can be assumed (no fluid-particle or particle-particle interaction occur). A suitable Stokes number is defined as a function of the particle features and of the laminar flame speed and thickness and burned/unburned gas temperature ratio. It is shown that the so-defined “flamelet” Stokes number is the suitable to describe the particle dynamics in the premixed flames. The DNS data are analyzed to address the effects of particle inertia on Particle Image Velocimetry measurements, in particular is observed that the particle inertia induces a time lag in particle to follow the v fluid acceleration across the flame front. This time lag generate a mismatch in the prediction of fluid velocity through the particle velocity. It is shown that the evaluation of high order velocity statistics needs particle with smaller and smaller flamelet Stokes number. The data are analyzed also to address the effects of the interaction between particle inertia and fluctuating flame front on the particle spatial distribution. Two statistical tools are used to this purpose, the Clustering Index, K, and the radial distribution function, g(r). K measures the departure of the actual distribution from the Poissonian distribution, g(r) is the probability to find a couple of particle at a certain distance r. An important outline consists in the presence of particle clusters in the flame brush. In particular also quasi-Lagrangian particles are characterized by not Poissonian spatial distribution as a consequence of the intermittent fluctuation of the instantaneous thin flame front separating two regions with different particle concentration. With the increasing of the Stokes number the cluster intensity increases experiencing a maximum value for order one flamelet Stokes number. All results concerning the particle dynamics are confirmed by experimental measurement on a Bunsen Methane-Air turbulent reacting jet. The obtained results are considered important both for experimental measurements and for soot dynamics and growth strongly influenced by the interaction of particle with the flame front and by their collision

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

Numerical Modeling Of Collision And Agglomeration Of Adhesive Particles In Turbulent Flows

Particle motion, clustering and agglomeration play an important role in natural phenomena and industrial processes. In classical computational fluid dynamics (CFD), there are three major methods which can be used to predict the flow field and consequently the behavior of particles in flow-fields: 1) direct numerical simulation (DNS) which is very expensive and time consuming, 2) large eddy simulation (LES) which resolves the large scale but not the small scale fluctuations, and 3) Reynolds-Averaged Navier-Stokes (RANS) which can only predict the mean flow. In order to make LES and RANS usable for studying the behavior of small suspended particles, we need to introduce small scale fluctuations to these models, since these small scales have a huge impact on the particle behavior. The first part of this dissertation both extends and critically examines a new method for the generation of small scale fluctuations for use with RANS simulations. This method, called the stochastic vortex structure (SVS) method, uses a series of randomly positioned and oriented vortex tubes to induce the small-scale fluctuating flow. We first use SVS in isotropic homogenous turbulence and validate the predicted flow characteristics and collision and agglomeration of particles from the SVS model with full DNS computations. The calculation speed for the induced velocity from the vortex structures is improved by about two orders of magnitude using a combination of the fast multiple method and a local Taylor series expansion. Next we turn to the problem of extension of the SVS method to more general turbulent flows. We propose an inverse method by which the initial vortex orientation can be specified to generate a specific anisotropic Reynolds stress field. The proposed method is validated for turbulence measures and colliding particle transport in comparison to DNS for turbulent jet flow. The second part of the dissertation uses DNS to examine in more detail two issues raised during developing the SVS model. The first issue concerns the effect of two-way coupling on the agglomeration of adhesive particles. The SVS model as developed to date does not account for the effect of particles on the flow-field (one-way coupling). We focused on examination of the local flow around agglomerates and the effect of agglomeration on modulation of the turbulence. The second issue examines the microphysics of turbulent agglomeration by examining breakup and collision of agglomerates in a shear flow. DNS results are reported both for one agglomerate in shear and for collision of two agglomerates, with a focus on the physics and role of the particle-induced flow field on the particle dynamics