Particle-turbulence Interaction Research Articles

Combustion in a chamber furnace with a tangentially swirling vortex

Relevance. The need to analyze the furnace processes in boiler units with vertical vortex when arranging pulverized coal combustion with the enhancement of environmental parameters through the installation of tertiary blast nozzles. Despite the carbon-free policy in modern power engineering coal remains one of the main sources of energy, so the reconstruction of boiler units in order to reduce harmful emissions is very relevant. Aim. To study aerodynamics and combustion processes of pulverized coal fuel in the furnace chamber of a boiler unit with a tangential arrangement of burner devices using a combustion scheme with tertiary air blowing. Methods. The Eulerian–Lagrangian approach was applied to numerical study of flow characteristics, heat transfer and combustion in a furnace with a tangential burner arrangement. Also, k-ε model of gas turbulence, particle-turbulence interaction, diffusion model of particle dispersion, P-1 radiation modeling method and pulverized coal particle combustion model based on global particle kinetics and experimental data were used for numerical calculations. Results. Based on numerical calculations and analysis, the dependences of combustion product concentrations, temperature, hydrodynamics at combustion of Kuznetsky coal grade D in the furnace chamber with tangential arrangement of burners for brown coal, as well as at installation of tertiary blast nozzles have been obtained. Insufficient redistribution of air between burners and tertiary nozzles is revealed, as for maintenance of stable twist of vertical vortex and accordingly presence of high values of velocities at the outlet of burner devices it is not possible to increase the portion of tertiary air without reconstruction of burners. It is also established that the volume of the furnace chamber below the burners is poorly involved in heat exchange.

Preferential Concentration of Particles in Forced Turbulent Flows: Effects of Gravity

The impact of gravity on the particle preferential concentration is investigated by direct numerical simulations in an Eulerian–Lagrangian framework for a large range of Stokes numbers Stη=0.01∼4. For particles with small Stokes numbers such as Stη=0.01, the gravity has minor effects on the particle spatial distribution in the turbulence. With increasing Stη, stripped structures of the high number density of particles appear and expand along the gravity direction. Different evaluation methods of particle preferential concentration are discussed such as the spatial distribution, the box index, and the probability density function. The number density of particles in the accumulating regions reduced under the influence of gravity. The reduction becomes prominent for the particle cloud at Stokes number Stη≈1, especially in the clusters of high particle number density. For large Stokes number Stη, the slip velocity significantly increases due to the particle gravity. Due to the gravity, the particle concentration reduces globally, particularly in the low vorticity regions. For the Stokes number range explored in this paper, gravity has a considerable impact on the particle-turbulence interaction.

A critical analysis of the CFD-DEM simulation of pharmaceutical aerosols deposition in extra-thoracic airways

Recent advances in the characterization of the human respiratory system and in multi-phase flow dynamics in complex geometries have led numerical simulations to play an expanding role for exploring aerosol deposition mechanisms in the lungs. However, the development of an efficient numerical and mathematical description is far from unique, and determining which aspects of the modelling are critical and which details are essentially irrelevant is indeed a difficult task. With the aim of addressing this lack of a rationalized framework, we propose a systematic analysis of pharmaceutical aerosols deposition in the extra-thoracic airways, focusing on several important modelling aspects whose related assumptions and approximations have not always been sufficiently discussed and clarified. We consider the importance of intrinsic time dependent fluctuations of the air flow, highlighting how their contribution in aerosol deposition is as important as the particle-turbulence interaction one. We show how sensitive the turbulence intensity can be to the meshing strategy and how aerosol deposition can be influenced by the latter choice. We demonstrate how a swirling air jet can enhance extra-thoracic deposition compared to a straight one, and how different the deposition patterns can be in case a realistic inhalation profile and aerosol plume are employed.

Superstatistical Wind Fields from Pointwise Atmospheric Turbulence Measurements

Accurate models of turbulent wind fields have become increasingly important in the atmospheric sciences, e.g., for the determination of spatiotemporal correlations in wind parks, the estimation of individual loads on turbine rotor and blades, or the modeling of particle-turbulence interaction in atmospheric clouds or pollutant distributions in urban settings. Because of the difficult task of resolving the fields across a broad range of scales, one oftentimes has to invoke stochastic wind field models that fulfill specific, empirically observed, properties. Whereas commonly used Gaussian random field models solely control second-order statistics (i.e., velocity correlation tensors or kinetic energy spectra), we explicitly show that our extended model emulates the effects of higher-order statistics as well. Most importantly, the empirically observed phenomenon of small-scale intermittency, which can be regarded as one of the key features of atmospheric turbulent flows, is reproduced with a very high level of accuracy and at considerably low computational cost. Our method is based on a multipoint statistical description of turbulent velocity fields that consists of a superposition of multivariate Gaussian statistics with fluctuating covariances. We propose a new and efficient sampling algorithm for this Gaussian scale mixture and demonstrate how such “superstatistical” wind fields can be constrained on a certain number of real-world measurement data points from a meteorological mast array.Received 12 April 2022Revised 26 July 2022Accepted 22 August 2022DOI:https://doi.org/10.1103/PRXEnergy.1.023006Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAtmospheric scienceBoundary layersShear flowsStochastic processesStructure & turbulence of boundary layersSustainabilityTurbulenceWind energyEnergy Science & TechnologyNonlinear DynamicsFluid DynamicsStatistical Physics

Effects of different characteristics of the dilute liquid-solid flow on the erosion in a 90° bend

A numerical model with a home-made interface code combining Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM) is built to simulate the erosion process in a 90° bend for transporting dilute liquid-solid flow. Different characteristics of flow and particle are defined in the code for analyzing the corresponding effects on the bend erosion. The simulated results indicate the bend erosion is continuously increased as the mass loading of particles is increased to 10% in volume fraction. Both the pressure gradient force and virtual mass force reduce the erosion along the bend extrados because they drive the particles toward the bend intrados and reduce the frequency of particle-wall collision. The effect of particle-turbulence interaction is minor for coarse particles regardless of the particle concentration. The sharper particle causes less erosion at given particle volume and concentration, because the impact angle and particle-wall collisions for the shaper particles are reduced.

Numerical Investigation of the Characteristics of Erosion in a Centrifugal Pump for Transporting Dilute Particle-Laden Flows

Erosion in centrifugal pumps for transporting flows with dilute particles is a main pump failure problem in many engineering processes. A numerical model combining the computational fluid dynamics (CFD) and Discrete Element Method (DEM) is applied to simulate erosion in a centrifugal pump. Different models of the liquid-solid inter-phase forces are implemented, and the particle-turbulence interaction is also defined. The inertial particles considered in this work are monodisperse and have finite size. The numerical results are validated by comparing the results with a series of experimental data. Then, the effects of particle volume fraction, size, and shape on the pump erosion are estimated in the simulations. The results demonstrate that severe erosive areas are located near the inlet and outlet of the pressure side of the impeller blade, the middle region of the blade, the corners of the shroud and hub of the impeller adjoining to the pressure side of the blade, and the volute near the pump tongue. Among these locations, the maximum erosion occurs near the inlet of the pressure side of the blade. Erosion mitigation occurs under the situation where more particles accumulate in the near-wall region of the eroded surface, forming a buffering layer. The relationship between the particle size and the erosion is nonlinear, and the 1 mm particle causes the maximum pump erosion. The sharp particles cause more severe erosion in the pump because both the frequency of particle-wall collisions and the impact angle increase with the increasing sharpness of the particle.

Two-way coupled particle-turbulence interaction: Effect of numerics and resolution on fluid and particle statistics

We show that statistics from simulations of particle-laden turbulent flows modeled by the point-particle Euler-Lagrange methodology diverge with mesh refinement when the undisturbed fluid velocity used in the drag force that couples the dispersed and continuous phases is not properly modeled. For decaying homogeneous isotropic turbulence, fluid and particle energies, particle acceleration, and energy dissipation rates are found to be grid insensitive only when the undisturbed fluid velocity is robustly modelled. Ultimately, this work provides insight into what types of questions are answerable using two-way coupled point-particle methods.

Particle-turbulence interaction of high Stokes number irregular shape particles in accelerating flow: A rocket-engine model

Metal particles in solid propellants enhance rocket engines performance. An interaction of particles with a high Reynolds number turbulent gas flow accelerating to a nozzle, has not been characterized thoroughly. We study the particle-turbulence interactions in a two-dimensional model of a rocket engine. Two-phase particle image/tracking velocimetry provides the flow velocity simultaneously with the velocities of irregularly shaped inertial particles (dp ~ 320 µm, Stokes St ~ 70, particle Reynolds number Rep ~ 300). We reveal the local augmentation of turbulent fluctuations in the particle wakes (up to 5 particle diameters downstream the particle). Despite the low mass fraction, the large response time of the particles leads to an increase of turbulent kinetic energy (TKE) everywhere in the chamber. The increase of local particle mass fraction near the nozzle, due to the mass conservation and converging streamlines, compensates for the dampening effect of the strong mean flow acceleration and further augments TKE at the nozzle inlet. Furthermore, this is accompanied by unexpectedly isotropic fluctuations in the proximity of the nozzle. The phenomenon of the isotropic, strongly enhanced turbulence in the proximity of the engine nozzle achievable with the low mass fraction of high St, Rep particles, can be used to improve the design of solid propellant rocket engines.

Effects of particle-fluid density ratio on the interactions between the turbulent channel flow and finite-size particles.

A parallel direct-forcing fictitious domain method is employed to perform fully resolved numerical simulations of turbulent channel flow laden with finite-size particles. The effects of the particle-fluid density ratio on the turbulence modulation in the channel flow are investigated at the friction Reynolds number of 180, the particle volume fraction of 0.84%, and the particle-fluid density ratio ranging from 1 to 104.2. The results show that the variation of the flow drag with the particle-fluid density ratio is not monotonic, with a larger flow drag for the density ratio of 10.42, compared to those of unity and 104.2. A significant drag reduction by the particles is observed for large particle-fluid density ratios during the transient stage, but not at the statistically stationary stage. The intensity of particle velocity fluctuations generally decreases with increasing particle inertia, except that the particle streamwise root-mean-square velocity and streamwise-transverse velocity correlation in the near-wall region are largest at the density ratio of the order of 10. The averaged momentum equations are derived with the spatial averaging theorem and are used to analyze the mechanisms for the effects of the particles on the flow drag. The results indicate that the drag-reduction effect due to the decrease in the fluid Reynolds shear stress is counteracted by the drag-enhancement effect due to the increase in the total particle stress or the interphase drag force for the large particle-inertia case. The sum of the total Reynolds stress and particle inner stress contributions to the flow drag is largest at the density ratio of the order of 10, which is the reason for the largest flow drag at this density ratio. The interphase drag force obtained from the averaged momentum equation (the balance theory) is significantly smaller than (but agrees qualitatively with) that from the empirical drag formula based on the phase-averaged slip velocity for large density ratios. For the neutrally buoyant case, the balance theory predicts a positive interphase force on the particles arising from the negative gradient of the particle inner stress, which cannot be predicted by the drag formula based on the phase-averaged slip velocity. In addition, our results show that both particle collision and particle-turbulence interaction play roles in the formation of the inhomogeneous distribution of the particles at the density ratio of the order of 10.

Vertical distribution of fluid velocity and suspended sediment in open channel turbulent flow

To predict the vertical distribution of streamwise fluid velocity and suspended sediment concentration profiles in an open channel turbulent flow, we derive a theoretical model here based on the Reynolds averaged Navier–Stokes equation and the mass conservation equations of solid and fluid phases. The model includes the effects of secondary current in terms of the vertical velocity of fluid, additional vertical velocity of fluid due to the suspended particles, mixing length of sediment-laden flow and settlement of the suspended particles due to gravitational force. We numerically solve our model as coupled differential equations and the obtained solution agrees well with a wide spectrum of experimental data. A detailed error analysis asserts the superior determination accuracy of our model in comparison to the traditional log-law and Rouse equation and other existing theoretical models. The significance of the turbulent features included in the model and the importance of their co-existence to compute velocity and concentration profiles are explained. In sharp contrast to the previous researchers, the present model has significant contribution in unveiling several latent phenomena of particle-turbulence interaction throughout the flow region. The model can also address various crucial phenomena of velocity and concentration profiles that occur during flow in real situation.

Lattice Boltzmann model for predicting the deposition of inertial particles transported by a turbulent flow

Deposition of inertial solid particles transported by turbulent flows is modelled in a framework of a statistical approach based on the particle velocity Probability Density Function (PDF). The particle-turbulence interaction term is closed in the kinetic equation by a model widely inspired from the famous BGK model of the kinetic theory of rarefied gases. A Gauss-Hermite Lattice Boltzmann model is used to solve the closed kinetic equation involving the turbulence effect. The Lattice Boltzmann model is used for the case of the deposition of inertial particles transported by a homogeneous isotropic turbulent flows. Even if the carrier phase is homogeneous and isotropic, the presence of the wall coupled with particle-turbulence interactions leads to inhomogeneous particle distribution and non-equilibrium particle fluctuating motion. Despite these complexities the predictions of the Lattice Boltzmann model are in very good accordance with random-walk simulations. More specifically the mean particle velocity, the r.m.s. particle velocity and the deposition rate are all well predicted by the proposed Lattice Boltzmann model.

Modification of near-wall coherent structures by inertial particles

Direct numerical simulations are combined with two-way coupled Lagrangian point particles to study the effect of Reynolds number on particle-turbulence interaction. Turbulent planar Couette flow is simulated at a constant dispersed phase mass loading of ϕm = 0.25 for particle Stokes numbers of StK = [O(1), O(10), O(100)] (based on the Stokes time scale of the particle and the Kolmogorov time scale of the flow) and bulk Reynolds numbers of Reb = [8100, 24000, 72000] (based on the plate velocity difference and separation distance). Statistics of swirling strength |λci| are used to evaluate the impact of particles on near-wall motions which are responsible for turbulent, wall-normal momentum transport. Instantaneously, the number of high-strength swirling motions near the wall decreases significantly in the presence of particles, and this trend is enhanced with increasing Reb. Conditional averages are computed using linear stochastic estimation, providing the average structures responsible for ejection events near the wall. These conditional eddies are weakened substantially by the presence of the dispersed phase, and this effect is again enhanced with increasing Reb. We propose a mechanism where particles, by interfering with the hairpin regeneration process near the wall, can influence turbulent fluxes in a way that increases with Reb despite only having direct interaction with scales on the same order as their small physical size. At the same time, turbulent momentum flux concentrated at higher wavenumbers with increasing Reb allows small particles to be effective agents for altering turbulent transport.

Numerical Investigation of Phase Distribution and Liquid Turbulence Modulation in Dilute Particle-Laden Flow

Studies on the motion of particles in turbulence and interactions between particles and turbulence are extremely significant, which can help us to improve the efficiency of industrial processes. In this article, we investigated the particle distribution and particle-turbulence interaction in a solid-liquid channel flow with the Euler-Lagrange two-way model. The liquid phase was solved using direct numerical simulation (DNS), and the particle motion was tracked by Newtonian equations of motion considering effects of drag force, pressure gradient force, and gravity. Two-way coupling was used to explain the effect of particles on the turbulence structure. The results show that the local void fraction of particles indicates the wall-peaked profile, particles scatter uniformly in the spanwise direction, and the injection of particles suppresses the turbulence activities in the near wall region. Suppression of the liquid turbulence is mainly caused by vortexes decay of different sizes.

APPLICATION OF LES-STOCHASTIC TWO-WAY MODEL TO TWO-PHASE BOUNDARY LAYER FLOWS

A particle / turbulence two-way coupling model, integrated with conventional stochastic and sub-grid stress models of three-dimensional Large Eddy Simulation (LES), has been applied to the particle-laden turbulent flow in a wave boundary layer developed over seabed with the aim to understand dynamic effects of the particle size and number density to the suspension process in shearing flow over the seabed. While the particle size affects local velocity fluctuations, the particle population significantly induces secondary large-scale flows varying over a scale of the wavelength, and intensifies the turbulent energy near the bed. The particle-induced turbulence may result in additional suspension from the bed, causing a recursive suspension process via the particle turbulence interaction in the boundary layer.

A numerical investigation of fine particle laden flow in an oscillatory channel: the role of particle-induced density stratification

Studying particle-laden oscillatory channel flow constitutes an important step towards understanding practical application. This study aims to take a step forward in our understanding of the role of turbulence on fine-particle transport in an oscillatory channel and the back effect of fine particles on turbulence modulation using an Eulerian–Eulerian framework. In particular, simulations presented in this study are selected to investigate wave-induced fine sediment transport processes in a typical coastal setting. Our modelling framework is based on a simplified two-way coupled formulation that is accurate for particles of small Stokes number (St). As a first step, the instantaneous particle velocity is calculated as the superposition of the local fluid velocity and the particle settling velocity while the higher-order particle inertia effect neglected. Correspondingly, only the modulation of carrier flow is due to particle-induced density stratification quantified by the bulk Richardson number, Ri. In this paper, we fixed the Reynolds number to be ReΔ = 1000 and varied the bulk Richardson number over a range (Ri = 0, 1 × 10−4, 3 × 10−4 and 6 × 10−4). The simulation results reveal critical processes due to different degrees of the particle–turbulence interaction. Essentially, four different regimes of particle transport for the given ReΔ are observed: (i) the regime where virtually no turbulence modulation in the case of very dilute condition, i.e. Ri ~ 0; (ii) slightly modified regime where slight turbulence attenuation is observed near the top of the oscillatory boundary layer. However, in this regime a significant change can be observed in the concentration profile with the formation of a lutocline; (iii) regime where flow laminarization occurs during the peak flow, followed by shear instability during the flow reversal. A significant reduction in the oscillatory boundary layer thickness is also observed; (iv) complete laminarization due to strong particle-induced stable density stratification.

RANS modeling of a particulate turbulent round jet

A complete and accurate model for the symmetric gas–solid turbulent round jet is accomplished using the Reynolds Averaged Navier–Stokes (RANS) equations. The two-fluid model was used to describe the averaged characteristics of the two phases, including the particle mass concentration, the turbulent kinetic energy and its dissipation in the mixture. Particle–turbulence interaction (turbulence modulation) is described by a two-way coupling model. The drag, lift and gravitation forces are incorporated into the system of equations using appropriate closure equations. A finite difference numerical scheme was used for the solution of the set of the governing equations and the results of the model were validated by comparison with data from several experiments. The influence of two types of particles, namely glass and electrocorundum, of different sizes and different loadings on the velocity and turbulence structure of the jet is examined. The computational results show the influence of the particulate phase on the velocity and turbulence structure of the jet. The significance of this study is that for the first time it presents explicitly the full RANS equations for a fluid jet with particles in an unabridged way and specifies the entire set of closure relations that are used for fluid–particle interactions including the equations for the extended k– ε model, the two-way particle–turbulence interactions and turbulence modulation as well as the inclusion of a lateral Saffman force.

Acceleration and transport of ions in turbulent current sheets: formation of non-maxwelian energy distribution

Abstract. The paper is devoted to particle acceleration in turbulent current sheet (CS). Our results show that the mechanism of CS particle interaction with electromagnetic turbulence can explain the formation of power law energy distributions. We study the ratio between adiabatic acceleration of particles in electric field in the presence of stationary turbulence and acceleration due to electric field in the case of dynamic turbulence. The correlation between average energy gained by particles and average particle residence time in the vicinity of the neutral sheet is discussed. It is also demonstrated that particle velocity distributions formed by particle-turbulence interaction are similar in essence to the ones observed near the far reconnection region in the Earth's magnetotail.

An effect of turbulent clustering on scattering of microwave radiation by small particles in the atmosphere

A coherent scattering of electromagnetic waves by clusters of inertial Rayleigh particles in atmospheric turbulence is considered. A preliminary estimate based on the Maxwell-Garnett theory and the Rayleigh approximation for single clusters demonstrates an importance of the coherent scattering contribution. It is confirmed by a general solution in a combination with theoretical estimates for the two-point probability density function for low-inertia spherical particles in isotropic turbulence. An approximate analytical expression for the coefficient characterizing effect of coherent scattering by the particle clusters is derived. The calculations for small Stokes numbers typical of water droplets in cumulus clouds yield an estimate of the coherent scattering effect on the microwave radar reflection. The model suggested allows solving the inverse problem to determine the pair correlation function for cloud particles. It is expected to be important for the investigations on particle–turbulence interaction in the atmosphere. The theoretical model developed is true not only in the limit of low-inertia particles and can be potentially used at arbitrary Stokes numbers in other applications.

Particle–turbulence interaction and local particle concentration in sediment-laden open-channel flows

The present study describes an experimental investigation on the effects of ejection and sweep motions to the local sediment concentration and turbulence modulation in suspended sediment-laden open-channel flows by using a combination between a discriminator particle tracking velocimetry (D-PTV) for sediment particles and a discriminator particle image velocimetry (D-PIV) for fluid tracers. Fluid tracers and sediment particles were discriminated by their occupied particle sizes, and the particle velocity and the fluid velocity were determined simultaneously. It is necessary to investigate coherent structure and particle–turbulence interaction in suspended sediment-laden open-channel flows for predicting the transport of suspended sediment in rivers. There are a lot of previous studies on velocity measurements and numerical simulations in these suspended sediment flows. However, there are little investigation on simultaneous measurements between particles and fluid in bursting phenomena of open-channel flow. So, this study focuses the particle–fluid interaction in the ejection and sweep events by using the discriminator PIV/PTV. The present study found that the turbulence in sediment-laden flows is enhanced or suppressed as compared with that of sediment-free (clear-water) flow. These enhancement and suppression of turbulence are determined by the critical particle diameter, which is correlated with the Kolmogoroff microscale rather than the macroscale of turbulence. The strength of the ejections and sweeps was changed by suspended sediment. The local sediment concentration became about 20–40% larger when the ejection motion occurred, whereas it became about 10–30% smaller when the sweep motion occurred.

Numerical study of particle turbulence interaction in liquid‐particle flows

A comprehensive numerical model that was previously developed to study the turbulent behavior of dilute gas-particle flows and also the effect of the particles in modulating the gas turbulence behind sudden expansion geometry1,2 was used to study the turbulent behavior of dilute liquid particle flow behind a sudden expansion geometry. The major endeavor of the study is to ascertain the response of the particles within the carrier liquid phase. The main aim prompting the current study is the density difference between the carrier and the dispersed phases which is less in terms of the liquid-particle flows in comparison to its counterpart gasparticle flow. The numerical simulations carried out in this study were validated against the experimental data of Founti and Klipfel3 and qualitative results have been obtained. Furthermore, their response to the carrier phase has been investigated both at the mean and at the turbulence level for a range of particle Stokes number. While the particulate velocity seems to increase with the corresponding increase in Stokes number amidst the carrier phase the particulate turbulence shows entirely a different pattern.