Turbulent Particle Research Articles

A Validation Study for RANS Based Modelling of Swirling Pulverized Fuel Flames

A swirling pulverized coal flame is computationally investigated. A Eulerian–Lagrangian formulation is used to describe the two-phase flow. Turbulence is modelled within a RANS (Reynolds averaged numerical simulation) framework. Four turbulence viscosity- (TV) based models, namely the standard k-ε model, realizable k-ε model, renormalization group theory k-ε model, and the shear stress transport k-ω model are used. In addition, a Reynolds stress transport model (RSM) is employed. The models are assessed by comparing the predicted velocity fields with the measurements of other authors. In terms of overall average values, the agreement of the predictions to the measurements is observed to be within the range 20–40%. A better performance of the RSM compared to the TV models is observed, with a nearly twice as better overall agreement to the experiments, particularly for the swirl velocity. In the second part of the investigation, the resolution of the discrete particle phase in modelling the turbulent particle dispersion (TPD) and particle size distribution (SD) is investigated. Using the discrete random walk model for the TPD, it is shown that even five random walks are sufficient for an accuracy that is quite high, with a less than 1% mean deviation from the solution obtained by thirty random walks. The approximation of the measured SD is determined by a continuous Rosin–Rammler distribution function, and inaccuracies that can occur in its subsequent discretization are demonstrated and discussed. An investigation on the resolution of the SD by discrete particle size classes (SC) indicates that 12 SC are required for an accuracy with a less than 1% mean deviation from the solution with 18 SC. Although these numbers may not necessarily be claimed to be sufficiently universal, they may serve as guidance, at least for SD with similar characteristics.

Effects of flow regime and conjugate heat transfer on particle deposition in heat exchange duct

Deposition of micron particles in heat exchange duct is commonly encountered in many thermal and HVAC engineering applications. Previous studies on this important issue always considered turbulent air flow regime without conjugate heat transfer (CHT). However, the flow regimes and the CHT may have significant influences on particle deposition behaviors. Therefore, this paper aims to study particle deposition characteristics in laminar flow (LF), developing turbulent flow (DTF) and fully developed turbulent flow (FDTF) regimes with and without CHT. The v′2‾−fturbulence model and discrete particle model (DPM) were used to predict gas phase and solid phase, respectively. Moreover, a modified discrete random walk method was adopted to model turbulent particle dispersion. The results showed that particle deposition velocities greatly increase for small particles (dp<20μm) but keep invariable for large particles (dp>20μm) due to thermophoretic deposition. WhenΔT=100Kanddp=1μm, the peak deposition velocity enhancement ratio reaches 3.8, 9.7 and 7.5 for LF, DTF and FDTF regimes respectively. Different flow regimes have obvious effects on the deposition characteristics. Particle deposition velocity for DTF regime is the highest, followed by FDTF case and LF case. Thermophoresis effect is reduced by the CHT, as the temperature difference decreases due to thermal conduction in solid domain. The CHT in DTF or FDTF regime has obviously larger effect on deposition characteristics, compared with the LF regime.

Vertical structure and microphysical observations of winter precipitation in an inner valley during the Cerdanya-2017 field campaign

Precipitation processes at windward and leeward sides of the mountains have been object of study for many decades. Instead, inner mountain valleys, where usually most mountain population lives, have received considerably less attention. This article examines precipitation processes during a winter field campaign in an inner valley of the Pyrenees (NE Spain) using, among other instruments, a K-band vertically pointing Doppler radar (Micro Rain Radar) and a laser-based optical disdrometer (Parsivel). A decoupling is found between the stalled air of the valley and the air of the free atmosphere above the mountain crest level, evidenced by an increase of turbulence and spectral width of precipitation particles. Wind shear layer may promote riming and aggregation of the ice and snow particles. Two main rainfall regimes are found during the campaign: (1) stratiform rainfall mostly produced by water vapour deposition processes, although sometimes riming and aggregation become important, and (2) weak convection with slight dominance of collision-coalescence processes. Precipitation characteristics at the bottom of the valley show typical continental features such as low Liquid Water Content, despite the valley is only about 100 km from the sea. This study demonstrates that inner valley may present distinct precipitation features with respect to windward and leeward precipitation.

Droplet and particle methods to investigate turbulent particle laden jets

The SARS-CoV-2 pandemic has heightened the interest in particle-laden turbulent jets generated by breathing, talking, coughing and sneezing, and how these can contribute to disease transmission. We present quantitative measurement methods for such flows, while exploring and offering improvements for common shortcomings. We generate jets consisting of either liquid droplets or solid particles in an isothermal, quiescent and electrically isopotential experimental chamber that was constructed to control the effects of ambient forcing on jet behavior. For liquid droplets, we find promise in surface deposition analysis based on fluorescent tracer use. For particles, we explore the performance of commercially available adhesive sampling strips and develop conductive grounded carbon tape based sampling strips. We explore ways in which the smallest of thermal gradients or electrostatic charge issues can affect particle dispersion, and suggest practical methods to address these issues. The developed methods are applied to study the simultaneous deposition of (Formula presented.) 25, 50 and 200 μm solid particles from a particle laden turbulent jet with a mean velocity of 33.2 m/s. The deposition location as a function of particle size was compared to results from a simple numerical RANS model, and illustrates ways in which imprecise initial or boundary conditions can lead to a notable deviation from experimental results. The differences in deposition pattern seen in experimental and numerical results despite a carefully controlled environment and characterized particle ejection indicate the need for a more stringent numerical model validation, especially when studying fate and transport of mid-range (neither purely aerosol or ballistic) sized particles.

Turbulent transport in the scrape-off layer of Wendelstein 7-X

Turbulent transport is widely considered to be the main driver for cross-field transport in the scrape-off layer (SOL) of toroidal magnetized plasmas. Here, reciprocating Langmuir probes are employed to measure both the plasma profiles and the turbulent particle transport in the SOL of the Wendelstein 7-X stellarator. The relation between turbulent radial particle flux Γr and the local pressure gradient is often approximately linear across the entire SOL width, indicating that radial turbulence spreading is absent. This observation holds across a wide range of magnetic configurations and different plasma heating and density scenarios. The magnitude of the turbulent transport for a given gradient reveals a dependence on the magnetic configuration and the position in the SOL, which we relate to the cross-spectral characteristics of multi-tip floating potential measurements. Magnetic islands can add further complexity due to non-monotonic SOL profiles and the breaking of the transport-gradient relation. Finally, anomalous diffusion coefficients are determined from the probe measurements.

Turbulent flame propagation of polymethylmethacrylate particle clouds in an O2/N2 atmosphere

The combustion of solid particle clouds is extensively used in many engineering areas. However, experimental data describing the turbulent flame propagation behavior and the combustion mechanism of solid particle clouds have remained limited. In this work, the combustion of polymethylmethacrylate (PMMA) particle clouds was studied by employing a unique fan-stirred constant-volume chamber. For the quasi-monodispersed particle clouds, the flame propagation velocity increased with the increase in the turbulence intensity and the decrease in the quasi-monodispersed particle size. However, the particle concentration had little effect on the flame propagation velocity, which is unique in a turbulent flow field. The consistency of the results between the current study of PMMA particle clouds and the previous study for coal particle clouds showed that the heterogeneous combustion of char particles had little effect on the turbulent flame propagation velocity of the solid particle clouds. Further, two types of quasi-monodispersed particles were mixed to study how the interactions between small and large (polydispersed) particles affect turbulent flame propagation. We found that the turbulent flame propagation velocity had a nonlinear relationship with the mass ratio of small particles (J-shaped curve). The turbulent flame propagation velocity slightly increased with low mass ratio of small particles, while it sharply increased with high mass ratio of small particles. Increasing the turbulence intensity and decreasing the primary particle (large particle) size can advance the starting point of the sharp increase. These unique features were explained by a mechanism considering the polydispersed interparticle interaction proposed by the authors. In the combustion of turbulent polydispersed particle clouds, the particle–particle agglomeration and the agglomeration break-up in the turbulent flow field affect turbulent flame propagation. To the best of our knowledge, this is the first report on the fundamental spherical turbulent flame propagation phenomenon and the mechanism of solid particle cloud combustion considering the polydispersed interparticle interactions.

Large-scale 3D flow investigations around a cyclically breathing thermal manikin in a 12 m³ room using HFSB and STB

Exhalation of small aerosol droplets and their transport, dispersion and (local) accumulation in closed rooms have been identified as the main pathway for indirect or airborne respiratory virus transmission from person to person, e.g. for SARS-CoV 2 or measles (Morawska and Cao 2020). Understanding airborne transport mechanisms of viruses via small bio-aerosol particles inside closed populated rooms is an important key factor for optimizing various mitigation strategies (Morawska et al. 2020), which can play an important role for damping the infection dynamics of any future and the ongoing present pandemic scenario, which unfortunately, is still threatening due to the spreading of several SARS-CoV2 variants of concern, e.g. delta (Kupferschmidt and Wadman 2021). Therefore, a large-scale 3D Lagrangian Particle Tracking experiment using up to 3 million long lived and nearly neutrally buoyant helium-filled soap bubbles (HFSB) with a mean diameter of ~ 370 µm as passive tracers in a 12 m³ generic test room has been performed, which allows to fully resolve the Lagrangian transport properties and flow field inside the whole room around a cyclically breathing thermal manikin (Lange et al. 2012) with and without mouth-nose-masks and shields applied. Six high-resolution CMOS streaming cameras, a large array of powerful pulsed LEDs have been used and the Shake-The-Box (STB) (Schanz et al. 2016) Lagrangian particle tracking algorithm has been applied in this experimental study of internal flows in order to gain insight into the complex transient and turbulent aerosol particle transport and dispersion processes around seated breathing persons.

Effect of Particle Inertia on the Alignment of Small Ice Crystals in Turbulent Clouds

AbstractSmall nonspherical particles settling in a quiescent fluid tend to orient so that their broad side faces down because this is a stable fixed point of their angular dynamics at small particle Reynolds number. Turbulence randomizes the orientations to some extent, and this affects the reflection patterns of polarized light from turbulent clouds containing ice crystals. An overdamped theory predicts that turbulence-induced fluctuations of the orientation are very small when the settling number Sv (a dimensionless measure of the settling speed) is large. At small Sv, by contrast, the overdamped theory predicts that turbulence randomizes the orientations. This overdamped theory neglects the effect of particle inertia. Therefore, we consider here how particle inertia affects the orientation of small crystals settling in turbulent air. We find that it can significantly increase the orientation variance, even when the Stokes number St (a dimensionless measure of particle inertia) is quite small. We identify different asymptotic parameter regimes where the tilt-angle variance is proportional to different inverse powers of Sv. We estimate parameter values for ice crystals in turbulent clouds and show that they cover several of the identified regimes. The theory predicts how the degree of alignment depends on particle size, shape, and turbulence intensity, and that the strong horizontal alignment of small crystals is only possible when the turbulent energy dissipation is weak, on the order of 1 cm2 s−3 or less.

Evolution of Heavy Ion Beam Probing from the Origins to Study of Symmetric Structures in Fusion Plasmas

The overview discusses development of the unique fusion plasma diagnostics—Heavy Ion Beam Probing (HIBP) in application to toroidal magnetic plasma devices. The basis of the HIBP measurements of the plasma electric potential and processing of experimental data are considered. Diagnostic systems for probing plasma in tokamaks TM-4, TJ-1, TUMAN-3M and T-10, stellarators WEGA, TJ-II and Uragan-2M are presented. Promising results of the HIBP projects for various existing modern machines, such as TCV, TCABR, MAST, COMPASS, GLOBUS-M2, T-15 MD and W7-X and the international fusion tokamak reactor ITER are given. Results from two machines with similar size and plasma parameters, but with different types of the magnetic con-figuration: axisymmetric tokamak T-10 and helically symmetric stellarator TJ-II are compared. The results of studies of stationary potential profiles and oscillations in the form of quasimonochromatic and broadband fluctuations, turbulent particle flux, fluctuations of density and poloidal magnetic field are presented. The properties of symmetric structures—zonal flows and geodesic acoustic modes of plasma oscillations as well as Alfvén Eigenmodes excited by fast particles from neutral beam injection heating are described. General trends in the behavior of electric potential and turbulence in magnetized fusion plasmas are revealed.

Lagrangian and Eulerian accelerations in turbulent stratified shear flows

The Lagrangian (LA) and Eulerian Acceleration (EA) properties of fluid particles in homogeneous turbulence with uniform shear and uniform stable stratification are studied using direct numerical simulations. The Richardson number is varied from $Ri=0$, corresponding to unstratified shear flow, to $Ri=1$, corresponding to strongly stratified shear flow. The probability density functions (pdfs) of both LA and EA have a stretched-exponential shape and they show a strong and similar influence on the Richardson number. The extreme values of the EA are stronger than those observed for the LA. Geometrical statistics explain that the magnitude of the EA is larger than its Lagrangian counterpart due to the mutual cancellation of the Eulerian and convective acceleration, as both vectors statistically show an anti-parallel preference. A wavelet-based scale-dependent decomposition of the LA and EA is performed. The tails of the acceleration pdfs grow heavier for smaller scales of turbulent motion. Hence the flatness increases with decreasing scale, indicating stronger intermittency at smaller scales. The joint pdfs of the LA and EA indicate a trend to stronger correlations with increasing Richardson number and at larger scales of the turbulent motion. A consideration of the terms in the Navier--Stokes equation shows that the LA is mainly determined by the pressure-gradient term, while the EA is dominated by the nonlinear convection term.

Settling behaviour of thin curved particles in quiescent fluid and turbulence

Abstract

A CFD study of particle deposition in three-dimensional heat exchange channel based on an improved deposition model

Airborne particle deposition in the heat exchange channel will cause serious heat transfer efficiency reduction. Better understanding of deposition characteristics is of significance for energy and thermal engineering application. In this study, particle deposition in three-dimensional heat exchange channel with surface ribs was conducted by Reynolds stress model and discrete phase model. An improved deposition model based on turbulent fluctuation correction and particle rebound was developed by user-defined function (UDF). After grid independence study and numerical verification, the flow structures, the secondary flow in the channel corners, particle deposition patterns and deposition efficiency on each wall were investigated and analyzed. The influences of several important parameters, such as particle diameter, the ratio of rib spacing to rib height p/e, and air flow velocity on the turbulent flow and deposition characteristics were investigated. The results showed that turbulent flow structures induced by the ribs, the secondary flow, the interception of rib windward surface, particle rebound and gravitational settling are the main mechanisms for particle deposition in ribbed channel. The main deposition locations are the windward surface of ribs and the cavity walls in front of the ribs, due to the interception of the ribs, particle rebound and flow eddies in the cavities. Moreover, the deposition efficiency of each wall is significantly affected by the influencing parameters.

Energy and flux budget closure theory for passive scalar in stably stratified turbulence

The energy and flux budget (EFB) closure theory for a passive scalar (non-buoyant and non-inertial particles or gaseous admixtures) is developed for stably stratified turbulence. The physical background of the EFB turbulence closures is based on the budget equations for the turbulent kinetic and potential energies and turbulent fluxes of momentum and buoyancy as well as the turbulent flux of particles. The EFB turbulence closure is designed for stratified geophysical flows from neutral to very stable stratification, and it implies that turbulence is maintained by the velocity shear at any stratification. In a steady-state, expressions for the turbulent flux of the passive scalar and the anisotropic non-symmetric turbulent diffusion tensor are derived, and universal flux Richardson number dependencies of the components of this tensor are obtained. The diagonal component in the vertical direction of the turbulent diffusion tensor is suppressed by strong stratification, while the diagonal components in the horizontal directions are not suppressed, but they are dominant in comparison with the other components of the turbulent diffusion tensor. This implies that any initially created strongly inhomogeneous particle cloud is evolved into a thin pancake in a horizontal plane with very slow increase in its thickness in the vertical direction. The turbulent Schmidt number (the ratio of the eddy viscosity and the vertical turbulent diffusivity of the passive scalar) linearly increases with the gradient Richardson number. The physics of such a behavior is related to the buoyancy force that causes a correlation between fluctuations of the potential temperature and the particle number density. This correlation that is proportional to the product of the vertical turbulent particle flux and the vertical gradient of the mean potential temperature reduces the vertical turbulent particle flux. Considering the applications of these results to the atmospheric boundary-layer turbulence, the theoretical relationships are derived, which allows us to determine the turbulent diffusion tensor as a function of the vertical coordinate measured in the units of the local Obukhov length scale. The obtained relations are potentially useful in modeling applications of particle dispersion in the atmospheric boundary-layer turbulence and free atmosphere turbulence.

Flocculation of suspended cohesive particles in homogeneous isotropic turbulence

Abstract

The Development and Application of a Kinetic Theory for Modeling Dispersed Particle Flows

Abstract This Freeman Scholar article reviews the formulation and application of a kinetic theory for modeling the transport and dispersion of small particles in turbulent gas-flows. The theory has been developed and refined by numerous authors and now forms a rational basis for modeling complex particle laden flows. The formalism and methodology of this approach are discussed and the choice of closure of the kinetic equations involved ensures realizability and well posedness with exact closure for Gaussian carrier flow fields. The historical development is presented and how single-particle kinetic equations resolve the problem of closure of the transport equations for particle mass, momentum, and kinetic energy/stress (the so-called continuum equations) and the treatment of the dispersed phase as a fluid. The mass fluxes associated with the turbulent aerodynamic driving forces and interfacial stresses are shown to be both dispersive and convective in inhomogeneous turbulence with implications for the build-up of particles concentration in near wall turbulent boundary layers and particle pair concentration at small separation. It is shown how this approach deals with the natural wall boundary conditions for a flowing particle suspension and examples are given of partially absorbing surfaces with particle scattering and gravitational settling; how this approach has revealed the existence of contra gradient diffusion in a developing shear flow and the influence of the turbulence on gravitational settling (the Maxey effect). Particular consideration is given to the general problem of particle transport and deposition in turbulent boundary layers including particle resuspension. Finally, the application of a particle pair formulation for both monodisperse and bidisperse particle flows is reviewed where the differences between the two are compared through the influence of collisions on the particle continuum equations and the particle collision kernel for the clustering of particles and the degree of random uncorrelated motion (RUM) at the small scales of the turbulence. The inclusion of bidisperse particle suspensions implies the application to polydisperse flows and the evolution of particle size distribution.

Review of hydrogen isotope effects on H-mode confinement in JT-60U

Hydrogen isotope effects on H-mode confinement in JT-60U are reviewed. The thermal energy confinement time becomes longer in deuterium by a factor of ∼1.4 than in hydrogen at a given absorbed power. When the absorbed power is fixed, the values of electron and ion temperature become evidently higher in deuterium than in hydrogen throughout the entire range of minor radius whereas the electron density profiles are nearly the same. Hence, the effective heat diffusivity becomes relatively lower in deuterium than in hydrogen. When the thermal stored energy is fixed, identical profiles for the electron density, electron temperature, and ion temperature are obtained for hydrogen and deuterium plasmas, whereas the required power clearly increases for hydrogen, which results in reduction of heat diffusivity for deuterium. The inverse of the ion-temperature-gradient (ITG) scale length required for a given ion heat diffusivity increased by a factor of approximately 1.2 for deuterium compared with that for hydrogen. Similar density profiles are formed for the hydrogen and deuterium plasmas despite the higher turbulent particle flux for the hydrogen case, suggesting that turbulent particle transport is more suppressed in the deuterium plasmas. The gyrokinetic calculation indicates that the stabilization effect of e-i collisionality on the TEM is more significant for deuterium plasmas and causes the difference in the quasilinear diffusivity between the two cases. The edge-core interplay leading to the overall H-mode confinement is also addressed where the effect of the hydrogen isotopic composition was involved. The correlation between edge pedestal pressure and global poloidal beta holds true consistently regardless of the difference in the isotopic composition. A higher value of global poloidal beta was obtained for deuterium because of its smaller ITG scale length and because of the additional stored energy in the thermal and fast ion components, the latter due to an increase of slowing down time with an increase of isotopic mass.

The significance of positive and negative inertial forces in Particle-Bubble interaction and their role in the general flotation kinetics model

In this study, a theoretical evaluation of the effect of inertial forces in particle-bubble interactions during the flotation process is presented and supported by the experimental data. The effects of positive and negative inertial forces were analyzed by comparing the differences between the models, which either consider or neglect the inertial forces. The Sutherland collision model and the Nguyen attachment model that completely ignore the effect of particle’s inertial forces (inertialess models) were implemented into the general flotation kinetic model. The modified model was then compared with one of the most accurate inertial models, which considers the Generalized Sutherland Equation (GSE) for collision efficiency along with the Dobby-Finch model for attachment efficiency. The flotation kinetics of chalcopyrite and galena particles were estimated using the general flotation kinetic model in order to demonstrate the effect of particle density on the model, which emphasizes the effect of inertial forces. The influence of positive and negative inertial forces on flotation kinetics was evaluated for various explicit parameters such as particle density, turbulence (energy dissipation), and bubble size and velocity. Obtained theoretical results clearly showed the potential of the particle density to counterbalance the negative effects of the inertial forces. The capability of the positive inertial forces for galena particles (high density) to overcome its negative effect was shown when the general flotation kinetic model was used. Theoretical calculations were further confirmed by experimental bubble loading measurements. It was shown that the inertial forces should not be omitted in any flotation model amidst concerns over the complexity.

Paths to caustic formation in turbulent aerosols

The dynamics of small, yet heavy, identical particles in turbulence exhibits singularities, called caustics, that lead to large fluctuations in the spatial particle-number density, and in collision velocities. For large particle, inertia the fluid velocity at the particle position is essentially a white-noise signal and caustic formation is analogous to Kramers escape. Here we show that caustic formation at small particle inertia is different. Caustics tend to form in the vicinity of particle trajectories that experience a specific history of fluid-velocity gradients, characterised by low vorticity and a violent strain exceeding a large threshold. We develop a theory that explains our findings in terms of an optimal path to caustic formation that is approached in the small inertia limit.

Mechanisms governing the settling velocities and spatial distributions of inertial particles in wall-bounded turbulence

We use theory and Direct Numerical Simulations (DNS) to explore the average vertical velocities and spatial distributions of inertial particles settling in a wall-bounded turbulent flow. The theory is based on the exact phase-space equation for the Probability Density Function describing particle positions and velocities. This allowed us to identify the distinct physical mechanisms governing the particle transport. We then examined the asymptotic behavior of the particle motion near the wall, revealing the fundamental differences to the near wall behavior that is produced when incorporating gravitational settling. When the average vertical particle mass flux is zero, the averaged vertical particle velocity is zero away from the wall due to the particles preferentially sampling regions where the fluid velocity is positive, which balances with the downward Stokes settling velocity. When the average mass flux is negative, the combined effects of turbulence and particle inertia lead to average vertical particle velocities that can significantly exceed the Stokes settling velocity, by as much as ten times. Sufficiently far from the wall, the enhanced vertical velocities are due to the preferential sweeping mechanism. However, as the particles approach the wall, the contribution from the preferential sweeping mechanism becomes small, and a downward contribution from the turbophoretic velocity dominates the behavior. Close to the wall, the particle concentration grows as a power-law, but the nature of this power law depends on the particle Stokes number. Finally, our results highlight how the Rouse model of particle concentration is to be modified for particles with finite inertia.

Mass flux of dispersed particles in turbulence: Representations and the influence of correlation structure in gravitational settling.

Different integral representations for the mass flux of inertial particles transported by turbulent gas flows have been proposed. These are discussed and analyzed. Each formulation provides its own insights into the underlying physical processes governing the resulting flux. However, none of the representations, as it stands, provides an explicit closed-form expression in terms of known statistical properties of the flow and parameters governing particle dynamics. We consider the representations in terms of their potential for reduction to closed-form models. To enable an analysis uncomplicated by the presence of many coupled interactions, we confine our attention to the classic test case of monodisperse particles in homogeneous, isotropic turbulent flows, and subject to a uniform gravitational field. The modification of the mean particle settling velocity resulting from their preferential sampling of fluid velocities is captured by the flux representations. A distribution-based symmetry analysis coupled with a correlation splitting technique is used to reduce and simplify the terms appearing in the flux integrals. This prompts a strategy for closure modeling of the resulting expressions in terms of correlations between the sampled fluid velocity and fluid strain-rate fields. Results from particle-trajectory-based simulations are presented to assess the potential of this closure strategy.