Fluid Turbulent Kinetic Energy Research Articles

Mitigation effects of lattice structure arrays on heat transfer deterioration of supercritical CO2

This study numerically investigates the suppression mechanism of heat transfer deterioration (HTD) of supercritical carbon dioxide (SCO2) in heated vertical channels with lattice structure arrays (LSA). The influences of lattice structure parameters on the velocity field, temperature field, heat transfer coefficient, buoyancy effect and flow acceleration effect are analyzed. The results demonstrate that compared to smooth channels, the tri-claw LSA can significantly reduce the maximum peak wall temperature by up to 57 K. The heat transfer coefficient substantially increases, and the fluid turbulent kinetic energy (TKE) quintuples. The vortex structures generated by lattice structures can enhance turbulence intensity and heat transfer, which effectively mitigates buoyancy effect and ultimately suppresses HTD. Increasing the lattice claws number can create more complex vortex structures, lengthening the lattice disrupts the laminar flow within the boundary layer, increasing the lattice diameter can produce a composite vortex that enhances the strength of flow mixing. An increase lattice number across the cross-section strengthens the suppression effect, but an excessive number may lead to instability. A dense lattice arrangement can continually suppress HTD, achieving an overall improvement in heat transfer within a certain range. This paper could support the design of heat exchangers in SCO2 Brayton cycles.

Unlocking the Thermal Efficiency of Irregular Open-Cell Metal Foams: A Computational Exploration of Flow Dynamics and Heat Transfer Phenomena

An open-cell metal foam has excellent characteristics such as low density, high porosity, high specific surface area, high thermal conductivity, and low mass due to its unique internal three-dimensional network structure. It has gradually become a new material for enhanced heat transfer in industrial equipment, new compact heat exchangers, microelectronic device cooling, etc. This research established a comprehensive three-dimensional structural model of open-cell metal foams utilizing Laguerre–Voronoi tessellations and employed computational fluid dynamics to investigate its flow dynamics and coupled heat transfer performance. By exploring the impact of foam microstructure on flow resistance and heat transfer characteristics, the study provided insights into the overall convective heat transfer performance across a range of foam configurations with varying pore densities and porosities. The findings revealed a direct correlation between convective heat transfer coefficient (h) and pressure drop (ΔP) with increasing Reynolds number (Re), accompanied by notable changes in fluid turbulence kinetic energy (e) and temperature (T), ultimately influencing heat transfer efficiency. Furthermore, the analysis demonstrated that alterations in porosity (ε) and pore density significantly affected unit pressure drop (ΔP/L) and convective heat transfer coefficient (h). This study identified an optimal configuration, highlighting a metal foam with a pore density of 20 PPI and a porosity of 95% as exhibiting superior overall convective heat transfer performance.

Numerical Simulation of Water-Sediment Two-Phase Seepage Characteristics and Inrush Mechanism in Rough Rock Fractures

The water-sediment two-phase flow in the rough fracture is one of the main causes of water-sediment inrush. In this study, numerical simulation models of the water-sediment two-phase flow in the smooth and rough fractures were established by ANSYS Fluent software based on the seepage theory; the mechanical properties of the water-sediment two-phase flow under different conditions were systematically investigated, and the influence laws of the surface morphology of the fracture on sediment volume concentration, sediment particle size, and sediment particle mass density were analyzed. In addition, the influence laws of the sediment volume concentration, sediment particle size, and sediment particle mass density on the absolute value of the pressure gradient, mean velocity of the fluid, and fluid turbulent kinetic energy were also illustrated from the perspective of sediment particle distribution. Research shows that during the water-sediment flow in the smooth fracture, the absolute value of pressure gradient Gp, the sediment volume concentrationФ, the sediment particle sizeDp, and the sediment mass densityρpare approximately linear, and the linearity ofGpandDpis the lowest; during the water-sediment flow in the smooth fracture, the mean velocityvof the continuous-phase fluid rarely changes withФ,Dp, andρp. However, during the water-sediment flow in the rough fracture,vis greatly affected byФ,Dp, andρp. During the water-sediment flow in the smooth fracture, the fluid turbulent kinetic energy kt decreases with the increase ofρpandФand decreases with the decrease of ρp. During the water-sediment flow in the rough fracture,ktis significantly affected byФ, Dp, andρp, which was manifested in the changes of curve shapes and deviation of the extreme points.

A combined multiscale modeling and experimental study on surface modification of high-volume micro-nanoparticles with atomic accuracy

Surface modification for micro-nanoparticles at the atomic and close-to-atomic scales is of great importance to enhance their performance in various applications, including high-volume battery, persistent luminescence, etc. Fluidized bed atomic layer deposition (FB-ALD) is a promising atomic-scale manufacturing technology that offers ultrathin films on large amounts of particulate materials. Nevertheless, nanoparticles tend to agglomerate due to the strong cohesive forces, which is much unfavorable to the film conformality and also hinders their real applications. In this paper, the particle fluidization process in an ultrasonic vibration-assisted FB-ALD reactor is numerically investigated from micro-scale to macro-scale through the multiscale computational fluid dynamics and discrete element method (CFD-DEM) modeling with experimental verification. Various vibration amplitudes and frequencies are investigated in terms of their effects on the fluid dynamics, distribution of particle velocity and solid volume fraction, as well as the size of agglomerates. Results show that the fluid turbulent kinetic energy, which is the key power source for the particles to obtain the kinetic energy for overcoming the interparticle agglomeration forces, can be strengthened obviously by the ultrasonic vibration. Besides, the application of ultrasonic vibration is found to reduce the mean agglomerate size in the FB. This is bound to facilitate the heat transfer and precursor diffusion in the entire FB-ALD reactor and the agglomerates, which can largely shorten the coating time and improve the film conformality as well as precursor utilization. The simulation results also agree well with our battery experimental results, verifying the validity of the multiscale CFD-DEM model. This work has provided momentous guidance to the mass manufacturing of atomic-scale particle coating from lab-scale to industrial applications.

Direct Numerical Simulation of Sediment Transport in Turbulent Open Channel Flow Using the Lattice Boltzmann Method

The lattice Boltzmann method is employed to conduct direct numerical simulations of turbulent open channel flows with the presence of finite-size spherical sediment particles. The uniform particles have a diameter of approximately 18 wall units and a density of ρp=2.65ρf, where ρp and ρf are the particle and fluid densities, respectively. Three low particle volume fractions ϕ=0.11%, 0.22%, and 0.44% are used to investigate the particle-turbulence interactions. Simulation results indicate that particles are found to result in a more isotropic distribution of fluid turbulent kinetic energy (TKE) among different velocity components, and a more homogeneous distribution of the fluid TKE in the wall-normal direction. Particles tend to accumulate in the near-wall region due to the settling effect and they preferentially reside in low-speed streaks. The vertical particle volume fraction profiles are self-similar when normalized by the total particle volume fractions. Moreover, several typical transport modes of the sediment particles, such as resuspension, saltation, and rolling, are captured by tracking the trajectories of particles. Finally, the vertical profiles of particle concentration are shown to be consistent with a kinetic model.

Numerical simulation of the variable diameter pipe based on abrasive flow machining

Based on the numerical analysis model of abrasive flow polishing, the effects of fluid turbulent kinetic energy, turbulent intensity and turbulent viscosity on the quality of polishing on polishing quality are studied and analyzed by taking variable diameter pipe as an example. The results of numerical simulation show that the polishing quality becomes better with the decrease of aperture. Therefore, the optimal processing parameters can be simulated according to the specific size of the workpiece, which can provide technical support for the production.

An Eulerian two-phase model for steady sheet flow using large-eddy simulation methodology

Abstract A three-dimensional Eulerian two-phase flow model for sediment transport in sheet flow conditions is presented. To resolve turbulence and turbulence-sediment interactions, the large-eddy simulation approach is adopted. Specifically, a dynamic Smagorinsky closure is used for the subgrid fluid and sediment stresses, while the subgrid contribution to the drag force is included using a drift velocity model with a similar dynamic procedure. The contribution of sediment stresses due to intergranular interactions is modeled by the kinetic theory of granular flow at low to intermediate sediment concentration, while at high sediment concentration of enduring contact, a phenomenological closure for particle pressure and frictional viscosity is used. The model is validated with a comprehensive high-resolution dataset of unidirectional steady sheet flow (Revil-Baudard et al., 2015, Journal of Fluid Mechanics, 767, 1–30). At a particle Stokes number of about 10, simulation results indicate a reduced von Karman coefficient of κ ≈ 0.215 obtained from the fluid velocity profile. A fluid turbulence kinetic energy budget analysis further indicates that the drag-induced turbulence dissipation rate is significant in the sheet flow layer, while in the dilute transport layer, the pressure work plays a similar role as the buoyancy dissipation, which is typically used in the single-phase stratified flow formulation. The present model also reproduces the sheet layer thickness and mobile bed roughness similar to measured data. However, the resulting mobile bed roughness is more than two times larger than that predicted by the empirical formulae. Further analysis suggests that through intermittent turbulent motions near the bed, the resolved sediment Reynolds stress plays a major role in the enhancement of mobile bed roughness. Our analysis on near-bed intermittency also suggests that the turbulent ejection motions are highly correlated with the upward sediment suspension flux, while the turbulent sweep events are mostly associated with the downward sediment deposition flux.

Direct numerical simulation of particle-laden turbulent channel flows with two- and four-way coupling effects: models of terms in the Reynolds stress budgets

In the first part of this study (Dritselis 2016 Fluid Dyn. Res. 48 015507), the Reynolds stress budgets were evaluated through point-particle direct numerical simulations (pp-DNSs) for the particle-laden turbulent flow in a vertical channel with two- and four-way coupling effects. Here several turbulence models are assessed by direct comparison of the particle contribution terms to the budgets, the dissipation rate, the pressure-strain rate, and the transport rate with the model expressions using the pp-DNS data. It is found that the models of the particle sources to the equations of fluid turbulent kinetic energy and dissipation rate cannot represent correctly the physics of the complex interaction between turbulence and particles. A relatively poor performance of the pressure-strain term models is revealed in the particulate flows, while the algebraic models for the dissipation rate of the fluid turbulence kinetic energy and the transport rate terms can adequately reproduce the main trends due to the presence of particles. Further work is generally needed to improve the models in order to account properly for the momentum exchange between the two phases and the effects of particle inertia, gravity and inter-particle collisions.

Large Eddy Simulation of Francis Turbine Flow Fields under Small Opening Condition

The vibration intensity is strong in Francis turbine occurred under the small opening conditions, such as Lijia Gorges and Three Gorges project. In paper we use large eddy simulation (LES) method base on Vreman SubGrid-Scale model to study the generation and evolution process of turbulence flow, capturing the details of the flow structures and the dissipation of the turbulent kinetic energy. The SIMPIEC algorithm is applied to solve the coupled equation of velocity and pressure. The result shows that the small guide vane opening conditions deviate the optimal conditions most. So some unstable flow characters been induced. Such as the turbulent kinetic energy of fluid in guide vanes zone, the blade passage and the draft tube are very strong. The unstable flow phenomenon including the swirl, flow separation, interruption and vortex strip. It can be deduced that the vibration of unit is induced by these flow characteristic.

Large Eddy Simulation of Francis Turbine Flow Fields under Small Opening Condition

The vibration intensity is strong in Francis turbine occurred under the small opening conditions, such as Lijia Gorges and Three Gorges project. In paper we use large eddy simulation (LES) method base on Vreman SubGrid-Scale model to study the generation and evolution process of turbulence flow, capturing the details of the flow structures and the dissipation of the turbulent kinetic energy. The SIMPIEC algorithm is applied to solve the coupled equation of velocity and pressure. The result shows that the small guide vane opening conditions deviate the optimal conditions most. So some unstable flow characters been induced. Such as the turbulent kinetic energy of fluid in guide vanes zone, the blade passage and the draft tube are very strong. The unstable flow phenomenon including the swirl, flow separation, interruption and vortex strip. It can be deduced that the vibration of unit is induced by these flow characteristic.

On turbulence closures for two‐phase sediment‐laden flow models

Recent experimental studies show that there exists a difference between horizontal velocity components of particles and of fluid in sediment‐laden open channel flows. For suspended particles with sufficient inertia, volume fraction profile cannot be represented without the introduction of empirical parameters in passive scalar sediment transport models. The aim of this paper is to propose a two‐phase model that allows representation of the main features of sediment‐laden flows: the existence of a horizontal velocity difference between particles and fluid, the dispersion of particles by fluid turbulent motion, and the damping of fluid turbulent kinetic energy. A 2‐D vertical numerical model for suspended particle transport in open channels using a two‐phase approach is used in conjunction with experimental measurements. Different turbulence models for both phases are presented. The fluid phase turbulence is modeled by a k − ɛ model. Two models for the solid phase turbulence are used: a first‐order model, based on the kinetic theory of granular flows, and an algebraic model based on an homogeneous, isotropic, and steady fluid turbulence assumption. We demonstrate that a modeling approach of the coupling between fluid and solid turbulence allows reproduction of the main features of sediment‐laden flows. We show this approach represents an improvement compared with the classical approach. We show that a simple algebraic model based on the homogeneous, isotropic, and steady fluid turbulence assumption for the solid phase turbulence allows the physical phenomenon to be reproduced with very slight differences compared with a first‐order model.

Homogeneous and isotropic turbulence modulation by small heavy ($St\sim 50$) particles

The interaction of a dilute dispersion of small heavy particles with homogeneous and isotropic air turbulence has been investigated. Stationary turbulence (at Taylor micro-scale Reynolds number of 230) with small mean flow was created in a nearly spherical sealed chamber by means of eight synthetic jet actuators. Two-dimensional particle image velocimetry was used to measure global turbulence statistics in the presence of spherical glass particles that had a diameter of 165 m, which was similar to the Kolmogorov length scale of the flow. Experiments were conducted at two different turbulence levels and particle mass loadings up to 0.3. The particles attenuated the fluid turbulence kinetic energy and viscous dissipation rate with increasing mass loadings. Attenuation levels reached 35–40% for the kinetic energy (which was significantly greater than previous numerical studies) and 40–50% for the dissipation rate at the highest mass loadings. The main source of fluid turbulence kinetic energy production in the chamber was the speakers, but the loss of potential energy of the settling particles also resulted in a significant amount of production of extra energy. The sink of fluid energy in the chamber was due to the ordinary viscous dissipation and extra dissipation caused by particles. The extra dissipation was greatly underestimated by conventional models.

Toward modeling turbulent suspension of sand in the nearshore

We present two depth‐ and phase‐resolving models, based on single‐ and two‐phase approaches for suspended sediment transport under water waves. Both models are the extension of a wave hydrodynamic model Cornell Breaking Wave and Structure (COBRAS). In the two‐phase approach, dilute two‐phase mass and momentum equations are calculated along with a fluid turbulence closure based on balance equations for the fluid turbulence kinetic energy kf and its dissipation rate εf. In the single‐phase approach the fluid flow is described by the Reynolds‐Averaged Navier‐Stokes equations, while the sediment concentration is calculated by an advection‐diffusion equation for the conservation of sediment mass. The fluid turbulence is calculated by kf‐εf equations that incorporate the essential influence of sediment, which can also be consistently deduced from the two‐phase theory. By adopting a commonly used sediment flux boundary condition near the bed the proposed models are tested against laboratory measurements of suspended sediment under nonbreaking skewed water waves and shoaling broken waves. Although the models predict wave‐averaged sediment concentrations reasonably well, the corresponding time histories of instantaneous sediment concentration are less accurate. We demonstrate that this is due to the uncertainties in the near‐bed sediment boundary conditions. In addition, we show that under breaking waves the near‐bed sediment pickup cannot be solely parameterized by the bottom friction, suggesting that other effects may also influence the near‐bed sediment boundary conditions.

The influence of particle inertia on the two-way coupling and modification of isotropic turbulence by microparticles

The objective of this paper is to examine the modulation of isotropic decaying turbulence by particles whose diameter is smaller than the Kolmogorov length scale, and their response time, τp, is smaller than the Kolmogorov time scale, τk (hence microparticles), and the influence of increasing the particle inertia on the two-way coupling. The particle volume fraction is considered small enough, so that particle–particle interactions are neglected. On the other hand, the particle material density ρp≫ρf, the fluid density, and the mass loading of the particles is large enough to modify the carrier flow. The particle Reynolds number is smaller than unity, and the gravitational settling of the particles is neglected. We obtain an asymptotic analytical solution describing the spectrum of the instantaneous two-way coupling source term, Ψp(k,t), in the equation for the fluid turbulence kinetic energy (TKE) spectrum, E(k,t), as a series in powers of the ratio (τp/τk). Recent results of Druzhinin and Elghobashi [Phys. Fluids 11, 602 (1999)] for particles whose τp≪τk show that to the zeroth order in (τp/τk), Ψp(k,t) is proportional to the fluid spectral dissipation function, ε(k,t). In the present paper, the asymptotic solution is extended up to the first order in (τp/τk) and is applicable for particles with small but finite inertia. We also perform direct numerical simulation (DNS) of particle-laden isotropic turbulence using the Eulerian–Lagrangian approach. The results obtained for particles whose τp⩽0.4τk show that both the TKE and its dissipation rate, ε(t), as well as the spectral transfer of the fluid kinetic energy, are increased by the two-way coupling as compared to the particle-free case, and the increase is more pronounced for smaller τp. The asymptotic solution for the two-way coupling source term spectrum, Ψp(k,t), is found in good qualitative and quantitative agreement with the numerical results. Both the asymptotic solution and the DNS results for the instantaneous source term spectrum, Ψp(k,t), show that as the particle response time is increased, the magnitude of the maximum of Ψp(k,t) is reduced and its location is shifted toward higher wave numbers, as compared to the limiting case τp≪τk. The DNS results also show that for particles with sufficiently high inertia (whose τp⩾0.5τk), a negative peak of Ψp(k,t) is created at low wave numbers, whereas the fluid spectral energy transfer is reduced, as compared to the one-way coupling case. The development of the negative peak of Ψp(k,t) is accompanied by a well-pronounced preferential accumulation of particles. The net two-way coupling effect is the reduction of the TKE by particles with sufficiently high inertia (whose τp=0.8τk in our DNS), as compared to the particle-free flow. In this case, our results are in qualitative agreement with the DNS results of Boivin et al. [J. Fluid Mech. 375, 235 (1998)], who considered particles whose τp⩾1.26τk. Therefore, our results show that there occurs a qualitative transition in the two-way coupling effect of particles on isotropic turbulence as the particle response time is increased from τp≪τk, in the limit of microparticles, to τp≃τk, for particles with finite inertia. In the case of microparticles (whose τp≪τk), the instantaneous spectrum of the two-way coupling source term, Ψp(k,t), is positive at all wave numbers so that the particles add the energy to the fluid motion and increase the turbulence kinetic energy, as compared to the one-way coupling case. On the other hand, in the case of particles with higher inertia (whose τp≃τk), the positive contribution of the source term, Ψp(k,t), is reduced at high wave numbers whereas a negative peak of Ψp(k,t) is created at low wave numbers. In this case, the net two-way coupling effect is the reduction of the TKE by the particles, as compared to the one-way coupling case.

Turbulence characteristics along the path of a heavy particle

The turbulence along the path of a particle in an isotropic turbulent flow is investigated using direct numerical simulation. The turbulence is simulated using a collocation spectral method on an equi-spaced three-dimensional grid of 128 3 points with a Taylor micro-scale Reynolds number of 65. The particulate phase is treated as a simple system where the flow is not affected by the presence of the particles. A total of 8192 particles are tracked through the flow using a Lagrangian advancement scheme with Stokes drag as the only force applied to the particles. The implementation of the forcing method used to maintain a constant level of kinetic energy within the flow is studied. The total fluid turbulence kinetic energy was found to be a function of particle Stokes number with smaller particles (St<1) seeing slightly higher average velocity fluctuations than a Lagrangian average of the turbulence and intermediate particles (1<St<10) seeing slightly lower average fluctuations. Likewise, the turbulence integral time scales of the fluid along the path of a particle appear slightly larger than both the Eulerian and Lagrangian integral time scales for these intermediate particles. All preferentially concentrated particles (0.1<St<10) saw a lower average vorticity and higher deformation of the fluid than a volume average of the turbulence. The preferential concentration of the particles peaked at a Stokes number of 1 with particles being on average 20% closer to the nearest neighboring particle than in a randomly distributed case. These data suggest that it is the vorticity measured along the path of a particle which is the dominant factor for variations of statistical properties in the particle Lagrangian reference frame.

Numerical simulation of dilute turbulent gas–solid flows

A new dilute turbulent gas–solid two-phase flow model in the grain inertia flow regime is developed in the present study. A set of time-averaged conservation equations for mass and momentum, and a two-equation multiscale k– ω closure are derived. The solid phase is composed of inelastic, frictional, uniform spheres. Each Lagrangian solid particle is tracked by integrating the particle equations of translational and rotational motion. The couplings in volume fraction, momentum and kinetic energy between the fluid and the solid phases are incorporated in this model. Turbulence modulation due to the solid particles is formulated on the basis of experimental observations. Interparticle collisions and particle–wall collisions are emulated by using a sticking–sliding collision model. The two-phase model is applied to study the steady state flow in a vertical pipe. Depending upon the particle size, mass loading and bulk carrier-fluid velocity, the two-phase system can experience turbulence attenuation, or augmentation, or a combination of both. In general, good agreement is found between the simulation prediction and the experimental data for the mean fluid and solid velocities. Furthermore, there is reasonable qualitative agreement for the fluid turbulence kinetic energy between the two. Many interesting numerical results for macroscopic flow properties of both fluid and solid phases are reported in this paper.

K-ε-T model of dense liquid-solid two-phase turbulent flow and its application to the pipe flow

To predict the characteristics of dense liquid-solid two-phase flow, K-e-T model is established, in which the turbulent flow of fluid phase is described with fluid turbulent kinetic energy K f and its dissipation rate ɛ f , and the particles random motion is described with particle turbulent energy K p and its dissipation rate ɛ p and pseudothermal temperature T p . The soverning equations are also derived. With K-e-T model, numerical study of dense liquid-solid two-phase turbulent up-flow in a pipe is performed. The calculated results are in good agreement with experimental data of Alajbegovic et al. (1994), and some flow features are captured.

Measurements of the particle-fluid velocity correlation and the extra dissipation in a round jet

Measurements of the particle/fluid velocity correlation and the dissipation of turbulent kinetic energy by particles were made in the near field of an axisymmetric jet laden with either 55 or 86 μm diameter glass beads. A digital particle-image-velocimetry system was adapted to measure simultaneously the velocity of fine tracer particles and larger dispersed particles. The particle/fluid velocity correlation was measured directly using the simultaneous measurements and indirectly using separate phase-locked measurements in an acoustically-forced jet. The direct measurements produced correlation values which were too low and the indirect measurements were too high, so the two techniques bracketed the correlation. Surprisingly, the correlation increased with increasing particle time-constant because the mean velocity of the heavier particles was closer to the convection velocity of the vortex rings in the jet. The dissipation of fluid turbulent kinetic energy by the particles was calculated using the phase-locked velocity measurements along with phase-locked particle concentration measurements. The results showed that the extra dissipation is large near the jet centerline, but quite small in the shear layer where the production is large.

On the two-way interaction between homogeneous turbulence and dispersed solid particles. I: Turbulence modification

The modification of decaying homogeneous turbulence due to its interaction with dispersed small solid particles (d/η&amp;lt;1), at a volumetric loading ratio φv≤5×10−4, is studied using direct numerical simulation. The results show that the particles increase the fluid turbulence energy at high wave numbers. This increase of energy is accompanied by an increase of the viscous dissipation rate, and, hence, an increase in the rate of energy transfer T(k) from the large-scale motion. Thus, depending on the conditions at particle injection, the fluid turbulence kinetic energy may increase initially. But, in the absence of external sources (shear or buoyancy), the turbulence energy eventually decays faster than in the particle-free turbulence. In gravitational environment, particles transfer their momentum to the small-scale motion but in an anisotropic manner. The pressure-strain correlation acts to remove this anisotropy by transferring energy from the direction of gravity to the other two directions, but at the same wave number, i.e., to the small-scale motion in directions normal to gravity. This input of energy in the two directions with lowest energy content causes a reverse cascade. This reverse cascade tends to build up the energy level at lower wave numbers, thus reducing the decay rate of energy as compared to that of either the particle-free turbulence or the zero-gravity particle-laden flow.