Graphite dust deposition in the HTR-10 steam generator

Graphite dust has an important effect on the safe operation of a high-temperature gas-cooled reactor (HTR). The present study analyzes temperature and flow field distributions in the HTR-10 steam generator. The temperature and flow field distributions are then used to study thermophoretic deposition and turbulent deposition. The results show that as the dust diameter increases, the thermophoretic deposition decreases, while the turbulent deposition first decreases and then increases. The thermophoretic deposition is higher at higher reactor powers, with turbulent deposition growing more rapidly at higher reactor power. For small particles, the thermophoretic deposition effect is greater than the turbulent deposition effect, while for large particles, the turbulent deposition effect is dominant.     

A stochastic Langevin model of turbulent particle dispersion in the presence of thermophoresis

Direct numerical simulation (DNS) and experimental data have shown that inertial particles exhibit concentration peaks in isothermal turbulent boundary layers, whereas tracer-like particles remain well mixed in the domain. It is therefore expected that the interactions between turbulence and thermophoresis will be strong in particle-laden flows where walls and carrier fluid are at significantly different temperatures. To capture turbulent particle dispersion with active thermophoresis, a coupled CFD-Lagrangian continuous random walk (CRW) model is developed. The model uses 3D mean flow velocities obtained from the Fluent 6.3 CFD code, to which are added turbulent fluid velocities derived from the normalized Langevin equation which accounts for turbulence inhomogeneities. The mean thermophoretic force is included as a body force on the particle following the Talbot formulation. Validation of the model is performed against recent integral thermophoretic deposition data in long pipes as well as the TUBA TT28 test with its detailed local deposition measurements. In all cases, the agreement with the data is very good. In separate parametric studies in a hypothetical cooled channel flow, it is found that turbulence strongly enhances thermophoretic deposition of particles with dimensionless relaxation times τ⁺ of order 1 or more. On the other hand, the thermophoretic deposition of very small inertia particles (τ⁺ < 0.2) in the asymptotic region far from the injection point tends to that which characterizes stagnant flow conditions, in agreement with the DNS results of Thakurta et al.     

An experimental and theoretical study of particle deposition due to thermophoresis and turbulence in an annular flow

The paper describes an experimental and theoretical study of the deposition of small particles from a turbulent annular-flow with cross-stream temperature variation, focusing on the effects of thermophoresis. Various expressions for the thermophoretic force on a spherical particle are critically discussed. The well-known composite formula of Talbot et al. (1980) does not include the ‘second mechanism of thermophoresis’ and it is concluded that the more recent theoretical approach of Beresnev and Chernyak (1995) is probably more reliable. New experimental measurements of particle deposition from a turbulent flow with cross-stream temperature gradients are then presented. The measurement technique is similar to the method of Liu and Agarwal (1974) but in the test section the aerosol flows vertically downwards in an annular gap between two concentric pipes. By heating the outer pipe and cooling the inner it is possible to establish a substantial, near-constant temperature difference between the two walls and hence a thermophoretic force which varies only with radius. Numerical calculations provide a comparison of theory with experiment. The theory is based on the turbulent deposition models of Young and Leeming (1997) and Slater et al. (2003) modified to include thermophoresis and the annular geometry. The theory of Beresnev and Chernyak gives good agreement with the experimental measurements.     

Modeling of dynamics, heat transfer, and combustion in two-phase turbulent flows: 1. Isothermal flows

This paper presents a review of authors' collective works in the field of two-phase flow modeling done in the past few decades. The paper is aimed at the construction of mathematical models for simulation of particle-laden turbulent flows. A kinetic equation was obtained for the probability density function (PDF) of the particle velocity distribution in turbulent flows. The proposed kinetic equation describes both the interaction of particles with turbulent eddies of the carrier phase and particle-particle collisions. This PDF equation is used for the derivation of different schemes describing turbulent momentum transfer in the dispersed particle phase. The turbulent characteristics of the gaseous phase are calculated on the basis of the k - turbulence model with a modulation effect of particles on the turbulence.

The constructed models have been applied to the calculation of various two-phase gas-particle turbulent flows in jets and channels as well as particle deposition in tubes and separators. For validating the theoretical and numerical results, a wide range of comparisons with experimental data from Russian and foreign sources has been done.     

Particle and droplet deposition in turbulent swirled pipe flow

In this work we study deposition of particles and droplets in non-rotating swirled turbulent pipe flow. We aim at verifying whether the capability of swirl to enhance particle separation from the core flow and the capability of turbulence to efficiently trap particles at the wall can co-exist to optimize collection efficiency in axial separators. We perform an Eulerian–Lagrangian study based on Direct Numerical Simulation (DNS) of turbulence, considering the effect of different swirl intensities on turbulence structures and on particle transfer at varying particle inertia. We show that, for suitably-chosen flow parameters, swirl may be superimposed to the base flow without disrupting near-wall turbulent structures and their regeneration mechanisms. We also quantify collection efficiency demonstrating for the first time that an optimal synergy between swirl and wall turbulence can be identified to promote separation of particles and droplets.     

A numerical study of bingham turbulent flow in sudden-expansion straight circular pipe

I.IntroductionBinghamfluidisonebranchofnon-Newtonianfluid,suchascrudeparaffinoil,highsediment--ladenwaterflow,highconcentrationmudandthelikewhicharetransportedinpipelinesinmanyindustries,soit'sofgreatsignificancetostudytheflowmechanismsofBinghamfluid.Tsaietal.II]studiedthelinkagebetweenBinghamfluidandpluggedflow.Wangetal.I2]measuredtheturbulencestructureofBinghammud.Mengetal.[3]researchedthekineticenergycorrectionfactorofBinghamfluidinacircularpipe.However,thestudyofBinghamfluidsofarisn't…     

Particle behavior in a turbulent pipe flow with a flat bed

Particle behavior in a turbulent flow in a circular pipe with a bed height h = 0.5R is studied at Re_b = 40,000 and for two sizes of particles (5 μm and 50 μm) using large eddy simulation, one-way coupled with a Lagrangian particle tracking technique. Turbulent secondary flows are found within the pipe, with the curved upper wall affecting the secondary flow formation giving rise to a pair of large upper vortices above two smaller vortices close to the pipe floor. The behavior of the two sizes of particle is found to be quite different. The 50 μm particles deposit forming irregular elongated particle streaks close to the pipe floor, particularly at the center of the flow and the pipe corners due to the impact of the secondary flows. The deposition and resuspension rate of the 5 μm particles is high near the center of the floor and at the pipe corners, while values for the 50 μm particles are greatest near the corners. Near the curved upper wall of the pipe, the deposition rate of the 5 μm particles increases in moving from the wall center to the corners, and is greater than that for the larger particles due to the effects of the secondary flow. The maximum resuspension rate of the smaller particles occurs above the pipe corners, with the 50 μm particles showing their highest resuspension rate above and at the corners of the pipe.     

流体黏性对输流管道运动方程及临界流速的影响

考虑实际流体黏性引起的管内流速非均匀分布,针对层流和两种不同的湍流流态,对理想流体情况下输流管道运动方程中的离心力项进行了修正,得到的修正系数分别为1.333(圆管层流)、1.020(光滑管壁圆管湍流)和1.037～1.055(粗糙管壁圆管湍流).根据修正后的运动方程得到的上述3种情况下的发散失稳临界流速比理想流体流动情况下依次分别低13.4%,1.0%和1.8%～2.6%.流体黏性对输流管道运动方程及临界流速的影响只与流态有关,雷诺数决定流态,而黏性系数通过雷诺数间接起作用.     

Influence of gravity and lift on particle velocity statistics and transfer rates in turbulent vertical channel flow

The present study reports detailed statistics for velocity and transfer rates of heavy particles dispersed in turbulent boundary layers. Statistics have been extracted from a homogeneous source of data covering a large target parameter space and are used here to analyze the effects of gravity and lift on particle dispersion and deposition in a systematic way. Datasets were obtained performing Direct Numerical Simulation (DNS) of particle-laden turbulent upward/downward flow in a vertical channel. Six values for the particle timescale (the particle Stokes number, St) ranging three orders of magnitude were considered to analyze the deposition process as the controlling mechanism was shifting from turbulent diffusion to inertia-moderated crossing trajectories. For the particle timescales examined, gravity and lift do not influence the qualitative behavior of particles even though velocity profiles and deposition coefficients are modified in a non-monotonic fashion, reaching an optimum for St ? 15. Physical mechanisms for the different behavior are discussed. Raw data and statistics obtained from the present DNS are made available at http://cfd.cineca.it (mirror site: http://158.110.32.35/download/database) and can be used to test simple models and closure equations for multiphase RANS and Large Eddy simulations.     

Effect of thermophoresis and other parameters on the particle deposition on a tilted surface

In this study a modified version of v2-f turbulence model (φ-α), is applied to simulate a non-isothermal air-flow. The φ-α model and a two-phase Eulerian approach complement each other to predict the rate of particle deposition on a tilted surface. The φ-α model can accurately calculate the normal fluctuations, which mainly represent the non-isotropic nature of turbulence regime near the wall. The Eulerian model was modified considering the most important mechanism in the particle deposition rate when compared to the experimental data. The model performance is examined by comparing the rate of particle deposition on a vertical surface with the experimental data in a turbulent channel flow available in the literature. The effects of lift force, turbophoretic force, thermophoreric force, electrostatic force, gravitational force and Brownian/turbulent diffusion were examined on the particle deposition rate. The results show that, using the φ-α model predicts the rate of deposition with reasonable accuracy. The results of modified particle model are in good agreement with the experimental data. This study highlights the paramount effect of thermophoretic force on the particle deposition rate and clearly shows that when the temperature difference exceeds a certain limit, the electrostatic force has insignificant effect on the particle deposition rate. Furthermore, it is indicated that even at small temperature differences, the effect of tilt angle on the particle deposition rate for intermediate-size particles is negligible.     

Thermophoretic effects on nano-particle deposition in channel flow

In this study, deposition of particles with diameters of 3, 5, and 10 nm in a finite-length heated channel flow is numerically studied under both molecular diffusion and thermophoretic effects. Two types of thermal conditions were examined. The first condition involved various inlet temperatures with a fixed wall temperature. The second condition involved various wall temperatures and a fixed inlet temperature. For a finite channel length, higher particle deposition can be obtained for the various inlet temperature and fixed wall temperature cases. However, for the same temperature ranges, complete particle collection on the wall can only be achieved under various wall temperatures and fixed inlet temperature cases when the channel length is long enough. This is because a temperature gradient appears in these cases. The temperature gradient in the various inlet wall temperatures and fixed wall temperature cases is zero when the flow is thermally fully developed.     

On the impedance of the pipe in laminar and turbulent pulsating flows

The response of a facility, consisting of a valveless reciprocating pump, a large settling chamber and a long straight smooth pipe, to a periodic change in the volume was analysed. The impedance of the pipe was estimated in both laminar and turbulent flow regimes under otherwise identical flow conditions. A good agreement with theory was obtained for the laminar flow. The estimate of the pipe impedance from the experimental data in turbulent flow was based on the momentum equation as well as on the measured resonant frequency of the system. These independent methods show that the inertance of the pipe has a qualitatively different behavior in laminar and turbulent flow regimes.     

A Modelling Study of Evolving Particle-laden Turbulent Pipe-flow

An Eulerian turbulent two phase flow model using kinetic theory of granular flows for the particle phase was developed in order to study evolving upward turbulent gas particle flows in a pipe. The model takes the feedback of the particles into account and its results agree well with experiments. Simulations show that the pipe length required for particle laden turbulent flow to become fully developed is up to five times longer than an unladen flow. To increase the understanding of the dependence of the development length on particle diameter a simple model for the expected development length was derived. It shows that the development length becomes shorter for increasing particle diameters, which agrees with simulations up to a particle diameter of 100 ??m. Thereafter the development length becomes longer again for increasing particle diameters because larger particles need a longer time to adjust to the velocity of the carrier phase.     

Thermophoretic augmentation of particle deposition in natural convection flow through a parallel plate channel

Present paper deals with temperature driven mass deposition rate of particles known as thermophoretic wall flux when a hot flue gas in natural convection flow through a cooled isothermal vertical parallel plate channel. Present study finds application in particle filters used to trap soot particles from post combustion gases issuing out of small furnaces with low technical implications. Governing equations are solved using finite difference marching technique with channel inlet values as initial values. Channel heights required to regain hydrostatic pressure at the exit are estimated for various entry velocities. Effect of temperature ratio between wall and gas on thermophoretic wall flux is analysed and wall flux found to increase with decrease in temperature ratio. Results are compared with published works wherever possible and can be used to predict particle deposition rate as well as the conditions favourable for maximum particle deposition rate.     

Particle deposition on a channel wall in a turbulent disperse flow with gradient

Kinetic ideas about the motion of a set of particles (droplets) in a turbulent gas flow with gradient are used to derive a Fokker-Planck equation for the case of sufficiently large particles (more than few microns). This equation describes the process in which they are deposited on the wall of a channel. Satisfactory agreement has been obtained between the numerical solution to this equation for the deposition rate and the experimental data published in the literature. Under the assumption that the parameters of the carrier gaseous flow vary fairly slowly, a generalized equation is derived for particle diffusion in turbulent flow. This takes into account the intensity gradient of transverse pulsations in the velocity of the carrier gaseous flow.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 57–63, July–August, 1985.     

纳米颗粒在人类鼻腔中沉降率的研究

借助欧拉和拉格朗日方法数值模拟了纳米颗粒在人类鼻腔中的输运和沉降. 在采用有限体积法以及k-w湍流模型求解流场的基础上, 通过单向耦合的拉格朗日方法得出了水动力、热泳力和布朗力等综合作用下的纳米颗粒的运动轨迹以及沉降率. 研究发现, 非常微小的纳米颗粒在鼻腔内的沉降率非常高; 粒径在1~10nm的纳米颗粒在鼻腔中的沉降率从80%降至18%; 粒径在10sim150nm之间的纳米颗粒在鼻腔中的沉降率变化很小,且其值介于15%~18%之间.      

Turbulent particle dispersion in arbitrary wall-bounded geometries: A coupled CFD-Langevin-equation based approach

A Lagrangian continuous random walk (CRW) model is developed to predict turbulent particle dispersion in arbitrary wall-bounded flows with prevailing anisotropic, inhomogeneous turbulence. The particle tracking model uses 3D mean flow data obtained from the Fluent CFD code, as well as Eulerian statistics of instantaneous quantities computed from DNS databases. The turbulent fluid velocities at the current time step are related to those of the previous time step through a Markov chain based on the normalized Langevin equation which takes into account turbulence inhomogeneities. The model includes a drift velocity correction that considerably reduces unphysical features common in random walk models. It is shown that the model satisfies the well-mixed criterion such that tracer particles retain approximately uniform concentrations when introduced uniformly in the domain, while their deposition velocity is vanishingly small, as it should be. To handle arbitrary geometries, it is assumed that the velocity rms values in the boundary layer can locally be approximated by the DNS data of fully developed channel flows. Benchmarks of the model are performed against particle deposition data in turbulent pipe flows, 90° bends, as well as more complex 3D flows inside a mouth-throat geometry. Good agreement with the data is obtained across the range of particle inertia.     

Modification of equation of motion of fluid-conveying pipe for laminar and turbulent flow profiles

Considering the non-uniformity of the flow velocity distribution in fluid-conveying pipes caused by the viscosity of real fluids, the centrifugal force term in the equation of motion of the pipe is modified for laminar and turbulent flow profiles. The flow-profile-modification factors are found to be 1.333, 1.015–1.040 and 1.035–1.055 for laminar flow in circular pipes, turbulent flow in smooth-wall circular pipes and turbulent flow in rough-wall circular pipes, respectively. The critical flow velocities for divergence in the above-mentioned three cases are found to be 13.4%, 0.74–1.9% and 1.7–2.6%, respectively, lower than that with plug flow, while those for flutter are even lower, which could reach 36% for the laminar flow profile. By introducing two new concepts of equivalent flow velocity and equivalent mass, fluid-conveying pipe problems with different flow profiles can be solved with the equation of motion for plug flow.     

Experimental Determination of Lagrangian Velocity Statistics in Turbulent Pipe Flow

The calculation of Lagrangian statistics out of experimentally determined data from homogeneously seeded inhomogeneous turbulent flows is far from straightforward since statistical properties are position-dependent, necessitating local sampling. Two solutions for the preferential sampling of faster particles at a certain position in the flow are proposed. The performance of both methods was tested using DNS calculations for turbulent pipe flow. Both methods show a good performance for various statistical properties, thus providing two reliable ways to analyze experimental data from inhomogeneous turbulent flows.