Particle-Wall Specularity Effects On Turbulent · PDF fileParticle-Wall Specularity Effects...
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Particle Particle - - Wall Specularity Effects Wall Specularity Effects Abstract Abstract Multiphase flows involving a high concentration of solids are encountered in a variety of applications. Currently computational fluid dynamics (CFD) using Eulerian two-fluid models is finding increasing application for predicting the velocity and concentration fields in such flows. In the two-fluid model, both phases are considered to be interacting, interpenetrating fluids. The constitutive equations for the solids phase are developed using kinetic theory as applied to granular flow. Most often the fluid flow is turbulent. The present research project considers the application of a two-fluid model to the prediction of turbulent vertical upward flow of air and sand particles in a rough-wall circular tube. The simulations explore the effect of surface roughness and the associated specularity on the predictions for the solids phase. The wall specularity factor measures the average tangential particle momentum transferred to the wall. Numerical simulations were performed for a range of values of the specularity factor. As the wall specularity increases, the change in momentum transfer at the wall begins to modify the flow characteristics of the particle phase. Motivation Motivation Multiphase flows are encountered in many different industrial and environmental applications. Hydro transport systems of the oil sands industry in northern Alberta are an example of liquid solid flow forming a high-concentration slurry. Clean coal technologies (CCT) offer many opportunities for the application of high solids concentration simulations. For example, pulverized coal combustion relies on pneumatic transport of finely ground powder to the boilers and the removal of particulate ash from flue gas. Further, all technologies available to reduce particulate emission, such as Electrostatic Precipitators, Fabric Filters, Inertial Collectors and Scrubbers that atomize and collect pollutants, can benefit from improved prediction of high-concentration, fully-developed, turbulent, gas-solid flow. On On Turbulent Turbulent Multiphase Flow Multiphase Flow Conclusion Conclusion Most two-fluid simulations have been applied assuming unrealistic smooth wall flow. It is shown that roughness significantly affects the particle phase behaviour. As the specularity and wall roughness increase, the wall shear stress due to the particles enhances causing the solid phase velocity fluctuations to become stronger. The increase in fluctuations smoothes the peak concentration toward the centre of the pipe. In a smooth pipe the particle phase velocity profile is flatter but as specularity increases the flow begins to behave as though it were obeying a no-slip condition and a more typical flow profile develops. Experimental studies on the effects of roughness on turbulent gas-solid flow is problematic but some literature is available on particle-laden flow in channels of different roughness 6 . It is found that wall roughness decreases the particle phase mean axial velocity and results in a more uniform distribution of velocity fluctuations. Increasing roughness further suppresses this result due to stronger turbulence dissipation and momentum loses at the wall. Studies on dimpled pipes 5 show similar trends. Future Work Future Work Having explored the matter of specularity, the next step in this research will be to add the hydrodynamic effects of roughness on the fluid phase. Research into non-vertical flow with particle phase sedimentation is the logical extension of this work. This summer project is part of a larger research program which is focused on the development of improved computational models for turbulent gas-solid and liquid-solid flows. Velocity Velocity • As specularity increases Fluid velocity profile becomes more sharply curved. Solid phase begins to flow more and more as if it obeys a no-slip condition. Velocity Fluctuations Velocity Fluctuations •Solid phase velocity fluctuations intensify as specularity increases. •This increases fluctuations of fluid near the centre. The energy shifts from the peak towards the centre. •Fluctuations cause average velocity to decay. Wall and particle collisions occur more frequently resulting enhanced energy dissipation lowering velocity and damping turbulence. Concentration Distribution Concentration Distribution • In smooth pipes particulate matter accumulates near the wall. • As the specularity increases the concentration profile rises at the centre. Wall Shear Stress Wall Shear Stress • As specularity increases, shear stress on the pipe due to particles increases. [1] S. V, [1] S. V, Patinikar Patinikar , Numerical Heat Transfer and Fluid Flow. , Numerical Heat Transfer and Fluid Flow. Hemisphere Publishing Corporation Hemisphere Publishing Corporation , New York, 1980. , New York, 1980. [2] D. J. [2] D. J. Tritton Tritton , Physical Fluid Dynamics. , Physical Fluid Dynamics. Oxford Science Publications Oxford Science Publications , Oxford, 1988. , Oxford, 1988. [3] E. J. [3] E. J. Bolio Bolio , J. A. , J. A. Yasuna Yasuna and J. L. Sinclair. Dilute turbulent gas and J. L. Sinclair. Dilute turbulent gas - - solid flow in risers and particle solid flow in risers and particle - - particle interactions. particle interactions. AIChE AIChE Journal Journal , 41 (6): 1375 , 41 (6): 1375 - - 1388, 1995. 1388, 1995. [4] S. L. Lee and F. Durst. On the motion of particles in turbul [4] S. L. Lee and F. Durst. On the motion of particles in turbul ent duct flows. ent duct flows. Int.J. Multiphase Flow Int.J. Multiphase Flow , 8 (2): 125 , 8 (2): 125 - - 146, 1982. 146, 1982. [5] A. Yu. [5] A. Yu. Varaskin Varaskin , M. V. , M. V. Protasov Protasov , T. F. , T. F. Ivanov Ivanov , and A. F. , and A. F. Polyakov Polyakov . An experimental investigation of the behaviour of solid partic . An experimental investigation of the behaviour of solid partic les during their motion in smooth and les during their motion in smooth and dimpled pipes. dimpled pipes. High Temperature High Temperature , 45 (2): 221 , 45 (2): 221 - - 226, 2007. 226, 2007. [6] J. [6] J. Kussin Kussin and M. and M. Sommerfeld Sommerfeld . Experimental studies on particle behaviour and turbulence . Experimental studies on particle behaviour and turbulence modificationin modificationin horizontal channel flow with different wall roughness. horizontal channel flow with different wall roughness. Experiments in Fluids Experiments in Fluids , 33: 143 , 33: 143 – – 159, 2002. 159, 2002. CFD CFD •Decomposes domain into grid of small control volumes (CV) •Reduces differential equations to discrete equations for each CV 1 . •Flow fields estimated as linear •Linear algebraic equations at each node solved iteratively. Turbulence Turbulence •Averaging Navier-Stokes results in additional term that acts as the turbulence component of the total mean stress, the Reynolds stress. Navier-Stokes: Averaged: Reynolds Stress •This suggests an eddy viscosity 2 . Total Viscosity = Molecular Viscosity + Eddy Viscosity Multiphase Flow Multiphase Flow Wall Boundary Conditions Wall Boundary Conditions • Gas phase: No Slip •Solid phase: Coefficient of restitution to describe normal momentum loss. Specularity Factor, ϕ, to describe average tangential momentum transfer. • Computational models of multiphase flow must bias their results by fitting the specularity coefficient so that predicted slip matches measured slip. Simulation Simulation •Modelling follows Bolio et al 3 •Utilises in-house code developed by A. Yerrumshetty. •Fine sand particles (d p = 200μm, ρ p = 2550 kg m -3 ) are carried vertically upward by air through a pipe (R = 17.3mm). This is a standard experimental setup 3-5 . •Specularity is fit until particle slip matches experimental value: ϕ = 0.002. •By varying specularity, numerical simulations explore effects of roughness on particle phase. fluc ave inst u U u ~ + = 0 ~ = fluc u F u u u u ρ ρ μ ρ 1 1 2 + ∇ + ∇ − = ∇ • + ∂ ∂ p t ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − ∂ ∂ ∂ ∂ + ∂ ∂ − = ∂ ∂ inst j inst i j ave i j i j ave j ave j u u r U r r P r U U ρ μ ρ ρ 1 1 inst j inst i u u ρ ≡ inst j inst i j ave i u u r U ρ μ − ∂ ∂ = Results Results T.N. T.N. Shendruk Shendruk Dr. D. J. Dr. D. J. Bergstrom Bergstrom 0 ~ ~ ≠ fluc fluc v u 0.0 0.2 0.4 0.6 0.8 1.0 0 1 2 3 4 5 6 7 8 9 Average Velocity Specularity Factor Fluid Phase Solid Phase 0.0 0.2 0.4 0.6 0.8 1.0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.0 0.2 0.4 0.6 0. 8 1.0 Fluid Velocity Fluctuations Specularity Factor r/R 0. 0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1. 0 Individually Normalized Fluid Veloc it y Specularity Factor r/R 0.0 0.2 0.4 0.6 0.8 1.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.0 0.2 0.4 0.6 0. 8 1.0 Individually Normalized Solid Phase Velocity Specularity Factor r/R 0.0 0.2 0.4 0.6 0.8 1.0 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.0 0.2 0.4 0. 6 0.8 1.0 Solid Phase Psuedo-Temperature Sp ecularity F actor r/R 0.0 0.2 0.4 0.6 0 .8 1.0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 Wall Shear Stress Specularity Total Wall Shear Stress Fluid Wall Shear Stress Solid Wall Shear Stress 0.0 0.2 0.4 0.6 0.8 1.0 0.00070 0.00072 0.00074 0.00076 0. 00078 0.00080 0.00082 0.0 0.2 0.4 0.6 0. 8 1.0 Concentration Speculari ty Factor r/R u U Velocity Time
Transcript of Particle-Wall Specularity Effects On Turbulent · PDF fileParticle-Wall Specularity Effects...
ParticleParticle--Wall Specularity EffectsWall Specularity Effects
AbstractAbstractMultiphase flows involving a high concentration of solids are encountered in a variety of
applications. Currently computational fluid dynamics (CFD) using Eulerian two-fluid models is finding increasing application for predicting the velocity and concentration fields in such flows. In the two-fluid model, both phases are considered to be interacting, interpenetrating fluids. The
constitutive equations for the solids phase are developed using kinetic theory as applied to granular flow. Most often the fluid flow is turbulent. The present research project considers the
application of a two-fluid model to the prediction of turbulent vertical upward flow of air and sand particles in a rough-wall circular tube. The simulations explore the effect of surface roughness and the associated specularity on the predictions for the solids phase. The wall
specularity factor measures the average tangential particle momentum transferred to the wall. Numerical simulations were performed for a range of values of the specularity factor. As the wall specularity increases, the change in momentum transfer at the wall begins to modify the
flow characteristics of the particle phase.
MotivationMotivationMultiphase flows are encountered in many different industrial and environmental applications. Hydro transport systems of the oil sands industry in northern Alberta are an example of liquid solid flow forming a high-concentration slurry. Clean coal technologies (CCT) offer many opportunities for the application of high solids concentration simulations. For example, pulverized coal combustion relies on pneumatic transport of finely ground powder to the boilers and the removal of particulate ash from flue gas. Further, all technologies available to reduce particulate emission, such as Electrostatic Precipitators, Fabric Filters, Inertial Collectors and Scrubbers that atomize and collect pollutants, can benefit from improved prediction of high-concentration, fully-developed, turbulent, gas-solid flow.
On On TurbulentTurbulent Multiphase FlowMultiphase Flow
ConclusionConclusionMost two-fluid simulations have been applied assuming unrealistic smooth wall flow. It is shown that roughness significantly affects the particle phase behaviour. As the specularity and wall roughness increase, the wall shear stress due to the particles enhances causing the solid phase velocity fluctuations to become stronger. The increase in fluctuations smoothes the peak concentration toward the centre of the pipe. In a smooth pipe the particle phase velocity profile is flatter but as specularity increases the flow begins to behave as though it were obeying a no-slip condition and a more typical flow profile develops.
Experimental studies on the effects of roughness on turbulent gas-solid flow is problematic but some literature is available on particle-laden flow in channels of different roughness6. It is found that wall roughness decreases the particle phase mean axial velocity and results in a more uniform distribution of velocity fluctuations. Increasing roughness further suppresses this result due to stronger turbulence dissipation and momentum loses at the wall. Studies on dimpled pipes5
show similar trends.
Future WorkFuture WorkHaving explored the matter of specularity, the next step in this research will be to add the hydrodynamic effects of roughness on the fluid phase. Research into non-vertical flow with particle phase sedimentation is the logical extension of this work. This summer project is part of a larger research program which is focused on the development of improved computational models for turbulent gas-solid and liquid-solid flows.
VelocityVelocity• As specularity increases
Fluid velocity profile becomes more sharply curved. Solid phase begins to flow more and more as if it obeys a no-slip
condition.
Velocity FluctuationsVelocity Fluctuations•Solid phase velocity fluctuations intensify as specularityincreases.•This increases fluctuations of fluid near the centre. The energyshifts from the peak towards the centre.•Fluctuations cause average velocity to decay. Wall and particle collisions occur more frequently resulting enhanced energy dissipation lowering velocity and damping turbulence.
Concentration DistributionConcentration Distribution• In smooth pipes particulate matter accumulates near the wall.• As the specularity increases the concentration profile rises at the centre.
Wall Shear StressWall Shear Stress• As specularity increases, shear stress on the pipe due to particles increases.
[1] S. V, [1] S. V, PatinikarPatinikar, Numerical Heat Transfer and Fluid Flow. , Numerical Heat Transfer and Fluid Flow. Hemisphere Publishing CorporationHemisphere Publishing Corporation, New York, 1980., New York, 1980.[2] D. J. [2] D. J. TrittonTritton, Physical Fluid Dynamics. , Physical Fluid Dynamics. Oxford Science PublicationsOxford Science Publications, Oxford, 1988., Oxford, 1988.[3] E. J. [3] E. J. BolioBolio, J. A. , J. A. YasunaYasuna and J. L. Sinclair. Dilute turbulent gasand J. L. Sinclair. Dilute turbulent gas--solid flow in risers and particlesolid flow in risers and particle--particle interactions. particle interactions. AIChEAIChE JournalJournal, 41 (6): 1375, 41 (6): 1375--1388, 1995.1388, 1995.[4] S. L. Lee and F. Durst. On the motion of particles in turbul[4] S. L. Lee and F. Durst. On the motion of particles in turbulent duct flows. ent duct flows. Int.J. Multiphase FlowInt.J. Multiphase Flow, 8 (2): 125, 8 (2): 125--146, 1982.146, 1982.[5] A. Yu. [5] A. Yu. VaraskinVaraskin, M. V. , M. V. ProtasovProtasov, T. F. , T. F. IvanovIvanov, and A. F. , and A. F. PolyakovPolyakov. An experimental investigation of the behaviour of solid partic. An experimental investigation of the behaviour of solid particles during their motion in smooth and les during their motion in smooth and dimpled pipes. dimpled pipes. High TemperatureHigh Temperature, 45 (2): 221, 45 (2): 221--226, 2007.226, 2007.[6] J. [6] J. KussinKussin and M. and M. SommerfeldSommerfeld. Experimental studies on particle behaviour and turbulence . Experimental studies on particle behaviour and turbulence modificationinmodificationin horizontal channel flow with different wall roughness. horizontal channel flow with different wall roughness. Experiments in FluidsExperiments in Fluids, 33: 143, 33: 143––159, 2002.159, 2002.
CFDCFD•Decomposes domain into grid of small control volumes (CV)
•Reduces differential equations to discrete equations for each CV1.
•Flow fields estimated as linear
•Linear algebraic equations at each node solved iteratively.
TurbulenceTurbulence
•Averaging Navier-Stokes results in additional term that acts as the turbulence component of the total mean stress, the Reynolds stress. Navier-Stokes: Averaged: Reynolds Stress
•This suggests an eddy viscosity2.Total Viscosity = Molecular Viscosity + Eddy Viscosity
Multiphase FlowMultiphase Flow
Wall Boundary Conditions Wall Boundary Conditions • Gas phase:
No Slip
•Solid phase:Coefficient of restitution to describe normal
momentum loss.
Specularity Factor, ϕ, to describe average tangential momentum transfer.
• Computational models of multiphase flow must bias their results by fitting the specularity coefficient so that predicted slip matches measured slip.
SimulationSimulation•Modelling follows Bolio et al3
•Utilises in-house code developed by A. Yerrumshetty.
•Fine sand particles (dp = 200μm, ρp = 2550 kg m-3) are carried vertically upward by air through a pipe (R = 17.3mm). This is a standard experimental setup3-5.
•Specularity is fit until particle slip matches experimental value: ϕ = 0.002.
•By varying specularity, numerical simulations explore effects of roughness on particle phase.
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