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August 10, 2015, 04:25 |
Wrong flow in ratating domain problem
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#1 |
Member
Sanyo
Join Date: Apr 2009
Location: India
Posts: 62
Rep Power: 17 |
Dear All,
Greetings. I am trying to simulate a solid-liquid decanter along with air. Decanter is an equipment which uses centrifugal force to separate solid & liquid phase. It is a rotating drum. Inside rotating drum there is rotating screw. Direction of rotation & RPM for both is identical. There is a small gap of 0.25-1 mm in drum & screw. On left side of drum there is a weir controlled liquid outlet while at right side elevated solid outlet. Both outlet are open to atmosphere. Screw rotation is such that it will push the solids towards solid outlet. In addition, there is dominant air volume which needs to be considered. I have modeled air & water as continuous phase & solid particles as discrete phase (not the DPM). Rotation of drum & screw is modeled using rotating domain & frozen rotor interface (There is stationary inlet pipe before drum). In reality, solids are collected from solid outlet while liquid passes to liquid outlet thru small gap between drum & screw. Weir at outlet controls the liquid level. However in simulation everything is passing thru solid outlet only due to conveying motion of screw. We have tried refining mesh in the gap, used stage & transient rotor approach, changed flow rates & RPM; but no use. Now its quite a time we are struggling with this problem & its frustrating now. Is there any way to solve this problem? Another problem with this simulation is mass balance is not achieved. Sometimes there is difference of 0.1% but sometimes the mass going out of the system is much more than the mass coming into the system. But I guess this might be because of dead air volume inside equipment which is causing accumulation of the mass. Thanks in advance. Regards, Sanyo |
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August 10, 2015, 06:02 |
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#2 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
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Please show some images of what you are modelling, your mesh and your CCL.
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August 11, 2015, 06:33 |
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#3 |
Member
Sanyo
Join Date: Apr 2009
Location: India
Posts: 62
Rep Power: 17 |
LIBRARY:
CEL: EXPRESSIONS: AreaIn = area()@INLET UpVFAir = 1-UpVFWater UpVFWater = step((Radius -rw)/1[m]) Vin = vol flow /AreaIn flow = 15 rw = 0.12 [m] vol flow = (flow /3600) [m^3 s^-1] END END COORDINATE FRAME DEFINITIONS: COORDINATE FRAME: Coord 1 Axis 3 Point = 0.0[m],0.0[m],1.0[m] Coord Frame Type = Cartesian Option = Axis Points Origin Point = 0.0[m],0.0[m],0.0[m] Plane 13 Point = 1.0[m],0.0[m],0.0[m] Reference Coord Frame = Coord 0 END END MATERIAL: Air at 25 C Material Description = Air at 25 C and 1 atm (dry) Material Group = Air Data, Constant Property Gases Option = Pure Substance Thermodynamic State = Gas PROPERTIES: Option = General Material EQUATION OF STATE: Density = 1.185 [kg m^-3] Molar Mass = 28.96 [kg kmol^-1] Option = Value END DYNAMIC VISCOSITY: Dynamic Viscosity = 1.831E-05 [kg m^-1 s^-1] Option = Value END MATERIAL: FLUID SOLIDS Material Description = Water (liquid) Material Group = Water Data,Constant Property Liquids Option = Pure Substance Thermodynamic State = Liquid PROPERTIES: Option = General Material EQUATION OF STATE: Density = 2200 [kg m^-3] Molar Mass = 18.02 [kg kmol^-1] Option = Value END DYNAMIC VISCOSITY: Dynamic Viscosity = 8.899E-4 [kg m^-1 s^-1] Option = Value END MATERIAL: Water Material Description = Water (liquid) Material Group = Water Data,Constant Property Liquids Option = Pure Substance Thermodynamic State = Liquid PROPERTIES: Option = General Material EQUATION OF STATE: Density = 1150 [kg m^-3] Molar Mass = 18.02 [kg kmol^-1] Option = Value END DYNAMIC VISCOSITY: Dynamic Viscosity = 0.0015 [Pa s] Option = Value END FLOW: Flow Analysis 1 SOLUTION UNITS: Angle Units = [rad] Length Units = [m] Mass Units = [kg] Solid Angle Units = [sr] Temperature Units = [K] Time Units = [s] END ANALYSIS TYPE: Option = Steady State EXTERNAL SOLVER COUPLING: Option = None END END DOMAIN: ROTATING II Coord Frame = Coord 0 Domain Type = Fluid Location = Auto Detected Volume 28,HEX,Auto Detected Volume 27 BOUNDARY: Air Opening Boundary Type = OPENING Frame Type = Stationary Location = AIR OPENING BOUNDARY CONDITIONS: FLOW DIRECTION: Option = Normal to Boundary Condition END FLOW REGIME: Option = Subsonic END MASS AND MOMENTUM: Option = Opening Pressure and Direction Relative Pressure = 0 [Pa] END TURBULENCE: Option = Medium Intensity and Eddy Viscosity Ratio END END FLUID: Air BOUNDARY CONDITIONS: VOLUME FRACTION: Option = Value Volume Fraction = 1 END END END FLUID: Liquid BOUNDARY CONDITIONS: VOLUME FRACTION: Option = Value Volume Fraction = 0 END END END FLUID: Solids BOUNDARY CONDITIONS: VOLUME FRACTION: Option = Value Volume Fraction = 0 END END END END BOUNDARY: Domain Interface 1 Side 2 Boundary Type = INTERFACE Location = INTERFACE INLET BOUNDARY CONDITIONS: MASS AND MOMENTUM: Option = Conservative Interface Flux END TURBULENCE: Option = Conservative Interface Flux END END END BOUNDARY: Domain Interface 2 Side 1 Boundary Type = INTERFACE Location = INTERFACE DISTRIBUTOR 01_2,INTERFACE DISTRIBUTOR \ 02_2,INTERFACE DISTRIBUTOR 03_2,INTERFACE DISTRIBUTOR 04_2 BOUNDARY CONDITIONS: MASS AND MOMENTUM: Option = Conservative Interface Flux END TURBULENCE: Option = Conservative Interface Flux END END END BOUNDARY: Domain Interface 2 Side 2 Boundary Type = INTERFACE Location = INTERFACE DISTRIBUTOR 01,INTERFACE DISTRIBUTOR 02,INTERFACE \ DISTRIBUTOR 03,INTERFACE DISTRIBUTOR 04 BOUNDARY CONDITIONS: MASS AND MOMENTUM: Option = Conservative Interface Flux END TURBULENCE: Option = Conservative Interface Flux END END END BOUNDARY: OUTLET LIQUID Boundary Type = OUTLET Frame Type = Rotating Location = OUTLET LIQUID BOUNDARY CONDITIONS: FLOW REGIME: Option = Subsonic END MASS AND MOMENTUM: Option = Static Pressure Relative Pressure = 0 [Pa] END END END BOUNDARY: OUTLET SOLID Boundary Type = OUTLET Frame Type = Rotating Location = OUTLET SOLID BOUNDARY CONDITIONS: FLOW REGIME: Option = Subsonic END MASS AND MOMENTUM: Option = Static Pressure Relative Pressure = 0 [Pa] END END END BOUNDARY: WALL DISTRIBUTER Boundary Type = WALL Frame Type = Rotating Location = DISTRIBUTOR BOUNDARY CONDITIONS: MASS AND MOMENTUM: Option = No Slip Wall END WALL ROUGHNESS: Option = Smooth Wall END END END BOUNDARY: WALL DRUM Boundary Type = WALL Frame Type = Rotating Location = DRUM,WALL HEX DRUM BOUNDARY CONDITIONS: MASS AND MOMENTUM: Option = No Slip Wall END WALL ROUGHNESS: Option = Smooth Wall END END END BOUNDARY: WALL OUTLETS Boundary Type = WALL Frame Type = Rotating Location = WALL LIQUID OUT,WALL SOLID OUT,WALL OTHER BOUNDARY CONDITIONS: MASS AND MOMENTUM: Option = No Slip Wall END WALL ROUGHNESS: Option = Smooth Wall END END END BOUNDARY: WALL SCREW Boundary Type = WALL Frame Type = Rotating Location = SCREW,WALL HEX SCREW BOUNDARY CONDITIONS: MASS AND MOMENTUM: Option = No Slip Wall END WALL ROUGHNESS: Option = Smooth Wall END END END BOUNDARY: WALL SHAFT Boundary Type = WALL Frame Type = Rotating Location = SHAFT BOUNDARY CONDITIONS: MASS AND MOMENTUM: Option = No Slip Wall END WALL ROUGHNESS: Option = Smooth Wall END END END DOMAIN MODELS: BUOYANCY MODEL: Option = Non Buoyant END DOMAIN MOTION: Angular Velocity = 2000 [rev min^-1] Option = Rotating AXIS DEFINITION: Option = Coordinate Axis Rotation Axis = Coord 0.1 END END MESH DEFORMATION: Option = None END REFERENCE PRESSURE: Reference Pressure = 1 [atm] END END FLUID DEFINITION: Air Material = Air at 25 C Option = Material Library MORPHOLOGY: Option = Continuous Fluid END END FLUID DEFINITION: Liquid Material = Water Option = Material Library MORPHOLOGY: Option = Continuous Fluid END END FLUID DEFINITION: Solids Material = FLUID SOLIDS Option = Material Library MORPHOLOGY: Mean Diameter = 50 [micron] Option = Dispersed Solid END END FLUID MODELS: COMBUSTION MODEL: Option = None END FLUID: Solids KINETIC THEORY MODEL: Option = None END SOLID BULK VISCOSITY: Option = None END SOLID PRESSURE MODEL: Option = None END SOLID SHEAR VISCOSITY: Option = None END END HEAT TRANSFER MODEL: Homogeneous Model = On Option = None END THERMAL RADIATION MODEL: Option = None END TURBULENCE MODEL: Option = SST END TURBULENT WALL FUNCTIONS: Option = Automatic END END FLUID PAIR: Air | Liquid INTERPHASE TRANSFER MODEL: Option = Free Surface END MASS TRANSFER: Option = None END SURFACE TENSION MODEL: Option = None END END FLUID PAIR: Air | Solids INTERPHASE TRANSFER MODEL: Option = Particle Model END MASS TRANSFER: Option = None END SURFACE TENSION MODEL: Option = None END END FLUID PAIR: Liquid | Solids INTERPHASE TRANSFER MODEL: Option = Particle Model END MASS TRANSFER: Option = None END SURFACE TENSION MODEL: Option = None END END INITIALISATION: Frame Type = Rotating Option = Automatic FLUID: Air INITIAL CONDITIONS: VOLUME FRACTION: Option = Automatic with Value Volume Fraction = UpVFAir END END END FLUID: Liquid INITIAL CONDITIONS: VOLUME FRACTION: Option = Automatic with Value Volume Fraction = UpVFWater END END END FLUID: Solids INITIAL CONDITIONS: VOLUME FRACTION: Option = Automatic with Value Volume Fraction = 0 END END END INITIAL CONDITIONS: Velocity Type = Cartesian CARTESIAN VELOCITY COMPONENTS: Option = Automatic with Value U = 0 [m s^-1] V = 0 [m s^-1] W = 0 [m s^-1] END STATIC PRESSURE: Option = Automatic with Value Relative Pressure = 0 [bar] END TURBULENCE INITIAL CONDITIONS: Option = Medium Intensity and Eddy Viscosity Ratio END END END MULTIPHASE MODELS: Homogeneous Model = On FREE SURFACE MODEL: Option = Standard END END END DOMAIN: STATIONARY I Coord Frame = Coord 0 Domain Type = Fluid Location = STATIONARY VOL BOUNDARY: Domain Interface 1 Side 1 2 Boundary Type = INTERFACE Location = INTERFACE INLET_2 BOUNDARY CONDITIONS: MASS AND MOMENTUM: Option = Conservative Interface Flux END TURBULENCE: Option = Conservative Interface Flux END END END BOUNDARY: INLET Boundary Type = INLET Location = INLET BOUNDARY CONDITIONS: FLOW REGIME: Option = Subsonic END MASS AND MOMENTUM: Normal Speed = Vin Option = Normal Speed END TURBULENCE: Option = Medium Intensity and Eddy Viscosity Ratio END END FLUID: Air BOUNDARY CONDITIONS: VOLUME FRACTION: Option = Value Volume Fraction = 0 END END END FLUID: Liquid BOUNDARY CONDITIONS: VOLUME FRACTION: Option = Value Volume Fraction = 0.9451 END END END FLUID: Solids BOUNDARY CONDITIONS: VOLUME FRACTION: Option = Value Volume Fraction = 0.0549 END END END END BOUNDARY: WALL INLET Boundary Type = WALL Location = WALL INLET BOUNDARY CONDITIONS: MASS AND MOMENTUM: Option = No Slip Wall END WALL ROUGHNESS: Option = Smooth Wall END END END DOMAIN MODELS: BUOYANCY MODEL: Option = Non Buoyant END DOMAIN MOTION: Option = Stationary END MESH DEFORMATION: Option = None END REFERENCE PRESSURE: Reference Pressure = 1 [atm] END END FLUID DEFINITION: Air Material = Air at 25 C Option = Material Library MORPHOLOGY: Option = Continuous Fluid END END FLUID DEFINITION: Liquid Material = Water Option = Material Library MORPHOLOGY: Option = Continuous Fluid END END FLUID DEFINITION: Solids Material = FLUID SOLIDS Option = Material Library MORPHOLOGY: Mean Diameter = 50 [micron] Option = Dispersed Solid END END FLUID MODELS: COMBUSTION MODEL: Option = None END FLUID: Solids KINETIC THEORY MODEL: Option = None END SOLID BULK VISCOSITY: Option = None END SOLID PRESSURE MODEL: Option = None END SOLID SHEAR VISCOSITY: Option = None END END HEAT TRANSFER MODEL: Homogeneous Model = On Option = None END THERMAL RADIATION MODEL: Option = None END TURBULENCE MODEL: Option = SST END TURBULENT WALL FUNCTIONS: Option = Automatic END END FLUID PAIR: Air | Liquid INTERPHASE TRANSFER MODEL: Option = Free Surface END MASS TRANSFER: Option = None END SURFACE TENSION MODEL: Option = None END END FLUID PAIR: Air | Solids INTERPHASE TRANSFER MODEL: Option = Particle Model END MASS TRANSFER: Option = None END SURFACE TENSION MODEL: Option = None END END FLUID PAIR: Liquid | Solids INTERPHASE TRANSFER MODEL: Option = Particle Model END MASS TRANSFER: Option = None END SURFACE TENSION MODEL: Option = None END END INITIALISATION: Option = Automatic FLUID: Air INITIAL CONDITIONS: VOLUME FRACTION: Option = Automatic with Value Volume Fraction = 0 END END END FLUID: Liquid INITIAL CONDITIONS: VOLUME FRACTION: Option = Automatic with Value Volume Fraction = 1 END END END FLUID: Solids INITIAL CONDITIONS: VOLUME FRACTION: Option = Automatic with Value Volume Fraction = 0 END END END INITIAL CONDITIONS: Velocity Type = Cartesian CARTESIAN VELOCITY COMPONENTS: Option = Automatic with Value U = -4.5 [m s^-1] V = 0 [m s^-1] W = 0 [m s^-1] END STATIC PRESSURE: Option = Automatic with Value Relative Pressure = 0 [bar] END TURBULENCE INITIAL CONDITIONS: Option = Medium Intensity and Eddy Viscosity Ratio END END END MULTIPHASE MODELS: Homogeneous Model = On FREE SURFACE MODEL: Option = Standard END END END DOMAIN INTERFACE: Domain Interface 1 Boundary List1 = Domain Interface 1 Side 1 2 Boundary List2 = Domain Interface 1 Side 2 Interface Type = Fluid Fluid INTERFACE MODELS: Option = General Connection FRAME CHANGE: Option = Frozen Rotor END MASS AND MOMENTUM: Option = Conservative Interface Flux MOMENTUM INTERFACE MODEL: Option = None END END PITCH CHANGE: Option = None END END MESH CONNECTION: Option = GGI END END DOMAIN INTERFACE: Domain Interface 2 Boundary List1 = Domain Interface 2 Side 1 Boundary List2 = Domain Interface 2 Side 2 Interface Type = Fluid Fluid INTERFACE MODELS: Option = General Connection FRAME CHANGE: Option = None END MASS AND MOMENTUM: Option = Conservative Interface Flux MOMENTUM INTERFACE MODEL: Option = None END END PITCH CHANGE: Option = None END END MESH CONNECTION: Option = GGI END END OUTPUT CONTROL: BACKUP RESULTS: Backup Results 1 File Compression Level = Default Option = Standard OUTPUT FREQUENCY: Iteration Interval = 50 Option = Iteration Interval END END RESULTS: File Compression Level = Default Option = Standard END END SOLVER CONTROL: Turbulence Numerics = High Resolution ADVECTION SCHEME: Blend Factor = 0.75 Option = Specified Blend Factor END CONVERGENCE CONTROL: Length Scale = 1 [m] Length Scale Option = Specified Length Scale Maximum Number of Iterations = 2000 Minimum Number of Iterations = 1000 Timescale Control = Auto Timescale Timescale Factor = 1.0 END CONVERGENCE CRITERIA: Residual Target = 0.00001 Residual Type = RMS END DYNAMIC MODEL CONTROL: Global Dynamic Model Control = Yes END |
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August 11, 2015, 06:45 |
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#4 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
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What stops the solids from flowing through the 1mm gap? They look like 50um particles and a 1mm gap so they would fit through.
Does the solids form a packed bed or agglomerate? CFX does not have models for this. What stops the liquid flowing out the solid outlet? Or does the screw push the solids above the fluid level, so they are pretty dry? |
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August 11, 2015, 10:56 |
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#5 |
Member
Sanyo
Join Date: Apr 2009
Location: India
Posts: 62
Rep Power: 17 |
Solid particles agglomerate and screw pushes particles above water level. We intend to follow SPH (solid particle harmonics) method in later stage of project for particle modelling. But for now liquid phenomenon is important.
Are we modeling rotations in wrong way? Is there any other method to model rotation? Or any other method to model the problem? |
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August 11, 2015, 19:06 |
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#6 |
Super Moderator
Glenn Horrocks
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Well, there's your problem. CFX has no agglomeration model, and no mechanism by which you can push the particles out of the water. So it appears CFX lacks two key bits of physics you need to model this device. So I don't think you can model your device with CFX - at least without developing a particle agglomeration model and a method of pushing particles out of the water.
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August 12, 2015, 02:13 |
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#7 |
Member
Sanyo
Join Date: Apr 2009
Location: India
Posts: 62
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Thanks for reply. I agree that it needs special treatment for particles. But at this moment, we are concerned about the water behavior. It should pass thru 1mm gap & leave the domain from liquid outlet which is at left side. But simulation shows liquid passing thru solid outlet (right side outlet). What could be the reason of this opposite behavior?
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August 12, 2015, 05:47 |
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#8 |
Super Moderator
Glenn Horrocks
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Please show images of your mesh and the flow results you are getting.
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August 12, 2015, 07:11 |
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#9 |
Member
Sanyo
Join Date: Apr 2009
Location: India
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Centreplane shows the liquid volume fraction accumulation.
When i try streamlines using velocity in stn frame, the streamlines are ending abruptly. i tried to increase the limits but not useful. |
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August 12, 2015, 19:28 |
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#10 |
Super Moderator
Glenn Horrocks
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This device lies horizontally, doesn't it? And gravity keeps the liquid at the bottom of the screw? The final image shows that liquid is right the way around the device. This appears to be a fundamental problem in the way you are modelling it.
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August 13, 2015, 05:44 |
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#11 |
Member
Sanyo
Join Date: Apr 2009
Location: India
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The device do lie horizontally. But the decanter separates the phases using centrifugal force. G force in decanter goes well beyond 1000-2000 times more than gravitational force making gravity insignificant.
CFX doesn't allow to solve rotating domain with gravity direction not aligned with rotation axis in steady state. I have to solve it as transient simulation. I think RFR approach is not suitable for this kind of problems. If I define the wall velocity instead of rotating domain, will it solve the problem? I know that I have to go for mesh deformation option but will it help to capture the physics? How wall velocity option treats the fluid? |
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August 13, 2015, 06:00 |
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#12 |
Super Moderator
Glenn Horrocks
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I see - so yes, in this case gravity is not important and can be ignored. So then I can see the solids will form a mud and the liquid will get centrifuged out and trickle along the outside wall. But what moves the solids towards the solids outlet?
Why do you think RFR is not suitable? It appears the only practical way of doing it to me. I think your problem is that you have not included important physical models, and nothing to do with RFR. I have already said CFX has no agglomeration model and no method of pushing solids out of the liquid - these seem like fundamental effects to me. But I do not understand the operating principle of this device yet so I may well be wrong. |
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August 13, 2015, 11:24 |
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#13 |
Member
Sanyo
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Solids are pushed out of device by screw movement. Yes we are not modeling the agglomeration. But if I do not model solid for the moment & only model air+water, in that case I should get the correct fluid behavior, right?? But in that case also, I am getting same behavior. Will it help by modeling the rotating domain differently say just near the screw & drum by few millimeters? In that case rest of the domain can be kept stationary.
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August 13, 2015, 18:20 |
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#14 |
Super Moderator
Glenn Horrocks
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Is the outer wall stationary and the screw moving inside? Or does it all move together?
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August 14, 2015, 00:44 |
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#15 |
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Sanyo
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The outerwall, screw, and distributor rotates at same speed & direction. Only inlet pipe is stationary.
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August 14, 2015, 08:27 |
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#16 |
Super Moderator
Glenn Horrocks
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Then what makes the solids move up to the solids outlet? They have to go against the centripetal force to get there (they need to go uphill) so they are not going to get there unless something pushes them. What is pushing the solids uphill?
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August 14, 2015, 10:04 |
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#17 |
Member
Sanyo
Join Date: Apr 2009
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Rep Power: 17 |
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August 15, 2015, 06:20 |
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#18 |
Super Moderator
Glenn Horrocks
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Looking at a few of the videos on the topic they say there is a speed differential between the scroll/screw and the shell. I cannot see how the solids would move without a speed differential between the screw and the shell.
It appears you have modelled the screw and the shell as having the same speed. This is a fundamental problem in your model, isn't it? Anyway, back to your original question: Why is the liquid going out the solids outlet? This would suggest the resistance in going through the gap is too large, or you have not run the simulation long enough. Can you show a cross section of your mesh through the gap region? Why not achieve mass balance? You might need to run a tighter convergence. But first determine whether it is important to get mass balance more accurate. If it is not required then don't bother to get it. |
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