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April 15, 2013, 10:21 
Low Mixing time Problem

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cholaole@gmail.com
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I am trying to reproduce the conditions form this paper: Verification and application of a mathematical model for the assessment of the effect of guiding walls on the hydraulic efficiency of chlorination tanks. First i ran a steadystate simulation to serve as the starting point for my transient analisys. I´m using the standard kepsilon model. I created a NaCl material with its standard characteristics. The top of the tank is a symmetry boundary condition. I create two separete simulations for the transient analisys: firste is the mixture (water+NaCl) entering my storage tank for 3 minutes (i use a 1s timestep); the second part is just water entering the domain (mass fraction of NaCl = 0, water as constraint). I have to outlet pipes that i have put opening boundary to conditions (to avoid the wall problem at the outlet), both with medium intensity (5%) and opening Pres. and Dirn = 0 Pa However the mixture is exiting the tank faster than expected, like the following graph shows: My results are the ones in green (with a time step equal to 1s in the second part of the simulation); and the purple one, time step equal to 4s at the second part of the simulation. where: C = mean NaCl concentration at the outlet Co = average tracer concentration t = current time T = theoretical detention time I really would like some help if i should change my turbulence model or if i should try perhaps a structured mesh (i generated mine with the ansys mesh generator using inflation at the walls) or a different time step. Here are my both CCL for the transient analysis: Part one (with NaCl): &replace FLOW: Flow Analysis 1 ANALYSIS TYPE: Option = Transient EXTERNAL SOLVER COUPLING: Option = None END INITIAL TIME: Option = Value Time = 0 [s] END TIME DURATION: Option = Total Time Total Time = 180 [s] END TIME STEPS: Option = Timesteps Timesteps = 1 [s] END END DOMAIN: Default Domain Coord Frame = Coord 0 Domain Type = Fluid Location = B41 BOUNDARY: Default Domain Default Boundary Type = WALL Create Other Side = Off Interface Boundary = Off Location = F294.41,F35.41,F36.41,F37.41,F38.41,F43.41,F44.41, F45.41,F46.41,F47.41,F48.41,F49.41 BOUNDARY CONDITIONS: MASS AND MOMENTUM: Option = No Slip Wall END WALL ROUGHNESS: Option = Smooth Wall END END END BOUNDARY: inlet Boundary Type = INLET Interface Boundary = Off Location = F293.41 BOUNDARY CONDITIONS: COMPONENT: Sodium Chloride Mass Fraction = 0.00234 Option = Mass Fraction END FLOW DIRECTION: Option = Normal to Boundary Condition END FLOW REGIME: Option = Subsonic END MASS AND MOMENTUM: Mass Flow Rate = 752.4 [kg s^1] Option = Mass Flow Rate END TURBULENCE: Option = High Intensity and Eddy Viscosity Ratio END END END BOUNDARY: outlet_1 Boundary Type = OPENING Interface Boundary = Off Location = F219.41 BOUNDARY CONDITIONS: COMPONENT: Sodium Chloride Mass Fraction = 0.0 Option = Mass Fraction END 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 END BOUNDARY: outlet_2 Boundary Type = OPENING Interface Boundary = Off Location = F220.41 BOUNDARY CONDITIONS: COMPONENT: Sodium Chloride Mass Fraction = 0.0 Option = Mass Fraction END 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 END BOUNDARY: simetria Boundary Type = SYMMETRY Location = F42.41 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: Dentro do Reservatorio Material = Water NaCl Option = Material Library MORPHOLOGY: Option = Continuous Fluid END END FLUID MODELS: COMBUSTION MODEL: Option = None END COMPONENT: Sodium Chloride Kinematic Diffusivity = 1.5e9 [m^2 s^1] Option = Transport Equation END COMPONENT: Water Option = Constraint END HEAT TRANSFER MODEL: Fluid Temperature = 25 [C] Option = Isothermal END THERMAL RADIATION MODEL: Option = None END TURBULENCE MODEL: Option = k epsilon END TURBULENT WALL FUNCTIONS: Option = Scalable END END END INITIALISATION: Option = Automatic INITIAL CONDITIONS: Velocity Type = Cartesian CARTESIAN VELOCITY COMPONENTS: Option = Automatic END COMPONENT: Sodium Chloride Mass Fraction = 0.0 Option = Automatic with Value END STATIC PRESSURE: Option = Automatic END TURBULENCE INITIAL CONDITIONS: Option = Medium Intensity and Eddy Viscosity Ratio END END END OUTPUT CONTROL: MONITOR OBJECTS: MONITOR BALANCES: Option = Full END MONITOR FORCES: Option = Full END MONITOR PARTICLES: Option = Full END MONITOR POINT: Concentracao_Outlet1 Expression Value = ConcOutlet1 Option = Expression END MONITOR POINT: Concentracao_Outlet2 Expression Value = ConcOutlet2 Option = Expression END MONITOR POINT: Mass_fraction_outlet1 Expression Value = MF1 Option = Expression END MONITOR POINT: Mass_fraction_outlet2 Expression Value = MF2 Option = Expression END MONITOR RESIDUALS: Option = Full END MONITOR TOTALS: Option = Full END END RESULTS: File Compression Level = Default Option = Standard END TRANSIENT RESULTS: Concentracoes File Compression Level = Default Include Mesh = No Option = Selected Variables Output Variables List = Velocity,Velocity u,Velocity v,Velocity w,Water.Mass Concentration,Water.Mass Fraction,Sodium Chloride.Mass Fraction,Sodium Chloride.Mass Concentration OUTPUT FREQUENCY: Option = Every Timestep END END END SOLUTION UNITS: Angle Units = [rad] Length Units = [m] Mass Units = [kg] Solid Angle Units = [sr] Temperature Units = [K] Time Units = [s] END SOLVER CONTROL: Turbulence Numerics = High Resolution ADVECTION SCHEME: Option = High Resolution END CONVERGENCE CONTROL: Maximum Number of Coefficient Loops = 10 Minimum Number of Coefficient Loops = 1 Timescale Control = Coefficient Loops END CONVERGENCE CRITERIA: Residual Target = 0.00001 Residual Type = RMS END EQUATION CLASS: continuity ADVECTION SCHEME: Option = High Resolution END END TRANSIENT SCHEME: Option = Second Order Backward Euler TIMESTEP INITIALISATION: Option = Automatic END END END END Part 2: &replace FLOW: Flow Analysis 1 ANALYSIS TYPE: Option = Transient EXTERNAL SOLVER COUPLING: Option = None END INITIAL TIME: Option = Value Time = 181 [s] END TIME DURATION: Option = Total Time Total Time = 5000 [s] END TIME STEPS: Option = Timesteps Timesteps = 1 [s] END END DOMAIN: Default Domain Coord Frame = Coord 0 Domain Type = Fluid Location = B41 BOUNDARY: Default Domain Default Boundary Type = WALL Create Other Side = Off Interface Boundary = Off Location = F294.41,F35.41,F36.41,F37.41,F38.41,F43.41,F44.41, F45.41,F46.41,F47.41,F48.41,F49.41 BOUNDARY CONDITIONS: MASS AND MOMENTUM: Option = No Slip Wall END WALL ROUGHNESS: Option = Smooth Wall END END END BOUNDARY: inlet Boundary Type = INLET Interface Boundary = Off Location = F293.41 BOUNDARY CONDITIONS: COMPONENT: Sodium Chloride Mass Fraction = 0 Option = Mass Fraction END FLOW REGIME: Option = Subsonic END MASS AND MOMENTUM: Normal Speed = 2.66 [m s^1] Option = Normal Speed END TURBULENCE: Option = High Intensity and Eddy Viscosity Ratio END END END BOUNDARY: outlet_1 Boundary Type = OPENING Interface Boundary = Off Location = F219.41 BOUNDARY CONDITIONS: COMPONENT: Sodium Chloride Mass Fraction = 0.00026494 Option = Mass Fraction END 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 END BOUNDARY: outlet_2 Boundary Type = OPENING Interface Boundary = Off Location = F220.41 BOUNDARY CONDITIONS: COMPONENT: Sodium Chloride Mass Fraction = 0.00028418 Option = Mass Fraction END 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 END BOUNDARY: simetria Boundary Type = SYMMETRY Location = F42.41 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: Dentro do Reservatorio Material = Water NaCl Option = Material Library MORPHOLOGY: Option = Continuous Fluid END END FLUID MODELS: COMBUSTION MODEL: Option = None END COMPONENT: Sodium Chloride Kinematic Diffusivity = 1.5e9 [m^2 s^1] Option = Transport Equation END COMPONENT: Water Option = Constraint END HEAT TRANSFER MODEL: Fluid Temperature = 25 [C] Option = Isothermal END THERMAL RADIATION MODEL: Option = None END TURBULENCE MODEL: Option = k epsilon END TURBULENT WALL FUNCTIONS: Option = Scalable END END END INITIALISATION: Option = Automatic INITIAL CONDITIONS: Velocity Type = Cartesian CARTESIAN VELOCITY COMPONENTS: Option = Automatic END COMPONENT: Sodium Chloride Mass Fraction = 0.0 Option = Automatic with Value END STATIC PRESSURE: Option = Automatic END TURBULENCE INITIAL CONDITIONS: Option = Medium Intensity and Eddy Viscosity Ratio END END END OUTPUT CONTROL: MONITOR OBJECTS: MONITOR BALANCES: Option = Full END MONITOR FORCES: Option = Full END MONITOR PARTICLES: Option = Full END MONITOR POINT: Concentracao_Outlet1 Expression Value = ConcOutlet1 Option = Expression END MONITOR POINT: Concentracao_Outlet2 Expression Value = ConcOutlet2 Option = Expression END MONITOR POINT: Mass_fraction_outlet1 Expression Value = MF1 Option = Expression END MONITOR POINT: Mass_fraction_outlet2 Expression Value = MF2 Option = Expression END MONITOR RESIDUALS: Option = Full END MONITOR TOTALS: Option = Full END END RESULTS: File Compression Level = Default Option = Standard END TRANSIENT RESULTS: Concentracoes File Compression Level = Default Include Mesh = No Option = Selected Variables Output Variables List = Velocity,Velocity u,Velocity v,Velocity w,Water.Mass Concentration,Water.Mass Fraction,Sodium Chloride.Mass Fraction,Sodium Chloride.Mass Concentration OUTPUT FREQUENCY: Option = Every Timestep END END END SOLUTION UNITS: Angle Units = [rad] Length Units = [m] Mass Units = [kg] Solid Angle Units = [sr] Temperature Units = [K] Time Units = [s] END SOLVER CONTROL: Turbulence Numerics = High Resolution ADVECTION SCHEME: Option = High Resolution END CONVERGENCE CONTROL: Maximum Number of Coefficient Loops = 10 Minimum Number of Coefficient Loops = 1 Timescale Control = Coefficient Loops END CONVERGENCE CRITERIA: Residual Target = 0.00001 Residual Type = RMS END EQUATION CLASS: continuity ADVECTION SCHEME: Option = High Resolution END END TRANSIENT SCHEME: Option = Second Order Backward Euler TIMESTEP INITIALISATION: Option = Automatic END END END END Thank you all in advance. Best regards Last edited by Mavier; April 15, 2013 at 11:09. 

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April 15, 2013, 19:05 

#2 
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Glenn Horrocks
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Can you confirm no multiphase stuff happens  so no droplets, bubbles, interfaces or anything like that? Both the wafer and NaCl dissolve in each other.
Your results are close so you do not seem to be too far off. The FAQ discusses general accuracy issues: http://www.cfdonline.com/Wiki/Ansys..._inaccurate.3F 

April 16, 2013, 16:04 

#3 
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cholaole@gmail.com
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Thank you for the reply Glenn.
My mixing conditions are the following: I created a PURE SUBSTANCE for the NaCl (that i called Sodium Chloride) with the following values: Molar Mass: 58.44 g/mol Density: 2.165 g/cm³ Specific Heat Capacity: 629.53 J/(kg.K) Then i created a VARIABLE COMPOSITION MIXTURE that consists of: Water + Sodium Chloride as na Ideal Mixture that i called "Water NaCl". Then at the domain i used this mixture of "Water NaCl" In the componet i specified the following: Sodium Chlorides as a Transport Equation with kinematic diffusity of: 1.5e9 m²/s (the NaCl diffusity in water at 25°C). Water as constraint. At the inlet i´m using a Mass Fraction for the Sodium Chloride of: 0.00234 Turbulence is High intensity (10%) There are a couple of things that the autor specifies in the paper that i don´t know where to put it, he mentions the following: The turbulent energy (k) and its dissipation rate (e) are assumed to be uniform, with values corresponding to an eddy viscosity at the inlet approximately 100 times the molecular viscosity of water. The tracer concentration, C, is assumed to be uniformly distributed at the inlet. At the outlet pipes the pressure is specified and the vertical gradients of k, e and C are set equal to zero. Is thera a palce in CFX that i can change the values for the turbulent energy, and dissipation rate. Anyway, thanks for the response again Glenn, best regards. 

April 16, 2013, 19:27 

#4 
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Glenn Horrocks
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The assumption that the k and e are uniform seems like a brave one to me. The whole point of CFD is to model these terms so you do not need to make assumptions like this. But if you want to reproduce their results then you can probably use a laminar flow model and increase the viscosity to be the sum of the molecular and turbulent viscosities. You would also need to increase the diffusivities of the heat and mass fraction equations as well.


April 28, 2013, 23:42 

#5 
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cholaole@gmail.com
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Thanlks for the answer Glenn.
I tried some alternatives here but the concentration is still leaving the tank early, do you think this could be mesh realted Glenn ? I´m using a nonstructured mesh, perhaps i should try using a structured one from icem. Also, i tried to simulate it using the SST turbulence model, but i couldn´t get it to converge during the steady state simulation and i think this this could also be due to the unstructured mesh that i´m using. Best Regards. 

April 29, 2013, 00:00 

#6 
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Glenn Horrocks
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Tags 
cfx, mixing, mixing time, water storage tank 
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