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-   -   Low Mixing time Problem (http://www.cfd-online.com/Forums/cfx/116219-low-mixing-time-problem.html)

 Mavier April 15, 2013 10:21

Low Mixing time Problem

Hello everybody.

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 steady-state simulation to serve as the starting point for my transient analisys.
I´m using the standard k-epsilon 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: http://img22.imageshack.us/img22/944...mixingtime.png

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 = 1.5e-9 [m^2 s^-1]
Option = Transport Equation
END
COMPONENT: Water
Option = Constraint
END
HEAT TRANSFER MODEL:
Fluid Temperature = 25 [C]
Option = Isothermal
END
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:
Length Units = [m]
Mass Units = [kg]
Solid Angle Units = [sr]
Temperature Units = [K]
Time Units = [s]
END
SOLVER CONTROL:
Turbulence Numerics = High Resolution
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
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 = 1.5e-9 [m^2 s^-1]
Option = Transport Equation
END
COMPONENT: Water
Option = Constraint
END
HEAT TRANSFER MODEL:
Fluid Temperature = 25 [C]
Option = Isothermal
END
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:
Length Units = [m]
Mass Units = [kg]
Solid Angle Units = [sr]
Temperature Units = [K]
Time Units = [s]
END
SOLVER CONTROL:
Turbulence Numerics = High Resolution
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
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

 ghorrocks April 15, 2013 19:05

Can you confirm no multi-phase 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.cfd-online.com/Wiki/Ansys..._inaccurate.3F

 Mavier April 16, 2013 16:04

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: 1.5e-9 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.

 ghorrocks April 16, 2013 19:27

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.

 Mavier April 28, 2013 23:42