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Old   March 24, 2014, 13:46
Default Water in stewpot
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onder
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Hello

I am trying to simulate stewpot on cookstove. I did analysis. When I check the result, I notice that water density in stewpot is constant. Therefore no flow and turbulance being in stewport. And the tepreture spread is not correct. Teperature is rising only near of fire border stewport. When you look the photo of result you will see. I have done little research on the internet and the forum I found table of water density depend on temperature of water. Then I wrote lineer equation. I dont have any problem with equation but I could not manage to write expression. I tried many ccl but cfx didnot accept them. İf someone write the right structure of expression which is for density depend on water temperature. Thanks , regards.
At analysis there is one air domain , one water domain and one stewpot domain. http://postimg.org/image/jmld71jgd/
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Old   March 24, 2014, 17:33
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Have a look at the buoyant flow examples in the CFX tutorial manual. You will have missed something you need. To do a buoyant flow you need:
* A gravity vector defined
* Either a boussinesq buoyancy model (incompressible fluid, buoyancy force applied as a source term proportional to temperature) or a density versus temperature model (eg ideal gas or variable properties fluid)
* Something to drive a temperature gradient
* Enough time to allow a temperature gradient to form.
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Old   March 24, 2014, 17:54
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When I wrote exprssion in material properties->equation of state -> densiyt = function 1 (T)
Function 1 (T) is table water density at temperatures like
0 997
20 996
40 995 like this.

when start run solver gives this error
PROBLEM ENCOUNTERED WHEN EXECUTING CFX EXPRESSION LANGUAGE |
| |
| The CFX expression language was evaluating: |
| Density |
| |
| The problem was: |
| Value below interpolation range |
| |
| FURTHER INFORMATION |
| |
| The problem was encountered in executing the expression for: |
| densty |
| The complete expression is: |
| Function 1(T ) |
| The error occurs on sub-expression: |
| ction 1(T |
| |
| BACKGROUND INFORMATION |
| |
| The error was detected at one location. The same problem may be |
| present at other locations - that has not been investigated. |
| The following values are for the first location which has the |
| problem. |
| |
| |
| END OF DIAGNOSTIC OUTPUT FOR CFX EXPRESSION LANGUAGE



When I wrote the expression in pre-cfx it gives this alert

Material 'Water at 25 C': Table generation controls are not set for this material. Please ensure that the default pressure and temperature tabulation ranges are appropriate for your setup.
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Old   March 24, 2014, 18:22
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I write another expression for densty
999[kg m^-3]-((T-273[K])/1[K])*0.385[kg m^-3]
with this expression I start the solver İt gave this eror

Domain Group: water1 domain

Buoyancy has been activated. The absolute pressure will include
hydrostatic pressure contribution, using the following reference
coordinates: ( 8.58744E-06, 0.00000E+00, 1.55602E+00).
Domain Group: water2 domain

This is a transient run with at least one compressible fluid
and no boundary pressure set. The pressure level is set
through the transient term in the continuity equation.
To accelerate convergence, the pressure level will also be
shifted dynamically to satisfy global mass conservation.
+--------------------------------------------------------------------+
| ****** Notice ****** |
| This is a multiphase simulation with a compressible phase. |
| Total pressure (used for post-processing or if total pressure |
| is specified at a boundary) is calculated assuming all phases |
| are incompressible. |
+--------------------------------------------------------------------+
+--------------------------------------------------------------------+
| ****** Notice ****** |
| This is a multiphase simulation in a closed system. |
| A global correction will be applied to the volume fractions to |
| accelerate mass conservation. |
+--------------------------------------------------------------------+
Domain Group: water2 domain

Buoyancy has been activated. The absolute pressure will include
hydrostatic pressure contribution, using the following reference
coordinates: ( 4.15488E-02,-2.40000E-02, 1.64800E+00).

Fatal bounds error detected
---------------------------
Variable: water.Density
Locale :

+--------------------------------------------------------------------+
| ERROR #001100279 has occurred in subroutine ErrAction. |
| Message: |
| Stopped in routine ENFORCE_BOUNDS |
| |
| |
| |
| |
| |
+--------------------------------------------------------------------+
+--------------------------------------------------------------------+
| An error has occurred in cfx5solve: |
| |
| The ANSYS CFX solver exited with return code 1. No results file |
| has been created. |
+---------------------------------------------------------------
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Old   March 24, 2014, 18:49
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Yes, that is another problem with your approach - if the domain is sealed then you will have problems with the density going bezerk as the system heats up.

To start off I recommend you use the simple incompressible fluid buoyancy model. You do not need to define a density versus temperature relationship for this which simplifies things. Have a look in the CFX tutorials for examples of how to set this sort of simulation up.

Also - this error message says this is a multiphase model. You did not mention you are doing a multiphase model! You have not got the basic buoyancy driven flow working yet so you should remove the multiphase bit and just get the buoyant flow working on pure water. Once you can model a pot full of water successfully then you can add the multiphase bits - which I presume are the meat and veges (yummy).
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Old   March 27, 2014, 15:03
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Thankyou so much ghorrock. I am working on it, I solved densty problem but still there are something wrong I willfix in soon time.
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Old   September 19, 2014, 09:46
Default density variation
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Hi mztcu
How u have solved density problem. How u have varied density with temperature using user function. I am also getting same error please help i have got stuck in my project
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