[Felggv]

Hi everyone,

Recently I've developed a preliminar model to simulate air conditioning.

In my model, we calculate the absolute humidity on the insuflation nozzle of the air conditioner by giving the abs humidity on the entrance of the air conditioner, temperature at the entrance and by making some assumptions.

Using this on CFX is giving me some solver errors because I set the abs humidity at the entrance of the air conditioner as the Mass Fraction at the Outlet of the room (the one that returns to the AC) by defining an expression that is ave(Water Vapour at 25 C.Mass Fraction)@Outlet since my fluid is Air + Water Vapour. I've also tried the @Default Domain location so CFX could read an initial value from Initialization at my Default Domain.

Since the abs humidity on the insuflation nozzle depends on the one from the entrance and the temperature at it depends on the abs humidity of the insuflation nozzle seems like the software can't get the initial input at the entrance and crashes saying:

Error processing expression 'Static Temperature'.
The expression is invalid because:
Mass Fraction is not available for use in this term

Error processing expression: Static Temperature = Ts

Just for you to know, Ts is my expression for the Temperature at the insuflation nozzle set on Inlet Boundary condition and as I said, it depends on the abs humidity at the insufl nozzle. The expression is: Ts = (15.66*ln(1000*ws)-21.70) [C] and again, ws depends on the abs humidity at the entrance (we) that seems to be unavaliable or cannot be read.

I'm wondering how can I make CFX read the Water Vapour Mass Fraction on each iteration and do all the calculations because from what I understood, Ts cannot be calculated because the entrance properties cannot be obtained.

My settings are: Air Ideal Gas (Constraint) and Water Vapour 25 C (Transport Equation) with the Water Vapour mass fraction at t=0 set to 0.

Thanks in advance and sorry for the long text.

Felipe.

[ghorrocks]

Does this work if you use a simple incompressible fluid for air? Why do you need air to be an ideal gas?

[Felggv]

Hello Glenn,

Are you just asking me if my simulation would work if I replaced air with some incompressible fluid like any other liquid?

If I did it, what benefit would this do? Wouldn't it just change my Air properties? Isn't it more realistic to make Air an ideal gas for AC applications?

My point is to study how humidity behaves so I can predict what will happen when we set up an AC system in any given room.

Thank you for the quick answer, I hope it'll help me!

Sorry if I misunderstood you, I'm a CFX begginer, specially when talking about CEL.

Felipe.

[ghorrocks]

For many HVAC applications modelling air as an incompressible fluid (with buoyancy) is adequate. A general rule in CFD is to use the simplest possible model, and only go to a more complex model (eg compressible air) when you have a clear reason to do so.

But regardless of whether you use comp or incomp air in the final solution, just as a debug experiment use incompressible air to see if it ersolves the error. It will give you some ideas of where the problem may lie.

[Felggv]

Thank you Glenn,

I'll do it and come back to post if it worked, but first gotta find out how to model air as incompressible.

[ghorrocks]

heat transfer model = total energy and the fluid having variable density (eg ideal gas) means compressible.

heat transfer model = none, isothermal or thermal energy = incompressible.

If you want incompressible with heat transfer and buoyancy you want the thermal energy model, with gravity defined.

[Felggv]

Mr. Horrocks,

I always use the Thermal Energy model on HVAC, so I was unconsciously modeling air as an imcompressible fluid.

When I set my entrance abs humidity (we) to a constant value instead of ave(Water Vapour at 25 C.mf)@Outlet, solver points another error instead of the error that Mass Fraction is not avaliable, that I strongly believe is an Outlet problem because solver says that a wall had been placed on the outlet to prevent fluid from flowing back to my domain and suggests to switch to an opening boundary (Overflow issue).

My material is called AirH2O and is composed by Air Ideal Gas + Water Vapour at 25 C, I've set Air as Constraint and Water Vapour as Tansport Equation with its Initial mass fraction of 0 and its Inlet mass fraction as the abs humidity of the Air Conditioner nozzle.

Then, this abs humidity of the nozzle depends on the abs humidity on the exhaust "hole" of the Air Conditioner which I assumed to be the Water Vapour mass fraction of the Outlet (ave(Water Vapour at 25 C.mf)@Outlet). My main problem is that the CFX says that this ave water vapour.mf isn't avaliable and therefore everything that depends on this mass fraction cannot be calculated.

Why isn't it avaliable if I set the initialization properly?

Sorry for another huge post, but I felt like better explaining my problem so it could be solved faster.

Thanks in advance,

Felipe.

[Felggv]

Hello everyone,

Now I come with a simple question, I've had some other problems and fixed them. Like problems with using Temperature in [C].

Now my question is that:

We can request the CFX-Pre to use the expression ave(T)@Outlet to get us the average temperature at each iteration and use it for any purpose we want, right?

Is the expression ave(Water Vapour at 25 C.Mass Fraction)@Outlet possible to use for requesting the Mass Fraction of a determined component of my fluid (Air + Water Vapour) in CFX-Pre or I can use it only in CFD-Post?

When I try to use it, the Solver crashes and says it's not avaliable, but the expression is recognized in CEL.

If this command does not exist, how can I make CFX-Pre get the mass fraction of this component at each iteration just like I did with the temperature?

I want the Solver to do the calculations iteratively and not just set a constant value for the Mass Fraction, otherwise it would lack precision.

Thanks

[ghorrocks]

We can request the CFX-Pre to use the expression ave(T)@Outlet to get us the average temperature at each iteration and use it for any purpose we want, right?

Yes - some finer points though - massflowAve(T)@Outlet is often better as the massflow average is proportional to the heat flux leaving the domain (for constant properties fluids ). ave(T)@Outlet does not have physical meaning. Also note all these functions are evaluated on the value from last iteration. As you approach convergence this does not matter as it does not change much.

You can use ave(fluid.Mass Fraction)@... in the solver. But the above comment applies - massflowave()@... is a better function in most cases.

If the solver is saying this is not available - is it the ave function or your variable (Water Vapour at 25C) which does not exist? Please post the output file so we can see the error message.

[Felggv]

Horrocks,

Here's what the Solver points:

Error processing expression 'Static Temperature'.
The expression is invalid because:
Mass Fraction is not available for use in this term

Error processing expression: Static Temperature = Ts
+--------------------------------------------------------------------+
| An error has occurred in cfx5solve: |
| |
| C:\Program Files\ANSYS Inc\v130\CFX\bin\winnt-amd64\ccl2flow.exe |
| was interrupted by signal QUIT (3) |
+--------------------------------------------------------------------+


This run of the ANSYS CFX Solver has finished.

Explaining the error in Static Temperature = Ts:

I've set the Inlet temperature as Static Temperature and enabled the expression button by setting it like

Ts = 15.66*ln(1000*ws) - 21.7

ws is the abs humidity... and it depends on the Outlet Mass Fraction which is the ave(Water Vapour at 25 C.Mass Fraction)@Outlet that I need!

Briefly, I need the Outlet Mass Fraction at each iteration to calculate ws and then calculate Ts.

Thanks.

[ghorrocks]

Please post the output file so we can see the error message. Your snippet does not tell us much.

[Felggv]

Here's the whole output report:

This run of the CFX-13.0 Solver started at 10:21:48 on 16 Jul 2012 by
user 69560 on PC5 (intel_xeon64.sse2_winnt) using the command:

"C:\Program Files\ANSYS Inc\v130\CFX\bin\perllib\cfx5solve.pl" -batch
-ccl runInput.ccl -fullname "Malha Helton Teste_001"

Setting up CFX Solver run ...

+--------------------------------------------------------------------+
| |
| CFX Command Language for Run |
| |
+--------------------------------------------------------------------+
LIBRARY:
CEL:
EXPRESSIONS:
A = - 2940.94 [kJ/kg]
B = \
(1243+(1.82*(15.66*ln(1000*we)-21.70))+2500.8+5203.2*we+2.92+2500.9) \
[kJ/kg]
C = (-Q/0.547 [kg/s]) + (-1.005 [kJ kg^-1]*Te) + (-1.82 [kJ kg^-1] * \
we * Te) + (-1.82 [kJ kg^-1] * we * Tsat) + (-2500.8 [kJ/kg] * we) + \
(-1.233 [kJ/kg]) + (-5.16 [kJ/kg] * we) + (-2500.9 [kJ/kg] * we)
Q = -10.55 [kW]
Ta = (massFlowAve(T)@Outlet / 1 [K]) - 273.15
Te = Ta
Ts = (15.66*ln(1000*ws)-21.695) [C]
Tsat = (15.66*ln(1000*we)-21.695)
wa = massFlowAve(Water Vapour at 25 C.Mass Fraction)@Outlet
we = wa
ws = -B*(2*A)^-1 + ((B^2 - 4*A*C)^0.5)*(2*A)^-1
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
SPECIFIC HEAT CAPACITY:
Option = Value
Specific Heat Capacity = 1.0044E+03 [J kg^-1 K^-1]
Specific Heat Type = Constant Pressure
END
REFERENCE STATE:
Option = Specified Point
Reference Pressure = 1 [atm]
Reference Specific Enthalpy = 0. [J/kg]
Reference Specific Entropy = 0. [J/kg/K]
Reference Temperature = 25 [C]
END
DYNAMIC VISCOSITY:
Dynamic Viscosity = 1.831E-05 [kg m^-1 s^-1]
Option = Value
END
THERMAL CONDUCTIVITY:
Option = Value
Thermal Conductivity = 2.61E-02 [W m^-1 K^-1]
END
ABSORPTION COEFFICIENT:
Absorption Coefficient = 0.01 [m^-1]
Option = Value
END
SCATTERING COEFFICIENT:
Option = Value
Scattering Coefficient = 0.0 [m^-1]
END
REFRACTIVE INDEX:
Option = Value
Refractive Index = 1.0 [m m^-1]
END
THERMAL EXPANSIVITY:
Option = Value
Thermal Expansivity = 0.003356 [K^-1]
END
END
END
MATERIAL: AirH2O
Material Group = User
Materials List = Air at 25 C,Water Vapour at 25 C
Option = Variable Composition Mixture
Thermodynamic State = Gas
END
MATERIAL: Water Vapour at 25 C
Material Description = Water (saturated vapour) at 25 C
Material Group = Water Data, Constant Property Gases, Interphase Mass \
Transfer
Option = Pure Substance
Thermodynamic State = Gas
PROPERTIES:
Option = General Material
EQUATION OF STATE:
Density = 0.02308 [kg m^-3]
Molar Mass = 18.02 [kg kmol^-1]
Option = Value
END
SPECIFIC HEAT CAPACITY:
Option = Value
Specific Heat Capacity = 1911.6 [J kg^-1 K^-1]
Specific Heat Type = Constant Pressure
END
REFERENCE STATE:
Option = Specified Point
Reference Pressure = 3.169E-2 [bar]
Reference Specific Enthalpy = -1.3424249E+7 [J/kg]
Reference Specific Entropy = 1.0475125E+04 [J/kg/K]
Reference Temperature = 25 [C]
END
DYNAMIC VISCOSITY:
Dynamic Viscosity = 9.8626E-06 [kg m^-1 s^-1]
Option = Value
END
THERMAL CONDUCTIVITY:
Option = Value
Thermal Conductivity = 0.01854 [W m^-1 K^-1]
END
ABSORPTION COEFFICIENT:
Absorption Coefficient = 1.0 [m^-1]
Option = Value
END
SCATTERING COEFFICIENT:
Option = Value
Scattering Coefficient = 0.0 [m^-1]
END
REFRACTIVE INDEX:
Option = Value
Refractive Index = 1.0 [m m^-1]
END
THERMAL EXPANSIVITY:
Option = Value
Thermal Expansivity = 0.003356 [K^-1]
END
END
END
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 = Transient
EXTERNAL SOLVER COUPLING:
Option = None
END
INITIAL TIME:
Option = Automatic with Value
Time = 0 [s]
END
TIME DURATION:
Option = Total Time
Total Time = 10 [s]
END
TIME STEPS:
Option = Timesteps
Timesteps = 1 [s]
END
END
DOMAIN: Default Domain
Coord Frame = Coord 0
Domain Type = Fluid
Location = BODY
BOUNDARY: Default Domain Default
Boundary Type = WALL
Location = INLET1,PAREDES,PISO,Primitive 2D,Primitive 2D A,Primitive \
2D B,Primitive 2D C,Primitive 2D D,Primitive 2D E,Primitive 2D \
F,Primitive 2D G,Primitive 2D H,Primitive 2D I,Primitive 2D \
J,Primitive 2D K,TETO
BOUNDARY CONDITIONS:
HEAT TRANSFER:
Option = Adiabatic
END
MASS AND MOMENTUM:
Option = No Slip Wall
END
WALL ROUGHNESS:
Option = Smooth Wall
END
END
END
BOUNDARY: Inlet
Boundary Type = INLET
Location = INLET2
BOUNDARY CONDITIONS:
COMPONENT: Water Vapour at 25 C
Mass Fraction = ws
Option = Mass Fraction
END
FLOW DIRECTION:
Option = Normal to Boundary Condition
END
FLOW REGIME:
Option = Subsonic
END
HEAT TRANSFER:
Option = Static Temperature
Static Temperature = Ts
END
MASS AND MOMENTUM:
Mass Flow Rate = 0.546 [kg s^-1]
Option = Mass Flow Rate
END
TURBULENCE:
Option = Medium Intensity and Eddy Viscosity Ratio
END
END
END
BOUNDARY: Outlet
Boundary Type = OUTLET
Location = OUTLET
BOUNDARY CONDITIONS:
FLOW REGIME:
Option = Subsonic
END
MASS AND MOMENTUM:
Option = Static Pressure
Relative Pressure = 0 [Pa]
END
END
END
DOMAIN MODELS:
BUOYANCY MODEL:
Buoyancy Reference Density = 1.2 [kg m^-3]
Gravity X Component = 0 [m s^-2]
Gravity Y Component = -g
Gravity Z Component = 0 [m s^-2]
Option = Buoyant
BUOYANCY REFERENCE LOCATION:
Option = Automatic
END
END
DOMAIN MOTION:
Option = Stationary
END
MESH DEFORMATION:
Option = None
END
REFERENCE PRESSURE:
Reference Pressure = 1 [atm]
END
END
FLUID DEFINITION: AirH2O
Material = AirH2O
Option = Material Library
MORPHOLOGY:
Option = Continuous Fluid
END
END
FLUID MODELS:
COMBUSTION MODEL:
Option = None
END
COMPONENT: Air at 25 C
Option = Constraint
END
COMPONENT: Water Vapour at 25 C
Option = Transport Equation
END
HEAT TRANSFER MODEL:
Option = Thermal Energy
END
THERMAL RADIATION MODEL:
Option = None
END
TURBULENCE MODEL:
Option = k epsilon
BUOYANCY TURBULENCE:
Option = None
END
END
TURBULENT WALL FUNCTIONS:
Option = Scalable
END
END
INITIALISATION:
Option = Automatic
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
COMPONENT: Water Vapour at 25 C
Mass Fraction = 0.01
Option = Automatic with Value
END
STATIC PRESSURE:
Option = Automatic with Value
Relative Pressure = 0 [Pa]
END
TEMPERATURE:
Option = Automatic with Value
Temperature = 25 [C]
END
TURBULENCE INITIAL CONDITIONS:
Option = Medium Intensity and Eddy Viscosity Ratio
END
END
END
END
OUTPUT CONTROL:
MONITOR OBJECTS:
MONITOR BALANCES:
Option = Full
END
MONITOR FORCES:
Option = Full
END
MONITOR PARTICLES:
Option = Full
END
MONITOR POINT: MP 1 Mass Fraction Outlet
Cartesian Coordinates = 2.56 [m], 2.24 [m], 2.56 [m]
Option = Cartesian Coordinates
Output Variables List = Water Vapour at 25 C.Mass Fraction
END
MONITOR POINT: MP 2 Mass Fraction Inlet
Cartesian Coordinates = 2.56 [m], 2.32 [m], 2.24 [m]
Option = Cartesian Coordinates
Output Variables List = Water Vapour at 25 C.Mass Fraction
END
MONITOR POINT: MP 3 Temperature Outlet
Cartesian Coordinates = 2.56 [m], 2.24 [m], 2.56 [m]
Option = Cartesian Coordinates
Output Variables List = Temperature
END
MONITOR POINT: MP 4 Temperature Inlet
Cartesian Coordinates = 2.56 [m], 2.32 [m], 2.24 [m]
Option = Cartesian Coordinates
Output Variables List = Temperature
END
MONITOR RESIDUALS:
Option = Full
END
MONITOR TOTALS:
Option = Full
END
END
RESULTS:
Extra Output Variables List = Water Vapour at 25 C.Mass Fraction
File Compression Level = Default
Option = Standard
END
TRANSIENT RESULTS: Transient Results 1
File Compression Level = Default
Option = Standard
OUTPUT FREQUENCY:
Option = Every Timestep
END
END
END
SOLVER CONTROL:
Turbulence Numerics = First Order
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 = 1.E-4
Residual Type = RMS
END
TRANSIENT SCHEME:
Option = Second Order Backward Euler
TIMESTEP INITIALISATION:
Option = Automatic
END
END
END
END
COMMAND FILE:
Version = 13.0
Results Version = 13.0
END
SIMULATION CONTROL:
EXECUTION CONTROL:
EXECUTABLE SELECTION:
Double Precision = Off
END
INTERPOLATOR STEP CONTROL:
Runtime Priority = Standard
MEMORY CONTROL:
Memory Allocation Factor = 1.0
END
END
PARALLEL HOST LIBRARY:
HOST DEFINITION: pc5
Host Architecture String = winnt-amd64
Installation Root = C:\Program Files\ANSYS Inc\v%v\CFX
END
END
PARTITIONER STEP CONTROL:
Multidomain Option = Independent Partitioning
Runtime Priority = Standard
EXECUTABLE SELECTION:
Use Large Problem Partitioner = Off
END
MEMORY CONTROL:
Memory Allocation Factor = 1.0
END
PARTITIONING TYPE:
MeTiS Type = k-way
Option = MeTiS
Partition Size Rule = Automatic
END
END
RUN DEFINITION:
Run Mode = Full
Solver Input File = Malha Helton Teste.def
END
SOLVER STEP CONTROL:
Runtime Priority = Standard
MEMORY CONTROL:
Memory Allocation Factor = 1.0
END
PARALLEL ENVIRONMENT:
Number of Processes = 1
Start Method = Serial
END
END
END
END

Error processing expression 'Static Temperature'.
The expression is invalid because:
Mass Fraction is not available for use in this term
Error processing expression: Static Temperature = Ts
+--------------------------------------------------------------------+
| An error has occurred in cfx5solve: |
| |
| C:\Program Files\ANSYS Inc\v130\CFX\bin\winnt-amd64\ccl2flow.exe |
| was interrupted by signal QUIT (3) |
+--------------------------------------------------------------------+

This run of the ANSYS CFX Solver has finished.

[cfdgremlin]

The problem is that in CFX the Dependency List for the parameter Static Temperature is restricted to a small set of solution variables (you can see these by looking in the RULES file). The list does not include mass fraction.

You can work around the problem by including a CCL snippet in your command file to extend the list; however, whilst this may get around the check it may produce unpredicatable results in the solver (these lists are usually restricted for good reason).

CG

[Felggv]

And do you know how can I add this variable there?

I have no idea how to edit the command list to include it.

Thanks for the help!

[cfdgremlin]

You need to add:

RULES:
PARAMETER: Static Temperature
Dependency List = {original dependency list}, mf
END
END

by editing your CCL file manually. The {original dependency list} can be found by looking in the RULES file.

CG

[Felggv]

Gremlin,

I've modified the RULES file, it worked!

Now I gotta fix another error here.

Thank you.

[ghorrocks]

Maybe, but do not assume that it has fully worked without checking. There is a good reason why it is not in there by default.

[Felggv]

That's true, I'm monitoring it to make sure it won't give me any wrong value. Otherwise I will need to find another way to solve my problem.

Thanks for the reminder

[Felggv]

Hello guys,

Another doubt came up to me... Is it possible for me to request a propertie or variable value at a determined time step on CFX-Pre?

For instance, get the Mass Fraction at t=5s and use it for any purpose.

[ghorrocks]

There are many ways to do this, you can define monitor points and export it after the run, you can extract things during post processing.

What are you trying to do?