CFX Intro 14.5 WS11 Room Temperature Study

8/13/2019 CFX Intro 14.5 WS11 Room Temperature Study

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2012 ANSYS, Inc. December 17, 2012 1 Release 14.5

14. 5 Release

Introduction to ANSYS

CFX

Workshop 11

Room Temperature Study


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In this workshop you will be analyzing the effect of computers and workers on

the temperature distribution in an office. In the first stage airflow through the

supply air ducts will be simulated and the outlet conditions for the duct will be

used to set the inlet conditions for the room.

Although both components could be analyzed together, separating the two

components allows different room configurations to be analyzed without

solving the duct flow again.

Introduction

Introduction Setup Solving Postprocessing Summary


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Operating Conditions

The operating conditions for the flow are given below:

The working fluid is Air Ideal Gas Fluid Temperature = 21 [C]

Inlet: 0 [atm] Total Pressure

Outlet: 0.225 [kg/s] (per vent)

Inlet

vent1

vent2



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Starting CFX in Workbench

Open Workbench

Drag a CFXsystem into the Project Schematic from the Component Systemstoolbox

Change the name of the system to duct

Save the project as RoomStudy.wbpjin your working directory

Double-click Setup



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Importing the Mesh

Right-click on Meshin theOutlinetree and select Import

Mesh > ICEM CFD

Select the file duct_mesh.cfx5

(workshop_input_files\WS_11_

Room Temperature Study) Make sure Mesh Units are in m

and click Opento import the

mesh

The first step is to import a mesh



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Domain

Double-click on Default Domain in theOutlinetree to edit the domain

On the Basic Settings tab, set the Fluid 1

MaterialtoAir Ideal Gas

Switch to the Fluid Models tab

Set the Heat Transfer Optionto

Isothermal

Set the Fluid Temperature to 21 [C]

Click OKto commit the changes to the

domain

You can now create the computational domain

Heat Transfer is not modeled but since theworking fluid is an ideal gas we need to provide atemperature so its properties can be calculated



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Boundary Conditions

Now create the following boundary

conditions

INLET Boundary Condition

Name: INLET

Boundary Type: Inlet

Mass and Momentum Option: TotalPressure (stable)

Relative Pressure:0 [Pa]

VENT2 Boundary Condition

Name: VENT2

Boundary Type: Outlet

Location: VENT2

Mass and Momentum Option: MassFlow Rate

Mass Flow Rate: 0.225 [kg/s]

VENT1 Boundary Condition

Name: VENT1


Location: VENT1

Mass and Momentum Option: MassFlow Rate





Solver Control

Double-click on Solver Controlfrom the Outlinetree

Enable the Conservation Targettoggle

Click OKto commit the settings


The default Conservation Target is 1%. This means that the global imbalance for eachequation must be less than 1% (i.e. (flux influx out)/flux in < 1%). The solver will not stopuntil both the Residual Target and the Conservation Target have been met or Max.Iterations is reached.



Monitor Point

Double-click on Output Controlfrom the

Outlinetree

Switch to the Monitortab and enable the

Monitor Objectstoggle Under Monitor Points and Expressionsclick

theAdd new item button

Keep the default name Monitor Point 1

Set the Optionto Expression

In the Expression Valuefield type:areaAve(Velocity w)@VENT1

Click OKto create the monitor point


Monitor points are used to monitor quantities of interest during the solution. Theyshould be used to help judge convergence. In this case you will monitor the velocity

of the air that exits through one of the vents. One measure of a converged solution iswhen this air has reached a steady-state velocity.



Solution

Close CFX-Preto return to the Project

Schematicwindow

Save the project

Right-click Solutionand select Edit

Click Start Runwhen the CFX-Solver

Managerappears

Examine the User Point.The velocity

becomes steady toward the end of the run.

Close the CFX-Solver Manager

In Workbenchright-click Resultsand select

Edit


You can now save the project and proceed to write a definition file for the solver


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Export

Select File> Export

Change the file name to vent1.csv

Use the browse icon to set an appropriate

directory

Set Typeas BC Profileand Locationsas VENT1

Leave Profile Type as Inlet Velocityand click Save

Similarly export a BC profile of VENT2 to the file

named vent2.csv

Close CFD-Postand return to the Project

Schematic


Now we will export a Boundary Condition profile from the outlet regions for use in the

next simulation.


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Summary

Details of the next simulation:

The working fluid is Air Ideal Gas

The temperature of the computer monitors is30 [C]

The flow through the vent of each computer is0.033 [kg/s] at 40 [C]

For the ceiling vents the velocity profile data are

used and the temperature is 21 [C]


The first part of the workshop simulated some upstream ductwork and theexit velocity profiles from the ductwork exit were exported.

Now those profiles will be used as the inlet conditions to a larger simulationinvolving a room with heat sources

outlet

vent1

vent2


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Room Simulation Setup in Workbench

Drag a CFXsystem into the Project Schematicfrom the Component Systems

toolbox

Change the name of the system to room

Double-click Setupin the room system



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Importing the Mesh

Right-click on Meshin the

Outlinetree and select Import

Mesh> ICEM CFD

Select the file room.cfx5

Make sure Mesh Unitsare m

and click Opento import the

mesh

The first step is to import a mesh



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Domain

Edit Default Domainfrom the Outlinetree

On the Basic Settings tab set the Fluid 1Materialsetting toAir Ideal Gas

Set the Buoyancy Option to Buoyant. Set the buoyancy settings as

shown:

Gravity X Dirn. = 0 [ m s^-2 ]

Gravity Y Dirn. = 0 [ m s^-2 ]

Gravity Z Dirn. = -g (first click the Enter Expression icon )

Buoy. Ref. Density = 1.185 [ kg m^-3 ]

You can now create the computational domain

Buoyancy must be included in order to model natural convection due to density

variations. The buoyancy force is represented by a momentum source, which isa function of density differences relative to the buoyancy reference density. The

buoyancy reference density should be chosen so that the source is not large. For

a single-phase simulation the reference density should be an average value for

the domain.



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Domain

Switch to the Fluid Models tab Change the Heat Transfer Option to Thermal Energy

Change the Turbulence Model Option to Shear Stress Transport

Switch to the Initializationtab

Check the Domain Initializationbox

Set the Temperature Option toAutomatic with Value andTemperature to

21 [C]

Click OKto commit the changes to the domain



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Profile Data Initialization

Select Tools> Initialize Profile Dataand choose

the Data File as vent1.csv.

Click OK

CFX-Prereads the file and creates functions thatpoint to the variables available in the file (see the

User Functions section in the Outlinetree). These

functions can be used in the definition of

boundary conditions.

Similarly initialize profile data for vent 2 by

choosing vent2.csv



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Boundary Conditions

vent1 boundary condition: Name: vent1


Location: VENT1

Select Use Profile Dataand choose VENT1 as theProfile Name

Click Generate Values

Switch to the Boundary Detailstab

Change the Option for Mass And Momentum to

Cart. Vel. Components. Expressions based on

the function VENT1 automatically appear

Set the Heat Transfer Option to Static

Temperature with a value of 21 [C]


Now create the following boundary conditions


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Boundary Conditions

vent2 boundary condition:

Name: vent2 Boundary Type: Inlet

Location: VENT2

Select Use Profile Data and choose VENT2 as the Profile Name

Click Generate Values

Mass And Momentum Option: Cart. Vel. Components Heat Transfer Option: Static Temperature

Static Temperature: 21 [C]

workers boundary condition

Name: workers

Boundary Type: Wall

Location: WORKERS

Heat Transfer Option: Temperature

Fixed Temperature: 37 [C]



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Boundary Conditions

outlet boundary condition:

Name: outlet Boundary Type: Opening

Location: OUTLET

Mass and Momentum Option: Opening Pres. And Dirn

Relative Pressure: 0 [Pa]

Heat Transfer Option: Opening Temperature Opening Temperature: 21 [C]

monitors boundary condition

Name: workers

Boundary Type: Wall

Location: MONITORS

Heat Transfer Option: Temperature

Fixed Temperature: 37 [C]



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Boundary Conditions

computerVent boundary condition: Name: computerVent


Location: COMPUTER1VENT, COMPUTER2VENT, COMPUTER3VENT,COMPUTER4VENT (to select multiple locations click on and then hold down the

Ctrlbutton while making selection)

Mass and Momentum Option: Mass Flow Rate


Heat Transfer Option: Static Temperature

Static Temperature: 40 [C]



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Boundary Conditions

computerIntake boundary condition:

Name: computerIntake


Location: COMPUTER1INTAKE, COMPUTER2INTAKE, COMPUTER3INTAKE,COMPUTER4INTAKE

Mass and Momentum Option: Mass Flow Rate


Mass Flow Update Option: Constant Flux

This enforces a uniform mass flow across the entire boundary region rather than letting a

natural velocity profile develop. It is used here to make sure the flow rate through each

intake is the same.



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Solver Control

Edit Solver Control from the Outlinetree

Increase the Max. Iterations to 750

Change the Timescale Control to Physical Timescale

Set a Physical Timescale of 2 [s]

Enable the Conservation Target toggle

The default value 0.01 is applicable here

Click OKto commit the settings



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Monitor Point

Edit Output Control from the Outlinetree

Switch to the Monitortab and enable the

Monitor Objectstoggle

Under Monitor Points and Expressionsclick theAdd new item icon

Enter the Nameas temp

Set the Optionto Expression

In the Expression Value field type in:massFlowAve(Temperature)@outlet

Click OKto create the monitor point


You will monitor the temperature of the air that exits through the outlet. Onemeasure of a converged solution is when this air has reached a steady temperature.


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Solution

Select File> Importfrom the main menu in

Workbench

Set the file filter to CFX-Solver Results File

Select the results file provided with this

workshop, room_001.res

Change the name of the system to roomresults


Save the project and write a definition file:

Close CFX-Preto return to the Project Schematicwindow and save the project

The solution will take several hours to solve on one processor. To save time a results file isprovided with this workshop. The Project Schematic shows that the roomSolution has notbeen completed, so you cannot view the results in CFD-Post yet. To view the results for thefile provided youll need to add the results to the project


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CFX Solver Manager

Right-click on Solutionin the room

results system and select Display

Monitors

Examine the residual plots for

Momentum and Mass, Heat Transfer and

Turbulence(shown on next slide)

The residual target of 1e-4 was met atabout 290 iterations, but the solver did not

stop because the conservation target had

not been met

Examine the User Points plot

Air temperature leaving through the outletdid not start to reach a steady temperature

until >650 iterations. Using residuals as the

only convergence criteria is not always

sufficient.


Now you can view the solution for the previously solved case.


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CFX Solver Manager


Residual plots for Mass and Momentum and Heat Transfer


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CFX Solver Manager

Check the Domain Imbalances at the

end of the .out file for each equation

You can right click in the text monitor,select Findand search for Domain

Imbalanceto find the appropriate

section

An imbalance is given for the U-Mom, V-

Mom, W-Mom, P-Massand H-Energyequations

It took 722 iterations to satisfy theConservation Target of 1% for the H-

Energyequationsee the Plot Monitor 1

tab

Close the CFX-Solver Manager

View the results in CFD-Postby

double-clickingResultsfor the same

roomResultssystem



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Temperature Plot

Select Location> Planefrom the

toolbar

On the Geometrytab in the

Detailswindow set the Method

toZX Plane

Set Yto 1.2 [m]

On the Colortab set Modeto

Variable

Set Variableto Temperature

Set Rangeto Localand clickApply

Observe how the warm aircollects under the table


Start by creating a ZX Plane at Y = 1.2 [m]


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Temperature Plots

ZX Planeat Y= 2 [m]

ZX Plane at Y= 5.1 [m]

XY Plane atZ= 0.25 [m]

When finished observing thetemperature distribution, uncheck

the visibility boxes of the planes that

you created


Using the same procedure, create several other planes displaying thetemperature profile:


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Vector Plots

Insert> Vector

On the Geometrytab in

the Detailswindow, set

Locationto Plane 2

On the Symboltab, setthe Symbols Sizeto 3

ClickApply

After observing the flow

behavior on Plane 2,

switch the LocationtoPlane 4

ClickApply


Plot vectors plots on the planes that you created:


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Further Steps

Observe the density variation at various planes

Create a streamline from each of the vents

You may want to adjust the values on the Limitstab (Max. Segments)

Animate the streamlines

Right-click on the Streamlinesin the 3D Viewer and selectAnimate

Create an isosurface based on different temperatures (e.g., 22 [C], 24 [C],

etc.)

Calculate the areaAve of Wall Heat Flux on the workers

Click Tools> Function Calculator


Time permitting, you may want to try the following:


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Summary

This workshop has shown the steps needed to set up a simulation that

includes:

Profile Boundary Condition export and import

Buoyant flow

Heat Transfer

Of particular note is that, for heat transfer problems, it is very importantto consider the domain imbalance in a system. In this case the solution

needed to proceed for more than double the number of iterations that

would have been needed to converge to 1e-4.


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CFX Intro 14.5 WS11 Room Temperature Study

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