Electronics Cooling with Natural Convection and Radiation Part A

Goals

In this workshop, you will model the heat dissipation from a hot electronics component fitted to a printed circuit board (PCB) via a finned heat sink.

The PCB is fitted into an enclosure which is open at the top and bottom.

Initially only the heat transfer via convection and conduction will be calculated. The effect of thermal radiation will then be included as a later stage.

Mesh Import (Workbench)

This workshop can be done either inside or outside of ANSYS Workbench.
If working outside of Workbench, you should skip this page.

Open a new Workbench session and select a new FLUENT session from Component Systems

Use Save As to save the session.

Import the the mesh file.

– Right-click on the Setup cell.

– Change Files of Type to Fluent
Mesh File

– Select the mesh file heatsink.msh

– Click Open.

Launch FLUENT using the default options.

Mesh Import (Stand-alone)

Start a 3D FLUENT session from the icon or from the Windows Start menu

Select either

– File ? Read ? Mesh from the top
menu

– Open File icon from toolbar

Open the file heatsink.msh

Check the grid to verify that there are no errors in the mesh.

View the model:

Display the mesh and color the faces by ID:

– Select Graphics and Animations

– Highlight Mesh, then Setup just below

Set Faces to on, and Edges to Feature

Deselect all currently selected faces

Select Surface Types Wall, Pressure Outlet and Velocity Inlet (note effect on Surfaces list)

Select Colors and Color by ID.

Display

– Select the Lights button, and turn on headlight.

– Make the outer walls transparent

Use Scene button

Select wall_left, wall_right and wall_top

Select Display and set transparency to roughly 50

Apply and close Display Property panel

Apply and close Scene Description panel

– Redisplay the image (Use Setup and Display buttons as above)

Model setup

1. Display the mesh and adjust the display settings.

a) Highlight Mesh and click Setup.

i. Select Feature and Edges. Set Edge Type to Feature.

ii. Deselect all currently selected faces

iii. Select Surface Types Wall, Pressure Outlet and Velocity Inlet (note effect on Surfaces list)

iv. Select Colors and Color by ID.

v. Click Display

1. Change temperature units to °C Define ? Units

1. Select Temperature as a Quantity
2. Select c as the temperature units.
3. Close the panel.

3. Enable the energy equation.

a.Select the Models tree item

b.Double-click on Energy and enable the equation.

Comments on Model setup

General

– It is good practice to display the grid after import to check for any boundary zone misassignment and that you have opened the correct model.

– Workbench uses SI units (meters, kg etc) but if importing a mesh from another source check the scale and dimensions are correct.

– Check mesh is used to confirm the mesh is suitable for use in a CFD simulation.

– Report Quality is a backup to the quality tools available within the meshing application.

By default the energy equation is not solved to reduce CPU load because many problems are isothermal. In this case, temperature must be calculated so the energy equation needs to be enabled.

The onset of turbulence is specified by the Reynolds Number (pipe flow) or Rayleigh Number (natural convection). Calculating these numbers using boundary conditions indicates that the flow will be laminar.

Material properties

The air density needs to change with temperature (but not pressure)

– Select Materials à Air à Create/Edit

– Change density to incompressible
ideal gas

– All other properties remain unchanged

– Click Change / Create then close the fluid materials window.

Define two additional solid materials (for the board and the heat sink).

– Select Materials à Solid à Create/Edit

– Click the FLUENT Database button.

– Change Type to Solid

– Select Copper

– Copy then close the database window

Modify the copper material to produce two different materials.

– The PCB is made of material Fr-4.

Change Name to fr-4

Delete the chemical formula

All other properties remain unchanged

Click Change/Create.

Click No when prompted to overwrite copper.

– Selecting No will create a new material Fr-4, but copper remains in the material list.

– Selecting Yes will overwrite the copper material for the current case only.

http://www.cadfamily.com/html/Article/Electronics%20Cooling%20with%20Natural%20Convection%20and%20Radiation%20Part%20A_886_1.htm

Room Temperature Study(Part 2)

Room: Operating Conditions

The operating conditions for the flow at room are:

-The working fluid is Air

-Worker Temperature = 310 K

-Computer Monitor Temperature = 303 K

-Computer Vent: 0.033 kg/s @ 313 K (per computer)

-Ceiling Vents: profile data, Temperature=294 K

Room Geometry and Details

Starting FLUENT in Workbench

1. Return to the Project window
2. Drag FLUENT into the Project Schematic
3. Change the name to Room
4. Double click on Setup
5. Choose 3D and Double Precision under Options and retain the other default settings

Import Mesh

This starts a new FLUENT session and the first step is to import the mesh that has already been created:

1. Under the File menu select Import> Mesh
2. Select the file duct.msh and click OK to import the mesh
3. After reading the mesh, check the grid using Mesh>Check option

or by using Check under Problem Setup>General

Reading the Profiles

Read the profile files that were written in the Duct’s case at Vent Boundaries

1. Under the File menu select Read> Profile
2. Select the file vent1.prof and click OK to read the profile
3. Similarly read vent2.prof file

Models

1. Select Pressure Based, Steady state solver
   Problem Setup>General>Solver

1. Specify turbulence model
   Problem Setup > Models > Viscous
   Double click and Select k-omega (2 eqn) under Model and SST under k-omega model and retain the default settings for the other parameters

1. Enable the Energy Equation.
   Problem Setup > Models> Energy

Materials

Define the materials.

Problem Setup > Materials

1. Double click on air to open Create/Edit Materials panel
2. Select incompressible-ideal-gas from the dropdown menu of Density
3. Retain other default values of Specific heat and Viscosity. Select ‘Change/Create’ to implement the changes then Close

NOTE: The incompressible ideal gas law for density is used when pressure variations are small enough that the flow is fully incompressible but you wish to use the ideal gas law to express the relationship between density and temperature

Operating Conditions

Problem Setup >Cell Zone Conditions

Click on Operating Conditions… and set the Operating Pressure (Pascal) to 101325

Enable Gravity and specify Z-component of Gravitational Acceleration as -9.81 m/s2

Enter Operating Density as 1.225 kg/m3

Note: Enabling gravity will allow the solver to take into account the buoyancy effect due to the change in the density of the air.

Boundary Conditions

Under Problem Setup > Boundary Conditions

1. Select vent1 under Zone and choose velocity-inlet from the drop down menu under Type. For this boundary we will specify the parameters using the previously read profile file
2. Now double click on vent1 under Zone
3. Go to Momentum tab, set Components as Velocity Specification Method
4. Select vent1 x-velocity from the dropdown menu for X-Velocity. (make sure you select the velocity variable “vent1 x-velocity” not the grid variable”vent1 x”. Do likewise for all the other variables (y-velocity, z-velocity, turbulent kinetic energy and specific dissipation rate).

5. In the Thermal tab, set a constant Temperature of 294K:

Under Problem Setup > Boundary Conditions

1. Similarly, select vent2 under Zone and set all the quantities. This time choose the profile quantities starting with vent2

Under Problem Setup > Boundary Conditions

1. Select outlet under Zone and choose Pressure-outlet from the drop down menu under Type. For this boundary we will specify the parameters using the previously read profile file
2. Now double click on outlet under Zone
3. Go to Momentum tab, set Gauge Pressure (Pascal) as 0
4. Set the backflow conditions for the turbulence quantities to have a Backflow Turbulent Intensity and Backflow Turbulent Viscosity Ratio of 5% and 5 respectively
5. In the Thermal tab, set a constant Backflow Total Temperature of 294 K

Under Problem Setup > Boundary Conditions

1. Select computer1intake under Zone and choose Mass-Flow inlet from the drop down menu under Type.
2. Set the Mass Flow Rate as 0.033 kg/s and keep the Direction Specification Method as Outward Normals
3. Set Turbulent Intensity (fraction) and Turbulent Viscosity Ratio as 5% 10 respectively

To save time, the conditions for computer1 can be copied over to the boundary conditions for the other 3 computers in the simulation.

1. Make sure that the inlets for the other computers are all of type mass-flow-inlet

2. In the Boundary Conditions Panel, click the Copy... button. This will open the Copy BCs panel

3. In the From Zone list, select the zone that has the conditions you want to copy: computer1intake

4. In the To Zones list, select the zones to which you want to copy the conditions to: computer2intake, computer3intake, computer4intake

5. Click Copy. FLUENT will set all of the boundary conditions for the zones selected in the To Zones list to be the same as the conditions for the zone selected in the From Zone list.

Under Problem Setup > Boundary Conditions

1. Repeat the instructions on the previous 2 slides in order to set the conditions for the computer vents.
2. So, first make sure all vents are of type ‘mass-flow-inlet’.
3. Set the conditions for computer1vent as in the image below.
4. In the Thermal tab, set a constant temperature of 313 K
5. Copy this boundary condition from computer1vent to the other 3 computers.

Under Problem Setup > Boundary Conditions

1. Select monitors under Zone and choose wall from the drop down menu under Type.
2. Now double click on monitors under Zone
3. Go to Momentum tab, set it as Stationary wall with No Slip
4. In the Thermal tab, set a constant Temperature of 303 K

Under Problem Setup > Boundary Conditions

1. Select workers under Zone and select wall from the drop down menu under Type.
2. Double-click on workers under Zone.
3. On the Momentum tab, specify a stationary wall with no slip.
4. On the Thermal tab, set a constant wall temperature of 310 K.

http://www.cadfamily.com/html/Article/Room%20Temperature%20Study(Part%202)_889_1.htm

Room Temperature Study(Part 1)

In this introductory workshop you will be analyzing the effect of computers and workers on the temperature distribution in an office. In the first stage, the simulation of airflow through the duct will be carried out and then the outlet conditions for the duct will be saved and provided as the profile data for the inlet condition(s) of the room

Duct Simulation: Description

The operating and boundary conditions for the flow are:

-The working fluid is Air

-Fluid Temperature = 294 K

-Inlet: 0.45 kg/s @ 294 K

-Outlet: 0.225 kg/s (per vent)

Starting FLUENT in Workbench

1. Open the Workbench (Start > Programs > ANSYS 12.0 > ANSYS Workbench)
2. Drag FLUENT into the project schematic
3. Change the name to Duct
4. Double click on Setup
5. Choose 3D and Double Precision under Options and retain the other default settings

Import Mesh

This starts a new FLUENT session and the first step is to import the mesh that has already been created:

1. Under the File menu select Import> Mesh
2. Select the file duct.msh and click OK to import the mesh
3. After reading the mesh, check the grid using Mesh>Check option or by using Check under Problem Setup>General

Setting up the Models

1. Select Pressure Based, Steady state solver Problem Setup>General>Solver
2. Specify Turbulence model

Problem Setup > Models > Viscous

Double click and Select k-omega (2 eqn) under Model and SST under k-omega model and retain the default settings for the other parameters

1. Make sure that the Energy Equation is disabled

Problem Setup > Models> Energy

Materials

Define the materials.

Problem Setup > Materials

1. Double click on air to open Create/Edit Materials panel
2. By default, Density and Viscosity of air are set as 1.225 kg/m3 and 1.7894e-05 kg/(m-s) respectively
3. Retain those values and close the panel

Operating Conditions

Under Problem Setup >Cell Zone Conditions (operating conditions are also in BC panel)

Click on Operating Conditions… and set the Operating Pressure (Pascal) to 101325

Boundary Conditions

Under Problem Setup > Boundary Conditions

1. Select inlet under Zone and choose Pressure-Inlet from the drop down menu under Type
2. Now double click on inlet under Zone

Input all the parameters in Momentum tab as shown below

Under Problem Setup > Boundary Conditions

1. Select vent1 under Zone and choose mass-flow-inlet from the drop down menu under Type
2. Now double click on vent1 under Zone

Input all the parameters in Momentum tab as shown below

Under Problem Setup > Boundary Conditions

1. Select vent2 under Zone and choose mass-flow-inlet from the drop down menu under Type and set the conditions similar to that of vent1

NOTE: Under the Direction Specification Method, we may also use Outward Normal condition for both the vents

Solution Methods

Set the Solution methods which decides the Pressure-Velocity coupling.

Under Solution>Solution Methods setup the parameters as shown in the image.

http://www.cadfamily.com/html/Article/Room%20Temperature%20Study(Part%201)_888_1.htm

Introductory FLUENT Training-Transient Flow Modeling

Motivation

Nearly all flows in nature are transient!

– Steady-state assumption is possible if we:

Ignore transient fluctuations

Employ ensemble/time-averaging to remove unsteadiness (this is what is done in modeling turbulence)

In CFD, steady-state methods are preferred

– Lower computational cost

– Easier to postprocess and analyze

Many applications require resolution of transient flow:

– Aerodynamics (aircraft, land vehicles,etc.) – vortex shedding

– Rotating Machinery – rotor/stator interaction, stall, surge

– Multiphase Flows – free surfaces, bubble dynamics

– Deforming Domains – in-cylinder combustion, store separation

– transient Heat Transfer – transient heating and cooling

– Many more.

Origins of Transient Flow

Naturally occurring transients

– transient flow due to growth of instabilities within the fluid or a non-equilibrium initial fluid state

– Examples: natural convection flows, turbulent eddies of all scales, fluid waves (gravity waves, shock waves)

Forced transients

– Time-dependent boundary conditions, source terms drive the transient flow field

– Examples: pulsing flow in a nozzle, rotor-stator interaction in a turbine stage

 

Transient CFD Analysis

Simulate a transient flow field over a specified time period

– Solution may approach:

Steady-state solution – Flow variables stop changing with time

Time-periodic solution – Flow veriables fluctuate with repeating pattern

– Your goal may also be simply to analyze the flow over a prescribed time interval.

Free surface flows

Moving shock waves

Etc.

Extract quantities of interest

– Natural frequencies (e.g. Strouhal Number)

– Time-averaged and/or RMS values

– Time-related parameters (e.g. time required to cool a hot solid, residence time of a pollutant)

– Spectral data – fast Fourier transform (FFT)

Transient Flow Modeling Workflow

Enable the transient solver.

Set up physical models and boundary conditions as usual.

– Transient boundary conditions are possible – you can use either a UDF or profile to accomplish this.

Prescribe initial conditions

– Best to use a physically realistic initial condition, such as a steady solution.

Assign solver settings and configure solution monitors.

Configure animations and data output/sampling options

Select time step and max iterations per time step

Prescribe the number of time steps.

Run the calculations (Iterate)

Enabling the Transient Solver

To enable the transient solver, select the Transient button on the General problem setup form:

Before performing iterations, you will need to set some additional controls.

– Solver settings

– Animations

– Data export / Autosave options

Selecting the Transient Time Step Size

Time step size, Dt, is set in the Run Calculation form.

– Dt must be small enough to resolve time-dependent features; make sure the convergence is reached within the number of Max Iterations per Time Step

– The order or magnitude of an appropriate time step size can be estimated as:

 

– Time step size estimate can also be chosen so that the transient characteristics of the flow can be resolved (e.g. flow within a known period of fluctuations)

To iterate without advancing in time, specify zero time steps. This will instruct the solver to converge the current time step only.

The PISO scheme may aid in accelerating convergence for many transient flows (set in the Solution Methods form).

http://www.cadfamily.com/html/Article/Introductory%20FLUENT%20Training-Transient%20Flow%20Modeling_872_1.htm

Post processing with FLUENT and CFD-Post Part B

Generating Tables and Charts

Tables and Charts can be created to format and present results

Tables

Select Insert > Table or use the toolbar icon to create a new table

– 3D Viewer will switch over to the Table Viewer

Tables allow you to display data and expressions in a tabular view

Tables are automatically added to the Report

Cells can contain expressions or text

– Begin with “=“ to distinguish

– Expressions are evaluated and updated when variables and/or locations they depend on change

This is not a spreadsheet

– Cannot reference other cells

Charts

Plot a relationship between two variables along a line/curve

– Need to create the line first

– Polyline, Boundary Intersection curve, Contour line, etc.

Charts are automatically added to the Report

Chart Points are not necessarily evenly spaced

– Data points usually correspond to where the line/curve intersects a mesh face

Multiple lines can be plotted on a single chart

Charts: Type

Charts can be one of three types:

– XY

Standard XY plots based on line locators

– XY – Transient or Sequence

Plots an expression (usually Time) versus a variable at a point locator

Typically used to show the transient variation of a variable at a point

– Histogram

Can be based on any locator that contains multiple data locations – lines, surfaces, planes, domains (but not points)

Plots a variable divided into discrete bands on the X Axis versus the frequency of occurrence on the Y Axis

Charts: Data Series and Axes

Each data series corresponds to a location (line, point, etc.) which corresponds to a curve on the chart

Use the X and Y Axis tabs to set the variables on the axes

The remaining tab are for various display options

http://www.cadfamily.com/html/Article/Post%20processing%20with%20FLUENT%20and%20CFD-Post%20Part%20B_878_1.htm

http://www.cadfamily.com/html/Article/Post%20processing%20with%20FLUENT%20and%20CFD-Post%20Part%20B_878_2.htm

http://www.cadfamily.com/html/Article/Post%20processing%20with%20FLUENT%20and%20CFD-Post%20Part%20B_878_3.htm

Post processing with FLUENT and CFD-Post Part A

Overview

There are two ways to postprocess CFD results from FLUENT

– FLUENT postprocessing tools – legacy tools integrated in the FLUENT solver.

– ANSYS CFD-Post application – a state-of-the-art postprocessor for ANSYS CFD products. CFD-Post can run as a standalone postprocessor, or within Workbench

Both postprocessors include many tools for analyzing CFD results

– Isosurfaces

– Vector Plots

– Contour plots (shaded and graded)

– Streamlines and pathlines

– XY plotting

– Animation creation

Contains the ability to postprocess any user-defined quantity either defined through custom field functions or user-defined memory (UDF).

Postprocessing in FLUENT

Many post processing tools are available in FLUENT:

– Surface creation

– Display types

– Rendering options

– Plots of solution data

– Flux reports and Integral calculations

Post processing functions typically operate on surfaces, which can be automatically created by FLUENT from existing zones or by the user.

For more information, please refer to the web-based lecture, “Post-Processing in FLUENT” on the FLUENT User Services Center www.fluentusers.com

Surface Creation

Fluent allows you to select portions of the CFD domain, called surfaces, to be used for visualizing and plotting the flow field.

There are a variety of ways to create surfaces:

– Zone surfaces (surfaces automatically created by solver from zones)

– Plane surfaces (specifying a specific plane in the domain)

– Iso-surfaces (surfaces that have constant value for a specified variable)

– Clipping Surfaces (iso-surfaces trimmed within specified range of values)

– Point surfaces (specifying a particular location in the domain)

– Line and Rake Surfaces (used for display of particle path lines)

Surfaces can be renamed, deleted or moved and used to write profile files.

Post-Processing Rendering Options

The rendering options in FLUENT allow control of the look-and-feel of the post-processing plots, including:

– Views and display options

– Colormaps for contour/vector plots

– Shading on surfaces using Lights

– Annotation of plots

– Surface manipulation

– Scene Composition using plot overlays, different colors, shading, transparency

– Scene animation (fly throughs)

Plots

FLUENT provides tools to generate data plots of the solution:

– XY plots of solution variables

– Histograms to illustrate frequency of distribution

– Fast Fourier Transforms (FFT)

– Residuals

You can modify the colors, titles, legend, axis and curve attributes to customize your plots.

Other data files (experimental, computational) can also be read in to compare results.

Reports

Flux reports

– Net flux is calculated.

– Total Heat Transfer Rate includes radiation.

Surface integrals

– slightly less accurate on user-generated surfaces due to interpolation error.

Volume integrals

Postprocessing with CFD-Post

Starting CFD-Post

Within ANSYS Workbench

– Drag the CFD-Post icon in the Component Systems list to the project tree.

– OR, create a standalone CFD-Post session.

From the Start Menu or Command Line

– Start > Programs > ANSYS 12.0 > ANSYS CFD-Post

CFD-Post can also be started from the CFX-Solver Manager or the CFX Launcher

GUI Layout

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http://www.cadfamily.com/html/Article/Post%20processing%20with%20FLUENT%20and%20CFD-Post%20Part%20A_877_2.htm

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