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© 2011 ANSYS, Inc. June 26, 2014 1
Tips and tricks for Thermal Analysis
Ankit Adhiya Lead Engineer
© 2011 ANSYS, Inc. June 26, 2014 2
• Introduction
• General tips: Model building
• General tips: Meshing
• General tips: Convergence and stability
• General tips: Post processing
• ANSYS R.15 Updates
Content
© 2011 ANSYS, Inc. January 27, 2012 3 ANSYS Icepak – Validation 1
• Aim: The goal of the model must be clearly defined. This often gives direction to the subsequent steps.
• Some examples: – Optimize thermal vias inside a
package to minimize die temperature
–Board layout design to get the target packages die temperature under a specified value
– Selection and positioning of fans to maintain the required flow in the telecom box
Steps in a CFD Project
Define Goal
Solve
Analyze results
Next step!
Model building
Discretization
Numeric model
Define Goal
© 2011 ANSYS, Inc. January 27, 2012 4 ANSYS Icepak – Validation 1
Tips and Tricks are applied in CFD projects to improve modelling efficiency, reduce design cycle and increase productivity.
Overview: Tips and Tricks
In CFD Simulations Tips and Tricks are used for
– Rapid model creation – Achieve high quality mesh – Improve solution accuracy – Better analyse and
postprocess the results
5 © 2014 ANSYS, Inc. June 26, 2014 ANSYS Confidential
Workflow in ANSYS Icepak
Post Processing
Solve
Flexible Meshing
Model Building
MCAD Through DM
ECAD
© 2011 ANSYS, Inc. January 27, 2012 6 ANSYS Icepak – Validation 1
Define Your Goals
– What results you are looking for?
• Flow structure, Heat transfer, Temperature limit
– Degree of detail required
• System level Vs Board level simulation
– Is Radiation, Gravity important
• Natural convection Vs Forced Convection
– Laminar Vs Turbulence flow
• Reynolds number
General Tips: Before You Start
© 2011 ANSYS, Inc. January 27, 2012 7 ANSYS Icepak – Validation 1
• Identify the domain:
– Use smallest possible domain without compromising the results.
– Can you use symmetry or periodic boundary?
– Use hollow blocks to change the shape of the domain and reduce mesh count.
– Apply correct boundary conditions o Constant T, convective BC, constant q”, …
General Tips: Domain Identification
© 2011 ANSYS, Inc. January 27, 2012 8 ANSYS Icepak – Validation 1
General Tips: Model creation using Primitive Object
• Goal: Reduce run time and increase accuracy. – Start with a simple model or few components and then populate
the model. – Can you simplify or reduce the components?
• Replace irregular (Polygonal) shapes with regular (prism and cylinder) shapes.
– Can you reduce number of components? • Use ‘individual sides’ specification instead of using plates or
annular blocks.
© 2011 ANSYS, Inc. January 27, 2012 9 ANSYS Icepak – Validation 1
General Tips: Model building using Macros
• ANSYS Icepak includes many macros designed to automate common model-building tasks.
• Icepak Macros can be used to assist creating different types of packages, PCBs, heat sinks, heat pipes, thermoelectric coolers, JEDEC test configurations and DELPHI package characterization
© 2011 ANSYS, Inc. January 27, 2012 10 ANSYS Icepak – Validation 1
General Tips: Importing ECAD Files
• Finalize your analysis type – System, Board or Package Level
• No need to Import any Ecad (Trace) files for system level
• Try using IDF import for Board level – Allegro from Cadence, Boardstation from
Mentor, Visula from Zuken-Redac – Pro/Engineer, SCRC/IDEAS, Unigraphics
• Try including traces for Package level – BRD file ,Gerber files, NJB file, .BOOL file,
.TCB file, .ANF file
© 2011 ANSYS, Inc. January 27, 2012 11 ANSYS Icepak – Validation 1
Workflow in ANSYS Icepak
Post Processing
Solve
Flexible Meshing
Model Building
MCAD Through DM
ECAD
© 2011 ANSYS, Inc. January 27, 2012 12 ANSYS Icepak – Validation 1
Flexible Meshing
• Powerful automated grid generation
• Types of mesh
o Hexa Unstructured o Hexa Cartesian o Hexa Dominant (HD) o Mixed meshing allowed
• Flexible mesh refinement o Domain level controls – applicable to all
objects o Individual object level controls o Multilevel meshing capability
© 2011 ANSYS, Inc. January 27, 2012 13 ANSYS Icepak – Validation 1
General Tips: Meshing
• Start with a coarse mesh and always look for the warnings in the Icepak message windows. Any problem will be caught quickly.
• Ensure that all objects are meshed, especially fans, openings, vents, and
resistance plates.
• If some objects/face are not in the mesh – check whether any other objects (with high meshing priority) are occupying the same location.
• Invest time in creating a good quality and efficient (less number of cells with required resolution) mesh.
© 2011 ANSYS, Inc. January 27, 2012 14 ANSYS Icepak – Validation 1
General Tips: Non Conformal Mesh
• Use non-conformal mesh where ever possible to reduce cell counts and to get a good quality mesh.
• Make sure all the non-conformal
rules (discussed in following slides) are satisfied during defining slack values.
© 2011 ANSYS, Inc. January 27, 2012 15 ANSYS Icepak – Validation 1
General Tips: Non Conformal Mesh
Conformal Mesh Mesh Count=1,424,828
Non-conformal Mesh Mesh Count=386,667 (73% reduction)
Regions with mesh bleeding
Mesh isolated in regions of interest
Mesh secluded from outside interference
© 2011 ANSYS, Inc. January 27, 2012 16 ANSYS Icepak – Validation 1
General Tips: Non Conformal Mesh
Conformal mesh Non-conformal mesh
V = 2 m/s
Tmax = 94.9 C
V = 2 m/s
Tmax = 93.85 C
Similar result with much smaller mesh count
© 2011 ANSYS, Inc. January 27, 2012 17 ANSYS Icepak – Validation 1
• Model with care – Eliminate unnecessary small gaps/ overlaps
– Use Model: Snap to Grid and other co-ordinate snap tools as needed
– Helps minimize mesh count
• Local refinement applications:
– Openings/ grilles/ fan annulus - 4 to 6 elements per side
– Flow channels/gaps (example - between heat sink fins) - apply 3 to 5 cells
– Lumped PCBs - 3 or 4 elements in thickness
– Power dissipating objects - apply 3 elements per side
– Heat sinks - 4 to 8 count in Fin height + 3 count in Heat sink base thickness
Mesh Guidelines
© 2011 ANSYS, Inc. January 27, 2012 18 ANSYS Icepak – Validation 1
• Mesh Verification – Ensure that mesh captures Design Intent – Ensure correctness of Meshing Priorities to achieve Design Intent – Minimum gaps - Apply suitable values to ensure all necessary
geometries remain – Quality Measures - check for recommended values
Mesh Guidelines : 2
© 2011 ANSYS, Inc. January 27, 2012 19 ANSYS Icepak – Validation 1
Workflow in ANSYS Icepak
Post Processing
Solve
Flexible Meshing
Model Building
MCAD Through DM
ECAD
© 2011 ANSYS, Inc. January 27, 2012 20 ANSYS Icepak – Validation 1
General Tips: Numerical Model (Solver)
• Equations to solve: select only the required equations. – Conduction only problem – check only temperature
– Forced convection – use ‘sequential solve of flow and energy’ (advanced solve) for better and quicker convergence, gravity and radiation may not be necessary
– Natural convection – flow and energy must be solved together, radiation and gravity need to be included.
• Under Relaxation Factor:
– Pressure 0.3 and Momentum 0.7 for forced convection
– Pressure 0.7 and Momentum 0.3 for natural convection, complex forced convection and models with ‘more nonlinearity’, e.g. many fans, large velocity or pressure gradients, closed domain ...
• Always use monitor points to monitor convergence of solution.
© 2011 ANSYS, Inc. January 27, 2012 21 ANSYS Icepak – Validation 1
General Tips: Radiation
• When is Radiation Important ? – Radiation is important if thermal resistance due to convection
is of the same order of radiation – Important in natural convection problems mostly
• A general rule of thumb is given below – Estimate the ambient/bulk fluid temperature in the model – Estimate the maximum temperature from the
solution(roughly) – Stefan Boltzmann law states the radiation flux is proportional
to σ*(Tsur4 – Tref
4 )
• As value of σ is very less, radiation flux will not be dominant unless the (Tsur
4 – Tref 4 ) is significant.
© 2011 ANSYS, Inc. January 27, 2012 22 ANSYS Icepak – Validation 1
Workflow in ANSYS Icepak
Post Processing
Solve
Flexible Meshing
Model Building
MCAD Through DM
ECAD
© 2011 ANSYS, Inc. January 27, 2012 23 ANSYS Icepak – Validation 1
CFD Post : Case Comparison
• Multiple cases can be post processed simultaneously by enabling “Keep Current cases loaded” option
• Feature useful for comparison studies
© 2011 ANSYS, Inc. January 27, 2012 24 ANSYS Icepak – Validation 1
Existing Simulation Knowledge Resources
© 2011 ANSYS, Inc. January 27, 2012 25 ANSYS Icepak – Validation 1
Existing Knowledge Resource
© 2011 ANSYS, Inc. January 27, 2012 26 ANSYS Icepak – Validation 1
Existing Knowledge Resource
© 2011 ANSYS, Inc. January 27, 2012 27 ANSYS Icepak – Validation 1
ANSYS Icepak R15.0 Updates
© 2011 ANSYS, Inc. January 27, 2012 28 ANSYS Icepak – Validation 1
Content
• Icepak parameters published to the Workbench parameter manager – Allows design points, design of experiments, and design
optimization with DesignXplorer
• Q3D Extractor – Icepak one-way coupling
• Graphics enhancements
• Solver enhancements
• Post-processing enhancements
• Miscellaneous Enhancements
© 2011 ANSYS, Inc. January 27, 2012 29 ANSYS Icepak – Validation 1
• Icepak parameters can be published to the Workbench parameter manager
• Allows design points, design of experiments, and optimization with ANSYS DesignXplorer
• Enables multiple simultaneous design point solutions with HPC parametric packs
Icepak Parameters Published to WB
Design point study of heat sink in wind tunnel, DesignModeler geometry.
© 2011 ANSYS, Inc. January 27, 2012 30 ANSYS Icepak – Validation 1
Icepak Parameters Published to WB
Icepak and DesignModeler parameters are now available in the Workbench parameter manger for design point studies and/or optimization with DX. Design point solutions can also be “exported” to review contour results.
© 2011 ANSYS, Inc. January 27, 2012 31 ANSYS Icepak – Validation 1
• Design of Experiments solution to obtain response surface and MOGA (Multiple Objective Generic Algorithm) used to find power values from measured temperatures for PBGA package model
Icepak-DesignXplorer PBGA Example
© 2011 ANSYS, Inc. January 27, 2012 32 ANSYS Icepak – Validation 1
• Import volumetric and surface losses from Q3D into
Icepak – Import volumetric and/or surface losses from Q3D into
Icepak via the Workbench project schematic
One-way Coupling with Q3D
© 2011 ANSYS, Inc. January 27, 2012 33 ANSYS Icepak – Validation 1
Q3D-Icepak Coupling
Temperature contours and velocity vectors from steady-state solution
Q3D geometry linked to DesignModeler and simplified for Icepak
Forced convection cooling of a coaxial cable
DC simulation with Q3D to compute losses
© 2011 ANSYS, Inc. January 27, 2012 34 ANSYS Icepak – Validation 1
• “Show metal fractions” display the percentage of copper for each trace layer of a package substrate or printed circuit board.
• The Metal fraction display provides a preview of the resulting thermal conductivity based on the metal fractions, which is useful for model verification.
View Metal Fraction
Metal fraction on top layer
Metal fraction on bottom layer
Trace import on a block
© 2011 ANSYS, Inc. January 27, 2012 35 ANSYS Icepak – Validation 1
Display Objects by Type
Select Model > Show Objects by Type
Resulting display shows object by type and sub type.
© 2011 ANSYS, Inc. January 27, 2012 36 ANSYS Icepak – Validation 1
Problem Setup Wizard
Right-click on Problem Setup to launch Problem Setup Wizard
Wizard guides users through a series of choices with guidance to properly setup the models physics
© 2011 ANSYS, Inc. January 27, 2012 37 ANSYS Icepak – Validation 1
• Heat sink example • Couple P-V formulation 2.6x
faster versus segregated solver
• 163 versus 500 iterations • More robust convergence
behavior versus segregated solver with similar results
Coupled Pressure-Velocity Formulation
SIMPLE Formulation (Icepak default)
Coupled P-V Formulation
© 2011 ANSYS, Inc. January 27, 2012 38 ANSYS Icepak – Validation 1
• New parallel option to enable GPU computing for certain solver operations
• Select using Parallel settings panel
• Accelerates view factor computations using ray-tracing radiation
• Accelerates Algebraic Multigrid (AMG) method for coupled pressure based solver
• ANSYS HPC license required for GPU computing
GPU Computing
© 2011 ANSYS, Inc. January 27, 2012 39 ANSYS Icepak – Validation 1
• New summary report options to report conduction, convection and/or radiation heat flow for objects
• Both Icepak objects and CAD objects supported
• Easier to determine heat balance for objects
Report Conduction, Convection and Radiation for Objects
© 2011 ANSYS, Inc. January 27, 2012 40 ANSYS Icepak – Validation 1
Happy Computing