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Best Practices Overview for
Electronics Thermal
Simulations
Geometry Preparation
Mesh
Materials
Conditions
Solution
Results
STAR-CCM+ Simulation Process
http://www.cd-adapco.com/webcasts (Industry = Electronics)
STAR-CCM+ Electronics Thermal Seminars
Natural convection series
– Best practices
– Small internal air gaps
– Radiation
Forced convection series
– Best practices, part 1
– Best practices, part 2
– Complex heat sinks
– Geometry preparation
• Assumes 3D-CAD → Parts → Regions
• Composite parts Geometry
• Pre-defined parts-based mesh (PBM) operations Mesh
• Air (ideal gas or Boussinesq)
• Solids (common in electronics) Materials
• Pre-defined boundaries
• Forced & natural convection
• Field functions for ambient temperature & altitude Conditions
• Segregated flow & energy with under-relaxation
• Gravity & radiation (natural convection)
• Stopping criteria Solution
• Temperature report
• Geometry & mesh scenes
• Temperatures with velocity vectors on section planes Results
STAR-CCM+ Template Simulation File
Solids
– Eliminate mechanical connectors
(screws, rivets, springs, etc.)
– Fill holes
– Simplify individual parts
– Sheet metal modifications
– Eliminate interferences
– Fill undesired gaps
Best Practices: Geometry Preparation
Air
– Internal: Fill the empty space
– External
• Natural: Sphere or hemisphere
• Forced: Short inlet, extended outlet
– Tools (3D-CAD): Extract Internal / External Volume, Boolean
Best Practices: Geometry Preparation
Best Practices: Geometry Preparation
Best Practices: Geometry Preparation
Ideally: Conformal polyhedral
mesh
– Strongly recommended for natural
convection (because of radiation)
– Good for forced convection
Option: Polyhedral (conformal)
air, trimmed (non-conformal)
solids
– Suitable for forced convection
Part-Based vs Regions-Based
– Preference
– Conformal thin mesh not yet
available with PBM
Typical mesh: 500,000 –
5,000,000 cells
Best Practices: Meshing
Best Practices: Meshing
Air
– Ideal gas with temperature-
varying properties always suitable
– Boussinesq sufficient for natural
convection
Solids
– Isotropic solids
– Orthotropic solids (e.g. PCBs)
• Separate continua
• Properties in the region
• Typical PCB: kplanar ~ 10 W/m-K,
kthrough plane ~ 0.5 W/m-K
– Components
• Can use contact resistances on
interface to model as 2-resistor.
• Otherwise aluminum oxide (k ~ 25
W/m-K) common.
Best Practices: Materials
Heat sources
– Temperature on all inlets & outlets
– If no air surrounding the enclosure in the model (common in forced
convection), to model heat loss to the ambient add convection on boundary:
• External natural convection: h ~ 5 – 10 W/m2-K
• External forced convection: h > 20 W/m2-K
– Heat power on all dissipating components*
Best Practices: Conditions
Heat Electrical
power supplied
Component
(e.g. IC, IGBT,
MOSFET,
LED,…)
Electrical power
delivered
RF energy,
visible light
• “Wall power”
• Max power (power budget)
• Measured power?
• Duty-cycled?
• What is the efficiency?
FORCED
CONVECTION Flow inlet Flow outlet
Flow “pushed”
into the system
• Specified positive flow speed
velocity), positive pressure,
positive mass flow rate, or
fan pressure jump
• Ambient temperature
• Pressure outlet (0 Pa)
• Ambient temperature (for any
reverse flow)
Flow “pulled”
through the
system
• Stagnation inlet (0 Pa)
• Ambient temperature
• Specified negative flow speed
velocity), negative pressure,
negative mass flow rate, or
fan pressure jump
• Ambient temperature (for any
reverse flow)
Fan inside the
system:
Internal
Interface fan +
• Stagnation inlet (0 Pa)
• Ambient temperature
• Pressure outlet (0 Pa)
• Ambient temperature (for any
reverse flow)
Best Practices: Conditions
Internal Interface Fan
– Only the circular or annular faces used as boundaries in fan definition
– Flow direction: From Boundary-0 to Boundary-1 (Swap Boundaries on the
interface as needed)
– Fan curve
• Define in the fan interface as a polynomial, OR
• Input fan curve to Tools > Tables & then select the curve.
Best Practices: Conditions
Natural Convection: Conditions on the exterior air boundary
– Convection
• Stagnation Inlet (0 Pa)
• Total temperature = Ambient temperature
– Radiation
• Boundary transparency = 1.0
• Radiation temperature is specified in the air continua
– Inside a room: Radiation temperature = wall temperature
– Outdoors: Turn on solar if device exposed to the sun during the day, at night Radiation Temperature = sky radiation temperature
Best Practices: Conditions
Solvers > Segregated Energy
– Fluid Under-Relaxation = 0.99
(default is 0.9)
– Solid Under-Relaxation = 0.9999
(default is 0.99)
Best Practices: Solution
Stopping Criteria
– Often convergence in 300 – 500
iterations.
– Observed residuals (non-
normalized)
• Energy residual < 1E-5
• Momentum residuals < 1E-8
– Convergence requires more
iterations for a finer mesh.
Scalability
– For typical size scales well to ~8
cores.
– I typically run with 2 or 4 cores.
Best Practices: Results
Best Practices: Results
Rthermal = (Tcenter of base – Tambient)
Heat power
Report (expression) from field functions:
($ThermocoupletemperatureReport -
$Tambient_K)/$Heat_power
Natural Convection
– Best practices: http://www.cd-adapco.com/webinar/electronics-best-practices-session-1-
simulating-natural-convective-airflow-electronic
– Small gaps: http://www.cd-adapco.com/webinar/electronics-best-practices-session-2-natural-
convection-analyses-thin-air-gaps
– Radiation: http://www.cd-adapco.com/webinar/electronics-best-practices-session-3-natural-
convection-analyses-thermal-radiation
Forced Convection
– Best practices, part 1: http://www.cd-adapco.com/webinar/best-practices-forced-convection-
simulations-series-1-part-1
– Best practices, part 2: http://www.cd-adapco.com/webinar/best-practices-forced-convection-
simulations-series-1-part-2
– Modeling complex heat sinks: http://www.cd-adapco.com/webinar/efficient-modeling-
complex-heat-sinks-series-2-part-1
– Geometry preparation: http://www.cd-adapco.com/webinar/geometry-preparation-electronics-
thermal-simulations-series-2-part-2
More Information: Web Seminar Recordings
