best practices: electronics cooling - mdx · pdf file•thermal (including radiation)...

Best Practices: Electronics Cooling

Ruben Bons - CD-adapco

Best Practices Outline

Geometry

MaterialsMesh Conditions

Solution

Results

Design exploration /

Optimization

Best Practices Outline

Geometry• Solids

• Simplification

• Preparation

• Air

• Forced convection

• Natural convection

Mesh• Trimmed / Polyhedral

• Conformal / Non-conformal

• Thin solids

• Prism layers in air

• Mesh operations

Materials• Solids

• Air (fluid)

• “Devices”

• Chips

• PCBs

• Porous media, perf plates

• Heat pipes

• Thermoelectric devices

Conditions• Physics: Flow & heat transfer

• Environment

• Inlet(s)

• Outlet(s)

• Thermal (including radiation)

• Heat sources

• Fans & blowers

Solution• Physics models

• Reference values / Initial conditions

• Segregated or Coupled

• Under-relaxation

• Convergence

Results• Temperature

• Velocity

• Field functions

Geometry

Geometry Mesh Materials

ConditionsSolutionResults

Geometry

Solids: Simplification

– Simplify the assembly by removing “unnecessary” parts

• Nuts, bolts, screws, washers, springs, rivets

– Simplify individual parts by removing “unnecessary” features

• Bolt / screw / rivet holes

• Connectors

– “Unnecessary” = not significant to both the flow & thermal

Geometry Mesh Materials Conditions Solution Results

Geometry Geometry Mesh Materials Conditions Solution Results

Geometry

Solids: Preparation

– CAD = “as-manufactured”;

Simulation prefers “as-

assembled” model

• Remove interferences (e.g. from

press fits)

• Close gaps, especially those

closed during assembly (e.g.

sheet metal flanges)

– Modify geometry where solids

contact to ease meshing

• “Coincident faces”

• Clean (“perfect”) fit (e.g.

clamshell molded parts)

• Tangencies that cause sliver air

gaps

– Seal internal air spaces


Geometry

Air: General

– Physical boundaries must be represented

• Enclosure

• Surroundings

– Boundary conditions should not alter the ‘natural’ flow patterns

– Want accurate results as quickly as possible


Geometry

Air: Forced Convection

– Often the internal air + venting is sufficient

• If desired, model exterior heat loss with boundary condition (e.g. heat transfer

coefficient)

• Conservative to ignore the exterior heat loss

– Identify inlet(s) & outlet(s)

• Inlet: Typically slightly extend (<1D) from the assembly

• Outlet: Extend from the assembly, as much as 5-10D


Geometry

Air: Natural Convection

– To simulate air flow & heat transfer on the exterior, model the surrounding air

(use a sphere as the baseline, diameter ~3-5X the bounding box diagonal).

– To model the heat transfer on the exterior, add boundary conditions (e.g. heat

transfer coefficient)


Mesh



Mesh

Cell topology

– Polyhedral

• Conformal

• Non-conformal

– Trimmed hexahedral

• Non-conformal

Approaches

– Parts-based

– Regions-based

Specialty options

– Prism-layer mesher

– Thin mesher

– Extruded mesher

Basic setting: Mesh sizing

Conformal vs Non-conformal

– Conformal possible only with

polyhedral cells

– Non-conformal an option with

polyhedral, trimmed hexahedral

– Accuracy

• Fully conformal is best (no

interpolation at interfaces)

• Non-conformal with similar

surface mesh sizes: Tests show

very small (<0.5%) difference

than fully-conformal results.

• Non-conformal with disparate

mesh sizes: Accuracy degrades

as surface size variance

increases

– Meshing speed

• Non-conformal is fastest

• Serial & parallel option for both

• Concurrent option for non-conf


Mesh

Parts-based vs Regions-based

– Personal preference

– Parts-based has advantages for

complex mesh sequences

– New thin mesher in PBM

Thin mesher (for solids)

– 1-2 layers for conducting-only

solids (no heat dissipation)

– 3+ layers for thin solids that

dissipate heat

Methodology

– Surface mesh all geometry in 1-

step (e.g. 1 PBM operation)

• Base size: 2 - 5% of bounding

box diagonal

• Min surface size: 0.01 – 0.001%

of base

• Curvature: 16 points / circle

• Proximity: 0.25 points in gap

Produces conformal surface

mesh

– Volume mesh

• Conformal or non-conformal

• Poly or trimmed hex or mixed

• Conformal polyhedral

recommended for S2S radiation

• 2-4 prism layers at all fluid walls

(e.g. fluid-solid interfaces,

exterior fluid boundaries)


Mesh Geometry Mesh Materials Conditions Solution Results

Conformal solid-solid interface

Fluid prism layers

Non-conformal fluid-solid interface

Mesh Geometry Mesh Materials Conditions Solution Results

Materials



Materials

Solids

Air (fluid)

“Devices”

– Chips

– PCBs

– Porous media, perforated plates

– Heat pipes

– Thermoelectric devices

Most material specifications

are detailed in the

corresponding continua

– Pick from the default library

– Customize, save to library

Some require details in the

corresponding region

Solids

– Isotropic properties by default

• Thermal conductivity can be

anisotropic – set Method of

Thermal Conductivity in continua

• Set values in appropriate region

– No temperature variation by

default

• Change in the continua

• Specific heat: Polynomial in T

• Thermal conductivity:

Polynomial in T, table(T), field

function


Materials Geometry Mesh Materials Conditions Solution Results

Source: Incropera & De Witt, Fundamentals of Heat and Mass Transfer, Third

Edition (New York: John Wiley & Sons, 1990), pg. A15.

Materials

Fluid

– Most commonly air

– Liquid cooling with water,

ethylene-glycol solution, etc.

Properties & appropriate

physics specified in the

continua

– Properties

• Density

• Viscosity

• Specific heat

• Thermal conductivity

– Physics

• Laminar or turbulent

• Turbulence model

Properties: Air

– Density

• For buoyancy (natural

convection), density must vary

with temperature (+ gravity)

• Ambient pressure strongly

affects air density (e.g. at

altitude)

– Viscosity can significantly vary

with temperature

Properties: Water

– Density

• Variation with temperature

important only with natural

convection (rare cases)

• Little variation with pressure

– Viscosity variation with

temperature can be significant



Common

temperature range

in electronics

Materials

Laminar or Turbulent (for air)

– Forced convection: Generally

turbulent

• Internal: Transition @ Re ~

2500 – 10,000

• External: Transition @ Re ~

500,000

– Natural convection: Generally

laminar

• Turbulent if Rah > 109 (vertical

flat plate)

• Assume

– Tw = 85 oC

– T∞ = 50 oC

• Properties @ 70 oC

• hcritical = 0.83 m

Turbulence model

– Many options in STAR-CCM+,

consult the help for details

• k-ε

• k-ω

• Reynolds stress

• Spalart-Allmaras

• DES

• LES

– Realizable k-ε with two-layer all-

y+ wall treatment seems to work

well for a wide range of models

• Forced convection

• Natural convection

– Compared a laminar run with a

k-ε run

– Essentially identical flow &

thermal results


𝑹𝒂𝒉 =𝒈𝜷 𝑻𝒘 − 𝑻∞ 𝒉𝟑

𝝊𝜶

Materials

Device: Chips

– Solid (isotropic) material

– 2-resistor

• High conductivity solid (e.g. Cu)

• Separate boundaries (in the

region) for top & bottom surfaces

• Assign resistivity to interfaces to achieve ϴjb & ϴjc.

• Resistivity ρ = t / k = Rt*Acontact.

Device: PCBs

– Equivalent thermal properties

• Orthotropic equivalent properties

computed from geometric details

(easiest in a spreadsheet)

• Commonly kin-plane ~ 10 W/m-K ~

20*kthrough-thickness.

– Detailed trace modeling

• Computationally costly

• 2D or 3D traces


Materials

Device: Porous media

– Fluid region, Type = Porous

Region

– Set Inertial &/or Viscous

resistance values under Region

Physics Values

• Viscous: ΔP α V (e.g. fibrous

filter)

• Inertial: ΔP α V2 (e.g. perf plate)

Device: Heat pipes

– Rarely are the full physics

(evaporation, condensation,

surface tension, etc.) modeled.

– Typically 3-part assembly

• Pipe wall (k = material

conductivity)

• Wick (k = 30-40 W/m-K)

• Vapor space (k > 10,000 W/m-K)


Materials

Device: Thermoelectric

devices

– Extract parameters from

datasheet values (Tc, Qmax, Tmax,

Relectrical).

– 3-part assembly (don’t mesh the

middle part)

– Field functions to iteratively

compute & apply Qc(Tc, Th) & Qh

(Tc, Th).

Device: Contact resistance

– Every solid-solid interface

physically has contact

resistance.

– Default in STAR-CCM+ is Rc = 0.

– To change, assign resistivity (ρc)

to the interface (in Physics

Values)

• ρc = Rc*Acontact.


Conditions



Conditions

Physics

– Air flow

– Heat transfer

• Conduction

• Convection

• Radiation

Environment

– Inlet(s)

– Outlet(s)

– Thermal (including radiation)

Heat sources

Fans & blowers

Air- (or water- or …) flow

– Computed if you have a fluid

region

– Navier-Stokes equations

Heat transfer

– Conduction computed in all

regions (solids & fluids)

– Convection computed anywhere

a fluid & solid touch (interface)

– Radiation needs to be activated

• In fluid region

• In transparent solid regions

• Methods

– Surface-to-surface (S2S)

– Discrete Ordinate Method

(DOM)

• Solar radiation

– Available with S2S

• More later…


Conditions

What are you trying to determine? What is the goal of the

simulation?

Are you simulating a test or usage?

What do you know about the conditions?

– Which variables are controlled?

– What are the unknowns you are trying to measure?

Fluid (momentum) Heat (thermal energy)

Flow “driver”

• Where does air enter & exit?• What causes the air to flow?

• Fan (on boundary or internal)• Pressure differential• Supplied flow rate• Buoyancy

• Where does heat enter & exit the system?

• What is dissipating heat?• What are the thermal paths through

the model?

Inlet(s)• Stagnation inlet• (Positive) Velocity, mass flow, or pressure

• Ambient temperature• Heat generation (volumetric, surface)

Outlet(s)• Pressure outlet• (Negative) Velocity, mass flow, or

pressure

• Ambient temperature• Convection on exterior surfaces (h = 5

– 10 W/m2-K) – no exterior air


Conditions

Radiation: Base setup

– Continua: Activate radiation for

air continua & any transparent

solids.

– Regions > Boundaries

• Air: Set ε on the interface boundaries (ρ is computed)

• Transparent solids: Set ε on the

interface boundaries that interface with the air (ρ is

computed)

– Interfaces

• Set τ values only for interfaces

between air & transparent solids.

Radiation exchange with the

environment (exterior)

– Set conditions on exterior air boundary (ε & τ, ρ is computed)

– Exterior environment (“outside”

the computational domain) is

treated as a blackbody

• Radiation temperature is set in

the continua (under Models >

Thermal Radiation > Thermal

Environments)

– Solar radiation

• Activate “Solar Loads” in

continua (with radiation already

activated)

• Set factors (e.g. date, time,

location, orientation) in Models >

Solar Loads for the continua


Conditions Geometry Mesh Materials Conditions Solution Results

No radiation

ε = 0.3

(Tmax -12%)

ε = 0.8

(Tmax -26%)

Conditions

How do we know the heat dissipation to specify for a component?

Apply the heat dissipation to the appropriate region

– Activate the Energy Source Option in Physics Conditions

– Assign the Heat Source in Physics Values

– Value assigned applies to the entire region (even if it consists of multiple

parts).


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?

Heat

Conditions Geometry Mesh Materials Conditions Solution Results

Fan Model

No CAD needed Fewer cells Short runtime Less accurate

Steady (MRF)

CAD needed More cells Moderate runtime More accurate

Unsteady

CAD needed More cells Long runtime Most accurate

Fan Curve

dP

Q

Fan Simulation Options

Conditions

Fan models in STAR-CCM+

(immersed fans)

– Volume momentum source

– Interface momentum source

Recommendation: Interface

– Geometry with faces where the

interface is desired.

– Set interface Type = Fan

Interface.

– Input the desired fan curve

Boundary fans (inlet and/or

outlet) also available

STAR-CCM+ iterates to find the

flow rate / pressure drop

combination at the intersection

of the fan curve & the system

resistance curve.

Blowers are modeled as a

special interface type

– Centrifugal fan

– Impeller fan


Solution



Solution

Air continuum

– Models

• Segregated Fluid Temperature

• Ideal gas or Boussinesq

recommended for natural

convection

• Gravity (activated)

– Reference values

• Gravity (vector direction) for

natural convection

• Reference altitude

• Reference density = density at

Tambient (based on ideal gas)

– Initial conditions

• Pressure = 0 (gage)

• Static temperature = Tambient

• Velocity = 0

Continua settings

– Physics models



Solution settings

– Under-relaxation

– Convergence

Solids continua

– Models

• Segregated Solid Energy


• None


• Static temperature = Tambient


Solution Geometry Mesh Materials Conditions Solution Results

Fluid energy: Change to 0.99 (default = 0.9)

Solid energy: Change to 0.9999 (default =

0.99)

Effect:

– Convergence in fewer iterations (~5X fewer)

– Stable, even with radiation

Results



Results Geometry Mesh Materials Conditions Solution Results

Results Geometry Mesh Materials Conditions Solution Results

STAR-View+

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Thank you!

Ruben Bons / [email protected] / +1-760-536-8122

mailto:[email protected]

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