Best Practices: Electronics Cooling Ruben Bons - CD-adapco Best - - PowerPoint PPT Presentation

Best Practices: Electronics Cooling

Ruben Bons - CD-adapco

Best Practices Outline

Geometry Materials Mesh 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 Conditions Solution Results

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 Mesh Materials Conditions Solution Results

Geometry

Geometry Mesh Materials Conditions Solution Results

Geometry

Geometry Mesh Materials Conditions Solution Results

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 Mesh Materials Conditions Solution Results

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 Mesh Materials Conditions Solution Results

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)

Geometry Mesh Materials Conditions Solution Results

Geometry

Geometry Mesh Materials Conditions Solution Results

Mesh

Geometry Mesh Materials Conditions Solution Results

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

Geometry Mesh Materials Conditions Solution Results

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%
• f 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)

Geometry Mesh Materials Conditions Solution Results

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

Geometry Mesh Materials Conditions Solution Results

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

Geometry Mesh Materials Conditions Solution Results

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

Geometry Mesh Materials Conditions Solution Results

Materials

Geometry Mesh Materials Conditions Solution Results

Common temperature range in electronics

Materials

Geometry Mesh Materials Conditions Solution Results

Materials

Geometry Mesh Materials Conditions Solution Results

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

Geometry Mesh Materials Conditions Solution Results

𝑺𝒃𝒊 = 𝒉𝜸 𝑼𝒙 − 𝑼∞ 𝒊𝟒 𝝋𝜷

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

Geometry Mesh Materials Conditions Solution Results

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)

Geometry Mesh Materials Conditions Solution Results

Materials

Device: Thermoelectric devices

– Extract parameters from datasheet values (T

c, Qmax, Tmax,

Relectrical). – 3-part assembly (don’t mesh the middle part) – Field functions to iteratively compute & apply Qc(T

c, Th) & Qh

(T

c, 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.

Geometry Mesh Materials Conditions Solution Results

Conditions

Geometry Mesh Materials Conditions Solution Results

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…

Geometry Mesh Materials Conditions Solution Results

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

Geometry Mesh Materials Conditions Solution Results

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

Geometry Mesh Materials Conditions Solution Results

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).

Geometry Mesh Materials Conditions Solution Results

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

• utlet) also available

STAR-CCM+ iterates to find the flow rate / pressure drop combination at the intersection

• f the fan curve & the system

resistance curve. Blowers are modeled as a special interface type

– Centrifugal fan – Impeller fan

Geometry Mesh Materials Conditions Solution Results

Solution

Geometry Mesh Materials Conditions Solution Results

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

T

ambient (based on ideal gas)

– Initial conditions

• Pressure = 0 (gage)
• Static temperature = T

ambient

• Velocity = 0

Continua settings

– Physics models – Reference values – Initial conditions

Solution settings

– Under-relaxation – Convergence

Solids continua

– Models

• Segregated Solid Energy

– Reference values

• None

– Initial conditions

• Static temperature = T

ambient

Geometry Mesh Materials Conditions Solution Results

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

Geometry Mesh Materials Conditions Solution Results

Results

Geometry Mesh Materials Conditions Solution Results

Results

Geometry Mesh Materials Conditions Solution Results

STAR-View+

Thank you!

Ruben Bons / ruben.bons@cd-adapco.com / +1-760-536-8122