Best Practices: Electronics Cooling Ruben Bons - CD-adapco
Best Practices Outline Geometry Mesh Materials Conditions Solution Design exploration / Optimization Results
Best Practices Outline Geometry Mesh Materials • Solids • Trimmed / Polyhedral • Solids • Simplification • Conformal / Non-conformal • Air (fluid) • Preparation • Thin solids • “Devices” • Air • Prism layers in air • Chips • Forced convection • Mesh operations • PCBs • Natural convection • Porous media, perf plates • Heat pipes • Thermoelectric devices Results Solution Conditions • Temperature • Physics models • Physics: Flow & heat transfer • Velocity • Reference values / Initial • Environment conditions • Field functions • Inlet(s) • Segregated or Coupled • Outlet(s) • Under-relaxation • Thermal (including • Convergence radiation) • Heat sources • Fans & blowers
Geometry Geometry Mesh Materials Results Solution Conditions
Geometry Geometry Mesh Materials Conditions Solution Results 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 Geometry Mesh Materials Conditions Solution Results
Geometry Geometry Mesh Materials Conditions Solution Results 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 Geometry Mesh Materials Conditions Solution Results
Geometry Geometry Mesh Materials Conditions Solution Results
Geometry Geometry Mesh Materials Conditions Solution Results 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 Geometry Mesh Materials Conditions Solution Results 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 Geometry Mesh Materials Conditions Solution Results 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 Geometry Mesh Materials Conditions Solution Results
Mesh Geometry Mesh Materials Results Solution Conditions
Mesh Geometry Mesh Materials Conditions Solution Results Cell topology Conformal vs Non-conformal – Polyhedral – Conformal possible only with polyhedral cells • Conformal – Non-conformal an option with • Non-conformal – Trimmed hexahedral polyhedral, trimmed hexahedral – Accuracy • Non-conformal • Fully conformal is best (no Approaches interpolation at interfaces) – Parts-based • Non-conformal with similar – Regions-based surface mesh sizes: Tests show very small (<0.5%) difference Specialty options than fully-conformal results. – Prism-layer mesher • Non-conformal with disparate – Thin mesher mesh sizes: Accuracy degrades – Extruded mesher as surface size variance increases Basic setting: Mesh sizing – Meshing speed • Non-conformal is fastest • Serial & parallel option for both • Concurrent option for non-conf
Mesh Geometry Mesh Materials Conditions Solution Results Parts-based vs Regions-based Methodology – Personal preference – Surface mesh all geometry in 1- step (e.g. 1 PBM operation) – Parts-based has advantages for • Base size: 2 - 5% of bounding complex mesh sequences box diagonal – New thin mesher in PBM • Min surface size: 0.01 – 0.001% Thin mesher (for solids) of base – 1-2 layers for conducting-only • Curvature: 16 points / circle solids (no heat dissipation) • Proximity: 0.25 points in gap – 3+ layers for thin solids that Produces conformal surface mesh dissipate heat – 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 Fluid prism layers Non- Conformal conformal solid-solid fluid-solid interface interface
Mesh Geometry Mesh Materials Conditions Solution Results
Materials Geometry Mesh Materials Results Solution Conditions
Materials Geometry Mesh Materials Conditions Solution Results Solids Solids – Isotropic properties by default Air (fluid) • Thermal conductivity can be “Devices” anisotropic – set Method of – Chips Thermal Conductivity in continua – PCBs • Set values in appropriate region – Porous media, perforated plates – No temperature variation by default – Heat pipes • Change in the continua – Thermoelectric devices • Specific heat: Polynomial in T • Thermal conductivity: Most material specifications Polynomial in T, table(T), field are detailed in the function corresponding continua – Pick from the default library – Customize, save to library Some require details in the corresponding region
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 Geometry Mesh Materials Conditions Solution Results Fluid Properties: Air – Most commonly air – Density – Liquid cooling with water, • For buoyancy (natural convection), density must vary ethylene-glycol solution, etc. with temperature (+ gravity) Properties & appropriate • Ambient pressure strongly physics specified in the affects air density (e.g. at continua altitude) – Viscosity can significantly vary – Properties with temperature • Density • Viscosity Properties: Water • Specific heat – Density • Thermal conductivity • Variation with temperature – Physics important only with natural convection (rare cases) • Laminar or turbulent • Little variation with pressure • Turbulence model – Viscosity variation with temperature can be significant
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 Geometry Mesh Materials Conditions Solution Results Laminar or Turbulent (for air) Turbulence model – Forced convection: Generally – Many options in STAR-CCM+, turbulent consult the help for details • Internal: Transition @ Re ~ • k- ε 2500 – 10,000 • k- ω • External: Transition @ Re ~ • Reynolds stress 500,000 • Spalart-Allmaras – Natural convection: Generally • DES laminar • LES • Turbulent if Ra h > 10 9 (vertical – Realizable k- ε with two-layer all- flat plate) y+ wall treatment seems to work 𝑺𝒃 𝒊 = 𝒉𝜸 𝑼 𝒙 − 𝑼 ∞ 𝒊 𝟒 well for a wide range of models 𝝋𝜷 • Forced convection • Assume • Natural convection – T w = 85 o C – Compared a laminar run with a – T ∞ = 50 o C k- ε run • Properties @ 70 o C – Essentially identical flow & • h critical = 0.83 m thermal results
Materials Geometry Mesh Materials Conditions Solution Results Device: Chips Device: PCBs – Solid (isotropic) material – Equivalent thermal properties – 2-resistor • Orthotropic equivalent properties computed from geometric details • High conductivity solid (e.g. Cu) (easiest in a spreadsheet) • Separate boundaries (in the • Commonly k in-plane ~ 10 W/m-K ~ region) for top & bottom surfaces 20*k through-thickness . • Assign resistivity to interfaces to – Detailed trace modeling achieve ϴ jb & ϴ jc . • Computationally costly • Resistivity ρ = t / k = R t *A contact . • 2D or 3D traces
Materials Geometry Mesh Materials Conditions Solution Results Device: Porous media Device: Heat pipes – Fluid region, Type = Porous – Rarely are the full physics Region (evaporation, condensation, surface tension, etc.) modeled. – Set Inertial &/or Viscous – Typically 3-part assembly resistance values under Region Physics Values • Pipe wall (k = material • Viscous: Δ P α V (e.g. fibrous conductivity) • Wick (k = 30-40 W/m-K) filter) • Inertial: Δ P α V 2 (e.g. perf plate) • Vapor space (k > 10,000 W/m-K)
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