Best Practices: Electronics Cooling Ruben Bons - CD-adapco
Best Practices Outline Geometry Mesh Materials 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 Results Temperature Velocity Field functions Solution Physics models Reference values / Initial conditions Segregated or Coupled Under-relaxation Convergence Conditions Physics: Flow & heat transfer Environment Inlet(s) Outlet(s) Thermal (including 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 asassembled 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 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 Geometry Mesh Materials Conditions Solution Results 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 Fluid prism layers Nonconformal fluid-solid interface Conformal solid-solid interface
Mesh Geometry Mesh Materials Conditions Solution Results
Materials Geometry Mesh Materials Results Solution Conditions
Materials Geometry Mesh Materials Conditions Solution Results 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 Geometry Mesh Materials Conditions Solution Results 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
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) Forced convection: Generally turbulent Internal: Transition @ Re ~ 2500 10,000 External: Transition @ Re ~ 500,000 Natural convection: Generally laminar Turbulent if Ra h > 10 9 (vertical flat plate) Ra h = gβ T w T h 3 υα Assume T w = 85 o C T = 50 o C Properties @ 70 o C h critical = 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 ally+ 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
Materials Geometry Mesh Materials Conditions Solution Results 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 ϴ jb & ϴ jc. Resistivity ρ = t / k = R t *A contact. Device: PCBs Equivalent thermal properties Orthotropic equivalent properties computed from geometric details (easiest in a spreadsheet) Commonly k in-plane ~ 10 W/m-K ~ 20*k through-thickness. Detailed trace modeling Computationally costly 2D or 3D traces
Materials Geometry Mesh Materials Conditions Solution Results 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 V 2 (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 Geometry Mesh Materials Conditions Solution Results Device: Thermoelectric devices Extract parameters from datasheet values (T c, Q max, T max, R electrical ). 3-part assembly (don t mesh the middle part) Field functions to iteratively compute & apply Q c (T c, T h ) & Q h (T c, T h ). Device: Contact resistance Every solid-solid interface physically has contact resistance. Default in STAR-CCM+ is R c = 0. To change, assign resistivity (ρ c ) to the interface (in Physics Values) ρ c = R c *A contact.
Conditions Geometry Mesh Materials Results Solution Conditions
Conditions Geometry Mesh Materials Conditions Solution Results 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 Geometry Mesh Materials Conditions Solution Results 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 Inlet(s) Outlet(s) Where does air enter & exit? What causes the air to flow? Fan (on boundary or internal) Pressure differential Supplied flow rate Buoyancy Stagnation inlet (Positive) Velocity, mass flow, or pressure Pressure outlet (Negative) Velocity, mass flow, or pressure Where does heat enter & exit the system? What is dissipating heat? What are the thermal paths through the model? Ambient temperature Heat generation (volumetric, surface) Ambient temperature Convection on exterior surfaces (h = 5 10 W/m 2 -K) no exterior air
Conditions Geometry Mesh Materials Conditions Solution Results 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 (T max -12%) ε = 0.8 (T max -26%)
Conditions Geometry Mesh Materials Conditions Solution Results How do we know the heat dissipation to specify for a component? Electrical power supplied Wall power? Max power (power budget)? Measured power? Duty-cycled? What is the efficiency? Component (e.g. IC, IGBT, MOSFET, LED, ) Electrical power delivered Heat RF energy, visible light 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).
Conditions Geometry Mesh Materials Conditions Solution Results Fan Curve dp Q Fan Model Steady (MRF) Unsteady Fan Simulation Options No CAD needed Fewer cells Short runtime Less accurate CAD needed More cells Moderate runtime More accurate CAD needed More cells Long runtime Most accurate
Conditions Geometry Mesh Materials Conditions Solution Results 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 Geometry Mesh Materials Results Solution Conditions
Solution Geometry Mesh Materials Conditions Solution Results 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 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
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 Results Solution Conditions
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