Discrepancy in results based on air flow direction

Numerical methods and mathematical models of Elmer
Post Reply
tibich72
Posts: 63
Joined: 07 Dec 2009, 05:16

Discrepancy in results based on air flow direction

Post by tibich72 »

I'm trying to simulate some 3D models of electronic components. One of the test cases is to have a small PCB board with a single component on top; the component should running pretty hot, at about 150C (based on measurements on the actual component). The simulation I'm using is creating the PCB and the component, and then immersing them into a large box of air (about 1.3 meters on X/Y/Z dimensions). There is a very slow airflow through the box (3cm/s) -- from what I've learned, this is the flow of air in a sealed room. Basically, the experiment tries to simulate the system in an closed room.

If the airflow is from left to right (i.e. the x- boundary of the box is open and the flow is 3cm/s, the x+ boundary is also open, but no air speed is set, and all the other boundaries are walls), then the results are pretty close to what's been measured. However, if the flow is from top to bottom, (i.e. Z+ boundary open and constant flow, Z- open, all others are walls), the temperature suddenly jumps by about 85C. This is way too much, considering that the airflow is very small.

The basic structure of the .sif file is inherited from someone that has left our company. I've tried to make some small changes here and there, but the results were the same. So, I have two questions:
- By any chance, is this jump in temperature to be expected? My instinct says no, but I'm not an expert in thermal simulations.
- I'm attaching the .sif file, maybe there's something there glaringly wrong. This .sif file is for the "left to right" simulation; the structure for the "top to bottom" simulation is exactly the same, except that the boundary conditions at the end refer to different physical surfaces.

Any suggestion is appreciated.
Tibi

Code: Select all

Header
  CHECK KEYWORDS Warn
  Mesh DB "." "demo"
  Include Path ""
  Results Directory ""
End

Simulation
  Max Output Level = 40
  Coordinate System = Cartesian
  Coordinate Mapping(3) = 1 2 3
  Simulation Type = Steady State
  Steady State Max Iterations = 1
  Output Intervals = 1
  Timestepping Method = BDF
  BDF Order = 1
  Solver Input File = demo.sif
End

Constants
  Gravity(4) = 0 -1 0 9.82
  Stefan Boltzmann = 5.67E-08
  Permittivity of Vacuum = 8.8542E-12
  Boltzmann Constant = 1.3807E-23
  Unit Charge = 1.602E-19
End

Solver 1
  Equation = Navier-Stokes
  Procedure = "FlowSolve" "FlowSolver"
  Variable = Flow Solution[Velocity:3 Pressure:1]
  Exec Solver = Always
  Stabilize = True
  Bubbles = False
  Lumped Mass Matrix = False
  Optimize Bandwidth = True
  Steady State Convergence Tolerance = 0.05
  Nonlinear System Convergence Tolerance = 0.05
  Nonlinear System Max Iterations = 5
  Nonlinear System Newton After Iterations = 10
  Nonlinear System Newton After Tolerance = 0.001
  Nonlinear System Relaxation Factor = 1
  Linear System Solver = Iterative
  Linear System Iterative Method = BiCGStabL
  BiCGStabL Polynomial Degree =  Integer 4
  Linear System Max Iterations = 1000
  Linear System Convergence Tolerance = 1E-06
  Linear System Preconditioning = ILUT
  Linear System ILUT Tolerance = 0.1
  Linear System Abort Not Converged = True
  Linear System Residual Output = 1
  Linear System Precondition Recompute = 1
End

Solver 4
  Equation = KEpsilon
  Procedure = "KESolver" "KESolver"
  Variable = KE[Kinetic Energy:1 Kinetic Dissipation:1]
  Exec Solver = Never
  Bubbles = True
  Lumped Mass Matrix = False
  Optimize Bandwidth = True
  Steady State Convergence Tolerance = 0.05
  Nonlinear System Convergence Tolerance = 0.05
  Nonlinear System Max Iterations = 20
  Nonlinear System Newton After Iterations = 30
  Nonlinear System Newton After Tolerance = 0.001
  Nonlinear System Relaxation Factor = 0.25
  Linear System Solver = Iterative
  Linear System Iterative Method = BiCGStabL
  BiCGStabL Polynomial Degree =  Integer 4
  Linear System Max Iterations = 1000
  Linear System Convergence Tolerance = 1E-08
  Linear System Preconditioning = ILUT
  Linear System ILUT Tolerance = 0.01
  Linear System Abort Not Converged = True
  Linear System Residual Output = 1
  Linear System Precondition Recompute = 1
End

Solver 2
  Equation = Heat Equation
  Procedure = "HeatSolve" "HeatSolver"
  Variable = -dofs 1 Temperature
  Exec Solver = Always
  Stabilize = True
  Bubbles = False
  Lumped Mass Matrix = False
  Optimize Bandwidth = True
  Steady State Convergence Tolerance = 0.05
  Nonlinear System Convergence Tolerance = 0.01
  Nonlinear System Max Iterations = 1
  Nonlinear System Newton After Iterations = 10
  Nonlinear System Newton After Tolerance = 0.001
  Nonlinear System Relaxation Factor = 1
  Linear System Solver = Iterative
  Linear System Iterative Method = BiCGStab
  BiCGStabL Polynomial Degree = Integer 2
  Linear System Max Iterations = 1000
  Linear System Convergence Tolerance = 1E-08
  Linear System Preconditioning = ILUT
  Linear System ILUT Tolerance = 0.01
  Linear System Abort Not Converged = True
  Linear System Residual Output = 1
  Linear System Precondition Recompute = 1
End

Solver 3
  Exec Solver = After Simulation
  Procedure = "ResultOutputSolve" "ResultOutputSolver"
  Output File Name = "..\output"
  Output Format = "vtu"
End

Equation 1
  Name = "Solids"
  Active Solvers(1) = 2
  Convection = None
End

Equation 2
  Name = "Air"
  Convection = Computed
  NS Convect = False
  Active Solvers(3) = 1 4 2
End

Material 1
  Name = "boardFront_top"
  Heat Capacity = 1210
  Density = 1970
  Heat Conductivity = 0.525
End

Material 2
  Name = "boardFront_bot"
  Heat Capacity = 1210
  Density = 1970
  Heat Conductivity (3) = 0.525 0.525 0.525
End

Material 3
  Name = "Copper"
  Heat Capacity = 385
  Density = 8960
  Heat Conductivity = 401
End

Material 4
  Name = "MoldCompound"
  Heat Capacity = 800
  Density = 3500
  Heat Conductivity = 0.7
End

Material 5
  Name = "Q1_body_bot_Composite"
  Heat Capacity = 779.25
  Density = 3773
  Heat Conductivity = 20.715
End

Material 6
  Name = "Silicon"
  Heat Capacity = 711
  Density = 2330
  Heat Conductivity = 124
End

Material 7
  Name = "Air"
  Heat Capacity = 1005
  Density = 1.1831
  Heat Conductivity = 0.02605
  Compressibility Model = Incompressible
  Viscosity = 1.54075E-05
  Heat Expansion Coefficient = 0
  Specific Heat Ratio = 1.4
  Reference Temperature = 25
  KE Cmu = 0.09
  KE C1 = 1.44
  KE C2 = 1.92
  KE SigmaK = 1
  KE SigmaE = 1.3
  KE Clip = 1E-06
  Viscosity Model = KE
  Heat Conductivity Model = KE
End

Body Force 2
  Name = "Source boardFront_top"
  Heat Source = 0.00362895868118333
End

Body Force 3
  Name = "Source Q1_die"
  Heat Source = 718569.88955897
End

Initial Condition 1
  Name = "Initial Condition"
  Velocity 2 = -0.03302
  Temperature = 25
  Kinetic Energy = 2.725801E-06
  Kinetic Dissipation = 1.88309811478612E-08
End

Body Force 1
  Name = "Buoyancy"
  Boussinesq = True
End

Body 2
  Name = "boardFront_top"
  Equation = 1
  Material = 1
  Target Bodies(1) = 2
  Body Force = 2
End

Body 3
  Name = "boardFront_bot"
  Equation = 1
  Material = 2
  Target Bodies(1) = 3
End

Body 4
  Name = "Q1_thPad"
  Equation = 1
  Material = 3
  Target Bodies(1) = 4
End

Body 5
  Name = "Q1_body_top"
  Equation = 1
  Material = 4
  Target Bodies(1) = 5
End

Body 6
  Name = "Q1_body_bot"
  Equation = 1
  Material = 5
  Target Bodies(1) = 6
End

Body 7
  Name = "Q1_die"
  Equation = 1
  Material = 6
  Target Bodies(1) = 7
  Body Force = 3
End

Body 8
  Name = "Pad1_wholeBody"
  Equation = 1
  Material = 3
  Target Bodies(1) = 8
End

Body 1
  Name = "airbox_full"
  Equation = 2
  Body Force = 1
  Initial Condition = 1
  Material = 7
  Target Bodies(1) = 1
End

Boundary Condition 1
  Target Boundaries(1) = 1
  Name = "Interior Wall"
  Normal-Tangential Velocity = True
  Velocity 1 = 0
End

Boundary Condition 2
  Target Boundaries(1) = 2
  Name = "Airbox Boundary 2"
  Normal-Tangential Velocity = True
  Velocity 1 = 0
  Wall Law = True
  Boundary Layer Thickness = 0.0196345140758292
  Surface Roughness = 9
  Temperature = 25
End

Boundary Condition 3
  Target Boundaries(1) = 7
  Name = "Airbox Boundary 3"
  Normal-Tangential Velocity = True
  Velocity 1 = 0
  Wall Law = True
  Boundary Layer Thickness = 0.0196345140758292
  Surface Roughness = 9
  Temperature = 25
End

Boundary Condition 4
  Target Boundaries(1) = 3
  Name = "Airbox Boundary 4"
  Normal-Tangential Velocity = True
  Temperature = 25
End

Boundary Condition 5
  Target Boundaries(1) = 5
  Name = "Airbox Boundary 5"
  Normal-Tangential Velocity = True
  Velocity 1 = -0.03302
  Velocity 2 = 0
  Velocity 3 = 0
  Temperature = 25
End

Boundary Condition 6
  Target Boundaries(1) = 6
  Name = "Airbox Boundary 6"
  Normal-Tangential Velocity = True
  Velocity 1 = 0
  Wall Law = True
  Boundary Layer Thickness = 0.0196345140758292
  Surface Roughness = 9
  Temperature = 25
End

Boundary Condition 7
  Target Boundaries(1) = 4
  Name = "Airbox Boundary 7"
  Normal-Tangential Velocity = True
  Velocity 1 = 0
  Wall Law = True
  Boundary Layer Thickness = 0.0196345140758292
  Surface Roughness = 9
  Temperature = 25
End

Post Reply