conjugate heat transfer simulations with cfd€¦ · the cfd approach • slightly simplified...
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Conjugate Heat Transfer
Simulations with CFD
Edmond Lam (BE-RF-MK)
MSc student at ETH Zurich
Trainee at CERN, Sep 2017 – Feb 2018
CLIC Project Meeting #28
08.12.2017
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My Work at CERN
• Two-beam module
• Analytical modelling for heat from the module to surrounding soil
through the tunnel wall
• Finite-element analysis (FEA) for structural stress-strain simulations
• Computational fluid dynamics (CFD) simulations for heat dissipation
from the module to air and cooling water*
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Two-beam Module
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Goals for simulation/ modelling:
- Heat dissipation from the module to air
- Heat dissipation from the module to the
cooling water
- Heat dissipation from the module to the
outside of the tunnel wall
→ appropriate cooling solution for uniformity
and stability of temperature over the entire
length
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Super-accelerating Structure
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- Take a part of the module as the first case
- Various scenarios tested to find suitable
models for fluid flow and radiation
- Move to the full module after gaining
insights
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Solution Approaches
• Experiment (Alex, Markus, Vishnu)• Results for only specific variables (e.g. temperature)
• Results for only preselected points (not a field)
• Essential for solution validation, given low measurement errors
• Analytical Model (Alex, Markus)• Highly simplified geometry
• Modelled (coefficients – involves assumptions)
• FEA (Antti)• Actual geometry
• Simulated: Conduction within the module
• Modelled (represented by coefficients – involves assumptions): • Convection to air and water; Radiation
• CPU time ~ minutes
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The CFD Approach
• Slightly simplified geometry (remove small geometrical features)
• Fully-coupled simulation:• Conduction within the module
• Convection to air and water (by simulating air and water flow profiles)
• Radiation
• No manual input of coefficients required – only required inputs are material properties
• CPU time ~ hours
• But significant time and labour in pre- and post-processing• Geometry repair and simplification, meshing, mesh independence studies
• ~ days
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tuning of coefficients for convection
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Experiment
CFD
Temperatures at pre-selected points
Different variable fields for every fluid/ solid domain
Temperature field for solid and heat dissipation values
validation
external models
for assumptions
(e.g. tunnel wall)
Combined Approach
Obtained ResultsApproach
FEA
Analytical M. Temp. at specific points and heat dissipation values
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The CFD Approach
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Simplification and Repair
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- Most of the small features do not matter for
fluid flows (screws, fillets, etc.), but increase
the number of elements dramatically
- Gaps left for manufacture and assembly have
to be located and removed, or:
- Large number of elements in the gaps
- Long meshing time/ errors
- Low accuracy (no thermal contact when
there should be)
- High element skewness → Low mesh
quality
10 μm of gap (not visible in this
screenshot) in the CAD geometry
which needs to be removed
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Problem Definition
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- Only one super-accelerating structure
- In “free” space
- Domain boundary 1 m from the structure
- Artificial pipes
Materials:
- Copper for the structure and pipes
- Air (variable ρ, incompressible ideal gas law)
- Water (constant ρ)
1m
1m
1m
1m
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Problem Definition
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- Heaters generate a total heat of 780 W
- Location of heaters correspond to that in the
experiment
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Meshing
08.12.2017 CHT Simulations with CFD 12
Mesher: ANSYS ICEM CFD v17.2
Type of elements: tetrahedron
Number of elements:
43 million for the finest mesh
Coarser meshes were generated
to determine mesh independence
Minimum size of elements: 1 mm
a cross section of the mesh
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Meshing
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Zoomed in
Yellow: Heating element
Sky blue: Structure
Magenta: Water
Purple: Air
These fluid/ solid zones were all
coupled in the solution process
and solved simultaneously.
a cross section of the mesh
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Meshing
08.12.2017 CHT Simulations with CFD 15
a cross section of the mesh
Cross section where the pipe
enters and leaves the structure.
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Meshing
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Surface mesh(not the same colours as previous slides)
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Boundary Conditions
- Specified velocity at air inlet
- Specified mass/ volume flow rate at
water inlet
Domain boundary:
- No shear stress on air
- Transparent to radiation
- External radiation at ambient
temperature → computes radiation
heat exchangeair
0.4 m/s
21 – 35 °C
water
1.3 L/min
27 °C
radiation exchange
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Solving
• Solver: ANSYS Fluent v17.2 on CERN cluster (32c, 256GB RAM)
• Time: steady-state
• Turbulence model: SST k-ω
• Radiation model: Discrete Ordinates
• Iterations required for convergence: ~ 500
• CPU time (for the finest mesh): ~ 6 hours
• Convergence criteria• Residuals
• Stability of monitored values (heat convection to air and water, increase in water temperature from inlet to outlet, etc.)
• Mass conservation
• Energy conservation
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Heat Dissipation Results
-50
-30
-10
10
30
50
70
90
110
130
150
15 20 25 30 35 40 45
Heat
(W)
“Ambient” or Air Inlet Temperature (°C)
Heat Dissipated by Convection to Air + Radiation
CFD FEA Experiment Linear (CFD) Linear (FEA)
- Convection to air and radiation to
external surfaces grouped to one
value
- FEA results assuming an air
convective heat transfer coefficient
of 6 W/m2 K (adjusted), and a water
convective heat transfer coefficient
~ 4000 W/m2 K
- Deviation from experiment
measurements likely from
overestimation of emissivity of
copper
Plot and FEA values by Antti; Experiment values by Alex and Vishnu
Total heat generated: 780 W
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08.12.2017 CHT Simulations with CFD 22
Heat Dissipation Results
600
650
700
750
800
850
15 20 25 30 35 40 45
Heat
(W)
“Ambient” or Air Inlet Temperature (°C)
Heat Dissipated by Convection to Water
CFD FEA Experiment Linear (CFD) Linear (FEA)
Plot and FEA values by Antti; Experiment values by Alex and Vishnu
Total heat generated: 780 W
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Visualisations
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Temperature field at a cross section
Structure temperature at ~ 36 °C
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Visualisations
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Temperature distribution at the
surface of the structure
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Visualisations
08.12.2017 CHT Simulations with CFD 26
Temperature of water in the pipe
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Visualisations
08.12.2017 CHT Simulations with CFD 27
Velocity field of air at a cross
section around the structure
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Visualisations
08.12.2017 CHT Simulations with CFD 28
Velocity field of air as a vector plot
at a cross section around the
structure
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Comments
• First case• Proof-of-concept for integrating CFD
• General workflow established after three months• Integrating the advantages from experiment ↔ analytical model ↔ FEA ↔ CFD
• Providing good estimation for the coefficients used in FEA and analytical model
• Not fully developed flow• Constant velocity of air at inlet → case only when the air first hits the structure
• Fully developed air flow not possible without considering tunnel wall
• Add the tunnel wall in the next step
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Now: moving to the Two-beam Module
In progress:
- Entire main beam part of the module
- Addition of tunnel wall
Analytical model in development for assumptions that can be made
for the tunnel wall in steady-state CFD or FEA simulations
Current Progress
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Challenges:
- Problem-specific techniques required in
meshing
- Solving for the full module may take a
full day instead of several hours
Completion of a CFD simulation for the
main beam part aimed at around Feb 2018
(partially simplified)
Current Progress
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