Conjugate Heat Transfer Simulations with CFD€¦ · The CFD Approach • Slightly simplified...

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

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*

08.12.2017 CHT Simulations with CFD 2

Two-beam Module


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

Super-accelerating Structure


- 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

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


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


tuning of coefficients for convection


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

The CFD Approach

Simplification and Repair


- 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

Problem Definition


- 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

Problem Definition


- Heaters generate a total heat of 780 W

- Location of heaters correspond to that in the

experiment

Meshing


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

Meshing


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.


Meshing



Cross section where the pipe

enters and leaves the structure.

Meshing


Surface mesh(not the same colours as previous slides)


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

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



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


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

Visualisations


Temperature field at a cross section

Structure temperature at ~ 36 °C

Visualisations


Temperature distribution at the

surface of the structure

Visualisations


Temperature of water in the pipe

Visualisations


Velocity field of air at a cross

section around the structure

Visualisations


Velocity field of air as a vector plot

at a cross section around the

structure

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



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


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

Conjugate Heat Transfer Simulations with CFD€¦ · The CFD Approach • Slightly simplified...

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