Use of CFD to investigate microchannel two-phase flows ...

Use of CFD to investigate microchannel

two-phase flows with phase change for

electronic cooling application

Laboratoire de Transfert de Chaleur et de Masse (LTCM),

Ecole Polytechnique Fédérale de Lausanne (EPFL),

EPFL-STI-IGM-LTCM, Station 9, CH-1015 Lausanne

Dr. Mirco Magnini

[email protected]

ANSYS Conference & 11ème Forum CADFEM, 10 septembre 2014, Lausanne, EPFL

Outline

2/18

1) Presentation:

The Laboratory of Heat and Mass Transfer

On chip cooling technology

Introduction to two-phase flows

2) CFD simulations of two-phase flows: why?

3) A case study: developing the solver (ANSYS Fluent) to

simulate microchannel slug flow boiling

Numerical model development

Validation

Results

4) Conclusions

Use of CFD to investigate microchannel two-phase flows – Dr M. Magnini

Laboratory of Heat and Mass Transfer (LTCM)

• Ecole Polytechnique Fédérale de Lausanne, Switzerland.

• Director: Prof. John R. Thome.

• Staff: one 1st assistant, 6 post-doc researchers, 7 PhD students.

Main fields of research: two-phase flows in narrow channels (8.8 mm down to

85 mm) for electronic cooling applications

from fundamental physics …

(flow visualization, CFD)

… to the whole system!

(1D flow models, experiments)

http://ltcm.epfl.ch

3/18 Use of CFD to investigate microchannel two-phase flows – Dr M. Magnini

Two-phase flows in microchannels: applications

Microchannel evaporator with 20 channels 0.45x4.0 mm

Microchannel evaporator sealed on the chip die

Cooling of high power density electronic devices (modern CPUs):

Advantages of two-phase flow cooling:

• Cooling capability: more than 300 W/cm2 (traditional air cooling ≈ 100 W/cm2 )

• Accurate control of the chip temperature (Tmax≈85 °C)

• Positive feedback to hot-spots


Two-phase flow

When liquid at the saturation temperature is heated,

evaporation begins.

Flow patterns:

Bubbly flow

Slug flow

Annular flow


CFD simulations of two-phase flows

Interests:

• Have access to fluid and thermal dynamics where experiments can not

• Generate a numerical database useful to test/develop prediction methods for the

most relevant thermal-hydraulic flow parameters (pressure drop, heat transfer,…)

Methodology:

• ANSYS Fluent version 14.5

• Volume Of Fluid method to track the vapor-liquid interface

• User-Defined Functions to implement peculiar two-phase flow effects

• Post-processing with Paraview (visualization) and Matlab (data reduction)


Case study

Development of ANSYS Fluent standard package to simulate microchannel slug flow

boiling and update the available heat transfer performance prediction methods.

Numerical model development with UDF

Numerical model validation with experimental/analytical solutions

Numerical set-up for microchannel slug flow boiling simulation

Results and comparison with existing models

Development of a new heat transfer model


Numerical model

The Volume Of Fluid method: cell volume fraction

occupied by a primary phase 0

0 0 0

0 0

1

111

1 1 1 1

0.520.14

0.860.41

0.610.03

0.72

Example: volume fraction field

vapor

liquid

Volume fraction field across a liquid-vapor interface

vapor

liquid

Volume fraction

1,0

0

1

α

if primary phase

if secondary phase

if interface cell

energy source term

TTct

Tc

pt

t

p

p

T

m

u

guuuuu

u

u

volume fraction source term

surface tension force

mass source term

Flow equations: (incompressible, newtonian fluid)

Source terms: User-Defined Functions


Validation (1)

Validation of surface tension model: simulations of adiabatic flow of

elongated air bubbles in water and glycerol liquid flow within a microchannel

(D=0.5 mm), and comparison with in-house experimental results.

Flow domain and boundary conditions

Horizontal circular channel (D=0.5 mm)

2D axisymmetric formulation (no gravity force)

Channel length L=(10-20)D

Liquid inlet velocity and bubble volume from experiments

Schematic of flow domain and boundary conditions

0u

212)( RrUru c

0

z

p 0

r

0

z

u

0p


Validation (2)

Air-glycerol flow

Air-water flow

Comparison of the bubbles shape for different bubbles volume and liquid velocity

(Blue: experimental, red:numerical)

Air-glycerol flow Air-water flow

In

creasin

g liq

uid

velo

city


Validation (4)

Validation of the evaporation model: simulation of the growth of a vapor

bubble for different fluids and comparison with an analytical solution

0.1 mm

axis

0.4

mm

0.8 mm

liquidp

8

TL=TSAT( )+5°Cp

8

vapor

TV=TSAT( )p

8

thermal

boundary layer

Schematic of initial and boundary conditions

Results and comparison

Snapshots of a growing bubble


Slug flow boiling: simulation set-up

Simulation set-up

Uniform computational mesh with about 2 million square cells

Inlet condition: saturated liquid inflow

t>0: vapor bubbles generated with constant frequency

Working conditions

Channel size: D=0.5 mm,

Ltot=(45-60)D, Lheated=22D

Fluid: R245fa

Saturation temperature: Tsat=31 °C

Heat flux: q=5-20 kW/m2

Mass flux G=400-700 kg/(m2s)

Bubble frequency: fbub=72-209 1/s

Inlet vapor quality xin=(1.4-4.2)·10-3

Fig.7: Initial temperature field


Slug flow boiling: results (1)

Velocity field

Temperature field

sp thermal layer

Bubble wake region Liquid film region

T-Tsat [K]

Streamlines


Slug flow boiling: results (2)

Comparison of bubble velocity and liquid film thickness with existing prediction methods

Data from simulations also validate the

available prediction methods as

experimental results are not available !


New heat transfer model (1)

Cooling capability of the two-phase flow: heat transfer coefficient

Best performing prediction method available: Thome et al. (2004)

satw

wtp

TT

qh

Thome et al. (2004) model flow domain decomposition

• Bubble zone: steady-state heat conduction

• Liquid zone: heat convection developing flow

New model developed with CFD results:

New model flow domain decomposition

• Bubble zone: transient heat conduction

• Liquid zone: heat convection recirculating flow


New heat transfer model (2)

Cooling capability of the two-phase flow: heat transfer coefficient satw

wtp

TT

qh

Thome et al. (2004):

• Bubble zone: steady-state heat conduction

• Liquid zone: heat convection developing flow

New model developed with CFD results:

• Bubble zone: transient heat conduction

• Liquid zone: heat convection recirculating flow


Conclusions

CFD simulations are an effective tool for investigation of two-phase flows within

microchannels…

… although modifications of the standard solvers are needed to improve accuracy

In absence of experimental data, CFD provides data to validate existing prediction

methods for two-phase flow parameters

CFD results can be used as well to advance existing prediction methods

CFD applied to two-phase flows is useful for fundamental studies

as well as for developing models for design applications


Merci de votre attention!

Dr. Mirco Magnini

[email protected]

Laboratoire de Transfert de Chaleur et de Masse (LTCM),

Ecole Polytechnique Fédérale de Lausanne (EPFL),

EPFL-STI-IGM-LTCM, Station 9, CH-1015 Lausanne

http://ltcm.epfl.ch

ANSYS Conference & 11ème Forum CADFEM, 10 septembre 2014, Lausanne, EPFL

http://ltcm.epfl.ch/

Relevant publications list

• M. Magnini, CFD modeling of two-phase boiling flows in the slug flow regime with an

interface capturing technique, PhD Thesis, Università di Bologna, Italy.

(available free at http://amsdottorato.cib.unibo.it/4437/ )

• M. Magnini, B. Pulvirenti and J. R. Thome, Numerical investigation of hydrodynamics

and heat transfer during flow boiling in a microchannel, Int. J. of Heat and Mass Transfer

49, pp. 451-471, 2013.

• M. Magnini, B. Pulvirenti and J. R. Thome, Numerical investigation of the influence of

leading and sequential bubbles on slug flow boiling within a microchannel, Int. J. of

Thermal Sciences 71, pp. 36-52, 2013.

• S. Szczukiewicz, M. Magnini and J. R. Thome, Proposed models, ongoing experiments,

and latest numerical simulations of microchannel two-phase flow boiling, Int. J. of

Multiphase Flow 59, pp. 84-101, 2014.

Use of CFD to investigate microchannel two-phase flows – Dr M. Magnini

http://amsdottorato.cib.unibo.it/4437/

Use of CFD to investigate microchannel two-phase flows ...

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