Multiphase-Simulation of Membrane Humidifiers for PEM … · Multiphase-Simulation of Membrane...

Multiphase-Simulation of Membrane Humidifiers for PEM Fuel Cells

STAR Global Conference

Sebastian Bilz, Vladimir Buday, Carolus Gruenig, Thomas von Unwerth

Vienna, March 17-19, 2014

© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers 1

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CFD Simulation of Fuel Cell Humidifiers Overview

© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers

Introduction

Simulation Methodology

Single-Duct Modelling

Modelling of a Complete Humidifier

Conclusion/Summary

Outlook/Further Work

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Introduction




Conclusion/Summary


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• Electrolyte of the fuel cell has to be hydrated to ensure proton conductivity

• An option: humidifier

• But: water management of a PEM fuel cell has to be well controlled to secure power

in every operating point → CFD simulation

Introduction Task/ Purpose


proton exchange membrane

hydrogen

electrical load gas with high

relative humidity

air gas diffusion layer

electrode with catalyst

anode cathode bipolar plates

cf: A. Vlath: Dreidimensionale dynamische Modellierung und Berechnung von PEM- Brennstoffzellen-systemen. 2009 cf: Perma Pure

Development of a methodology to simulate a fuel cell humidifier with Star-CCM+

Examination of this methodology under different operating conditions

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Introduction Functional Principle of the Humidifier

Compressor Humidifier

Anode

Cathode

Fuel cell

Dry Humid

Humid exhaust Dry exhaust

Air

H2

Position of the humidifier in the cathode stream:

Setup of the humidifier:


Dry air Humid air

Liquid water or gas

with high humidity

Bundle of pipes

Exhaust stream

Transport of moisture

through the membrane

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Introduction




Conclusion/Summary


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Example water-to-gas-humidifier:

Δx

Δa

aA=f(λ)

aI=f(λ)

Liquid water

Membrane

Low humidity High humidity

Simulation Methodology Calculation of the Resulting Flux


Procedure:

1. Writing the activity of both sides into tables → interpolate them → conditions are available on

both sides of the membrane

2. The resulting flux is calculated by assuming a linear gradient

3. This flux is added to the inner region and subtracted from the outside

Air

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Introduction




Conclusion/Summary


9

Geometrical data is taken from the humidifier:

Single-Duct Modelling Design of the Simulation Model


• Inner fluid: cathode stream, fluid region with shell region on the wall for evaporation

• Membrane: properties of Nafion 115® for heat transfer, solid region

• Outer fluid: liquid water or cathode exhaust, fluid region (shell region – condensation)

254mm

4m

m

1,4

mm

0,1

27

mm

H2O

H2O

Air

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• Example: Temperature of water 40°C

Volume flow of the cathode stream 357 l/min

• Mass Fraction (flow direction Z):

Single-Duct Modelling Results of the Water-to-Gas-Humidifier

Gas inlet Gas outlet


Mass f

raction o

f H

2O

[-]

Coordinate Z [m]

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Gas inlet Gas outlet

Single-Duct Modelling Results of the Water-to-Gas-Humidifier


• Example: Temperature of water 40°C

Volume flow of the cathode stream 357 l/min

• Temperature (flow direction Z):

Coordinate Z [m]

Ga

s tem

pera

ture

[K

]

R

ela

tive

Hu

mid

ity [

-]

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Variation of water temperature and volume flow of the cathode stream:

Volume flow of the cathode stream [l/min]

Ma

ss f

ractio

n o

f w

ate

r a

t th

e o

utle

t [-

] Single-Duct Modelling Validation of the Water-to-Gas-Humidifier


TH2O = 80°C

TH2O = 60°C

TH2O = 40°C

Low change in temperature of the inner fluid means negligible error

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• Example: Temperature of the exhaust 70°C,

Volume flow of the cathode stream 60l/min

• Mass fraction (flow direction Z):

Inlet of the

cathode stream

Single-Duct Modelling Results of the Gas-to-Gas-Humidifier

• Temperature (flow direction Z):


Outlet of the

cathode stream

Inlet of the

cathode stream Outlet of the

cathode stream

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Single-Duct Modelling Validation of the Gas-to-Gas-Humidifier


Variation of temperature of the outer gas with constant mass fraction

Mass fraction H2O at the outlet of

cathode stream Deviation to validation data

Mass fraction of the inner gas is influenced by the mass fraction of the outer gas

Agrees with validation data

Simulation

Data sheet

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Introduction




Conclusion/Summary



• Realistic description of the outer fluid (flow profile)

• Interaction of the pipes

Transfer of the moisture in dependence of the position of the pipe

Increase of accuracy

Model: regular array Reality: irregular array

Modelling of a Complete Humidifier Motivation

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Outlet:

air

Inlet: air

Inlet: water

Outlet:

water

• Fluid region water: 150 pipes modelled

• Solid/fluid region air: 4 representing pipes, transfer of the results to the surrounding

pipes

Modelling of a Complete Humidifier Design of the Simulation Model


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Mass fraction: Temperature:

Ma

ss fra

ctio

n H

2O

at th

e o

utle

t [-

]

Volume flow of the cathode stream in [l/min] Volume flow of the cathode stream [l/min] Te

mp

era

ture

at th

e o

utle

t [K

]

Modelling of a Complete Humidifier Results in Comparison with the Single Duct


data sheet

single duct

humidifier

single duct

humidifier

Temperature of gases at the outlet agree with data of a single duct,

But: constant offset in mass fraction of 0.02

No significant difference between the results of the positions of the 4 pipes

Prospect: study of a gas-to-gas-humidifier

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Introduction




Conclusion/Summary


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• Implementation of a suitable simulation approach

• Computation of multiphase flow with condensation and evaporation

• Calculation of the mass transfer through a semipermeable membrane with the

help of field functions and tables

• Calculation of both operational strategies (gas-to-gas, water-to-gas) possible

• Achieved result accuracy

• Trend of the results agree with validation data, error sources are known

• But: no validation data for temperature or relative humidity

• Modelling/ simulation of complete humidifier

• Not feasible in terms of require computational efforts

• No advantage in simulation of the entire humidifier in operation with liquid

water in comparison to the single-duct strategy

• Calculation with the help of the water content of the membrane → matches water

management in a fuel cell → first step to simulate an entire fuel cell

CFD Simulation of Fuel Cell Humidifiers Conclusion / Summary


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Introduction




Conclusion/Summary


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• Measurement to get more validation data (heat transfer, temperature, relative

humidity)

• Further development of the methodology to reduce errors

Extension to consider other physical effects (membrane swelling, crossover of

gases)

• Computation of an entire gas-to-gas-humidifier, comparison with single-duct-model

CFD Simulation of Fuel Cell Humidifiers Outlook / Future Work


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Thank you very much

Sebastian Bilz

IAV GmbH

Rockwellstraße 16, 38518 Gifhorn

[email protected]

www.iav.com



Appendix Comparison of Parameter: Model and Data Sheet

Parameter Data Sheet Model

Length 10‘‘=254 mm 10‘‘=254 mm

Inner diameter of the pipe - Measured: 1,4 mm

Membrane material Nafion Nafion 115 (based on membrane

thickness)

Properties of the membrane (porosity,

tortuosity, Diffusion coefficient)

- taken from literature, empirical

equations

Volume flow of air 71-500 l/min 71-500 l/min

Inlet air temperature - 298,15 K

Volume flow of water - 4,5E-3 (negligible)

Inlet water temperature1 40, 60, 75, 80°C 40, 60, 80°C

Air temperature or relative humidity at

outlet

- computed

Mass fraction of water of air at outlet Dew point temperature computed

Water temperature1 at the outlet - computed

Mass fraction of water of the exhaust

stream at the outlet2

- computed

1: temperature of exhaust stream in case of a gas-to-gas humidifier

2: only in case of gas-to-gas humidifier


Appendix Interpolating of Activities

Activity is written into a table at the inner membrane surface

Interpolated with field function

Values pertain for each volume element and change in z-direction (from inlet to outlet)

Values are equal in the radial direction

Resulting flux can be calculated in the inner and outer fluid region to subtract and add the

same value

Inner Activity

Inner fluid

Membrane

Outer fluid


Small volume flow:

Large volume flow:

Inlet

Outlet

Outlet

Appendix Comparison of the Fluid Film Thickness

Small volume flow → fluid film thickness grows → saturated gas near the outlet

Large volume flow → minor fluid film thickness, remains constant → unsaturated

gas near the outlet

Inlet

Multiphase-Simulation of Membrane Humidifiers for PEM … · Multiphase-Simulation of Membrane...

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