A Technique for Improved Water Removal from PEM Fuel Cells viaNatural Frequency Excitation of Free Surfaces

By

Andrew M. Schafer

A THESIS

Submitted in partial ful�llment of the requirements for the degree of

MASTER OF SCIENCE

(Mechanical Engineering)

MICHIGAN TECHNOLOGICAL UNIVERSITY

2010

Copyright c© 2010 Andrew M. Schafer

This thesis, �A Technique for Improved Water Removal from PEM Fuel Cells viaNatural Frequency Excitation of Free Surfaces,� is hereby approved in partial ful�ll-ment for the requirements for the Degree of MASTER OF SCIENCE IN MechanicalEngineering.

Department of Mechanical Engineering � Engineering Mechanics

Advisor:Professor Je�rey S. Allen

Committee Member:Professor Dennis Meng

Committee Member:Professor Robert Kolkka

Department Chair:Professor William W. Predebon

Date:

Abstract

A Technique for Improved Water Removal from PEM Fuel Cells via Natural Fre-quency Excitation of Free Surfaces

Andrew M. SchaferMichigan Technological University, 2010

Advisor: Professor Je�rey S. Allen

The accumulation of water drops in the reactant �ow channels of proton exchangemembrane (PEM) fuel cells can result in the formation of water plugs that can causean uneven distribution of reactants, leading to decreased e�ective surface area andcorrosion of electrodes. Currently, water drops and plugs are removed from the re-actant �ow channels through gas (reactant) �ow. In addition, a number of surfacetreatments and channel geometries have been used to enhance the e�ectiveness ofwater removal by gas �ow. While e�ective at high gas �ow rates, liquid plugs canform at low gas �ow rates plugs, blocking channels.

Previous research has shown that water droplets can be excited at their naturalfrequency and will oscillate with little energy input. The oscillating drop generatessu�cient inertia, even at very small length scales, to overcome the contact line pinningon the gas di�usion layer (GDL) surface thereby allowing the water drop to be expelledfrom the channel at low gas �ow rates.

An experimental fuel cell �ow �eld has been fabricated, consisting of four parallel,one-millimeter square channels over a gas di�usion layer. Nitrogen gas, fed througha �ow modulator, passed through four channels. The gas �ow was modulated using aspeaker attached to a function generator and an ampli�er. The �ow modulation wasmeasured using microphones at the inlet and outlet of the �ow �eld. Water dropswere injected into a single channel through the GDL. The behavior of the injectedwater subjected to gas �ow was observed using a stereo microscope and a high-speedcamera.

Acknowledgments

I would like thank Dr. Je�rey Allen for his outstanding dedication and support as bothan advisor and professor, without which this work would not have been possible. Iwould also like to thank all of the students in Michigan Tech's MNIT research group,along with the faculty and sta� of Michigan Tech, in and out of the MechanicalEngineering department, who provided guidance and assistance for this and all of myendeavors. I would also like to thank my parents, Stephen and Linda and my newbride Emily for their endless support and encouragement.

vii

Contents

Abstract v

Acknowledgments vii

Table of Contents ix

List of Figures 1

1 Introduction 3

1.1 Proton Exchange Membrane Fuel Cell Overview . . . . . . . . . . . . 31.2 Water Management in Fuel Cells . . . . . . . . . . . . . . . . . . . . 5

1.2.1 Factors Impeding Water Removal . . . . . . . . . . . . . . . . 71.2.2 Current Techniques for Water Removal . . . . . . . . . . . . . 8

1.3 Flow Oscillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2 Experiment 13

2.1 Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.2 Flow Field Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.2.1 Plexiglass Flow Field . . . . . . . . . . . . . . . . . . . . . . . 132.2.2 PDMS Flow Fields . . . . . . . . . . . . . . . . . . . . . . . . 17

2.3 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.3.1 MiDAS DA . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.4 Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3 Results 25

3.1 Flow Rate Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . 253.2 Power Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4 Conclusions 33

Appendices 36

ix

x CONTENTS

A Calculations 39

1.1 Bond Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391.2 Natural Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401.3 Minimum Pressure Drop . . . . . . . . . . . . . . . . . . . . . . . . . 411.4 Flow Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

List of Figures

1.1 Basic Construction of a PEM Fuel Cell (Wikimedia [2008]) . . . . . . 41.2 Advancing and Receding Contact Angles on a Droplet Allen et al. [2009] 91.3 Contact Angle vs. Droplet Velocity & Contact Angle Hysteresis . . . 9

2.1 Top View of Plexiglass Channel Setup . . . . . . . . . . . . . . . . . 152.2 Bottom View of Plexiglass Channel Setup . . . . . . . . . . . . . . . 162.3 Circular PDMS Channel Setup . . . . . . . . . . . . . . . . . . . . . 192.4 Experimental Setup Schematic . . . . . . . . . . . . . . . . . . . . . . 212.5 Flow Modulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.1 Average nitrogen �ow rate to expel water (1 mm square plexiglasschannels against GDL) excited with 94 Hz sine wave. . . . . . . . . . 26

3.2 Average nitrogen �ow rate to expel water (1.93 mm dia. circular PDMSChannels) excited with 83 Hz sine wave. . . . . . . . . . . . . . . . . 28

3.3 Average power required to expel water (1 mm Square Plexiglass chan-nels) excited with 94 Hz sine wave . . . . . . . . . . . . . . . . . . . . 30

3.4 Average power required to expel water (0.079 inch dia. circular PDMSchannels) excited with 83 Hz sine wave . . . . . . . . . . . . . . . . . 31

1

Chapter 1. Introduction

1.1 Proton Exchange Membrane Fuel Cell Overview

A proton exchange membrane fuel cell (hereafter referred to as a PEM fuel cell)

operates on the basic principle that hydrogen gas enters the cell on the anode side

and oxygen enters on the cathode side. Within the cell a chemical reaction is used to

generate electricity with water and heat as the biproducts. On the anode side of the

cell hydrogen sheds an electron, which travels through an electric circuit in the form

of electricity while the remaining portion of the hydrogen molecule becomes protons

that pass through the membrane to the cathode. On the cathode side of the cell the

protons combine with oxygen ions to form water. In order to increase the current

the available reaction sites are increased by increasing the surface area of the cell,

either by using larger individual cells or by wiring the cells in parallel. To increase

the voltage of a fuel cell stack cells are wired in seriesLarminie and Dicks [2003].

The basic structure of a PEM fuel cell is illustrated in Figure 1.1 and is described

as follows. The outer layer of an individual cell consists of a bipolar plate on either

side. While the form of bipolar plates may vary greatly, they all serve the same

purpose. Bipolar plates are used to evenly distribute the reactant gases across the

cell and also serve to collect current over the surface area of the cell for extraction

3

4 1.1. PROTON EXCHANGE MEMBRANE FUEL CELL OVERVIEW

Figure 1.1. Basic Construction of a PEM Fuel Cell (Wikimedia [2008])

1.2. WATER MANAGEMENT IN FUEL CELLS 5

occurring typically at the edges. In a fuel cell stack the plates may have additional

channels that are not connected to the reactant channels. These channels are used

to disperse cooling �uid over the cell to regulate cell temperature. The bipolar plate

design must be optimized to provide adequate contact area to the cell for current

extraction, and adequate channel area to allow for maximum reactant contact to the

cell. Moving inwards the next layer in a PEM fuel cell is the gas di�usion layer (GDL)

also called the porous transport layer (PTL). This layer also serves multiple purposes

in that it must further distribute reactants across the cell, while allowing for product

water to escape from the cell and it must conduct electricity away from the reaction

sites to the bipolar plates. A GDL is typically a porous and non-wetting carbon paper.

The heart of the PEM fuel cell consists of the proton exchange membrane (PEM).

The PEM is an acidic membrane and is always either coated with a catalyst layer, or

sandwiched between a catalyst layer. This catalyst layer is generally the same on both

the cathode and anode side and serves to enhance the chemical reaction, allowing low

temperature reactions (below 90 ◦C). The catalyst layer typically consists of carbon

powder with platinum particles attached to the carbon particles. The center of the

fuel cell, consisting of the electrode and the catalyst layer is called the membrane

electrode assembly (MEA). For further reading on PEM fuel cells the reader can refer

to Larminie and Dicks [2003].

1.2 Water Management in Fuel Cells

Water management is a critical aspect in the operation of PEM fuel cells. In a

PEM fuel cell, conduction of ions through the membrane electrode assembly (MEA)

6 1.2. WATER MANAGEMENT IN FUEL CELLS

is proportional to water content of the membrane. Therefore the membrane must

be adequately hydrated in order to serve as a conductor Larminie and Dicks [2003].

Fortunately, water is a byproduct of fuel cell operation and aids in membrane hydra-

tion. If additional water is needed for ion conduction, it is supplied by humidifying

the inlet air to the cathode Larminie and Dicks [2003]. To make most e�ective use of

the MEA the ion conduction should be maximized with adequate water content.

If too much water accumulates in the MEA it will block hydrogen and oxygen from

reaching reaction sites, decreasing the e�ective surface area for reaction Larminie and

Dicks [2003], Tuber et al. [2003]. Blocking of reaction sites occurs when water forms

in the MEA faster than it may be removed through the catalyst layers, the GDL and

the �ow channels. Blocking of reaction sites occurs as a result of water trapped in

the catalyst, the GDL or even water plugs or droplets in the bipolar plate channels,

e�ectively blocking reaction in entire portions of the cell Allen et al. [2009], Tuber

et al. [2003].

Local starvation of reactants directly a�ects the performance and longevity of the

fuel cell. A local starvation of reactants over active fuel cell reactant sites directly

results in a phenomenon known as carbon corrosion in which the carbon supporting

the platinum catalysts is corroded Allen et al. [2009].

Water management issues are especially prevalent in fuel cells operated at ambient

pressures and low temperatures (below 30 ◦C) occurring primarily during transient

and outdoor operation Tuber et al. [2003]. This phenomenon has been observed visu-

ally in transparent fuel cells and is veri�ed by exit gas streams reaching 100% relative

humidity and increased pressure drop through the cathode Tuber et al. [2003]. The

problem is increasingly detrimental at temperatures below freezing during transient

operation such as in the automotive environment where water trapped in these chan-

1.2. WATER MANAGEMENT IN FUEL CELLS 7

nels may result in ice formation, further blocking reactant channelsAllen et al. [2009].

1.2.1 Factors Impeding Water Removal

A typical fuel cell �ow �eld consists of a common inlet manifold which is split into

many parallel �ow channels, all of which terminate in a common exit manifold Zhang

et al. [2010]. Many di�erent bipolar plate con�gurations have been created, all with

di�erent �ow �eld designs intended to primarily optimize water management as well as

the other bipolar plate functions Li and Sabir [2005]. This channel layout exacerbates

the removal of product water through gas �ow by assuring constant pressure drop

across all parallel channels. If a water droplet or plug occurs in one channel, the

pressure drop is equalized throughout all of the channels by an increase in gas �ow

into the non-�ooded channels and a corresponding decrease in the plugged channel.

Other factors that need to be considered for e�ective water removal are capillary

in nature. To assess the importance of capillary phenomena the pertinent length

scales of the system should be compared to the capillary length and Bond number.

The capillary length is represented below as Lc and is de�ned in terms of gravitational

constant (g), the surface tension (σ) and the liquid density (ρ). The Bond number is

de�ned in terms of Lc and the characteristic length scale of the system (L), which is

the channel diameter.

Lc =

√σ

ρg(1.1)

Bo =

(L

Lc

)2(1.2)

8 1.2. WATER MANAGEMENT IN FUEL CELLS

In order to remove trapped water in microchannels (de�ned as channel diameter is

equal to or less than capillary length) the shear and pressure forces have to overcome

two types of capillary phenomena relevant to �uid �ow on a scale corresponding to

a low Bond number (less than 1) Allen et al. [2009]. The �rst capillary phenomenon

is contact line pinning, which occurs at any chemical or physical discontinuity. At

the three-phase intersection around the drop or plug is point of contact will resist

motion allowing the free surfaces to deform while this interface remains stationary.

This phenomenon creates essentially an elastic restoring force that serves to resist the

forces created by the reactant �ow.

The second phenomenon is known as contact angle hysteresis. This occurs when

the advancing and receding contact angles of the water mass deform to resist net

�uid motion as shown in Figure 1.2. The di�erence between advancing and receding

contact angles correspond to the magnitude of the droplets resistance to net motion.

Hysteresis is a�ected by the drop deformation which is a result of the force applied,

appearing in relation to droplet velocity as demonstrated in Figure 1.3. Surfaces

with contact angles between 70 ◦ and 100 ◦ tend to exhibit the greatest contact angle

hysteresisAllen et al. [2009]. In order to create a net �uid motion both contact line

pinning on the advancing and receding edge of the plug or droplet and contact angle

hysteresis must be overcome by the reactant �ow.

1.2.2 Current Techniques for Water Removal

Currently excess water in a non-wetting fuel cell �ow �eld is removed by shear force

and pressure generated by reactant �ow Zhang et al. [2010]. The reactant �ow through

the channels must reach a critical velocity for a droplet of a given size to be swept

1.2. WATER MANAGEMENT IN FUEL CELLS 9

Figure 1.2. Advancing and Receding Contact Angles on a Droplet Allen et al. [2009]

Figure 1.3. Contact Angle vs. Droplet Velocity & Contact Angle Hysteresis

10 1.3. FLOW OSCILLATION

away. Various contact angle values around the contact line and knowledge of their

dynamic change while undergoing shear have been used to successfully determine the

droplet adhesion force and prediction of the point at which droplets lose contact from

the surface as a result of air �ow Theodorakakos et al. [2006].

1.3 Flow Oscillation

Capillary forces generated in fuel cell channels tend to resist net �uid motion, and

when the driving force consists of constant shear force and pressure as a result of

reactant �ow capillary restoring forces are undesirable. These droplet restoring forces

may be thought of as similar to that of a spring. While these phemomena resist �uid

motion downstream, these same phenomena will resist �uid motion upstream and can

serve to store energy in the droplet or plug. If the droplet rocks between a forward

moving hysteresis and one moving upstream, the contact angle hysteresis can serve to

propel the �uid mass forward for a portion of the rocking motion, allowing the �uid

inertia to assist in overcoming the contact line pinning holding the mass in place.

The combination of a rocking motion and a wettability gradient have been demon-

strated to produce a net droplet motion as a result of rocking the substrate on which

the droplet is placed Daniel and Chaudhury [2002]. Due to the wettability gradient,

there is a net contact angle hysteresis which serves to counteract the rocking motion in

one direction while assisting it in the other direction. A simple square wave oscillation

of the substrate allowed the drople to move across the substrate. The contact angle

hysteresis served to rectify half of the mechanical pulse waves to result in net motion.

A similar net droplet motion on a uniformly wettable surface was also achieved by

1.3. FLOW OSCILLATION 11

applying an axisymmetrical lateral vibration to the substrate Daniel et al. [2005]. In

this experiment the waveform used to excite the substrate consisted of a half period

sawtooth wave while the other half of the wave was a smooth sine wave. Di�erent

drop sizes were analyzed and velocity peaks for speci�c frequencies corresponded to

the drop sizes for which this frequency was the �rst resonant frequency. The lowest

natural frequency rocking mode resulted in the greatest drop velocity.

While both of these methods of oscillation have been demonstrated to be e�ective,

implementation in a fuel cell stack would be extremely di�cult. It is proposed that

the oscillation of inlet air may achieve the same net result with easier implementation.

Also, water formation occurs randomly in fuel cells and may take on an in�nite number

of positions and sizes of droplets, resulting in a broad range of excitation frequencies

required for their removal. To that end, this work focuses on liquid plugs, which will

have a constant interface size and a resonant frequency a�ected primarily by channel

diameter and secondarily by viscous e�ects as a result of plug length.

In order to aid removal of product water plugs, this work focuses on an experi-

mental investigation undertaken to determine the e�ectiveness of superimposing an

acoustic wave on the �ow of inlet air at the natural frequency of these plugs to de-

crease the amount of reactant �ow required for their removal. Similar to previous

work, capillary restoring forces are used to supplement a net directional force to

enhance motion of product water.

While water removal is the focus of this work, �ow oscillation may have another

bene�t to fuel cell applications. Enhanced di�usion has been demonstrated in PEM

fuel cells as a direct result of oscillation of reactant �ow, especially in the cathode.

Oscillation of reactant �ow was demonstrated to increase the concentration of oxygen

resulting in increased mass transfer and increased current density, similar to that of

12 1.3. FLOW OSCILLATION

forced reactant �ow from a blower or compressor Hwang et al. [2010].

The natural sloshing frequency of liquids was investigated in the 1960s by NASA

Salzman et al. [August 1967]. This experiment took place on a sled in a drop tower

where it experienced a period of weightlessness lasting 2.3 seconds. The liquid was

placed inside precision diameter glass chambers which received an impulse and the

resulting motion was observed with a high speed camera. The sloshing frequency (1/2

of the period of an interface oscillation) of a liquid with a 90 degree contact angle

was observed to follow the following formula: Salzman et al. [August 1967]

Ω2 = 6.255 + 1.841Bo (1.3)

where:

Ω2 =R3ω2oβ

(1.4)

R is the channel e�ective radius, ωo is the sloshing frequency and is therefore twice

the interface oscillation frequency used in this experiment and β is the speci�c surface

tension (σ/ρ). This formula was applied to assist with a resonant frequency descrip-

tion for both types of channels used in this experiment and the results are discussed

further in the experimental procedure section.

Chapter 2. Experiment

2.1 Concept

An experiment was undertaken to investigate acoustic excitation as a means to

accumulate su�cient energy to overcome contact line pinning. To this end the goal

was to evaluate the e�ectiveness of using an acoustic wave superimposed on reactant

gas �ow at the lowest resonant frequency of a trapped water plug. This reactant �ow

was supplied to a simple parallel channel �ow �eld of comparable size and composition

to a fuel cell. One channel in the �ow �eld is to be supplied with water at a constant

rate and the e�ectiveness is evaluated in reactant �ow required and power expended

to remove the water.

2.2 Flow Field Fabrication

2.2.1 Plexiglass Flow Field

An experimental ex-situ �ow �eld was fabricated, consisting of two sheets of plexiglass

compressing a strip of a 9% PTFE Toray T060 gas di�usion layer (GDL). This GDL

has a thickness of 0.23 mm and is surrounded by a plastic shim of 0.19 mm thickness.

13

14 2.2. FLOW FIELD FABRICATION

Into the top sheet of plexiglass four parallel, one-millimeter square channels have been

machined. Channels were approximately 15 cm in length and the water was injected

at a location 11 cm upstream from the channel exhaust. All channels were treated

with a non-wetting coating described later. Water was injected into a single channel

through the bottom sheet of plexiglass via a needle situated just underneath the GDL.

To correctly center the needle under the GDL the needle was inserted into a 1/8 inch

hose barb and held in place with epoxy. The hose barb was carefully threaded into

the lower plexiglass sheet until it was situated just against the GDL. Water passes

through the needle and GDL to forms droplets in the channel on the upper GDL

surface. A manifold located upstream of the water injection site delivers nitrogen gas

through the GDL and into all four channels simultaneously. The channel assembly

with gas manifold and water injection needle is depicted in Figures 2.1 and 2.2.

For early testing, Rain-X R© was used �rst to coat the channels. This coating was

measured to have a contact angle of 103 ◦. When using Rain-X R© it was observed

that the coating degraded after a relatively short period of time. Breakdown was

observed by water wicking into the channel corners, or wetting along a surface rather

than forming discrete droplets and plugs. After this wetting was observed the water

injection was moved to another parallel channel to continue testing until that coating

was observed to break down. This was repeated until testing concluded or the non-

wetting coating in all channels had broken down, in which case the channels were

recoated for further testing.

Later 3M FC721 was used to coat the channels. This coating was measured to

have a contact angle of 113 ◦. Due to the rapid drying nature of the coating, it

was applied by placing the channel vertical and introducing the coating quickly by

syringe, �lling the channel and moving downwards. The syringe was followed by dry

2.2. FLOW FIELD FABRICATION 15

Figure 2.1. Top View of Plexiglass Channel Setup

16 2.2. FLOW FIELD FABRICATION

Figure 2.2. Bottom View of Plexiglass Channel Setup

2.2. FLOW FIELD FABRICATION 17

nitrogen to blow o� any excess coating and the channel was left to cure at ambient

temperature for 24 hours. This method achieved the most uniform coating without

excessive buildup. FC721 seemed to last longer but still broke down before a complete

set of data was taken.

Next a 3M coating (EGC 1700) was used with a measured contact angle of 100 ◦

and was applied using the same method as the FC721 coating. This coating was

measured to have a contact angle of 100.2 and lasted noticably longer than the FC721

but still not long enough to gather a complete set of data and again this was mediated

by moving the injection location to subsequent channels or recoating. To overcome

contact line pinning at the ends of the channels as strip of paper towel was placed

against the plexiglass just underneath the exit of all channels to allow water to wick

away from the channel exit.

2.2.2 PDMS Flow Fields

To obtain a more consistent non-wetting channel setup, an aluminum mold was fabri-

cated and used as to create 1mm × 1mm square channels in a sheet of polydimethyl-

siloxane (PDMS) having the same dimensions as the plexiglass sheet. The PDMS

had a measured contact angle of 113 ◦. This PDMS sheet was compressed between

two sheets of plexiglass against the same GDL mentioned earlier and water was again

injected up through the GDL via a needle situated just underneath it. This con�gura-

tion also resulted in water wicking into the channel corners as earlier. The wicking is

attributed to a Concus-Finn conditionWeislogel [2001] in the corners of the channel.

In order to prevent corner wicking, a PDMS �ow �eld was created which employed

circular channels. This mold consisted of 1.93 mm outside diameter circular PTFE

18 2.2. FLOW FIELD FABRICATION

tubes adhesively attached to a plate of glass that was coated with 0.65 cSt silicon oil.

This method created a PDMS sheet similar that created using the aluminum mold

and was compressed against the GDL for testing in the same manner described for the

square-channel PDMS setup. Due to the PDMS detaching itself from the GDL along

the lower surface this setup once again resulted in even more pronounced wicking as

a result of the Concus-Finn criterion.

Finally, a test setup was created with channels with entirely circular cross sections,

made entirely of PDMS as depicted in Figure 2.3. To create the PDMS channels,

the same circular tubes had glass pipets inserted to keep them straight and were

suspended on sections of PDMS above a sheet of plexiglass. This was framed with

glass slides and the tubes and glass slides were coated with the same silicon oil. It

is not necessary to coat the plexiglass with silicon oil as PDMS will not adhere to

it. PDMS was poured into this mold and allowed to cure. After curing, the PDMS

was detached from the plexiglass sheet and glass slides before carefully pulling out the

circular tubes to create four parallel circular channels. A manifold was created on one

end of the channels by reinserting the circular tubes and attaching them to a short

strip of Tygon c©tubing. PDMS was poured around these tubes to seal the manifold

to the four channels already created. The tubes were removed and a 1/8-inch hose

barb adapter was sealed to the PDMS using silicon sealant. To inject water into the

channels the needle was inserted into the upper side of the channels until it protruded

into the channels and was pulled partially out to create a hole for the water to enter

the channels but minimizing distortion of the channel surface.

2.2. FLOW FIELD FABRICATION 19

Figure 2.3. Circular PDMS Channel Setup

20 2.3. SETUP

2.3 Setup

The parallel �ow channels were incorporated into the experiment setup as shown

in Figure 2.4. Ultra high purity nitrogen was regulated to 20 psi before passing

through a rotameter to obtain �ow rates. An acoustic wave was superimposed on the

dry nitrogen �ow by passing the nitrogen through a �ow modulator which consisted

of a sealed speaker attached to a Tektronix CFG250 function generator and an am-

pli�er. The �ow modulator is depicted in Figure 2.5 The �ow modulator was located

downstream of the rotameter and upstream of the manifold. The acoustic wave was

monitored with a multimeter measuring the speaker input frequency and at times

by using microphones situated at the inlet and outlet of the �ow �eld. The micro-

phone outputs were synchronized with video captured using MiDAS DA hardware

and software, described later. Distilled water was supplied to the injection needle

from a syringe on a Harvard Apparatus model 975 syringe pump. The behavior of

the injected water subjected to gas �ow was observed from above the �ow �eld using

a Nikon SMZ1500 stereo microscope and a high-speed camera (Photron APX-RS).

2.3.1 MiDAS DA

In order to synchronize video frames with audio data a new software was used called

MiDAS DA. This system was installed on a PC and used with the Photron APX-RS

camera in an attempt to attach acoustic data to individual video frames. The software

requires that both the data acquisition (MiDAS) and video (Photron) are setup to

record the same time interval, with triggers positioned identically. The camera trigger

is used to trigger the data acquisition and a video �le is created using the camera

2.3. SETUP 21

Figure 2.4. Experimental Setup Schematic

2.4. PROCEDURE 23

software while data is collected by the MiDAS software. The video must be uploaded

to the MiDAS software and once that is done the video and data (in graphical form)

may be played back simultaneously. The �les may only be played back in a MiDAS

player. The goal of attaching acoustic amplitudes to individual video frames was

beyond the capability of this system and unfortunately this system could not be put

to e�ective use in this experiment.

2.4 Procedure

To obtain the natural frequency of the water plugs, water was manually injected

with the syringe until it formed the smallest possible plug in the channel. The channel

setup was impacted and the resulting drop motion was observed with the high speed

camera. From the video obtained the lowest natural frequency of oscillation for this

liquid plug was obtained. This was repeated multiple times until the natural frequency

of the plug was obtained with con�dence.

To obtain the lowest nitrogen �ow rate necessary to prevent the water from plug-

ging the channels the following procedure was followed. Water was injected into the

channels and the rotameter was opened up to the maximum �ow rate. The water

injection rate was held constant and the nitrogen �ow rate was slowly decreased until

plugs were observed to form and grow in the channels to the extent that the nitrogen

was insu�cient to expel them before another plug formed behind it. The nitrogen

�ow rate at which the channels plugged was recorded and the oscillator was switched

on and the procedure was repeated. After obtaining the minimum nitrogen �ow rate

with both the oscillator on and o� this procedure was repeated two more times for

24 2.4. PROCEDURE

the same water �ow rate to obtain three measurements for each case. Once data was

gathered the water �ow rate was increased to the next setting on the syringe pump

and the procedure was repeated. In the case of the coated channels, the channel coat-

ing condition was carefully monitored and if it was observed to break down the water

injection was moved to an identical position in another channel and the procedure

was repeated. For the PDMS channels the wetting characteristic of the channel was

constant and all data was collected by injecting the water into the same channel for

all tests. If the needle was removed and reinserted into a previously used channel the

needle was inserted slightly downstream to avoid interface deformation or pinning at

the previous injection site.

Chapter 3. Results

3.1 Flow Rate Comparison

For square plexiglass channels situated against a GDL a dramatic improvement

in water removal was observed by a lower nitrogen �ow rate required to expel liquid

plugs. The results are shown in Figure 3.1. This plot illustrates the nitrogen �ow rate

at which the channels no longer clear, for a given water injection rate. At nitrogen

�ow rates above the plotted values the channels will shed water plugs and not �ood.

Below this rate water will accumulate in the channel and the channel will �ood.

Channel �ooding occurs either when the water plug fails to start moving down the

channel, or when another plug forms before a plug has exited the channel, causing

both plugs to pin unless a much greater nitrogen �ow rate is used to clear them.

The lack of a smooth curve in Figure 9 re�ects the di�culty in obtaining a reliable

coating and a lack of consistency in actual contact angles. Thus, Figure 3.1 shows

the average of several test runs. As a result of switching the channel into which

water was injected and coating degradation within a given channel, the amount of

nitrogen required to evacuate the channels �uctuated. These �uctuations resulted

from continuous coating degradation, condition of the GDL at the injection site and

�aws inherent to each individual channel and coating application. However, in all

25

26 3.1. FLOW RATE COMPARISON

Figure 3.1. Average nitrogen �ow rate to expel water (1 mm square plexiglass channelsagainst GDL) excited with 94 Hz sine wave.

3.1. FLOW RATE COMPARISON 27

channels and coating conditions a dramatic decrease in required nitrogen �ow was

noticed as a result of �ow oscillation.

Next, data was collected in PDMS channels with a circular cross-section. Three

sets of data were collected, with the resonant frequency recalculated between the �rst

and second test runs. The �rst set of collected data was taken in freshly formed

channels and the next two data sets were taken two weeks later. Since a non-wetting

coating was not needed, all �ow rates were obtained from the same injection channel.

The contact angle of the channel did not change during the test, eliminating the need

to switch channels or recoat during data collection. As a result of testing in the same

channel, conditions and channel defects were identical for all tests resulting in a much

smoother curve. As can be seen in Figure 3.2, at low �ow rates an even more dramatic

decrease in required nitrogen �ow was observed.

Since the needle was required to puncture the channels for injection, the new needle

insertion point was moved slightly downstream (∼ 1 mm) for each repeated test,

resulting in a slightly shorter test section. In the two later tests slightly more nitrogen

was required than for the freshly formed PDMS. This may be a result of channel

contamination or changing contact angle of fully cured PDMS, but the dramatic

decrease in required nitrogen �ow was still observed.

28 3.1. FLOW RATE COMPARISON

Figure 3.2. Average nitrogen �ow rate to expel water (1.93 mm dia. circular PDMSChannels) excited with 83 Hz sine wave.

3.2. POWER COMPARISON 29

3.2 Power Comparison

To determine overall system energy consumption, the total power required for

water plug removal with the oscillator on and o� were compared. To determine the

speaker power consumed the RMS voltage and speaker resistance were measured and

the following formula was applied:

P = cosφV 2rmsR

(3.1)

For AC power this formula obtains the apparent power of the system. To de-

termine the actual power consumption of the system the power factor (cosφ) must

be known. The power factor is de�ned as the cosine of the phase shift between the

current and voltage. To obtain the power factor the line current was observed us-

ing a Hall e�ect current sensor. The voltage was observed simultaneously across the

speaker, both on di�erent channels of the same oscilloscope. The current and voltage

were observed to be in phase, so the power factor was unity. The power consumed

by the speaker (�ow modulator) was determined to be 1 Watt.

To determine the required compressor power to drive the gas �ow through the

channels, the isothermal expansion power formula was used in the form shown below.

Ẇ = ρ∀̇(RT

M

)ln

(P2P1

)(3.2)

∀̇ is the �ow rate in L/s, M is the atomic mass of the gas, and ρ is the density

in kg/m3. To compare power consumption the expansion power was added to the

speaker power for the oscillator-on case and just the expansion power was used when

30 3.2. POWER COMPARISON

Figure 3.3. Average power required to expel water (1 mm Square Plexiglass channels)excited with 94 Hz sine wave

the oscillator was o�. The results were plotted and are shown below in Figures 3.3

and 3.4.

In square channels the �ow rate was dramatically less, but since the �ow rates were

already so low for each case the total power is higher with the oscillator on. This

occurs because the oscillator was always set at the maximum amplitude allowable

without distortion. While the maximum oscillator amplitude was used for testing it

was observed that much lower speaker amplitudes still resulted in observable �uid

oscillation. It remains unclear how much of the oscillation energy is dissipated within

the oscillation chamber and in the lines upstream of the trapped water.

3.2. POWER COMPARISON 31

Figure 3.4. Average power required to expel water (0.079 inch dia. circular PDMSchannels) excited with 83 Hz sine wave

In the larger circular channels it is evident that �ow oscillation drastically reduced

the power necessary to purge water from the channels at low liquid injection rates. At

higher liquid injection rates the power required is comparable to that of �ow without

oscillation.

For a �nal comparison, the pressures generated by the speaker oscillation were

compared with the pressures required to move the plug down the channel. To obtain

the pressure di�erential across the channel the hydraulic resistance of the channel

was calculated Bruus [2008] and all calculations are located in Appendix A.3. These

calculations were performed at the lowest nitrogen �ow rates recorded during testing.

32 3.2. POWER COMPARISON

For plexiglass channels the pressure drop was 2.196 kPa and for PDMS channels the

presusre drop was 525 Pa.

Next, the pressure di�erentials were calculated across the meniscus using the fol-

lowing equation.

∆P =2σ cos θ

r(3.3)

For the above equation σ is again surface tension, θ is the measured contact angle,

and r is the e�ective channel radius. The pressure di�erential generated as a result

of the meniscus curvature for plexiglass channels was 50 Pa and for PDMS channels

was 58 Pa.

Finally the pressure was measured using a pressure transducer downstream from

the oscillator and the peak-to-peak pressure di�erential during oscillation was 60

Pa. The minimum pressure drop across the channel as a result of nitrogen �ow

were compared to the miniscus pressure di�erential and the oscillation peak to peak

pressure di�erential. It is apparent that pressures generated by the speaker are close

to those capable of being stored by the meniscus and much less than the minimum

pressure di�erential across the channel as a result of nitrogen �ow. From these results

it is apparent that lower levels of energy, when applied at the resonant frequency of

the plug are capable of moving the plug down a channel.

Chapter 4. Conclusions

A drastic decrease in gas �ow required to remove pinned water plugs by superimposing

a sine wave on gas �ow �ow has been demonstrated repeatably in both square and

round channels on the scale of roughly 1 mm diameter. This decrease is most dramatic

in a hydrophobic channel and with gas �ow superimposed with a sine wave at the

resonant frequency of the plug. The e�ect is most pronounced when water forms

into plugs with a consistent surface interface shape. The interface shape can be

a�ected by channel geometry and hydrophobic characteristics. Therefore, a consistent

hydrophobic coating is essential to assure consistent interface shapes and therefore

consistent resonant frequency of the water plug. Water plug interface shape can also

be a�ected by channel contaminants and defects in channel shape, both of which can

impede motion of plugs. With both the oscillator on and o� water plugs are slowed

by these contaminants but �ow oscillation still overcomes these at a lower �ow rate

than without oscillation Acoustic modulation was e�ective not only for a single plug

but multiple plugs as evident in decreased reactant �ow rates.

33

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in the cathode of a transparent pem fuel cell. Journal of Power Sources, 124:

403�414, 2003.

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dedigns. International Journal of Hydrogen Energy, 30:359�371, 2005.

35

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APPENDICES

37

Appendix A. Calculations

1.1 Bond Number

The Bond numbers of all channels used were calculated using the following for-

mulas.

Lc =

√σ

ρg(1.1)

Bo =(L

Lc

)2(1.2)

For water ρ=1000 kg/m3, σ= 0.072 N/m, g= 9.8 m/s2 and Lc for water is 2.73mm.

L is the characteristic length scale, or the e�ective radius of the channel. For square

channels L is 0.5 mm and the resulting Bond number is 0.034. For the circular PDMS

channels the e�ective length scale used was again the radius of the circular channels.

L was 0.96 mm and the resulting Bond number was 0.51.

39

40 1.2. NATURAL FREQUENCY

1.2 Natural Frequency

The natural frequency of a liquid sloshing in a cylinder with a contact angle of 90

degrees is given by equation 1.3. Salzman et al. [August 1967]

Ω2 = 6.255 + 1.841Bo (1.3)

where:

Ω2 =R3ω2oβ

(1.4)

β is the speci�c surface tension (σ/ρ). ωo is the lowest natural frequency, or the

sloshing frequency and R is the channel radius. For water at STP σ is 0.072 N/m is

and ρ is 998 kg/m3.

Sloshing natural frequency corresponds to an interface shape of 1/2 of a period,

so the duration of one oscillation is twice the value obtained. The frequency obtained

from this formula must be halved for an interface oscillating in the longitudinal di-

rection of a low-Bond number channel.

For the rectangular plexiglass channels Ω2 = 6.318 resulting in ωo=305.2 Hz. Since

the sloshing natural frequency was calculated and the interface oscillating frequency

is desired the frequency is halved so ωo=152 Hz. The actual measured frequency was

ωo,m=94 Hz. A lower measured frequency could be a result of the interaction of two

meniscii in close proximity to one another.

For the circular PDMS channels Ω2 = 7.196 resulting in ωo=121 Hz. This natural

frequency was again halved so ωo=60.5 Hz. The actual measured frequency was

ωo,m=94 Hz. The cause of this di�erence has not been determined and could possibly

be attributed again to the interaction of two meniscii.

1.3. MINIMUM PRESSURE DROP 41

1.3 Minimum Pressure Drop

The minimum pressure drop along a given channel length as a result of hydraulic

resistance and volumetric �ow rate is given by the �uids analog to Ohm's Law de�ned

in 1.5. For further reference see Bruus [2008]

∆P = Rhyd∀̇ (1.5)

The minimum pressure drop across channels during testing was calculated by using

the lowest measured nitrogen volumetric �ow rate for each channel type.

For a square channel of width h, the hydraulic resistance is:

Rhyd = 28.4ηL1

h4(1.6)

where η is the absolute viscosity. For water, η=1.78 × 10−5 and in the case of

the plexiglass channels h=1 mm, and L=0.11 m. The resulting hydraulic radius

is Rhyd=5.56 × 107 Pa-s/m3. For a volumetric �ow rate of 3.95 × 10−5 m3/s the

resulting pressure drop is calculated to be 2.196 kPa.

For the circular PDMS channels the hydraulic resistance is calculated using the

following formula:

Rhyd =8πηL 1

a4

Where η is the same as earlier L is again the channel length and a is the channel

radius. For this case a=.96 mm, and L=0.11 m. The resulting hydraulic radius

is Rhyd=6.12 × 106 Pa-s/m3. For a volumetric �ow rate of 8.58 × 10−5 m3/s the

resulting pressure drop is calculated to be 525 Pa.

42 1.4. FLOW WORK

1.4 Flow Work

Flow work is de�ned as the energy required to generate the nitrogen �ow through

all four channels in the test setup in terms of isothermal expansion/compression.

Since the system total pressure drop (pressure di�erence from nitrogen tank regulator

to channel outlet) is equal for all tests a multiplier was calculated to convert the

volumetric �ow rate into power (Watts). Isothermal compressor work is calculated

using the following formula:

W = zRT ln(P2P2

)

Where z is de�ned as the compressibility actor (unity), P1 is the pressure at the

channel outlet (Patm=101,325 Pa). P2 is the pressure at the outlet of the nitrogen

tank regulator and is equal to Patm + 20 psi = 239, 225 Pa. Using these values, the

compressor work was determined to be W = 2589 Jmol

.

Power in terms of Watts was desired, and the �ow rate was measured in L/min

so the following calculation was performed to obtain a factor used to obtain watts

when multiplied by �owrate. This factor was used for ease of plot generation and

data analysis.

From EES ρ = 2.749 kg/m3, obtained by using T=293K and P=239.2kPa. Using

the molecular weight of nitrogen (28.01 g/mol) the work was determined to be 9.24

J/kg. The density of nitrogen was then used to convert the work to 254 J/L.

Flow rate is in L/min and we desire this factor to obtain watts so the preceding

value is divided by 60 to obtain 4.23 Watts × min/L.

This factor was used to generate plots in the results section. The symbolic equiv-

alent of the preceding exercise is:

1.4. FLOW WORK 43

Ẇ = ρ∀̇(RT

M

)ln

(P2P1

)(1.7)

Abstract Acknowledgments Table of Contents List of Figures1 Introduction1.1 Proton Exchange Membrane Fuel Cell Overview1.2 Water Management in Fuel Cells1.2.1 Factors Impeding Water Removal1.2.2 Current Techniques for Water Removal

1.3 Flow Oscillation

2 Experiment2.1 Concept2.2 Flow Field Fabrication2.2.1 Plexiglass Flow Field2.2.2 PDMS Flow Fields

2.3 Setup2.3.1 MiDAS DA

2.4 Procedure

3 Results3.1 Flow Rate Comparison3.2 Power Comparison

4 ConclusionsAppendicesA Calculations1.1 Bond Number1.2 Natural Frequency1.3 Minimum Pressure Drop1.4 Flow Work