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8/10/2019 H. Epstein - Shirtbutton-sized Gas Turbines . the Engineering Challenges of Micro High Speed Rotating Machinery
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R E P O R T
DOCUMENTATION
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Approved
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ncluding the
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2 . RE P O RT
DA TE
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3 .
RE P O RT T YP E ANDDATESC O V E R E D
FINAL
0 1
May
95
-
31
ul
00
4.
TITLE
AND SUBTITLE
Micro
Gas
Turbine
Generators
5 .
FUNDING
N U M B E R S
G r antD AAH0 4 -9 5 -1 -0 0 9 3
6 .
A U T H O R S
A.
Epstein,
.
Breuer,J.Lang,
M.Schmidt,
S.
Senturia,
M.
Spearing,
C.Tan,.Waitz
7 . P E RFO RM I NGORGANIZATION
NAM E ( S)ANDA D D R E S S ( E S )
Massachuset tsInst itute
o f
Techno l ogy
77
Massachuset tsAve.,
31- 264
Cam br idg e ,
M A
0 2 1 3 9
8 . P E RFO RM I NG ORGANIZATION
RE P O RT
N U MB ER
9 .
SP O NSO RI NG
/
MONITORING
AG EN C Y
NAME(S) AND
ADDRE SS( E S)
U.S.
Arm y
Research
Office
P.O.Bo x12211
Research
Triangle
Park,
NC
7709-2211
1 0 . SPONSORING/MONITORING
AG E NCY
R E P O R TNUM BE R
ARO 33888.2-CH-MUR
1 1 . SUP P L E M E NT ARYNOTE S
12a.
^3TR,BUT,O
O6
M|NT
ATEwENTA
Approved for
Publ ic Release
Distr ibut ion
Unlimited
12b. DISTRIBUTIONCO DE
1 3 .
A B S T R A C T
Ma x i mum
20 0
words)
MI T
ha s
developed
th e
technology
fo r
micro-gas
turbine
generators.
hesear emill imeter-to centimeter-size
heat
engines
fabricated with
semiconductor
industry
micromachining
techniques
(MEMS),
ultimately
capable
of
producing10-100
W
of
p o we r
in
es s
than
a
cubic
cent imeter.
Applicat ionsincludecompactpowersources offeringenergy an dpow erdensit iesanorderof
magni tudebetter
thancurrentbattery technology;
propulsionfor small
ai r
vehicles;
an davarietyo f
microb lowers ,
compressors ,
an d
heat
pumps.The
wo rk
w as
d iv ided
nt o8
microscale
disciplinary
areas:
1)
engine
systemsdes ign,
(2 )
turbomachinery fluiddynamics ,
(3)
combust ion,
(4 )
structures,
(5 )
bearings,
(6)
elect romechanics ,
(7)
silicon
fabricationtechnology,
an d
(8)microfabricat iono f
high
temperaturematerials
an d
structures.Advancesinthe
disciplinary
technologies enabledth e
design
an dconstruction
o f
a
proof-of-principle
d em o
engine .
hi s
2 0m msquareby
4m m
thicksimple
cycle
ga s
turbine
is
designed
to
produce
about1 1
gramso f
thrustor 17 wattsof shaft
power.
he design turbine
inlet
temperatureis
1600
K
an d
therotationalspeeds
1.2M
rpm. tth e
conclusionof
thisMURI,th efirstengines
had
beenbuilt
an d
were
justbeginningtesting.
companionmicroturbogenerator isa
fe w
monthsbehind
th e ga s
turbine.
1 4 . S U B J E C TT E R M S
M E M S ,
compact
power ,microturbine,
micromotor,
microcombustion
1 5 . NUM BE ROFP A GE S
2 1
1 6 . PRICECO DE
17 .
ECURITY
CLASSIFICATION
O F
RE P O RT
UNCLASSIFIED
18 .
ECURITY
CLASSIFICATION
OFTHISPAGE
UNCLASSIFIED
19.
ECURITY
CLASSIFICATIONO F
A B S T R A C T
UNCLASSIFIED
20 . LIMITATIONOFABST RACT
UL
NS N7540-01-280-5500
StandardForm2 98
(Rev.
2-89)
Prescribedby ANSIStd.Z39-1298-102
DTicqui
^
LJ AXJ
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2 0 0 1 0 1 1 64 3
8/10/2019 H. Epstein - Shirtbutton-sized Gas Turbines . the Engineering Challenges of Micro High Speed Rotating Machinery
2/23
GasTurbine
Laboratory
Department
ofAeronauticsandAstronautics
Massachusetts
Institute
of
Technology
Cambridge,
MA
02139
FinalTechnicalProgressReport
on
Grant
#DAAH04-95-1-0093
entitled
MICRO
GAS
TURBINE
GENERATORS
prepared
for
US
Army
Research
Office
P.O.
Box
12211
ResearchTrianglePark,NC7709
ATTN:r.
RichardPaur
CO-INVESTIGATORS:
Dr..
K.S.Breuer
Prof.
A.H.
Epstein
(Principal
Investigator)
Prof.
J.H.
Lang
Prof.
M.A.
Schmidt
Prof.
S.D.
Senturia
Prof.
S.M.
Spearing
D r.
C.S.T an
Prof.I.A.
Waitz
PERIOD
COVERED:
pril1,1996-
July
31 ,
2000
- -_
mM1
^
W
M
.
DISTRIBUTION STATEMENT
A
Approved
fo r
Public Release
December
2000 is tr ibut ionUnl imi ted
8/10/2019 H. Epstein - Shirtbutton-sized Gas Turbines . the Engineering Challenges of Micro High Speed Rotating Machinery
3/23
1.0
REPORT
OUTLINE
Thisisthefinaltechnicalprogressreport
on
AROGrant
DAAH04-95-1-0093,
a
five-
year
MultidisciplineUniversityResearchInitiative
(MURI)
program.
ecausetheprogram
has
generated
lengthly
annualtechnicalreports
an da
large
numberof
technicalpublications
an dgraduatetheses
(whichareavailableupon
request)
this
final
technicalreport
is
relatively
brief.
t
consistsof four
sections
in
addition
to
this
one:
(2 )
Ashort
summary
an d
list
of
accomplishments,
(3)
a
listofparticipants,
(4)a
list
of publicationsand
theses,and
(5)
a
technical
paper
which
gives
a
more
detailedoverviewofthetechnology.
2.0SUMMARYANDACCOMPLISHMENTS
Under
this
MURI
support,
M IT has
developed
thetechnologyfo rmicro-gasturbine
generators.
hesearemillimeter-
to
centimeter-sizeheatenginesfabricatedwith
semiconductorindustry
micromachining
techniques.
s
such,
they
are
micro
electrical
an d
mechanical
systems
(MEMS)devices.
hey
also
representthefirstapplication
of
a
new
technical
field
conceivedunder
this
program,
Power
M E M S .
alculations
showthat
these
microdevicesmayultimately
becapableofproducing
10-100
Wofpower
or
10-50
gramsof
thrust
in
lessthan
acubiccentimeter.
pplicationsincludecompact
powersourcesoffering
energyan d
power
densities
an
order
of
magnitude
better
thancurrent
batterytechnology;
propulsionfo rsmallai r
vehicles;
an davarietyof
microblowers,compressors,
and
heat
pumps.
Much
of
this
technology
is
also
applicable
to
M E M S
microrocketengines
(which
are
the
enabling
technology
forverysmall
launchvehicles
and
missiles),
the
technologyof
whichcan
be
considered
derivative
of
this
program.
he
promise
of
Power
M E M S
issufficientthat
DA RPA
an dtheJapanese
governmenthavestartedprograms
in
thisareaandthatPower
M EM S
internationalworkshops
and
meetings
arenow
held.
This
technicalefforthas
been
an
admixture
of
research
and
engineering
design
since
theprojectgoalsaredevice-oriented.
he
work
is
nominally
dividedinto8principal
microscaledisciplinary
areas:
(1)
engine
systems
design,
(2 )
turbomachineryfluiddynamics,
(3)combustion,
(4)
structures,
(5)
bearings,
(6 )electromechanics,(7 )siliconfabrication
technology,
(8 )microfabricationof
hightemperaturematerials
andstructures.
T he
centerpiece
of
this
effort
ha s
been
the
design
and
construction
of
the
world's
first
M E M S
micro-gas
turbine
engine.
ealization
of
such
adevicehas
necessitated
significant
advancesinmany
disciplinary
technologies.
Some
specifictechnical
achievements
include
theestablishment
of
the
enablingbasictechnologyforan ddemonstrationof:
hefirst
M E M S
micro-gasbearing
technology,
with
speeds
above1.4M
rp m
demonstrated.
icro-high-speedturbomachinery,
transoniccentrifugalturbinesan dcompressors.
8/10/2019 H. Epstein - Shirtbutton-sized Gas Turbines . the Engineering Challenges of Micro High Speed Rotating Machinery
4/23
he
first
highpower
density
M E M SH
2
an d
hydrocarbonfuelmicrocombustor
technology;
400 wattsofthermalenergyin200
m m
3
hasbeen
demonstrated.
he
firstcooled
silicon
microturbine
airfoils,
which
have
operatedinagastemperatures
above
the
melting
point
of
silicon,
1800
K.
icromotor
performance
OOx
higherthanpreviously
achieved.
dvances
in
the
SO
A
ofhighaspect
ratio
silicon
etching
bya
factor
of 3-5.
he
first
multi-wafer(5-6)precision-aligned(1-2
micron)
silicon
structures.
ackagingfo rveryhigh
temperature
silicon
chips,including
high
pressurefluid
interconnections.
These
advances
in
disciplinary
technology
enabled
the
design
an d
constructionof
a
so-
called"demoengine",asaproof-of-principle.
hi s20
m m
squareby4m mthicksimple
cycle
gas
turbine
is
designedto
produce
about1 1
grams
of
thrust
or
17
watts
of
shaft
power
T he
designturbineinlet
temperature
is
1600
K
an d
therotationalspeed
is
1.2M
rpm.
t
the
conclusionof
this
MURI,
the
first
engines
ha d
beenbuiltan dwerejustbeginningtesting.
companion
microturbogenerator
is
a
few
monthsbehindthe
gas
turbine.
U S A
3
FIRSTSUPEft
US 3
S&- 3
Dem o
Gas
Turbine
Engine
This
success
of
the
technological
advances
made
inthisM U R I
promptedDA RPAto
match
MURIfunding.
iscussionswith
the
Army
suggest
that
further
6.1
an d
6.2
support
m ay
be
available
both
tocontinuethe
basicresearchand
to
beginthetechnologytransitionprocess.IT
ha sinitiated
discussionswithseveralpotentialindustrial
partnersinterested
indeveloping
the
technology
further.
8/10/2019 H. Epstein - Shirtbutton-sized Gas Turbines . the Engineering Challenges of Micro High Speed Rotating Machinery
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3.0
CONTRIBUTINGTECHNICAL
PERSONNEL
Name
Faculty:
Prof.
Kenneth
Breuer
Prof.John
Brisson
Prof.
Alan
H.Epstein
Prof.Jeffrey
H.Lang
Prof.
MartinA.
Schmidt
Prof.
Stephen
D .
Senturia
Prof.
MarkS.Spearing
Prof.
IanA.
Waitz
Technical
Staff.
D r.
G.K.Ananthasuresh
(Post
D o c )
D r.
A.
Ayon(Post
Doc)
D r.
Christopher
Cadou
(Post
D o c )
D r.
FredricF.
Ehrich
(Senior
Lecturer)
EricEsteve(Visiting
Eng.)
D r.Anthony
Forte
D r.
GautamGauba
(PostDoc)
D r.Reza
Ghodssi
D r.
YifangGong(PostDoc)
D r.PaulHolke
(Post
Doc)
D r.
Eugene
W .
Huang
(LL
Tech.
Staff)
D r.
Stuart
A.
Jacobson
(Engineer)
D r.
Ravi
Khanna
(ResearchEng.)
D r.
CarolLivermore(Post
Doc)
StevenLukachkoResearchEng.)
D r.
Paul
Maki
(L L
Tech
Staff)
D r.
JamesPaduano(PrincipalEng.)
LarryRetherford,
Jr .
LLTech.Staff)
D r.
Choon
S.
T an(Principal
Eng.)
D r.Steven
Umans
D r.RichardWalker(C.S.Draper
Labs)
Paul
Warren
D r.
WenjingYe(Post
D o c )
Patrick
Yip
(L LTech
Staff)
D r.
Xin
Zhang
(Post
Doc)
Primary
Discipline
Fluids,Instrumentation
Thermal
Systems,
Heat
Transfer
EngineDesign,
Fluids
Electromechanics
(iFab,
Processes
(jFab,Processes
&
Materials
Structures,Materials
Combustion
jiFab
Modeling
uFab,Processes
Fluids,Combustion
RotorDynamics ,
Design
Fluids,
Engines
^Fabrication
Combustion
^Fabrication
Turbomachinery
^Fabrication
Structures
Fluids
^Fabrication
Electromechanics
Combustion
^Fabrication
Controls
Packaging
Turbomachinery
Electromechanics
Gas
Bearings
Electronics
Structures
M AVAvionics
(^Fabrication
8/10/2019 H. Epstein - Shirtbutton-sized Gas Turbines . the Engineering Challenges of Micro High Speed Rotating Machinery
6/23
Graduate
Students:
Dye-ZoneChen
Kuo-Shen
Chen
DongwonChoi
Luc
Frechette
T od
Harrison
KashifKhan
Jin-Wook
Lee
Chuang-Chia
Lin
ChunmeiLiu
Kevin
Lohner
AdamLondon
AmitMehra
BrunoMiller
Jose
Miranda
Hyug-SooMoon
Steve
Nagle
DJ.Orr
BaudoinPhilippon
Ed
Piekos
John
Protz
Nicholas
Savoulidis
GregoryShirley
Chris
Spadaccini
ShaunSullivan
DavidTang
Sheng-Yang
Tzeng
Douglas
Walters
Chee
W eiWong
^Fabrication,
Instrumentation
Structures
Structures
&
Materials
Turbomachinery
Systems
Structures,Packaging
Turbomachinery
Combustion
(^Fabrication
Turbomachinery
Structures
&
Materials
Packaging
Turbomachinery
Structures
Electric
Bearings
Structures
&Materials
ElectricMachinery
FluidBearings
Turbomachinery
Fluids
Modelling
Engine
Systems
Bearings
Turbomachinery
Combustion
Fluids,HeatTransfer
Instrumentation
Combustion
Structures
Bearings
8/10/2019 H. Epstein - Shirtbutton-sized Gas Turbines . the Engineering Challenges of Micro High Speed Rotating Machinery
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4.0
LIST
OF
PUBLICATIONSANDTHESES
1 .
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of
Position
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fo r
Microengine
Rotor,"
Technical
Report,
M ay1996.
2.
steve,E.,
"Secondary
Flow
SystemModeling,"TechnicalReport,1996.
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aitz,I.A.,
Gauba,G.an d
Tzeng,
Y.-S.,
"Combustorsfo r
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and
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1996.
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pearing,
S.M .,
Chen,K.S.,
"Micro-GasTurbineEngineMaterialsan dStructures",
presentedat21
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t
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pstein,
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H.,
an d
Senturia,
S.
D .,
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pstein,Senturia,Anathasuresh,
Ayon,
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Jacobson,Kerrebrock,Lang,Lin,London,Lopata,
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Mur
Miranda,Nagle,
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Tzeng,Waitz,
"Micro-Heat
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AIAA97-1773, 8th
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4thAIAAShear
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June
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Orr,
DJ.,Jacobson,
S.A.,
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1997.
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aitz,
I.A.,
Gautam,
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fo rMicro-GasTurbineEngines,"
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an d
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Jacobson,
S.A.,"Aerothermal
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29thAIAA
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Lin,C.C.,Braff,
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15.Ayn,A.A.,Ishihara,K.,
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the
Electrochemical
Society,
Vol.
146,
Number
1,
January
1999,
pp .
339-349.
24.
Ayon,A.A.,Ishihara,K,.
Braff,
R.A.,Sawin,H.H.,
Schmidt,
M.A.,
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the
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R. ,
Ayon,
A.
A.,
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1999.
26.Ayon,
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Epstein,A:H.,Frechette,
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Nagle,
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an d
Schmidt,
M .A.,
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an d
Controlling
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aDeep
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to
Transducers'99,
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1999.
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M .
A.,
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Wafer
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A.,
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A.,
an d
Schmidt,
M.
A.,"Microfabricationof High
Temperature
Silicon
Devices
Using
WaferBonding
and
Deep
Reactive
Io nEtching",
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Journal
ofMicroelectromechanical
Systems,
Vol.
,
No.
2,June
1999,
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152-160.
30.
Ayn,
A.
A.,
Chen,
D.-Z.,
Braff,
R.
A.,Khanna,
R. ,
Sawin,
H.
H.,
Schmidt,M.
A.,
"A
novel
IntegratedProcessUsingFluorocarbon
Films
DepositedwithaDeep
Reactive
Ion
Etching
(DRIE)
Tool,"
Fall
Meeting
of
the
Materials
Research
Society,
Boston,
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29
-
December
3,1999.
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Chen,K-S,Ayon,A.,andSpearing,S.M .,"Controllingan dTestingthe
FractureStrength
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Society,
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32 .
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Braff,
R.A.,
Bayt,
R. ,Sawin,H.H.,Schmidt,M.A.,"Influence
of
Coil
Power
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the
EtchingCharacteristics
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a
High
DensityPlasma
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146,
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7,
1999.
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Epstein,
A.H.,
Jacobson,S.A.,Protz,J.M.,Frechette,
L.G.,"Shirtbutton-Sized
Gas
Turbines:
heEngineering
Challenges
ofMicro
High
Speed
RotatingMachinery,"
Plenary
Lecture,
th
nternational
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on
Transport
Phenomena
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of
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Honolulu,
HI,
March2000.
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A.H.,"TheInevitabilityofSmall,"AerospaceAmerica,March2000,pp .30-37.
35.
Ayon
A.A.,
Zhang
X.,
andKhanna
R.,
"Ultra
Deep
AnisotropieSiliconTrenches
Using
Deep
Reactive
Io n
Etching
(DRIE),"
Hilton
Head
Solid-State
Sensor
Actuator
Workshop,
HiltonHeadIsland,SC,
June
4-9,
2000,
pp .
339-342.
36 .
Epstein,A.H.,Jacobson,S.A.,Protz,
Livermore,C,Lang,J. ,Schmidt,M.A.,
"Shirtbutton-
Sized,Micromachined,
GasTurbine
Generators,"
presentedat39
th
Power
Sources
Conference,CherryHill,NJ,June
2000.
37.
Ayon
A.A.,
Protz
J.M.,
Khanna
R,
Zhang
X.,
an d
Epstein
A.H.,
"Application
of
Deep
SiliconEtching
an d
Wafer
Bonding
in
the
MicroManufacturing
of
Turbochargers
an d
Micro-
Air-Vehicles,"the 47
th
International
Symposiumofthe
American
VacuumSociety,Boston,
M A,October
2-6,2000.
38 .
Zhang
X.,
GhodssiR. ,ChenK-S,AyonA.A.,an d
Spearing
S.M.,
"ResidualStress
Characterization
of
ThickPECVD TEOSFilmforPowerM E M SApplications,"HiltonHead
Solid-State
Sensor
Actuator
Workshop,
Hilton
Head
Island,
SC,June4-9,2000,
pp .316-
319.
39.Frechette,
L.G,
Jacobson,S.A.,
Breuer,
K.S.,
Enrich,
F.F.,
Ghodssi,
R. ,Khanna,
R. ,
Wong,
C.W.,
Zhang,
X.,
Schmidt,
M.A.,
an d
Epstein,
A.H.,
"Demonstration
of
a
Microfabricated
High-SpeedTurbine
Supported
on
Gas
Bearings,"
Hilton
Head
Solid-StateSensor
&
Actuator
Workshop,
HiltonHeadIsland,
SC,
June
4-9,2000,
pp .43-47.
40.
Orr,D.J,an dJacobson,S.A.,"High
Order
GalerkinModels
for
Gas
Bearings,"
submitted
to
the
Proceedings
oftheASME/STLE
Tribology
Conference,
paperASME/2000-TRIB-131,
Seattle,WA,
October2000.
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10/23
41 .
MehraA.,
Zhang
X.,
Ayon
A.A.,
Waitz
I.A.,an d
Schmidt
MA.,
"A
Through-Wafer
Electrical
InterconnectforMulti-Level
M EM SSevices," JournalofVaccumScience
and
Technology
B,
Vol.18 ,
No.5,
pp .2583-2589,September/October
2000.
42 .
MehraA.,Zhang
X.,
Ayon
A.A.,Waitz
I.A.,SchmidtM.A.,and
Spadaccini
CM.,
"A
6-
Wafer
Combuston
System
for
a
Silicon
Micro
Gas
Turbine
Engine,"
to
appear
in
Journal
of
MicroElectroMechanicalSystems,
December
2000.
43 .
Ayon,
A.A.,"T ime
Multiplexed
Deep
Etching,"
Sensors,
Vol.
16 ,No.
9,
September 2000,
pp.4-73.
44.Ayon,
A.A.,
Bayt,
R.L.,
Breuer,
K.S.,
"Deep
Reactive
Io n
Etching:
Promising
Technology
for
Micro
and Nanosatellites,"submitted toJournalofSmart
MaterialsandStructures:
SpecialIssueon MEMSinSpace,June
2000.
45 .GhodssiR.,
Frechette
L.G.,
Nagle
S.F.,Zhang
X.,
Ayon
A.A.,SenturiaS.D. ,an dSchmidt
M.A.,
"Thick
Buried
Oxide
inSilicon
(TBOS):An
IntegratedFabrication
Technology
for
Multi-Stack
Wafer-Bonded
M EM S
Processes,"
Proceedings
of
the1999
International
Conferenceon Solid-StateSensors
and
Actuators,
Sendai,
Japan,
June
7-10,1999,
pp.
1456-
1459.
46 .
Ghodssi
R,
ZhangX.,
Chen
K-S,
Spearing
S.M.,
an d
Schmidt
M.A.,
"Residual
Stress
Characterization
of
Thick
PECVD
OxideFilmforM EM S
Application,"the46
th
International
Symposiumofthe
American
Vacuum
Society,
Seattle,
WA,
October
25-29,
1999.
47 .
Chen
K-S,Zhang
X .,
an d
Ghodssi
R,"Residual
Stress
an d
Failure
Modeling
of
Thick
PECVD Oxide
Films
forM EM S
Application,"
Proceeding
ofthe
st
jointChina/Taiwan
Symposium
on Microsystem
Technology,Tainan,Taiwan,
M ay
2000,
pp .264-269.
48 .
Chen
K-S,
Zhang
X.,
an d
Spearing
S.M.,
"Processing
of
Thick
Dielectric
Films
for
Power
M EM S:Stressan d
Fracture,"
Materials
Scienceof
Microelectromechanical
System
(MEMS)
DevicesIII,
Materials
Research
SocietySymposium,
Boston,
M A,
November27-
December
1,2000.
49.
Khanna
R,
Zhang
X.,Protz
J.M.,andAyon
A.A.,
"Microfabrication
Protocols
fo r
Multi-
Stack
Projects
Involving
Deep
Reactive
Io n
Etchingand Wafer-Level
Bonding,"
Sensors,
accepted,willbe
published
in
March
issue
of
2001.
50.
Ayon
A.A.,ZhangX.,
an d
Khanna
R,
"AnisotropieSilicon
Trenches30 0um
to500um
DeepEmployingTime
Multiplexed
DeepEtching
(TMDE),"SensorsandActuators,
will
be
published
in
the
special
issue
for the
Hilton
Head
Solid-State
Sensor
&
Actuator
Workshop.
51.Zhang
X.,GhodssiR. ,Chen
K-S,
Ayon
A.A.,an d
SpearingS.M.,"Stress
an d
Fracture
in
ThickTetraethylorthosilicate
(TEOS)
Films,"
Sensors
and
Actuators,
will
be
published
in
the
specialissuefor
the
HiltonHead
Solid-State
Sensor
&
ActuatorWorkshop.
52 .
Savoulides,N.,
Breuer,
K.S.,
Jacobson,
S.,Ehrich,
F.F.,
"Low-OrderModels
fo r
VeryShort
Hybrid
Gas
Bearings,"
ASM E
Paper
2000-TRIB-12,
presented
at the
STLE/ASME
TribologyConference,
Seattle,
W A,
October
2000;also
to
appear
inJ.
ofTribology.
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11/23
4.1:
raduateTheses
1 .
roshenry,
C,
"PreliminaryDesignStudyof
a
Micro-GasTurbineEngine,"M.S.
Thesis,
M IT
Departmentof
Aeronautics
an d
Astronautics,
September
1995.
2.ehra,A.,"ComputationalInvestigationan dDesignofLow ReynoldsNumberMicro-
Turbomachinery,"
M.S.
Thesis,
M IT
Department
of
Aeronautics
an d
Astronautics,
June
1997.
3.
ur
Miranda,J.O.,
"Feasibility
of
ElectrostaticBearingsforMicroTurboMachinery,"
M.Eng.
Thesis,M IT
Department
ofElectrical
Engineering
an d
Computer
Science,
December
1997.
4.zeng,Y-S,"A nInvestigation
of
MicrocombustionThermalPhenomena, M.S.Thesis,
M IT
Department
of
Aeronauticsan dAstronautics,June
1997.
5.
hirley,G.,AnExperimental
Investigation
of
a
Low
ReynoldsNumber,High
Mach
NumberCentrifugal
Compressor,"
M.S.Thesis,M IT DepartmentofAeronautics
and
Astronautics,
September1998.
6.
hen,
K-S,
"MaterialsCharacterizationan dStructuralDesignofCeramicMicro
Turbomachinery, Ph.D.Thesis,M IT Departmentof
MechanicalEngineering,February
1999.
7.
in ,C.C.,"Development
of
a
MicrofabricatedTurbine-Driven
Air
Bearing
Rig,"Ph.D.
Thesis,
M IT Department
of
Mechanical
Engineering,
June
1999.
8.ohner,
K.,
"Microfabricated Refractory
CeramicStructures
for
Micro
Turbomachinery,"
M.S.
Thesis,
M IT
Department
ofAeronauticsan d
Astronautics,
June
1999.
9.
hen,D-Z,
"Designan d
Calibration
of
an
InfraredPositionSensor,"M.S.
Thesis,
M IT
Department
of
MechanicalEngineering,
June
1999.
10.Walters,
D .,"CreepCharacterization
of
SingleCrystal
Silicon
in
Support
of
the
M IT
Microengine
Project,"
M.S.
Thesis,
M IT Departmentof
MechanicalEngineering,June
1999.
11.Mehra,A.,"Development
of
a
HighPowerDensityCombustion
Systemfo raSiliconM icro
Gas
TurbineEngine,"Ph.D.Thesis,M IT
Department
of
Aeronauticsan dAstronautics,
February
2000.
12.Orr,D.J.,"Macro-ScaleInvestigationof
HighSpeed
Gas
Bearings
fo r
M E M S
Devices,"
Ph.D.Thesis,
M IT
Departmentof
Aeronauticsan dAstronautics,
February2000.
13.
Piekos,E.,"Numerical
Simulation
of
Gas-LubricatedJournalBearingsfo rMicrofabricated
Machines,"
Ph.D.
Thesis,
M IT Department
of
Aeronautics
an dAstronautics,
February
2000.
14.Savoulides,
N.,
"LowOrderModelsforHybridGas
Bearings,"M.S.Thesis,
M IT
Department
of
Aeronautics
an d
Astronautics,February2000.
15.Liu,C,"DynamicalSystem
Modelingofa
MicroGasTurbineEngine, M SThesis,M IT
Department
of
Aeronautics
an d
Astronautics,
June2000.
16.Lee,Jin-Wook,
"NumericalSimulation
of
a
HydrogenMicrocombustor, M.S.Thesis,
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12/23
M IT
Department
of
Aeronautics
an d
Astronautics,M ay
2000.
17.Nagle,S.F.,
"Analysis,
Design
an d
Fabrication
O fAnElectricInductionMicromotorfo r
a
MicroGas-Turbine
Generator",
Ph.D.Thesis,
M IT
Department
ofElectrical
Engineering,
October
2000.
18 .
Frechette,
L.G.,
"Development
of
a
Microfabricated
Silicon
Motor-Driven
Compression
System,"
Ph.D.
Thesis,
M IT
DepartmentofAeronauticsan dAstronautics,
September
2000.
10
8/10/2019 H. Epstein - Shirtbutton-sized Gas Turbines . the Engineering Challenges of Micro High Speed Rotating Machinery
13/23
5.0OVERVIEWTECHNICAL PAPER
SHIRTBUTTON-SIZED
GA S
TURBINES:
T HEENGINEERING
CHALLENGESOF
MICRO
HIGH
SPEED
ROTATINGMACHINERY
AlanH.Epstein,Stuart
A.
Jacobson,
Jon
M.Protz,LucG.
Frechette
GasTurbineLaboratory
MassachusettsInstitute
of
Technology
Cambridge,
MA
02139,USA
Fax
617-258-6093,
epstein@mit.edu
KEYWORDS: MEMS,
microturbine,microcombustion,
microbearings
ABSTRACT
MIT
sdeveloping
micro-electro-mechanical
ystems
(MEMS)-based
gas
turbine engines,
turbogenerators,
and
rocket
engines.Fabricatedin largenumbers
in parallel
using
semicon-
ductormanufacturing
techniques,
hese
engines-on-a-chip
re
basedon micro-highspeedrotatingmachinerywithpower den-
sities
approaching
those
of theirmore
familiar,
full-sized
breth-
ren.
T he
micro-gas
turbine
is
a
2
cm
diameter
by
3
mm
thick
Si
or
SiC
heat
engine
designed
toproduceabout
10
W
of
electric
poweror0.1
Nof
thrustwhile
consumingabout
15
grams/hr
of
H
2
ater
versions
may
produce
up
to
100
W
using
hydrocar-
bo nfuels.
hi s
paper
gives
an
overview
ofthe
project
and
dis-
cusses
he
challengesaced
inthedesign
an d
manufacture
of
high
speed
microrotating
machinery.
luid,structural,bearing,
and rotordynamics
design
issuesare reviewed.
INTRODUCTION
Highspeed
rotatingmachinery
comesin
manysizes.
n
recent
years
much
emphasis
has
been
placed
on
the
large
end
of
the
business
-0mdiameter
hydroelectric
turbines,
30 0
ton
ground-based
ga sturbinegenerators,
3
m
diameter
aircraft
en-
gines.
hese
machines
are
engineered
to
produce
hundreds
of
megawatts
of
power.
T he
focus
of
this paper
isthe oppositeen d
ofthe
rotating
machine
sizescale,devicesa
few
millimeters
in
diameter
and weighinga gram or
two.
These machinesareabout
one
thousandth
the linearscaleoftheir
largest
brethren an dthus,
since powerlevel
scales
withfluid
mass
flowrate
an dflowrate
scaleswith intakearea,they should produceaboutonemillionth
the
powerlevel,a few
tens
of
watts.
T heinterestinrotatingmachinery
of
thissize
rangeis
fu-
eled by
both
a
technology
pushan da user
pull.
T hetechnology
push
is
the
development
of
micromachining capability based
on
semiconductor
manufacturing
techniques.
This
enables
the
fab-
rication
of
complexsmall
parts
andassemblies
-devices
with
dimensions
in
the1-10,000
micron
size
range
with
micron
an d
evensubmicron
precision.
uc h
parts
are
produced
usingpho-
tolithographydefined
featuresan d
many
can
be
madesimulta-
neously,
holding
ou tthe
promiseof low
production
cost.
uc h
assemblies
re
knownasmicro-electrical-mechanicalystems
(MEMS)
an d
have
been
the
subject
of
thousands
of
publications
overthe lastdecade.arlywork
in
M E M Sfocusedonsensors
an dmanydevicesbased
on
this
echnology
are
inlarge
scale
production
(such
as
pressure
sensorsand
airbag
accelerometers
for
automobiles).
orerecently,workhasbeendoneon
actua-
tors
of
varioussorts.luid
handling
is
receiving
ttentions
well,
for
exampleMEMS
valvesare
commerciallyavailable.
T he
user
pull
is predominatelyone
of
electric
power.There
is
proliferation
ofsmall,
portable
electronics
-
computers,
digi-
talassistants,cell
phones,
GPS
receivers,
etc.
-
which
require
compact
energysupplies.hedemandisfo renergy
supplies
whoseenergy
and
power
density
exceed
that
of
thebest
batter-
ies
available
today.
Also,
the
continuing
advance
in
microelec-
tronics
permits
the
shrinking
of
electronic
subsystems
of
mobile
devices
uc h
as
robots
nd
ai r
vehicles.
hese
mall,
nd
in
some
cases
very
small,
systems
requireincreasing compact
power
an d
propulsion.
Fo rcompactpower
production,
hydrocarbonfuels
burned
inai rhave
20-30
timesthe
energy
density
of
thebestcurrent
lithium
chemistry-based batteries.
Thermalcycles
andhigh
speed
rotating
machinery
offerhigh
powerdensity
compared
to
other
power
productionschemes
and
MEMS
technology is
advancing
rapidly.
ecognizing
thesetrends,
group
at
M IT
began
re-
search
inthe mid1990's
ona
MEMS-based"micro-gas
turbine
generator"capable
of
producing tens
ofwatts
of
electrical power
8/10/2019 H. Epstein - Shirtbutton-sized Gas Turbines . the Engineering Challenges of Micro High Speed Rotating Machinery
14/23
from
acubic
centimeter-sized
package
Epstein
an d
Senturia,
1997;
Epstein
et
al
1997).
ince
that
time,relatedeffortshave
been
started
on
a
micromotor-driven
ai r
compressor
and
abi -
propellant,
liquid
rocket
motor
which
utilize
muchof
the
same
technology
as
the
gas
turbine.
he
attractiveness
of
these
de-
vicesispredicatedtoalargepartontheir
high
powerdensity.
T he
power
density
is
function
of
the
designof
the
rotating
machinery.hi s
paperreports
on
this
work
in
progress,
with
emphasison the
rotating
machinery
development.
SYSTEM
DESIGNCONSIDERATIONS
At
length
scalesofa
few millimeters,thermodynamiccon-
siderationsar eno
differentthanfo rmuchlargerdevices.
Thus,
high
power
densityfo ra
simple
Brayton
cycle
requires
high
com-
bustorexit
temperatures
(1400-1800K)
and
pressure
ratiosabove
2 and preferably
above
4.Thisca n
be
seeninthethermodynam-
ic s
ycle
calculation
llustratedinFigure whichshows
ha t
severaltensofwattscan
be
expected
froma
machine
with
a
m m
2
intakearea.
T he
power
density
ofrotatingmachinery,
both
fluid
an d
electric,
scale
with
the
square
of
the
peripheral
speed,
asdoesthestressin the rotor.Thus,
high
power
densityimplies
highly
stressedrotating
structures.
eripheralpeeds
of300-
600m/sare neededtoachieve pressureratiosinthe2:1-5:1per
stage
range,
assuming
centrifugalturbomachinery.This implies
rotorcentrifugal
stresses
on
the order of
hundreds
of
megaPascals.
These
peripheral
speeds
in
rotors
a
few
millimeters
in
diameter
requirerotationalratesonth eorderof
1-3
millionrpm.
hus,
low frictionbearingsare needed.Also, highspeed rotatingma -
chinery
generally
requires
high
precision
manufacturing
to main-
tain
tight
clearanceand
good
balance.For
millimeter-sized m a-
chines
tohave
th e
samefractional
precisionasmeter-sized
de-
vices,
th e
geometric
precision
requirementsare on theorder
of
a
micron.
ABraytoncycle
is
notth eonlychoiceforpowerproduc-
tionfrom
MEMS-based
thermodynamic
cycle.ankine,
Stirling,
an dOtto
cycles
canallbeconsideredcandidates.he
advantages
an d
disadvantages
of
each
differ.
Theprincipal
ad-
vantages
of
the
Brayton
cycle
ar e
simplicity
(only
on emoving
part,
a
rotor,
is
needed),highest
power
density
(duetothe
high
throughflowMach
numberan d
thus
high
mass flow
per
unitarea),
and the availability ofcompressedai r
for
coolingan d
other
uses.
T heprimary
disadvantage
is
ha t
aminimum
component
effi-
ciency
(on
the
order
of
40-50%)mustbemetfo r
the
cycle
to
be
Table
1:
Micro
vs .
MacroMaterial
Properties
1
3
.2 .
c
o
CO
0.8.
u.
o
.6-
'5
4:1
Pressure
Ratio
0.6
1200K
Q.
03 0.4-
Ni-based
Titanium
Macro Micro
Super
Alloys
Alloys
(Micro)
Ceramics
Silicon
Centrifugal
Stress
[V^TpJ
(m/s)
33 0 42 0
42 0
1000
(670)
ThermalStress
2 .7
x
10"
3
1.2
xlO
3
2. 0x10"
3
0.9 x
10"
3
[ctE/a
f/y
]
(1.1
xlO
-3
)
Stiffness
-26 -25
-95
-70
[E/p](MPa/Kgm"
3
)
M ax
Temp(C)
limiting
factor
-1000
(creep)
-300
(strength)
-1500
(oxidation)
-600
(creep)
10
0
0
0
ShaftP o w e rOutpu t(watts)pe rm m
2
Inlet
Area
50
Figure
1:
Simple
cycle
ga s
turbine
performance
with
H2fuel.
self-sustaining;onlythencanne t
power
he
produced.
As thisis
a
first-of-its-kind
effort
thatchallenges
the
capabilities
ofsev-
eral
disciplines,
especially
microfabrication,
simplicity
isa
very
desirable
irtue.
he
Brayton
ycle
eems
most
attractive
in
thisregard.
MechanicsScaling
Thermodynamic considerations
do not
changeas
machines
become
smaller,butmechanicsconsiderationso.tructural
mechanics,
fluid
mechanics,an d
electromechanicsall change
in
a
manner
important
to
machine performance
an ddesign as
length
scaleisdecreased by
a
factorof
order1000.
For
structural
mechanics,
it
is
th echangeinmaterialprop-
ertieswithlengthscalethat
is
mostimportant.
A relatively small
se tofmaterialsar eaccessibletocurrentmicrofabricationtech-
nology.
Themostcommonlyused
by
far
is
Silicon(Si),
while
SiliconCarbide(SiC)an dGallium
Arsinide(GaAs)areusedto
datemainlyin nicheapplications.ormanyof
themetrics
im -
portanttohighspeedrotating
machinery,
Si
an d
Si Caresupe-
riortomost
commonly
usedmetals
such
as
steel,ti tanium,an d
nickel-basedsuperalloys(Spearingan dChen,
1997).
Thisca n
be seenin Table whichcomparesmaterialsin termsof
proper-
tiesimportantforcentrifugalstress,thermalstress,
vibrations,
an d
hot
strength(Figure
2)
(Mehra,
2000).
Materials
such
as
Si
an d
S iC
are not
used
inconventional-sizedrotating
machinery
because
theyarebrittle.Theirusablestrengthisdominatedby
flaws
introduced
inmanufacturing
an dflaw
population
gener-
allyscales with
partvolume.However, thesematerialsare
avail-
able
intheformrequiredfo rsemiconductormanufacture
(thin
wafers)
withanessentiallyperfect,
singlecrystal
structure.
s
such,
they
have
high
usablestrength, values aftermicromachining
above
4
GP a
have
been
eported
Chen
etal
1998),
everal
times higher than
that
ofrotating
machinery
metallic
alloys.
This
higher strengthcan
be used
either to
realize
higher
rotationspeeds
(and
thus
higher
power
densities)
t
constant
geometry,
or
to
simplifythe
geometry
(and
thus
th e
manufacturing)
at
constant
peripheral
speed.
o
date,we haveadoptedth elaterapproach
for
expediency.
n
dditional
materials
consideration
is
ha t
thermal
shock
increaseswith
length
scale.Thus,materials
which
haveveryhightemperaturecapabilitiesbut
ar e
no t
considered
high
temperature
structural
ceramicssuch
as
lumina
or
sap-
phire)
du eto
their
susceptibility
to
thermal
shock,
ar e
viable
at
themillimeterandbelow
ength
scales. Since
these
have
no t
8/10/2019 H. Epstein - Shirtbutton-sized Gas Turbines . the Engineering Challenges of Micro High Speed Rotating Machinery
15/23
aW
CO
I1 0
2
10
C VD
Si C
/V\-HastelloyX
;n n
V
nconel
60 0
20 0
SI
1
\
60 000 040 0
Temperature
K )
\
C VD
SiC
25~10
5
-22S5+\
8/10/2019 H. Epstein - Shirtbutton-sized Gas Turbines . the Engineering Challenges of Micro High Speed Rotating Machinery
16/23
Starter/
Flame
ue
ue
enerator
Hnlrfpfs
u,Su
r Compressor /
olders
Man
,
d
lnject
Va nes
Blades / **
lnlet
Rotor
Turbine
urbine
xhaust
enterl ine
Nozzleotor
ozzle
f
Rotation
Va nes
Blades
Figure
4:
Baseline
design
microengine
cross-section.
fromhu b
to
tip.
urrent
technology
canyield
a
taper
unifor-
mityof
about
30-50:1
with
eitherapositiveornegativeslope.
The
constraints
onairfoilheights
are the
etch
rate(about3
m i-
crons
perminute)
an d
centrifugal
bending
stresstthe
blade
root.urbomachinesofsimilargeometryhavebeenproduced
with
blade
heights
ofover
40 0
microns.
The
effort
describedherein
has
been
ocussed
on
micrornachinery
which
ar e
producedwithsemiconductor
fabri-
cation
technology
(MEMS).ther
manufacturingtechniques
m ay
be
feasible
as
well,
especially
as
the
device
sizegrows
into
the
centimeter
range.
he
M E M S
approach
w as
chosen
here
because
it
is
intrinsically
highprecision
and
parallel
production,
offering
the promiseofverylow costinlarge
quantity
produc-
tion.
nitial
estimates
suggest
that
th e
cost
per
unit
power
might
ultimately
approach
that
oflargegas
turbine
engines.
GA STURBINE
ENGINE
Considerations
such
as
thosediscussed
above
led
in
1996
tothe
preliminary or "baseline"
gas
turbine
engine
designillus-
trated
in
Figure
4.
he
cm
diameter
engine
is
a
single-spool
arrangement with
a
centrifugal
compressor
an d
radial
inflowtur-
bine,separated
bya
hollowshaft
for
thermalisolation,and
sup-
ported
onai r
bearings.Ata
tip
speed
of
50 0m/s,
the
adiabatic
pressureratio
isabout4:1.
T he
compressor
is
shrouded
an dan
electrostaticstartergeneratorismountedonthetipshroud.T he
combustor
premixes
hydrogen
fuel
an d
ai r
upstream
of
flame
holdersan d
burns
lean
(equivalence
ratio
0.3-0.4)
so
that
th e
combustorexittemperatureis60 0K,withinthetemperature
capabilities
ofan
uncooled
S iC
turbine.
Thedesign philosophy
was
to
use
a high
turbine
inlet
temperature
to
achieve
acceptable
work
perunitai r
flow,
recognizingthatcomponent
efficiencies
wouldberelatively
low
an d
parasitic
losseshigh.Witha4mm
rotordiameter,theunitw assizedtopump
0.15
gram/secof
air
an d
produce
10-20
wattsof
power
at
2 .4
million
rpm.heen -
gine
is
constructed
from
8
wafers,
diffusion-bonded
together.
The
turbine
wafer
wa sassumed to
beSiC.
This
design
served
as
abaselinefo r
the
research
in componenttechnologiesdescribed
inlatersections.
On e
primary
goalofth e
projectis
to
show
that
aMEMS-
based
gas turbine
is
indeed possible,
bydemonstrating
benchtop
operation
ofsuchadevice.
This
impliesthat,for
afirst
demon-
stration,itwould
be
expedientto
trade
engineperformancefo r
simplicity,
especially
fabrication
simplici ty.
y
1998,
th e
req-
uisite
technologies
were
judged
sufficiently
advanced
tobegin
building
such
an
engine
with
th e
exception
of
fabrication tech-
nology
for
SiC.Since Si
rapidlylosesstrength
above 95 0
K,this
becomes
an
upper
limit
to
the
turbine
rotor
temperature.ut
95 0K
is
toolow
acombustorexittemperaturetoclosetheen-
gine
cycle
(i.e.
produce
net
power)
with
th e
component
efficien-
ciesavailable,
so
turbinecooling
is
required.Thesimplest
wa y
to
cool
the
turbine
ina
millimeter-sized
machine
is
toeliminate
the
shaft,an dthus
conductthe
turbineheatto
the compressor,
rejecting
the
heat
to
the
compressorfluid.hi s
has
thegreat
advantageofsimplici tyandthegreatdisadvantageoflowering
the
pressure
ratio
of
the
now non-adiabaticcompressor
from 4:1
to
2: 1
with
a
concomitantdecreasein
cyclepoweroutput
an d
efficiency.ThisexpedientarrangementisreferredtoastheH
2
demo
engine.
t
is
a
gas
generator/turbojet
designed
with
the
objectiveof
demonstrating
th econceptofaM E M Sgasturbine.
It doesno t
contain
electrical
machinery.
The
H
2
demo
enginedesign
is
hown
inFigure5.he
compressor
an d
turbine
rotor
diameters
re
8
m m
and
6
mm
respectively
(sincethe turbinedoesnotextractpowerto
drive
a
generator,
itssize
andthus
its
coolingloadcouldbereduced).
The
compressordischarge
ai r
wraps
around
the
outside
ofthe
combustor
to
cool
the
combustor
walls,capturing
the
waste
heat
an d
so
increasing
the combustorefficiencyand
reducing
the ex -
ternalpackage
temperature.
T herotorissupported
on
a
journal
bearing
on the
periphery
ofthe
turbine
and
by
thrust
bearings
on
the
rotor
centerline.The
peripheral
speed
ofthe
compressor
is
500m/ ssothatth erotationrateis
1.2Mrpm.xternalairis
usedto
start
the
machine.
With
400
microntallairfoils,the unit
is
sizedto
pump
0.36
grams/sec
of
air,
producing
11 rams
of
thrust
or17wattsof
shaft
power.
irst
testsofthis
engine
are
scheduledfo r
2000.
COMPONENT
TECHNOLOGIES
Given
the
overview ofthe system design
requirements
out-
lined
above,the
followingsectionsdiscuss
technical
consider-
ationofthe component
technologies.
For
each
component,the
overriding
designobjective
is
to
devisea
geometry
which
yields
th e
performance
required
by
the
cycle
while
being
consistent
with
near-term
realizablemicrofabricationtechnology.
A
problemcommonto
allofthecomponenttechnologies
is
that
of
instrumentation
an d
testing.
t
device
sizes
of
mi -
Start ing Fuel
.air
in
por t
Com pressor Diffuser
Inlet
\
an e
Combustor
Exhaust
Turbine
1mm
Nozzleguide
vane
Figure
5:
H
demoenginewith
silicon,
cooled
turbine.
8/10/2019 H. Epstein - Shirtbutton-sized Gas Turbines . the Engineering Challenges of Micro High Speed Rotating Machinery
17/23
crons
to
hundreds
of
microns,
instrumentation
cannotbe
pur-
chasedan dtheninstalled,ratheritmustbefabricatedintothe
devicefromthe
start.
While
technicallypossible,this
approach
ca neasily
doubleth e
complexity
ofthemicrofabrication,
nd
thesedevices
are
already
on
theedge
ofthestate-of-the-art.T o
expedite
the
development
process
therefore,
whenever
possible
development w as
done
insuperscalerigs,rigslarge
enough
for
conventional
instrumentation.
Bearings
Asinallhighspeedrotatingmachinery,th erotormust be
supported
fo r
all
radial
and
axialloads
seen
inservice.nnor-
ma l
operation
this
load
issimply
theweightoftherotortimes
theaccelerations
imposed
(9
g's
for
aircraftengines).
fa
small
device
isdropped
onahard
floor
from two meters,severalthou-
sand
g's
are impulsively applied.An additional
requirement
for
portableequipment
is
thatth esupport
beindependent
of
device
orientation.he
bearings
an danyassociated
equipment
must
also
be
compatiblewith
the microdevice's
environment,
high
temperature
in
the
case
of
the
gas
urbine
engine.
revious
M E M S
rotating
machines
havebeen
mainly
micromotors
turn-
in g
atsignificantlyowerspeedsha nofinteresthereand
so
couldmake
dowithdryfriction
bearingsoperating
forlimited
periods.
The
higher
speeds
needed
and
longer
lives
desired
for
micro-heatenginesrequirelow frictionbearings.Bothelectro-
magnetic
an d
ai rbearings
have beenconsidered
for
this
applica-
tion.
Electromagneticbearings
can
beimplementedwitheither
magnetic or
electric
fieldsproviding
therotorsupportforce.
Mag-
netic
bearings
have
tw o
disadvantages
fo r
this
application.
First,
magnetic materials
are
no t
compatible withmost
microfabrication
technologies,
limitingdevicefabricationoptions.
econd,Cu -
rie
point
considerations
limit
the
temperatures
twhich
mag-
netic
designscan
operate.ince
these
temperature
ar e
below
those
encountered
in
th e
micro-gas
turbine,
cooling
would
be
required.
Fo r
thesereasons,effortw asfirst concentrated on de-
signsmploying
electric
fields.hese
designs
examined
di d
not
appear
promisingin
that
th eforcesproduced
were
marginal
comparedtothe bearingloads
expected
(Miranda,
1997).
Also,
sinceelectromagnetic bearingsare
unstable,
feedbackstabiliza-
tion
is
needed,adding
tosystem
complexity.
Air
bearings
supporttheir
loadon
thin
ayersofpressur-
ized
air.
f
the
ai r
pressure
is
supplied
from
an
external
source,
the
bearingis
known
as
hydrostatic.ftheai r
ressure
is
de-
rivedfromthe
motion
ofthe rotor,thenthe designishydrody-
namic.
Hybridimplementations combiningaspectsofboth
ap-
proaches
are
also
possible.
Since
the
micromachines
in
question
includeai rcompressors, bothdesignsareapplicable.Eitherap-
proach
ca n
readilysupport
theloads
of
machines
in
this
size
rangeandca nbeused
on
hightemperaturedevices.ll
else
beingthe
same,the
relative
load-bearing capability
ofan
ai r
bear-
ing improvesassizedecreases
since
the surfacearea-to-volume
ratio(andthusthe
inertial
load)scalesinversely
with
size.Ro -
to ran dbearingdynamicsscalingis
more
complex,
however.
T he
simplest journal
bearing
is
a
cylindricalrotor
within
a
close-fittingcircularjournal
Figure
6) .
his
eometrywa s
adopted
firstasthe
easiest
to
microfabricate.
Other,
more
com-
plex
variations
might
include
wave
bearings
an d
foil
bearings.
The
relevant
physicalparametersdeterminingthe
bearingbe-
haviorare the length-to-diameterratio
(L/D)\
the
gap
between
th erotor
nd
journal
atioed
to
he
rotor
radiusc/R);nd
nondimensional
forms
oftheperipheralMach
numberofthe
rotor(a measure
of
compressibility),
th e
Reynolds
number,
an d
themassoftherotor.
ora
bearing
supported
onahydrody-
namic
film,he
oa d
bearing
capability
cales
nverselywith
(c/R)
5
which
tends
todominatethe design
considerations.
Load
bearing
alsoscales
withL/D
(Piekos
et
al.,
1997).
T he
designspace
available
fo rth emicrorotating
machin-
ery isconstrained
by manufacturing capability.
We havechosen
tofabricate
the rotorand journal
structure
atthe
same
time
to
facilitatelo w
cost,
volumemanufacturing.
Themost
important
constraint
is
the
etching
ofverticalsidewalls.
y
pushing
the
limitations
of
published
etching
technology,w ehave
been
able
toachievetaper ratiosofabout30:1
-50:1
on narrow etched
ver-
tical
channels
for
channel
depths
of
300-500
microns
as
shown
inFigure
7
(Lin
et
al.,
1999).
hi s
capability
definesthebear-
ing
lengthwhilethe taperratiodelimitsth ebearing
gap,
c.
T o
A
C
D
Hydrodynamic
pressureforce
Figure6 :Gasbearing
geometry
and nomenclature.
The
gap,
c ,
is
greatly exaggerated
in this
figure.
S-:-;7v,i
Figure7:Narrow trenches can
be
etchedto serveas
journal
bearings.
8/10/2019 H. Epstein - Shirtbutton-sized Gas Turbines . the Engineering Challenges of Micro High Speed Rotating Machinery
18/23
Air
Exhaust
Ar
(4)
Instrumentation
Port(4 )
LAYERS
Forward
Foundation
Ar
Inlet
-
Forward
Thrust
Bearing
Turbine
Aft
Thrust
.
Bearing
&Side
Pressurization
Aft
Foundation
Figure 8a:
Explodedviewof five
layers
comprising
the
turbine
bearing
rig.
minimize
gap/radius,the
bearing
should
be
on the
largest
diam-
eter
available,theperipheryof
the
rotor.
he
penalty
fo r
the
high
diameter
is relatively
high
area an dsurface
speed
(thusbear-
in g
drag)
an d
low
UD
(therefore
reduced
stability).
In
the
radial
turbineshowninFigure3,
the journal
bearing
is30 0
microns
long
with
an
UDof0.075,
c/Rof0.01,
an d
peripheral
Mach
number
of
1.
hi s
relativelyshort,
wide-gapped,
highspeed
bearing
is
well
outsideth e
range
ofanalytical
an dexperimental
resultsreportedinthe
gas
bearingliterature.
Stability
is
an
importantconsideration
fo rall
highspeed
rotatingmachines.
When
centered,
hydrodynamicbearings
are
unstable,speciallytlow rotational
speed.
Commonly,uc h
bearings arestabilized
by the
application ofaunidirectional
force
which
pushes
therotor
toward
the
journalwall,
as
measured
by
th e
eccentricity,
the minimumapproachdistanceofthe rotorto
th e
wall
as
a
fractionof th eaverage
gap
(0=
centered,
=wall
strike).At conventional
scale,
the
rotorweight
is
often
thesource
of
this
side
force.
Atmicro
scale,
(1 )
the rotorweightis
negli-
gible,
an d
(2 )
insensitivity
to
orientation
is
desirable,
so
we
have
adopteda
scheme
which
usesdifferential
gas
pressuretoforce
therotor
eccentric.
xtensive
numericalmodeling
of
these
microbearing
flows
have
shown
that
the
rotor
willbe
stableat
eccentricitiesabove0.8-0.9(Piekosan dBreuer,
1998).
For
th e
geometryof
the
turbinein Figure
3,
the
rotor
must
thus
operate
between
1-2 micronsfrom the journalwall(Piekos
et
al.,
1997).
Thisimplies
that
deviations
from
circularity
of
the
journal
an d
rotormustbesmallcomparedto
1micron.
T otestthese
ideas,
tw ogeometricallysimilarturbine-bear-
Thrust-bearing
supplyplenum
Forward
Exhaust
n
J
st
bearing
JSp Af tthrust
f|||| bearing
Pressurizat ion
plenum
Figure
8b :
Five-layermicroturbinebearingrigwith
4
mm
dia
rotor.
in gtest
rigs
have
been
built
and
tested
using
the
samebearing
geometry,
on e
atmicroscale witha4mm diameter
rotor
and the
otheramacroscale
unit
26
times
larger.T hemacroversion
wa s
extensivelyinstrumented for pressure
and
rotormotionmeasure-
ments
(Orr,
999).
T he
microturbine
bearing
test
rig,
hownin
Figure8,consistsoffivestackedlayers,eachfabricatedfroma
single
Si
wafer(Linet
at.,
1999).
T he
center
wafer
is
theradial
inflow turbineofFigure 3,with a4,200micron diameter,300mi -
cronthick
rotor.
he
turbinerotor
is
aparallel-sideddiskwith
bladescantileveredfrom
on e
side.Whilesuchasimpledesignis
viablein
siliconabove
500
m/s,
the
centrifugal
stresses
are
too
high
fo r
metals withouttapering ofthe disk (so the macro version
islimitedto400 m/s).The waferson eithersidecontainthe thrust
bearingsan d plumbing
for
the side
pressurizationneeded
to
oper-
ate
the
rotor
eccentrically.
T he
outside
waferscontain
the
intake,
exhaust,an dvent
holes.
n
this
test
device
the thrust
bearingsare
hydrostatic,
pressurized
by
external
air,
an d
the
journal
bearing
ca noperate
in
eitherhydrodynamicor
hydrostaticmode.Figure 9
isdata
taken
from
an
optical speedsensor
duringhydrostaticbear-
ing operation.
Turbomachinery FluidMechanics
In
many ways
the
fluid
mechanics
of
microturbomachinery
are
similarto
that
of
large
scalemachines,
for
example,
high
t-2.
Twice
synchronous
C D
Q
-3
n
t>
4 -
C D
Q.
W
-5
Ro t
synchro
Bearing
natural
frequency
a r
nous
1
6
a.
o>-7
_l
-B
I/JIMMJW*
JUAJUIA
H
20000
40000
60000
FrequencyHz )
Figure9:
Speeddatafrommicroturbine
at
1. 2Mrpm.
8/10/2019 H. Epstein - Shirtbutton-sized Gas Turbines . the Engineering Challenges of Micro High Speed Rotating Machinery
19/23
3
0)
>
Q Z
N
o
z
v
Compressor
Design
Point
.
\
8/10/2019 H. Epstein - Shirtbutton-sized Gas Turbines . the Engineering Challenges of Micro High Speed Rotating Machinery
20/23
2 .4
2 .2
2 .0
g
1- 8
o
3
1 6
0
a
1. 4
1. 2
1.0
fflh
-s
Square
Inlet,Experiment
-SmoothIn let,
Experiment
I S ES ,
2DCFD
*
Dawes,
3D
CF D
Fue l
Fue l
ArIn
0. 4
.6
.8
.0.2
CorrectedMassFlow(fractionof
design)
1.4
Figure
12 :
Data
and
simulations
on
a
40 0
m/s
tip
speed
compressoratRe=
20,000.
dence
time
an d
effectively sets theminimum
volume
ofthecom-
bustorfo ragiven
mass
flow.
hemixingtime
ca n
scalewith
device
ize
bu t
the
chemical
reaction
times
ono t
(typically
mixing accountsformore
than90 % ofcombustorresidence
time).
Thus,the combustorvolumeisagreaterfraction of
a
microengine
than
a
large
engine,
by
afactorofabout
40
fo rthe
devices
de -
signed
to
date.
Another
difference
between
large
an d
micro scale
machines
is
the increasedsurfacearea-to-volume ratio
at
small
sizes.
hi s
impliesincreasedheat
loss
frommicrocombustors
but offersmoreareafor catalysts.
T he
design
detailsaredependent
on
th e
fuel
chosen.
ne
design
approach
taken
ha s
been
to
separate
the
fuel-air
mixing
from thechemical
reaction.
This
is
accomplishedby premixing
the
fuelwith
the
compressor
dischargeair
upstream
of
the
com-
bustorflame holders.
This
permitsa
reduction of
the combustor
residencetime
by
a
factor
of
about10
from th e
usual 5-10
msec.
T hedisadvantage
to
this
approach
is
susceptibility
to
flash-
back
fromthe
combustor
intothe
premixzone,which
must
be
avoided.
T oexpedite
the
demonstration
of
a
micro-gas
turbine
engine,hydrogen
wa s
chosenas
theinitialfuelbecauseofits
wide
flammability
limits
an d
fastreaction
time(this
is
the
same
approach
taken
by
vo n
Ohain
when
developing
the
first
jet
en -
ginein
Germany
in the1930s).Specifically,hydrogen willbum
at
equivalence
ratios
aslow as0. 3whichyieldsadiabatic com-
bustion
temperatures
below500K.ydrocarbonfuels
must
be
operated
closer
to stoichiometric
an d
therefore
at
higher
tem-
peratures,
above
2000K.hereduced
heat
load
fromth elow
temperature
combustion,
combined
with
th e
high
thermal
con-
ductivityofsilicon,means
that
silicon
(whichmeltsat1600K)
is
a
viablestructuralmaterialforaH2 combustor(Waitzeted.,
1998;
Mehra
er
a/.,1999a).
Silicon
combustorshave
been
built
which
duplicate
thege -
ometryof the enginesin
Figures
4and 5,withthe
rotating
parts
replacedwith
stationary
swirlvanes
(Mehra
and
Waitz,998).
The combustorvolumesar e66 an d190 m m
3
,respectively(Fig-
ures13
an d
14).
T he
designs
take
advantage
of
microfabrication's
Combustion
Flame
Turbine
Chamber
Holders
Nozzles
Figure
13 :
Microcombustor
which
comprises
the
static
structure
of
th e
demo
engine
ofFigure
S.
ability to
produce
similar
geometric featuressimultaneously.
Fo r
example,
the
largercombustor
ha s
90
fuelinjection
ports,
each
12 0micronsindiameter,
to
promote
uniform
fuel-air
mixing.
T he
smaller combustoroperating
at
a
power
level
of
20 0
watts
is
shownin
Figure
14 along with
a
CF D
simulation
of
the
flowfield.
These
tests
demonstrated
that
conductively-cooled
silicon
tur-
bine
vanescan survive
at
1800
K ga stemperaturesfor5
hrs
with
little
degradation(Figure
15).easurementshave
shown
that
thesemicrocombustorsca nachieve
efficiencies
inthemi d90%
range(Mehraetal.,1999b).
Data
at various
equivalence
ratios
(0 )
are
shown
in
Figure
6.(The
gaps
inthe
inesredu eto
thermocouplesailing
t
high
emperature.)Ignitionhas
ot
proven
a
problem;
a
simple
ho t
wire
ignitorha s
provensuffi-
cient,
even
at
room pressure
(Mehra,
2000).
Hydrocarbons
are more
difficult
to
burn.
nitialtests
show
that theexistingcombustorscanbumethylene
an d
propane,bu t
at
reduced
efficiencies
since
the
residence
times
are
too
short
Figure
14 :
200wattmicrocombustor.
8/10/2019 H. Epstein - Shirtbutton-sized Gas Turbines . the Engineering Challenges of Micro High Speed Rotating Machinery
21/23
Figure
15 :
Silicon
turbine
vanes
as
built
and
after
5
hours
in
1800
K
combustor
outflow.
for
complete
combustion.Otherapproaches
being
pursued
here
includea
stoichiometric zone-dilutionschemesimilar
to
con-
ventional
gasturbinecombustors
an dcatalytic combustion.
ElectricalMachinery
Microelectrical
machinery
isrequiredfo r
power
genera-
tion
and
as
a
prime
mover
fo r
a
starter
or
various
pumps
an d
compressors.
here
is
an
extensiveliteratureonmicroelectric
motors,
which
will
no t
be
reviewed
here,
bu t
little
work
ongen-
erators.Therequirements forth edevicesofinterestherediffer
from
previouswork
in
that
th e
power
densities
needed
are
at
least twoordersof
magnitude
abovethosereportedin thelitera-
turetodate.Also,the thermalenvironment
isgenerally harsher.
Integrating
theelectric
machine
within
th e
device(gas
turbine,
compressor,etc.)offerstheadvantageof
mechanical
simplicity
inthat
no
additional bearingsor
structures
are
requiredover
that
neededfo r
the fluid
machine.Also,
there
is
a
supplyof
cooling
fluidavailable.
As
in
the
caseof
electricbearings,
bothelectrostatic
an d
magnetic
machinedesigns
can be
considered
and,
to
first
order,
bothpproachesan
yieldaboutequivalentpowerdensities.
Sincethe
magnetic
machines
ar e
material
property-limited
at
highemperaturend
ecausef
he
hallengesf
microfabricating
magnetic
materials, lectrostatic
commonly
referred
toaselectric)
designswere
first
examined.Powerden-
sity
scales
withelectricfield
strength
(torque),
frequency,
an d
rotational
speed.The
micromachinery
of
interest
hereoperates
at
peripheralvelocities
1-2 ordersof
magnitude
higherthan pre-
viously
reported
micromotors, andso yieldsconcomitantly more
power.
lectric
machines
ma y
be
configured
in
many
ways.
Here
an
inductiondesignw aschosen(Bart
an d
Lang,
1989).
T he
operation
of
anelectricinductionmachineca n
be
un-
derstoodwith
reference
to
Figure
17
(Nagle
an d
Lang,999).
T hemachine
consistsof
tw o
components,
a
rotoran d
a
Stator.
T he
rotor
is
comprised
ofa5-20urn
thick
goodinsulatorcov-
ered