056
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$%6%,/0-%.4 /& ! #!4!,94)# 3),)#/. -)#2/ #/-"534/2 &/2 (9$2/#!2"/. &5%,%$ 0/7%2 -%-3 # - 3PADACCINI 8 :HANGo # 0 #ADOUp . -IKI AND ) ! 7AITZ 'AS 4URBINE ,ABORATORY -ASSACHUSETTS )NSTITUTE OF 4ECHNOLOGY #AMBRIDGE -! 53! o$EPARTMENT OF -ANUFACTURING %NGINEERING AND &RAUNHOFER 53! #ENTER FOR -ANUFACTURING )NNOVATION "OSTON 5NIVERSITY "OSTON -! 53! p$EPARTMENT OF !EROSPACE %NGINEERING 5NIVERSITY OF -ARYLAND #OLLEGE 0ARK -$ 53!
THE OBSTACLES TO SATISFYING THESE REQUIREMENTS ARE DIFFERENT FOR A MICRO SCALE DEVICE !S lRST DESCRIBED BY 7AITZ ET AL ;= A MICRO SCALE COMBUSTOR IS MORE HIGHLY CONSTRAINED BY INADEQUATE RESIDENCE TIME FOR COMPLETE COMBUSTION AND HIGH RATES OF HEAT TRANSFER FROM THE COMBUSTOR -ICRO COMBUSTOR DEVELOPMENT ALSO FACES UNIQUE CHALLENGES DUE TO MATERIAL AND THERMODYNAMIC CYCLE CONSTRAINTS 4HIS FUNDAMENTAL TIME CONSTRAINT CAN BE QUANTIlED IN TERMS OF A HOMOGENEOUS $AMKHLER NUMBER THE RATIO OF THE RESIDENCE TIME TO THE CHARACTERISTIC CHEMICAL REACTION TIME
!"342!#4 4HIS PAPER REPORTS THE DEVELOPMENT OF A HIGH EFlCIENCY HYDROCARBON FUELED MICRO COMBUSTION SYSTEM FOR A MICRO SCALE GAS TURBINE ENGINE FOR POWER GENERATION AND MICRO PRO PULSION APPLICATIONS ! THREE WAFER CATALYTIC COMBUSTOR WAS FABRICATED AND TESTED %FlCIENCIES IN EXCESS OF WERE ACHIEVED FOR ETHYLENE AIR AND PROPANE AIR COMBUSTION ! FABRICATION PROCESS FOR A SIX WAFER CATALYTIC COMBUSTOR WAS DEVELOPED AND THIS DEVICE WAS SUCCESSFULLY CONSTRUCTED ).42/$5#4)/. -)4S 'AS 4URBINE ,ABORATORY AND -ICROSYSTEMS 4ECH NOLOGY ,ABORATORY HAVE BEEN DEVELOPING A MICRO GAS TURBINE ENGINE CAPABLE OF PRODUCING 7 OF POWER IN A VOLUME LESS THAN CM WHILE CONSUMING GRAMS OF FUEL PER HOUR ;= !PPLICATIONS INCLUDE POWER GENERATION PROPULSION SYS TEMS FOR SMALL AIR VEHICLES AND A VARIETY OF BLOWERS COM PRESSORS AND HEAT PUMPS 0OWER DENSITIES FOR THESE DEVICES ARE ESTIMATED TO BE AN ORDER OF MAGNITUDE GREATER THAN THOSE FOR CURRENT BATTERY TECHNOLOGY !S PART OF THIS PROGRAM SEVERAL MICRO FABRICATED SIL ICON GAS PHASE COMBUSTION SYSTEMS HAVE BEEN DESIGNED FABRICATED PACKAGED AND TESTED ; = 4HE DEVELOPMENT OF THE MICRO ENGINE COMBUSTION SYSTEM HAS BEEN PURSUED THROUGH A COMBINED PROGRAM OF EXPERIMENTS NUMERICAL SIM ULATIONS AND ANALYSIS 7ITH HYDROGEN FUEL THESE SYSTEMS REACHED GAS TEMPERATURES EXCEEDING + COMBUSTOR EFl CIENCIES GREATER THAN AND POWER DENSITIES ABOVE -7M WELL WITHIN THE REQUIREMENTS OF A MICRO GAS TURBINE THERMODYNAMIC CYCLE (OWEVER FOR HYDROCARBON FUELS SUCH AS PROPANE THE POWER DENSITIES DROP TO APPROXIMATELY ONE TENTH OF THAT FOR HYDROGEN 4HESE FUELS ARE MORE CHALLENG ING TO MICRO COMBUSTOR DEVELOPMENT DUE TO THE FOLD INCREASE IN REACTION TIME WHEN COMPARED TO HYDROGEN (OW EVER THEY ARE REQUIRED FOR PRACTICAL DEVICES DUE TO THEIR INCREASED ENERGY DENSITY AVAILABILITY AND EASE OF STORAGE 4HIS HAS MOTIVATED THE CURRENT STUDY OF CATALYTIC COMBUSTORS AS A MEANS OF DEVELOPING A HIGH EFlCIENCY HIGH POWER DEN SITY HYDROCARBON FUELED MICRO COMBUSTOR
4O ENSURE A $AH GREATER THAN UNITY AND COMPLETE COMBUS TION A DESIGNER OF A MICRO COMBUSTOR CAN EITHER INCREASE THE mOW RESIDENCE TIME OR DECREASE THE CHEMICAL REACTION TIME 4HE CHARACTERISTIC COMBUSTOR RESIDENCE TIME IS GIVEN BY THE BULK mOW THROUGH THE COMBUSTOR VOLUME
VOLUME
60
T RESIDENCE z VOLUMETRIC FLOW RATE M24
WHERE 6 IS VOLUME 0 IS PRESSURE M IS MASS mOW 4 IS TEM PERATURE AND 2 IS THE GAS CONSTANT 2ESIDENCE TIME CAN BE INCREASED BY INCREASING THE SIZE OF THE CHAMBER REDUCING THE MASS mOW RATE OR INCREASING THE OPERATING PRESSURE ! CHEMICAL REACTION TIME CAN BE APPROXIMATED BY AN !RRHENIUS TYPE EXPRESSION
; FUEL =
T REACTION z
!; FUEL =A ;/ =B E
%A
24
%A IS THE ACTIVATION ENERGY WHILE ! A AND B ARE EMPIRICALLY DETERMINED 2EACTION TIME IS PRIMARILY A FUNCTION OF FUEL PROPERTIES AND THE MIXTURE TEMPERATURE AND PRESSURE 3INCE HIGH POWER DENSITY REQUIREMENTS MANDATE HIGH MASS mOW RATES THROUGH SMALL CHAMBER VOLUMES THE MASS mOW RATE PER UNIT VOLUME CANNOT BE REDUCED WITHOUT COMPRO MISING DEVICE POWER DENSITY (ENCE THERE IS A BASIC TRADEOFF BETWEEN POWER DENSITY AND mOW RESIDENCE TIME 0OWER DENSITY t
#(!,,%.'%3 !.$ #/.342!).43 4HE FUNCTIONAL REQUIREMENTS OF A MICRO COMBUSTOR ARE SIMILAR TO THOSE OF A CONVENTIONAL GAS TURBINE COMBUSTOR 4HESE INCLUDE THE EFlCIENT CONVERSION OF CHEMICAL ENERGY TO mUID THERMAL AND KINETIC ENERGY WITH LOW TOTAL PRESSURE LOSS RELIABLE IGNITION AND WIDE mAMMABILITY LIMITS (OWEVER
0-7803-7185-2/02/$10.00 ©2002 IEEE
T $AH RESIDENCE T REACTION
M F ,(6 6
t
M R t 6 T RESIDENCE
WHERE ,(6 IS THE LOWER HEATING VALUE OF THE FUEL AND R IS THE mUID DENSITY &OR A GIVEN OPERATING PRESSURE AND THUS DEN SITY AND ASSUMING A $AH OF UNITY REDUCING THE CHEMICAL REAC TION TIME AND THUS REQUIRED RESIDENCE TIME IS THE ONLY MEANS OF ENSURING COMPLETE COMBUSTION WITHOUT COMPROMISING THE HIGH
228
!IR
(YDROGEN
4OP PLATE
&UEL MANIFOLD INJECTOR PLATE 3PACER INLET HOLES
0LATINUM COATED FOAM
A
#OMBUSTION CHAMBER
MM #ENTER LINE
RECIRCULATION JACKET
3PACERINLET HOLES &LOW
COMPRESSOR BLADES
FUEL INJECTOR
&UEL MANIFOLD INJECTORS
#OMBUSTION CHAMBER
AIR
WAFER
COMBUSTION CHAMBER TURBINE TURBINE BLADES
WAFER
MM
WAFER WAFER
"OTTOM PLATE
B
&IGURE %XPLODED VIEW OF THREE WAFER COMBUSTOR CAT ALYTIC INSERT AND INVAR PLATES
WAFER WAFER MM
INVAR PLATES !S A RESULT INSERTING CATALYST MATERIAL INTO THE COMBUSTION CHAMBER CAN BE DONE OUTSIDE THE CLEAN ROOM ENVIRONMENT &IGURE SHOWS AN EXPLODED VIEW OF THE THREE WAFER COMBUSTOR THE SURROUNDING INVAR PLATES AND THE CATA LYTIC INSERT &IGURE SHOWS A PHOTOGRAPH OF THE NICKEL FOAM SUPPORT AS IT IS lT INTO A THREE WAFER DEVICE 0REVIOUS GAS PHASE EXPERIMENTS WITH THE THREE WAFER COMBUSTOR SHOWED THAT HYDROCARBON COMBUSTION COULD NOT BE SUSTAINED IN THE DEVICE DUE TO INSUFlCIENT RESIDENCE TIME AND SIGNIlCANT HEAT LOSS FROM THE STRUCTURE 7ITH THE CATA LYTIC FOAM PIECE COMBUSTION OF BOTH PROPANE AND ETHYLENE WAS ACHIEVED &IGURE SHOWS THE OVERALL COMBUSTOR EFl CIENCIES FOR BOTH FUELS IN THIS DEVICE OVER A RANGE OF MIXTURE RATIOS %FlCIENCIES IN EXCESS OF WERE ATTAINED IN BOTH CASES 4HIS EXPERIMENT REPRESENTS THE lRST STABLE HYDROCAR BON COMBUSTION IN THIS DEVICE )N ADDITION IT IS IMPORTANT TO NOTE THAT HYDROCARBON COMBUSTION WAS STABILIZED AT THE SAME MASS mOW RATE AS GAS PHASE HYDROGEN AIR COMBUSTION 4HIS RESULTED IN POWER DENSITIES APPROXIMATELY HALF OF THAT OBTAINED WITH HYDROGEN RATHER THAN THE FOLD DECREASE EXPECTED FOR HYDROCARBON AIR REACTIONS INDICATING THAT THERE IS SIGNIlCANT CATALYSIS OCCURRING
&IGURE 3CHEMATIC OF COMBUSTORS USED FOR CATALYTIC TESTS A THREE WAFER COMBUSTOR C SIX WAFER COMBUSTOR POWER DENSITY OF THE DEVICE !S WE MOVE TOWARD THE DEVEL OPMENT OF A HYDROCARBON FUELED COMBUSTOR THIS BECOMES A SIGNIlCANT CONCERN 4HE RATE OF A HYDROCARBON AIR REACTION IS TYPICALLY TIMES SLOWER THAN THAT OF A HYDROGEN AIR REAC TION &OR CONSTANT MASS mOW RATE AND VOLUME THIS LEADS TO A CORRESPONDING DECREASE IN POWER DENSITY &OR PRACTICAL DEVICES HYDROCARBON FUELS SUCH AS PRO PANE AND ETHYLENE WILL BE REQUIRED 3PADACCINI ET AL ;= HAVE SHOWN THAT WITH THESE FUELS POWER DENSITIES DECREASED BY APPROXIMATELY A FACTOR OF TEN 4HIS WAS PRIMARILY DUE TO THE INCREASE IN REACTION TIME 4O UTILIZE THESE HYDROCARBON FUELS EFlCIENTLY WITHOUT INCREASING COMBUSTOR VOLUME OR TEMPERATURE CHEMICAL CON VERSION RATES MUST BE INCREASED SIGNIlCANTLY #ATALYSTS ACT TO INCREASE THE $AMKHLER NUMBER BY LOWERING THE ACTIVATION ENERGY ASSOCIATED WITH THE REACTION THEREBY INCREASING REAC TION RATE 4O ACHIEVE THIS WE HAVE BEEN PURSUING THE DEVEL OPMENT OF CATALYTIC COMBUSTION SYSTEMS WHERE PLATINUM IS PLATED ONTO A NICKEL FOAM SUBSTRATE 4O ENSURE A HIGH DEGREE OF CHEMICAL CONVERSION THE CATALYST AND ITS SUBSTRATE MUST lLL MOST OF THE COMBUSTOR VOLUME (OWEVER TO SATISFY THE GAS TURBINE THERMODYNAMIC CYCLE THE COMBUSTOR PRESSURE LOSS SHOULD NOT EXCEED &!"2)#!4)/. !.$ 4%34 2%35,43 )NITIAL FEASIBILITY TESTS WERE CARRIED OUT ON THE THREE WAFER LEVEL COMBUSTOR SHOWN IN &IGURE A .ICKEL FOAM WAS SELECTED AS A CONVENIENT CATALYST SUPPORT MATERIAL AND WAS CONVENTIONALLY MACHINED TO lT IN THE COMBUSTION CHAM BER !N ELECTRON BEAM DEPOSITION PROCESS WAS USED TO COAT THE SUBSTRATE WITH PLATINUM WHICH SERVES AS THE ACTIVE CATA LYTIC SURFACE 4HE COMBUSTORS PACKAGING SCHEME CONSISTS OF PLACING THE SILICON BETWEEN TWO CONVENTIONALLY MACHINED
0-7803-7185-2/02/$10.00 ©2002 IEEE
&IGURE 4HREE WAFER COMBUSTOR WITH CATALYTIC INSERT
229
%FFICIENCY
A 0ROPANE
GS GS GS GS GS
%FFICIENCY
( 'AS 0HASE
%QUIVALENCE 2ATIO
B %THYLENE
#( #AT
( 'AS 0HASE GS GS
&IGURE #ATALYTIC PIECES BEING INSERTED INTO SIX WAFER COMBUSTOR
GS GS GS
%QUIVALENCE 2ATIO
ICALLY CLEANED TO REMOVE LARGE PARTICLES AND DEBRIS 4O PROTECT THE BONDING SURFACES FROM THE INSERTS A hSHIELD WAFERv WAS FABRICATED 4HIS WAFER WAS ETCHED THROUGH WITH THE SHAPE OF THE INSERTS AND SILICON DIOXIDE WAS DEPOSITED ON ITS SURFACE VIA PLASMA ENHANCED CHEMICAL VAPOR DEPOSITION 0%#6$ 4HE FOURTH lFTH AND SIXTH WAFER LEVELS WERE FUSION BONDED PRIOR TO INSERTING THE CATALYST 4HE SHIELD WAFER WAS THEN ALIGNED AND CONTACTED WITH THESE THREE LEVELS 7HILE IN CONTACT THE CATALYTIC PIECES WERE PLACED INTO THE BONDED WAFER STACK THROUGH THE SHIELD WAFER &INALLY THE PROTECTIVE SHIELD WAFER WAS CAREFULLY REMOVED 4HE 0%#6$ OXIDE PREVENTED THE SHIELD WAFER FROM ADHERING TO THE STACK )NSERTION OF THE CATALYTIC PIECES CAN BE SEEN IN &IGURE 7AFER LEVELS ONE TWO AND THREE WERE FUSION BONDED FORMING ANOTHER THREE WAFER STACK 4HE lNAL BONDING OF THE TWO THREE WAFER STACKS WAS UNSUCCESSFUL 4HREE BOND ING ATTEMPTS RESULTED IN THE BELIEF THAT THE THICKNESS OF THE STACKS PREVENTED BONDING 4HE THICK THREE WAFER STACKS WERE VERY STIFF AND UNABLE TO COMPLY TO EACH OTHERS SHAPE 3MALL
#( #AT
&IGURE 4HREE WAFER CATALYTIC COMBUSTOR EFlCIENCY ! SIX WAFER LEVEL COMBUSTION SYSTEM COMPATIBLE WITH CURRENT MICRO ENGINE DESIGNS IS SHOWN IN &IGURE B &OR THIS DEVICE THE PLATINUM COATED NICKEL FOAM INSERTS MUST BE PLACED INTO THE EXISTING DESIGN DURING THE FABRICATION PRO CESS IN THE CLEAN ROOM ENVIRONMENT 4HIS POSES SEVERAL CHALLENGES DUE TO CLEANLINESS OF THE INSERTS PARTICULATE CON TAMINATION OF BONDING SURFACES AND DAMAGE TO BONDING SUR FACES VIA MISHANDLING 0LATINUM WAS DEPOSITED ONTO THE NICKEL FOAM SUBSTRATE BY DIPPING THE FOAM INTO A SOLUTION CONTAINING PLATINIC ACID 4HIS WAS DONE TO OBTAIN BETTER COVERAGE AND PENETRATION OF PLATI NUM ONTO THE CATALYST SUPPORT 4HE INSERTS WERE THEN ULTRASON 7AFER MICRONS MASK DEEP ETCH 7AFER MICRONS MASKS SHALLOW DEEP ETCHES 7AFER MICRONS MASKS DEEP ETCHES
7AFER MICRONS MASKS SHALLOW DEEP ETCHES
#ATALYTIC INSERT 3HIELD WAFER MICRONS 0%#6$ OXIDE
7AFER MICRONS MASKS DEEP ETCHES
7AFER MICRONS MASK DEEP ETCH
WAFER COMBUSTOR WITH CATALYST 3HIELD WAFER REMOVED PRIOR TO BONDING
BOND
&IGURE &ABRICATION ASSEMBLY AND BONDING PROCESS FOR SIX WAFER CATALYTIC COMBUSTOR
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230
A
B
&IGURE )NFRA RED IMAGES OF THE SIX WAFER STACK WITH CATALYTIC INSERTS A AFTER INITIAL CONTACT B AFTER lNAL ANNEAL AMOUNTS OF BOW IN EITHER BONDING SURFACE COULD NOT BE OVER COME AND STIFFNESS OF THE WAFERS PREVENTED UNIFORM CONTACT FROM BEING ESTABLISHED !S A RESULT A NEW BONDING SEQUENCE WAS DEVELOPED 2ATHER THAN BONDING TWO THICK THREE WAFER STACKS TOGETHER THE LEVEL THREE WAFER WAS INDIVIDUALLY CONTACTED TO THE STACK WITH THE CATALYTIC PIECE 4HIS WAS FOLLOWED BY INDIVID UALLY CONTACTING AND BONDING THE LEVEL TWO AND ONE WAFERS !S A RESULT WAFER BONDING ON THE lNAL BUILD PRODUCED GOOD BOND QUALITY OVER MOST OF THE WAFER 4HIS ASSEMBLYBONDING PROCESS IS SHOWN IN &IGURE &IGURE A SHOWS AN INFRA RED IMAGE OF THE SIX WAFER STACK IMMEDIATELY AFTER CONTACT ! SMALL WELL CONTACTED REGION CAN BE SEEN IN THE CENTER OF THE WAFERS !FTER SEVERAL HOURS OF COLD PRESSING AT ATM AND # THERMAL PRESSING THE BOND FRONT PROPAGATED RADIALLY OUTWARD RESULTING IN DEVICE YIELD !N INFRA RED IMAGE OF THE lNAL BONDED STACK IS SHOWN IN &IGURE B
ALSO GRATEFUL TO $R ! !YON 0ROFESSOR - 3CHMIDT AND 0RO FESSOR ! %PSTEIN FOR THEIR INSIGHTFUL SUGGESTIONS TO + "ROD ERICK AND , (O FOR HELP DURING THE MICROFABRICATION TO 3 !JMERA FOR ASSISTANCE WITH PLATINUM COATING AND TO ALL THE OTHER MEMBERS OF THE -)4 MICROENGINE TEAM FOR THEIR HELP AND SUPPORT 4HE EDITORIAL ASSISTANCE OF $ 0ARK IS ALSO GRATEFULLY ACKNOWLEDGED 4HIS WORK IS SUPPORTED BY THE !RMY 2ESEARCH /FlCE $!!' UNDER $R 4 $OLIGALSKI AND BY $!20! $!!' $!"4 # UNDER $R 2 .OWACK AND $R 2 2OSENFELD RESPECTIVELY 2%&%2%.#%3 ;= %PSTEIN ET AL h-ICRO (EAT %NGINES 'AS 4URBINES AND 2OCKET %NGINESv PRESENTED AT THE TH !)!! &LUID $YNAMICS #ONFERENCE ;= -EHRA ET AL h$EVELOPMENT OF A (YDROGEN #OMBUSTOR FOR A -ICROFABRICATED 'AS 4URBINE %NGINEv PRESENTED AT THE 3OLID 3TATE 3ENSOR AND !CTUATOR 7ORKSHOP AT (ILTON (EAD ;= -EHRA ! h$EVELOPMENT OF A (IGH 0OWER $ENSITY #OMBUSTION 3YSTEM FOR A 3ILICON -ICRO 'AS 4URBINE %NGINEv 0H$ 4HESIS -ASSACHUSETTS )NSTITUTE OF 4ECH NOLOGY ;= -EHRA ET AL h! 7AFER #OMBUSTION 3YSTEM FOR A 3ILI CON -ICRO 'AS 4URBINE %NGINEv )%%%!3-% *OURNAL OF -ICROELECTROMECHANICAL 3YSTEMS ;= 7AITZ ET AL h#OMBUSTORS FOR -ICRO 'AS 4URBINE %NGINESv !3-% *OURNAL OF &LUIDS %NGINEERING 6OL PP -ARCH ;= 3PADACCINI ET AL h(IGH 0OWER $ENSITY 3ILICON #OM BUSTION 3YSTEMS FOR -ICRO 'AS 4URBINE %NGINESv TO BE PRESENTED AT !3-% )NTERNATIONAL 'AS 4URBINE )NSTITUTE 452"/ %80/
#/.#,53)/. !.$ &5452% 7/2+ 4HREE WAFER COMBUSTORS WERE lTTED WITH PLATINUM COATED NICKEL FOAM INSERTS AND USED AS AN INITIAL CATALYTIC COMBUSTOR TEST BED "OTH ETHYLENE AIR AND PROPANE AIR REAC TIONS WERE SUSTAINED AT OVERALL EFlCIENCIES IN EXCESS OF WHEN PREVIOUSLY IT WAS NOT POSSIBLE TO BURN HYDROCARBON FUELS IN THIS DEVICE 4HIS RESULTED IN POWER DENSITIES APPROXI MATELY HALF OF THAT OBTAINED WITH HYDROGEN RATHER THAN THE FOLD DECREASE EXPECTED FOR HYDROCARBON AIR REACTIONS ! METHOD FOR FABRICATING A SIX WAFER COMBUSTOR lTTED WITH PLATINUM COATED NICKEL FOAM HAS BEEN DEVELOPED &ABRICATION OF THESE DEVICES WAS COMPLETED 0RESSUREMASS mOW CHARAC TERISTICS AND CHEMICAL AND OVERALL EFlCIENCY WILL BE MEASURED AND OPERATING SPACE OF THE CATALYTIC DEVICE WILL BE MAPPED !#+./7,%$'%-%.43 4HIS WORK STEMS FROM PIONEERING WORK BY $R ! -EHRA ET AL IN HYDROGEN FUELED MICRO COMBUSTORS 4HE AUTHORS ARE
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231
THE OBSTACLES TO SATISFYING THESE REQUIREMENTS ARE DIFFERENT FOR A MICRO SCALE DEVICE !S lRST DESCRIBED BY 7AITZ ET AL ;= A MICRO SCALE COMBUSTOR IS MORE HIGHLY CONSTRAINED BY INADEQUATE RESIDENCE TIME FOR COMPLETE COMBUSTION AND HIGH RATES OF HEAT TRANSFER FROM THE COMBUSTOR -ICRO COMBUSTOR DEVELOPMENT ALSO FACES UNIQUE CHALLENGES DUE TO MATERIAL AND THERMODYNAMIC CYCLE CONSTRAINTS 4HIS FUNDAMENTAL TIME CONSTRAINT CAN BE QUANTIlED IN TERMS OF A HOMOGENEOUS $AMKHLER NUMBER THE RATIO OF THE RESIDENCE TIME TO THE CHARACTERISTIC CHEMICAL REACTION TIME
!"342!#4 4HIS PAPER REPORTS THE DEVELOPMENT OF A HIGH EFlCIENCY HYDROCARBON FUELED MICRO COMBUSTION SYSTEM FOR A MICRO SCALE GAS TURBINE ENGINE FOR POWER GENERATION AND MICRO PRO PULSION APPLICATIONS ! THREE WAFER CATALYTIC COMBUSTOR WAS FABRICATED AND TESTED %FlCIENCIES IN EXCESS OF WERE ACHIEVED FOR ETHYLENE AIR AND PROPANE AIR COMBUSTION ! FABRICATION PROCESS FOR A SIX WAFER CATALYTIC COMBUSTOR WAS DEVELOPED AND THIS DEVICE WAS SUCCESSFULLY CONSTRUCTED ).42/$5#4)/. -)4S 'AS 4URBINE ,ABORATORY AND -ICROSYSTEMS 4ECH NOLOGY ,ABORATORY HAVE BEEN DEVELOPING A MICRO GAS TURBINE ENGINE CAPABLE OF PRODUCING 7 OF POWER IN A VOLUME LESS THAN CM WHILE CONSUMING GRAMS OF FUEL PER HOUR ;= !PPLICATIONS INCLUDE POWER GENERATION PROPULSION SYS TEMS FOR SMALL AIR VEHICLES AND A VARIETY OF BLOWERS COM PRESSORS AND HEAT PUMPS 0OWER DENSITIES FOR THESE DEVICES ARE ESTIMATED TO BE AN ORDER OF MAGNITUDE GREATER THAN THOSE FOR CURRENT BATTERY TECHNOLOGY !S PART OF THIS PROGRAM SEVERAL MICRO FABRICATED SIL ICON GAS PHASE COMBUSTION SYSTEMS HAVE BEEN DESIGNED FABRICATED PACKAGED AND TESTED ; = 4HE DEVELOPMENT OF THE MICRO ENGINE COMBUSTION SYSTEM HAS BEEN PURSUED THROUGH A COMBINED PROGRAM OF EXPERIMENTS NUMERICAL SIM ULATIONS AND ANALYSIS 7ITH HYDROGEN FUEL THESE SYSTEMS REACHED GAS TEMPERATURES EXCEEDING + COMBUSTOR EFl CIENCIES GREATER THAN AND POWER DENSITIES ABOVE -7M WELL WITHIN THE REQUIREMENTS OF A MICRO GAS TURBINE THERMODYNAMIC CYCLE (OWEVER FOR HYDROCARBON FUELS SUCH AS PROPANE THE POWER DENSITIES DROP TO APPROXIMATELY ONE TENTH OF THAT FOR HYDROGEN 4HESE FUELS ARE MORE CHALLENG ING TO MICRO COMBUSTOR DEVELOPMENT DUE TO THE FOLD INCREASE IN REACTION TIME WHEN COMPARED TO HYDROGEN (OW EVER THEY ARE REQUIRED FOR PRACTICAL DEVICES DUE TO THEIR INCREASED ENERGY DENSITY AVAILABILITY AND EASE OF STORAGE 4HIS HAS MOTIVATED THE CURRENT STUDY OF CATALYTIC COMBUSTORS AS A MEANS OF DEVELOPING A HIGH EFlCIENCY HIGH POWER DEN SITY HYDROCARBON FUELED MICRO COMBUSTOR
4O ENSURE A $AH GREATER THAN UNITY AND COMPLETE COMBUS TION A DESIGNER OF A MICRO COMBUSTOR CAN EITHER INCREASE THE mOW RESIDENCE TIME OR DECREASE THE CHEMICAL REACTION TIME 4HE CHARACTERISTIC COMBUSTOR RESIDENCE TIME IS GIVEN BY THE BULK mOW THROUGH THE COMBUSTOR VOLUME
VOLUME
60
T RESIDENCE z VOLUMETRIC FLOW RATE M24
WHERE 6 IS VOLUME 0 IS PRESSURE M IS MASS mOW 4 IS TEM PERATURE AND 2 IS THE GAS CONSTANT 2ESIDENCE TIME CAN BE INCREASED BY INCREASING THE SIZE OF THE CHAMBER REDUCING THE MASS mOW RATE OR INCREASING THE OPERATING PRESSURE ! CHEMICAL REACTION TIME CAN BE APPROXIMATED BY AN !RRHENIUS TYPE EXPRESSION
; FUEL =
T REACTION z
!; FUEL =A ;/ =B E
%A
24
%A IS THE ACTIVATION ENERGY WHILE ! A AND B ARE EMPIRICALLY DETERMINED 2EACTION TIME IS PRIMARILY A FUNCTION OF FUEL PROPERTIES AND THE MIXTURE TEMPERATURE AND PRESSURE 3INCE HIGH POWER DENSITY REQUIREMENTS MANDATE HIGH MASS mOW RATES THROUGH SMALL CHAMBER VOLUMES THE MASS mOW RATE PER UNIT VOLUME CANNOT BE REDUCED WITHOUT COMPRO MISING DEVICE POWER DENSITY (ENCE THERE IS A BASIC TRADEOFF BETWEEN POWER DENSITY AND mOW RESIDENCE TIME 0OWER DENSITY t
#(!,,%.'%3 !.$ #/.342!).43 4HE FUNCTIONAL REQUIREMENTS OF A MICRO COMBUSTOR ARE SIMILAR TO THOSE OF A CONVENTIONAL GAS TURBINE COMBUSTOR 4HESE INCLUDE THE EFlCIENT CONVERSION OF CHEMICAL ENERGY TO mUID THERMAL AND KINETIC ENERGY WITH LOW TOTAL PRESSURE LOSS RELIABLE IGNITION AND WIDE mAMMABILITY LIMITS (OWEVER
0-7803-7185-2/02/$10.00 ©2002 IEEE
T $AH RESIDENCE T REACTION
M F ,(6 6
t
M R t 6 T RESIDENCE
WHERE ,(6 IS THE LOWER HEATING VALUE OF THE FUEL AND R IS THE mUID DENSITY &OR A GIVEN OPERATING PRESSURE AND THUS DEN SITY AND ASSUMING A $AH OF UNITY REDUCING THE CHEMICAL REAC TION TIME AND THUS REQUIRED RESIDENCE TIME IS THE ONLY MEANS OF ENSURING COMPLETE COMBUSTION WITHOUT COMPROMISING THE HIGH
228
!IR
(YDROGEN
4OP PLATE
&UEL MANIFOLD INJECTOR PLATE 3PACER INLET HOLES
0LATINUM COATED FOAM
A
#OMBUSTION CHAMBER
MM #ENTER LINE
RECIRCULATION JACKET
3PACERINLET HOLES &LOW
COMPRESSOR BLADES
FUEL INJECTOR
&UEL MANIFOLD INJECTORS
#OMBUSTION CHAMBER
AIR
WAFER
COMBUSTION CHAMBER TURBINE TURBINE BLADES
WAFER
MM
WAFER WAFER
"OTTOM PLATE
B
&IGURE %XPLODED VIEW OF THREE WAFER COMBUSTOR CAT ALYTIC INSERT AND INVAR PLATES
WAFER WAFER MM
INVAR PLATES !S A RESULT INSERTING CATALYST MATERIAL INTO THE COMBUSTION CHAMBER CAN BE DONE OUTSIDE THE CLEAN ROOM ENVIRONMENT &IGURE SHOWS AN EXPLODED VIEW OF THE THREE WAFER COMBUSTOR THE SURROUNDING INVAR PLATES AND THE CATA LYTIC INSERT &IGURE SHOWS A PHOTOGRAPH OF THE NICKEL FOAM SUPPORT AS IT IS lT INTO A THREE WAFER DEVICE 0REVIOUS GAS PHASE EXPERIMENTS WITH THE THREE WAFER COMBUSTOR SHOWED THAT HYDROCARBON COMBUSTION COULD NOT BE SUSTAINED IN THE DEVICE DUE TO INSUFlCIENT RESIDENCE TIME AND SIGNIlCANT HEAT LOSS FROM THE STRUCTURE 7ITH THE CATA LYTIC FOAM PIECE COMBUSTION OF BOTH PROPANE AND ETHYLENE WAS ACHIEVED &IGURE SHOWS THE OVERALL COMBUSTOR EFl CIENCIES FOR BOTH FUELS IN THIS DEVICE OVER A RANGE OF MIXTURE RATIOS %FlCIENCIES IN EXCESS OF WERE ATTAINED IN BOTH CASES 4HIS EXPERIMENT REPRESENTS THE lRST STABLE HYDROCAR BON COMBUSTION IN THIS DEVICE )N ADDITION IT IS IMPORTANT TO NOTE THAT HYDROCARBON COMBUSTION WAS STABILIZED AT THE SAME MASS mOW RATE AS GAS PHASE HYDROGEN AIR COMBUSTION 4HIS RESULTED IN POWER DENSITIES APPROXIMATELY HALF OF THAT OBTAINED WITH HYDROGEN RATHER THAN THE FOLD DECREASE EXPECTED FOR HYDROCARBON AIR REACTIONS INDICATING THAT THERE IS SIGNIlCANT CATALYSIS OCCURRING
&IGURE 3CHEMATIC OF COMBUSTORS USED FOR CATALYTIC TESTS A THREE WAFER COMBUSTOR C SIX WAFER COMBUSTOR POWER DENSITY OF THE DEVICE !S WE MOVE TOWARD THE DEVEL OPMENT OF A HYDROCARBON FUELED COMBUSTOR THIS BECOMES A SIGNIlCANT CONCERN 4HE RATE OF A HYDROCARBON AIR REACTION IS TYPICALLY TIMES SLOWER THAN THAT OF A HYDROGEN AIR REAC TION &OR CONSTANT MASS mOW RATE AND VOLUME THIS LEADS TO A CORRESPONDING DECREASE IN POWER DENSITY &OR PRACTICAL DEVICES HYDROCARBON FUELS SUCH AS PRO PANE AND ETHYLENE WILL BE REQUIRED 3PADACCINI ET AL ;= HAVE SHOWN THAT WITH THESE FUELS POWER DENSITIES DECREASED BY APPROXIMATELY A FACTOR OF TEN 4HIS WAS PRIMARILY DUE TO THE INCREASE IN REACTION TIME 4O UTILIZE THESE HYDROCARBON FUELS EFlCIENTLY WITHOUT INCREASING COMBUSTOR VOLUME OR TEMPERATURE CHEMICAL CON VERSION RATES MUST BE INCREASED SIGNIlCANTLY #ATALYSTS ACT TO INCREASE THE $AMKHLER NUMBER BY LOWERING THE ACTIVATION ENERGY ASSOCIATED WITH THE REACTION THEREBY INCREASING REAC TION RATE 4O ACHIEVE THIS WE HAVE BEEN PURSUING THE DEVEL OPMENT OF CATALYTIC COMBUSTION SYSTEMS WHERE PLATINUM IS PLATED ONTO A NICKEL FOAM SUBSTRATE 4O ENSURE A HIGH DEGREE OF CHEMICAL CONVERSION THE CATALYST AND ITS SUBSTRATE MUST lLL MOST OF THE COMBUSTOR VOLUME (OWEVER TO SATISFY THE GAS TURBINE THERMODYNAMIC CYCLE THE COMBUSTOR PRESSURE LOSS SHOULD NOT EXCEED &!"2)#!4)/. !.$ 4%34 2%35,43 )NITIAL FEASIBILITY TESTS WERE CARRIED OUT ON THE THREE WAFER LEVEL COMBUSTOR SHOWN IN &IGURE A .ICKEL FOAM WAS SELECTED AS A CONVENIENT CATALYST SUPPORT MATERIAL AND WAS CONVENTIONALLY MACHINED TO lT IN THE COMBUSTION CHAM BER !N ELECTRON BEAM DEPOSITION PROCESS WAS USED TO COAT THE SUBSTRATE WITH PLATINUM WHICH SERVES AS THE ACTIVE CATA LYTIC SURFACE 4HE COMBUSTORS PACKAGING SCHEME CONSISTS OF PLACING THE SILICON BETWEEN TWO CONVENTIONALLY MACHINED
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&IGURE 4HREE WAFER COMBUSTOR WITH CATALYTIC INSERT
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%FFICIENCY
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%FFICIENCY
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&IGURE #ATALYTIC PIECES BEING INSERTED INTO SIX WAFER COMBUSTOR
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ICALLY CLEANED TO REMOVE LARGE PARTICLES AND DEBRIS 4O PROTECT THE BONDING SURFACES FROM THE INSERTS A hSHIELD WAFERv WAS FABRICATED 4HIS WAFER WAS ETCHED THROUGH WITH THE SHAPE OF THE INSERTS AND SILICON DIOXIDE WAS DEPOSITED ON ITS SURFACE VIA PLASMA ENHANCED CHEMICAL VAPOR DEPOSITION 0%#6$ 4HE FOURTH lFTH AND SIXTH WAFER LEVELS WERE FUSION BONDED PRIOR TO INSERTING THE CATALYST 4HE SHIELD WAFER WAS THEN ALIGNED AND CONTACTED WITH THESE THREE LEVELS 7HILE IN CONTACT THE CATALYTIC PIECES WERE PLACED INTO THE BONDED WAFER STACK THROUGH THE SHIELD WAFER &INALLY THE PROTECTIVE SHIELD WAFER WAS CAREFULLY REMOVED 4HE 0%#6$ OXIDE PREVENTED THE SHIELD WAFER FROM ADHERING TO THE STACK )NSERTION OF THE CATALYTIC PIECES CAN BE SEEN IN &IGURE 7AFER LEVELS ONE TWO AND THREE WERE FUSION BONDED FORMING ANOTHER THREE WAFER STACK 4HE lNAL BONDING OF THE TWO THREE WAFER STACKS WAS UNSUCCESSFUL 4HREE BOND ING ATTEMPTS RESULTED IN THE BELIEF THAT THE THICKNESS OF THE STACKS PREVENTED BONDING 4HE THICK THREE WAFER STACKS WERE VERY STIFF AND UNABLE TO COMPLY TO EACH OTHERS SHAPE 3MALL
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&IGURE 4HREE WAFER CATALYTIC COMBUSTOR EFlCIENCY ! SIX WAFER LEVEL COMBUSTION SYSTEM COMPATIBLE WITH CURRENT MICRO ENGINE DESIGNS IS SHOWN IN &IGURE B &OR THIS DEVICE THE PLATINUM COATED NICKEL FOAM INSERTS MUST BE PLACED INTO THE EXISTING DESIGN DURING THE FABRICATION PRO CESS IN THE CLEAN ROOM ENVIRONMENT 4HIS POSES SEVERAL CHALLENGES DUE TO CLEANLINESS OF THE INSERTS PARTICULATE CON TAMINATION OF BONDING SURFACES AND DAMAGE TO BONDING SUR FACES VIA MISHANDLING 0LATINUM WAS DEPOSITED ONTO THE NICKEL FOAM SUBSTRATE BY DIPPING THE FOAM INTO A SOLUTION CONTAINING PLATINIC ACID 4HIS WAS DONE TO OBTAIN BETTER COVERAGE AND PENETRATION OF PLATI NUM ONTO THE CATALYST SUPPORT 4HE INSERTS WERE THEN ULTRASON 7AFER MICRONS MASK DEEP ETCH 7AFER MICRONS MASKS SHALLOW DEEP ETCHES 7AFER MICRONS MASKS DEEP ETCHES
7AFER MICRONS MASKS SHALLOW DEEP ETCHES
#ATALYTIC INSERT 3HIELD WAFER MICRONS 0%#6$ OXIDE
7AFER MICRONS MASKS DEEP ETCHES
7AFER MICRONS MASK DEEP ETCH
WAFER COMBUSTOR WITH CATALYST 3HIELD WAFER REMOVED PRIOR TO BONDING
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&IGURE &ABRICATION ASSEMBLY AND BONDING PROCESS FOR SIX WAFER CATALYTIC COMBUSTOR
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&IGURE )NFRA RED IMAGES OF THE SIX WAFER STACK WITH CATALYTIC INSERTS A AFTER INITIAL CONTACT B AFTER lNAL ANNEAL AMOUNTS OF BOW IN EITHER BONDING SURFACE COULD NOT BE OVER COME AND STIFFNESS OF THE WAFERS PREVENTED UNIFORM CONTACT FROM BEING ESTABLISHED !S A RESULT A NEW BONDING SEQUENCE WAS DEVELOPED 2ATHER THAN BONDING TWO THICK THREE WAFER STACKS TOGETHER THE LEVEL THREE WAFER WAS INDIVIDUALLY CONTACTED TO THE STACK WITH THE CATALYTIC PIECE 4HIS WAS FOLLOWED BY INDIVID UALLY CONTACTING AND BONDING THE LEVEL TWO AND ONE WAFERS !S A RESULT WAFER BONDING ON THE lNAL BUILD PRODUCED GOOD BOND QUALITY OVER MOST OF THE WAFER 4HIS ASSEMBLYBONDING PROCESS IS SHOWN IN &IGURE &IGURE A SHOWS AN INFRA RED IMAGE OF THE SIX WAFER STACK IMMEDIATELY AFTER CONTACT ! SMALL WELL CONTACTED REGION CAN BE SEEN IN THE CENTER OF THE WAFERS !FTER SEVERAL HOURS OF COLD PRESSING AT ATM AND # THERMAL PRESSING THE BOND FRONT PROPAGATED RADIALLY OUTWARD RESULTING IN DEVICE YIELD !N INFRA RED IMAGE OF THE lNAL BONDED STACK IS SHOWN IN &IGURE B
ALSO GRATEFUL TO $R ! !YON 0ROFESSOR - 3CHMIDT AND 0RO FESSOR ! %PSTEIN FOR THEIR INSIGHTFUL SUGGESTIONS TO + "ROD ERICK AND , (O FOR HELP DURING THE MICROFABRICATION TO 3 !JMERA FOR ASSISTANCE WITH PLATINUM COATING AND TO ALL THE OTHER MEMBERS OF THE -)4 MICROENGINE TEAM FOR THEIR HELP AND SUPPORT 4HE EDITORIAL ASSISTANCE OF $ 0ARK IS ALSO GRATEFULLY ACKNOWLEDGED 4HIS WORK IS SUPPORTED BY THE !RMY 2ESEARCH /FlCE $!!' UNDER $R 4 $OLIGALSKI AND BY $!20! $!!' $!"4 # UNDER $R 2 .OWACK AND $R 2 2OSENFELD RESPECTIVELY 2%&%2%.#%3 ;= %PSTEIN ET AL h-ICRO (EAT %NGINES 'AS 4URBINES AND 2OCKET %NGINESv PRESENTED AT THE TH !)!! &LUID $YNAMICS #ONFERENCE ;= -EHRA ET AL h$EVELOPMENT OF A (YDROGEN #OMBUSTOR FOR A -ICROFABRICATED 'AS 4URBINE %NGINEv PRESENTED AT THE 3OLID 3TATE 3ENSOR AND !CTUATOR 7ORKSHOP AT (ILTON (EAD ;= -EHRA ! h$EVELOPMENT OF A (IGH 0OWER $ENSITY #OMBUSTION 3YSTEM FOR A 3ILICON -ICRO 'AS 4URBINE %NGINEv 0H$ 4HESIS -ASSACHUSETTS )NSTITUTE OF 4ECH NOLOGY ;= -EHRA ET AL h! 7AFER #OMBUSTION 3YSTEM FOR A 3ILI CON -ICRO 'AS 4URBINE %NGINEv )%%%!3-% *OURNAL OF -ICROELECTROMECHANICAL 3YSTEMS ;= 7AITZ ET AL h#OMBUSTORS FOR -ICRO 'AS 4URBINE %NGINESv !3-% *OURNAL OF &LUIDS %NGINEERING 6OL PP -ARCH ;= 3PADACCINI ET AL h(IGH 0OWER $ENSITY 3ILICON #OM BUSTION 3YSTEMS FOR -ICRO 'AS 4URBINE %NGINESv TO BE PRESENTED AT !3-% )NTERNATIONAL 'AS 4URBINE )NSTITUTE 452"/ %80/
#/.#,53)/. !.$ &5452% 7/2+ 4HREE WAFER COMBUSTORS WERE lTTED WITH PLATINUM COATED NICKEL FOAM INSERTS AND USED AS AN INITIAL CATALYTIC COMBUSTOR TEST BED "OTH ETHYLENE AIR AND PROPANE AIR REAC TIONS WERE SUSTAINED AT OVERALL EFlCIENCIES IN EXCESS OF WHEN PREVIOUSLY IT WAS NOT POSSIBLE TO BURN HYDROCARBON FUELS IN THIS DEVICE 4HIS RESULTED IN POWER DENSITIES APPROXI MATELY HALF OF THAT OBTAINED WITH HYDROGEN RATHER THAN THE FOLD DECREASE EXPECTED FOR HYDROCARBON AIR REACTIONS ! METHOD FOR FABRICATING A SIX WAFER COMBUSTOR lTTED WITH PLATINUM COATED NICKEL FOAM HAS BEEN DEVELOPED &ABRICATION OF THESE DEVICES WAS COMPLETED 0RESSUREMASS mOW CHARAC TERISTICS AND CHEMICAL AND OVERALL EFlCIENCY WILL BE MEASURED AND OPERATING SPACE OF THE CATALYTIC DEVICE WILL BE MAPPED !#+./7,%$'%-%.43 4HIS WORK STEMS FROM PIONEERING WORK BY $R ! -EHRA ET AL IN HYDROGEN FUELED MICRO COMBUSTORS 4HE AUTHORS ARE
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