Transcript PowerPoint Presentation - Integrated Micropower Generator

Integrated Micropower
Generator
Combustion, heat transfer, fluid
flow
Lead: Paul Ronney
Postdoc: Craig Eastwood
Graduate student: Jeongmin Ahn (experiments)
Graduate student: James Kuo (modeling)
University of Southern California
Collaborator: Prof. Kaoru Maruta (Tohoku Univ.,
Sendai, Japan) (catalytic combustion modeling)
Integrated Micropower Generator
Objectives
catalytic
combustor
• Thermal / chemical
management for SCFC
– Deliver proper
temperature,
composition, residence
time to SCFC
Products
– Oxidize SCFC products
Task progress
air/fuel reactants
• “Swiss roll” heat exchanger
/ combustor development
• Catalytic afterburner
SCFC
stack
Products out
- out
+ out
• Micro-aspirator
Air/fuel in Air in
Integrated MicroPower Generator
Inter-group pow-wow, June 24, 2002
Combustor development
• Prior results in Swiss-roll burners show surprising effects of
Flow velocity or Reynolds number (dual limits)
Catalyst vs. non-catalyst (reversal of limits)
Lean limits richer than stoichiometric (!) (catalytic only)
Wall material
Equivalence ratio at lean limit
–
–
–
–
3
1
0.8
0.6
0.4
0.2
Ceramic (no cat)
Ceramic (cat)
Inconel (no cat)
Inconel (cat)
Weinberg 4.5 turn CH4
Conventional
lean limit
Pr opane
10
Integrated MicroPower Generator
100
Reynolds number
1000
Inter-group pow-wow, June 24, 2002
Combustor development
• Limit temperatures much lower with catalyst
Center temp. at limit (ÞC)
1200
1000
Inconel (no cat)
Inconel (cat)
Ceramic (no cat)
Ceramic (cat)
Pr opane
800
600
400
200
10
Integrated MicroPower Generator
100
Reynolds number
1000
Inter-group pow-wow, June 24, 2002
Combustor development
• Temperature measurements confirm that catalyst can
inhibit gas-phase reaction
1000
No stable non-cataly tic reaction
C H -air
3
800
600
8
Re = 45
Center (cat)
1 turn out (cat)
2 turns out (cat)
Center (no cat)
1 turn out (no cat)
2 turns out (no cat)
Extinction
Temperature (ûC)
Stable non-cataly tic
react ion
400
200
Cent ered reaction
No ef f ect of cataly st
0
0.35
0.4
0.45
0.5
Cataly st inhibits
react ion in cent er
Cent ered reaction
Cataly st ret ards
react ion
0.55
0.65
0.6
0.7
Equivalence ratio
Integrated MicroPower Generator
Inter-group pow-wow, June 24, 2002
Mesoscale experiments
• Steady combustion obtained even at < 100˚C with Pt catalyst
• Sharp transition to lower T at low or high fuel conc., low or high flow
velocity - transition from gas-phase to surface reaction?
• Can’t reach as low Re as macroscale burner!
• Wall thick and has high thermal conductivity - loss mechanism?
1000
T (Re = 500)
T (Re = 458)
T (Re = 328)
T (Re = 229)
T (Re = 199)
T (Re = 159)
T (Re = 113)
Temperature (ÞC)
800
600
400
200
0
2
Integrated MicroPower Generator
3
4
5
6
7
Mole percent propane in air
8
Inter-group pow-wow, June 24, 2002
9
Mesoscale experiments
• Next generation mesoscale burner - ceramic rapid
prototyping using colloidal inks (Prof. Jennifer Lewis,
UIUC)
1.5 cm tall 2-turn alumina Swiss-roll combustor
Integrated MicroPower Generator
Inter-group pow-wow, June 24, 2002
Combustor development
• 4-step chemical model (Hauptmann et al.) integrated into FLUENT
(1) C3H8(3/2)C2H4 + H2
(2) C2H4 + O2  2CO + 2H2
(3) CO + (1/2)O2  CO2
(4) H2 + (1/2)O2  H2O
• Typical results (V = 20 cm/s, Re = 70, lean propane-air)
Temperature
Integrated MicroPower Generator
Heat release rate
Inter-group pow-wow, June 24, 2002
Combustor development
• Model predicts intermediates H2 and CO used
in electrochemical cell
H2
Integrated MicroPower Generator
CO
Inter-group pow-wow, June 24, 2002
Combustor development
• Individual reactions occur at different locations within Swiss
roll - possibility for in-situ reforming of C3H8 and O2 to CO and
H2 without a catalyst
1
3
Integrated MicroPower Generator
2
4
Inter-group pow-wow, June 24, 2002
Heat exchanger / combustor modeling
• Simple quasi-1D analytical model of counterflow
heat-recirculating burners developed including:
(1) heat transfer; (2) chemical reaction in WSR;
(3) heat loss to ambient; (4) streamwise thermal
conduction along wall
Heat loss coefficient to ambient =
Gas temperature = Te(x)
Channel height d
Reactants
T = Ti(0)
Surface temperature =
h2
Tw,i (x)
h1 Well-stirred
reactor
Wall thickness 
Wall temperature = Tw(x) = ( Tw,e(x) + Tw(x))/2
T = Te(1)
Heat transfer coefficient to wall = h1 Area = A R
Surface temperature = Tw,e(x)
Adiabatic
end walls
Products
T = Te(0)
Channel height d
x= 0
Heat transfer coefficient to wall =
Gas temperature = Te(x)
Heat loss coefficient to ambient =
Integrated MicroPower Generator
h2
x= 1
Inter-group pow-wow, June 24, 2002
Heat exchanger / combustor modeling
e
Reactor temperature T (1)
• Results show low-velocity limit requires heat loss (H > 0)
and wall heat conduction (B < ∞)
• Very different from burners without heat recirculation!
7
7
Da = 10
H = dimensionless
heat loss
B-1 = dimensionless
wall conduction
effect
Da = dimensionless
reaction rate
H = 0.05
B= °
6
Da = °
H= 0
B= °
H = 0.05
B = 1000
5
H = 0.05
B = 100
4
H= 0
B= °
3
0.01
Integrated MicroPower Generator
0.1
M (mass flux)
Inter-group pow-wow, June 24, 2002
1
Heat exchanger / combustor modeling
• High-velocity limit almost unaffected by wall heat conduction,
but low-velocity limit dominated by wall conduction
• Thin wall, low thermal conductivity material (ceramic vs.
steel) will maximize performance
T)
Temperature rise (
1.6
1.5
1.4
B = 100
1.3
B = 1000
1.2
B = 10000
1.1
1
0.001
Integrated MicroPower Generator
B=°
0.01
0.1
M (mass
flux) pow-wow, June 24, 2002
Inter-group
i
w,i
Heat transfer coefficient to wall = h1 Well-stirred
reactor
Wall thickness 
Wall temperature = Tw(x) = ( Tw,e(x) + Tw,i(x))/2
T = Te(L)
Heat transfer coefficient to wall = h1 Area = A R
Surface temperature = Tw,e(x)
Adiabatic
end walls
Heat exchanger / combustor modeling
Products
T = Te(0)
Channel height d
Gas temperature = Te(x)
Heat loss coefficient to ambient = h2
x=L
• Much worse performance found with conductive-tube
burner
x=0
Surface temperature = Tw,•(x)
Adiabatic
end wall
Wall thickness 
Wall temperature = Tw(x) = ( Tw,I(x) + Tw,•(x))/2
Surface temperature = Tw,i(x)
Channel half-height d/2
Reactants
T = Ti(0)
x=0
Line of symmetry
Integrated MicroPower Generator
Heat loss coefficient to ambient = h2
Heat transfer coefficient to wall = h1
Gas temperature = Ti(x)
T = Tw(L)
Well-stirred
reactor
T = Te(L)
Area = A R
Products
T = Tw(L)
x=L
Inter-group pow-wow, June 24, 2002
Catalytic combustion modeling
• Detailed catalytic combustion model integrated into
FLUENT computational fluid dynamics package
• Interactions of chemical reaction, heat loss, fluid flow
modeled in simple geometry at microscales
– Cylindrical tube reactor, 1 mm dia. x 10 mm length
– Platinum catalyst, CH4-air and CH4-O2-N2 mixtures
Wall boundary condition
H = 0, 5 or10 W/m 2ÞC
1 mm
diameter
Fuel/air
inlet
1 mm
non-catalytic wall
• Effects studied
9 mm
catalytic wall
– Heat loss coefficient (H)
– Flow velocity or Reynolds number (2.4 - 60)
– Fuel/air AND fuel/O2 ratio
Integrated MicroPower Generator
Inter-group pow-wow, June 24, 2002
Catalytic combustion modeling
• “Dual-limit” behavior similar to experiments
observed when heat loss is present
Integrated MicroPower Generator
Inter-group pow-wow, June 24, 2002
Catalytic combustion modeling
• Surface temperature profiles show effects of
heat loss at low flow velocities
Integrated MicroPower Generator
Inter-group pow-wow, June 24, 2002
Catalytic combustion modeling
•
Heat release inhibited by high O(s) coverage (slow O(s) desorption) at
low temperatures - need Pt(s) sites for fuel adsorption / oxidation
a
b
Heat release rates and gas-phase CH4 mole fraction
Integrated MicroPower Generator
Surface coverage
Inter-group pow-wow, June 24, 2002
Catalytic combustion modeling
• Computations with fuel:O2 fixed, N2 (not air) dilution
• Minimum fuel concentration and flame temperatures needed to
sustain combustion much lower for even slightly rich mixtures!
• Typical strategy to reduce flame temperature: dilute with excess
air, but slightly rich mixtures with exhaust gas dilution is a much
better operating strategy! (and consistent with SCFC operation)
Integrated MicroPower Generator
Inter-group pow-wow, June 24, 2002
Catalytic combustion modeling
• Behavior due to
transition from O(s)
coverage for lean
mixtures (excess O2)
to CO(s) coverage
for rich mixtures
(excess fuel)
Lean
Rich
Integrated MicroPower Generator
Inter-group pow-wow, June 24, 2002
Catalytic combustion modeling
900
T
max
3
(non-cat)
800
T
max
2.5
(cat)
700
2
600
Fuel % (cat)
500
1.5
Mole % propane at limit
Maximum temperature at limit (K)
• Predictions consistent with experiments (C3H8-O2-N2) in
2D Swiss roll at similar Re
• Opposite (conventional) fuel:O2 ratio effect seen in gasphase combustion
Fuel % (non-cat)
1
400
0.4
0.6
0.8
1
1.2
1.4
Equivalence ratio
Integrated MicroPower Generator
Inter-group pow-wow, June 24, 2002
Catalytic combustion modeling
• Similar behavior at other Re
Re = 35
3
680
2.5
660
T
max
(cat)
2
640
Fuel % (cat)
1.5
620
Mole % propane at limit
Maximum temperature at limit (K)
700
1
600
0.4
0.6
0.8
1
1.2
1.4
Equivalence ratio
Integrated MicroPower Generator
Inter-group pow-wow, June 24, 2002
Catalytic combustion modeling
• Also seen with methane - surprisingly low T
7
T
max
1000
(cat)
6.5
950
Fuel % (cat)
900
6
850
5.5
Re = 35
CH -O -N mixtures
800
4
2
Mole % methane at limit
Maximum temperature at limit (K)
1050
2
5
750
0.4
0.6
0.8
1
1.2
Equivalence ratio
Integrated MicroPower Generator
Inter-group pow-wow, June 24, 2002
Micro-aspirator
Air
P = ambient, V - 0
Fuel from supply tank
P = Psat, V - 0
Exit to burner
Fuel/air mixture
P > ambient
V>0
• FLUENT modeling being used
to design propane/butane
micro-aspirator
• Goal: maximize exit pressure
for given fuel/air ratio
• Unlike macroscale devices,
design dominated by viscous
losses
Propane mass fraction fields for varying
inner nozzle diameters (outside dia. 2 mm)
Integrated MicroPower Generator
Inter-group pow-wow, June 24, 2002
Future plans
• Build/test macroscale titanium “Swiss Roll” burner (2x
lower conductivity & thermal expansion coefficient)
• Test macroscale Ti Swiss Roll IMG
– H2, CO, H2/CO mixtures
– Hydrocarbons
• Meso/microscale "Swiss Roll”
– Optimized for SCFC use using FLUENT - determine the
conditions required for stable 2D combustor at target
operating temperature & composition
•
•
•
•
Number of turns
Wall thickness
Catalyst type & surface area
Reactant flow velocity and composition (fuel, air, exhaust
gas, bypass ratio)
– Build/test stand-alone Swiss roll, verify design
– Build/test IMG
• Design micro-aspirator
Integrated MicroPower Generator
Inter-group pow-wow, June 24, 2002