Integrated Micropower Generator

Integrated Micropower Generator

Combustion, heat transfer, fluid flow

Lead: Paul Ronney

Postdoc: Craig EastwoodGraduate student: Jeongmin Ahn (experiments)

Graduate student: James Kuo (modeling)Collaborator: Kaoru Maruta (Tohoku Univ.)

(Catalytic combustion modeling)

University of Southern California

Integrated MicroPower Generator Review, Oct. 18, 2002

Integrated Micropower Generator

Objectives• Thermal / chemical

management for SCFC– Deliver proper

temperature, composition, residence time to SCFC

– Oxidize SCFC products

Task progress• Catalytic “Swiss roll”

combustor experiments• Numerical modeling• Fuel cell testing Products out

Air inAir/fuel in

- out+ out

Products

air/fuel reactants

catalyticcombustor

SCFCstack


Lessons learned from earlier work

• Heat losses limit performance of Swiss roll (or any combustor) at low Re

• Heat transfer along dividing wall of Swiss roll limits burner performance, especially at low Re

• Catalytic combustion greatly aids low-Re performance

Emphasize low-Re catalytic combustion, minimize thermal losses, minimize wall thickness and conductivity


2D Inconel macroscale burner

• 3 turn, 3.5 mm channel width, 5 cm tall• 7 thermocouples (1 center, 1 each inlet & outlet turn)• Mass flow controllers, LabView data acquisition & control


0.1

1

10 100 1000

Extinction (lean)Extinction (lean, no catalyst)Out-of-center (lean)Out-of-center (rich)Weinberg (4.5 turn, methane)

Reynolds number

Propane-airBare metal Pt catalyst3 turn macro inconel burner

Rich limit at φ > 30!

Quenching limits in Swiss roll

• Dual limits - low-velocity (heat loss) and high-velocity (blow-off)• Out-of-center combustion regime (unstable operation)• Very low Re (< 4) possible with catalytic combustion• Lean limit can be richer than stoichiometric (!) (catalytic only)• Weinberg low-Re performance very poor (more heat losses?)



• Lower Re - flame always centered - heat recirculation needed to obtain sufficiently high temperature to sustain reaction

• Maximum temperatures near stoichiometric

0

100

200

300

400

500

600

700

2 4 6 8 10 30

TC1TC2TC3TC4 TC5 TC6 TC7

Mole percent propane in air

Re = 23 Stoichiometric



• Higher Re - flame not centered near stoichiometric - less heat recirculation needed to sustain combustion - reaction zone moves toward inlet

• Center cool due to heat losses

0

200

400

600

800

1000

1200

1 10

TC1TC2 TC3TC4 TC5TC6 TC7


Re = 70 Stoichiometric


0.1

1

10 100 1000

RichLeanLean, no catalyst

Reynolds number


Probable transition togas-phase combustion

Quenching limits - continued

• Ratio of (estimated) heat loss to heat generation ≈ constant for low Re (indicating heat loss induced extinction)

• Ratio decreases at higher Re (indicating “blow-off“ type extinction)


300

400

500

600

700800900

1000

10 100 1000

RichLeanLean, no catalyst

Reynolds number


Probable transition togas-phase combustion

Quenching limits - continued

• Temperatures dramatically lower with Pt catalyst - < 500 K possible even at Re < 4


Thermal behavior of Swiss roll

• Peak temperatures correlate well with heat

recirculation parameter = {Abs(Ti-Ti-1)/(Tad-T∞)}

-1

-0.5

0

0.5

1

1.5

0 1 2 3 4 5

Non-catalyticCatalytic

Heat transfer parameter



0

200

400

600

800

2 2.5 3 3.5 4 4.5 5 5.5 6

TC1 (Inconel)TC3 (Inconel)TC5 (Inconel)TC1 (Titanium)TC3 (Titanium)TC5 (Titanium)


Re = 23

Thermal performance

• Titanium burner - lower wall conductivity, same wall thickness & number of turns - higher peak temperatures

• Also lower coefficient of thermal expansion


inletinlet outletoutlet

Numerical model

• FLUENT, 2D, 8216 grid points• Conduction (solid & gas),

convection (gas), radiation (solid-solid only)

• Temperature-dependent gas properties

• 1-step chemistry (Westbrook & Dryer)

• Boundary condition:– Inlet: 300K, 3 m/s (Re = 700), 1

mole % propane in air (stoichiometric = 4.02%)

– Outlet: Pressure outlet– Heat loss: volumetric term to

simulate heat loss in 3rd dimension

• Radiation: discrete ordinates, unit emissivity on all surfaces


Model results - radiation & heat loss

• Reaction near center (centered for weaker mixtures near extinction limit)

• Peak T near peak reaction rate


Model results - radiation, no heat loss

• Minor effect of heat loss for high Re (=700) case shown, much greater effect for lower Re


Model results - no radiation, heat loss

• Extreme case - low Re (23), high fuel concentration (3.0%)• Thin reaction zone (laminar flame), anchored near inlet

(doesn’t need heat recirculation to exist), rest of burner acts as heat sink


Model results - no radiation, heat loss

• Higher temperatures (by ≈150K) without radiation), more nearly isothermal

• Radiation transfers heat between walls but not directly to gas - similar effect as increasing wall thermal conductivity

• Important for scale-down - radiation will be less significant at smaller scales due to higher gradients for conduction

• Boltzman number T3d/


Numerical modeling - 3D• 3D modeling initiated with 4-step

chemical model (Hauptmann et al.) – (1) C3H8 (3/2)C2H4 + H2

– (2) C2H4 + O2 2CO + 2H2

– (3) CO + (1/2)O2 CO2

– (4) H2 + (1/2)O2 H2O• 3D simulation (217,000 cells) confirms

most of heat loss is in axial (z) direction• Use 3D model to calibrate/verify 2D

model with heat loss coefficient


• SCFC in macroscale Ti Swiss roll, 1 turn from center, inlet side• Pt catalyst in center, use fuel % & Re to control T• First tests: performance poor (probably due to fuel cell connection

method), but it’s probably the world’s smallest self-sustaining SOFC!• Power peaks at ≈ 2x stoichiometric fuel concentration

Fuel cell testing

1

10

0.002

0.004

0.006

0.008

0.01

0.03

0.05

380 400 420 440 460 480 500 520 540

Temperature (˚C)

Voltage (mV)

Current (mA/cm2)

Power (mW/cm2)

6.0%

8.0%12.0%20.0% C3H

8

Re = 47


Future plans

• Near-term– Continue SCFC testing in macroscale Swiss roll– Consider UIUC customized ceramic Swiss rolls as an

alternative to wire-EDM parts– Complete validation of numerical model

• Longer term– Design mesoscale Swiss roll guided by numerical model

(with inputs from SCFC experiments & modeling)• Number of turns• Wall thickness• Catalyst type & surface area• Reactant flow velocity and composition (fuel, air, exhaust

gas, bypass ratio)– Fabricate/test mesoscale Swiss roll– Integrate/test SCFC in mesoscale Swiss roll

• H2, CO, H2/CO mixtures• Hydrocarbons


Mesoscale burners

• Possible next generation mesoscale burner - ceramic ( ≈ 1 W/mK) rapid prototyping using colloidal inks (Prof. Jennifer Lewis, UIUC)

1.5 cm tall 2-turn alumina Swiss-roll combustor


Mesoscale burners• Wire-EDM fabrication• Tungsten carbide, 10% Co ( ≈ 20 W/mK)

Integrated Micropower Generator

Documents

Transcript of Integrated Micropower Generator