Integrated Micropower Generator Combustion, heat transfer, fluid flow Lead: Paul Ronney Postdoc:...

Integrated Micropower Generator

Combustion, heat transfer, fluid flow

Lead: Paul Ronney

Postdoc: Craig EastwoodGraduate 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 Inter-group pow-wow, June 24, 2002

Integrated Micropower Generator

Objectives

• Thermal / chemical management for SCFC– Deliver proper

temperature, composition, residence time to SCFC

– Oxidize SCFC products

Task progress

• “Swiss roll” heat exchanger / combustor development

• Catalytic afterburner

• Micro-aspirator

Products out

Air inAir/fuel in

- out+ out

Products

air/fuel reactants

catalyticcombustor

SCFCstack


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

0.2

0.4

0.60.8

1

3

10 100 1000

Ceramic (no cat)Ceramic (cat)Inconel (no cat)Inconel (cat)Weinberg 4.5 turn CH4

Reynolds number

Conventionallean limit

Propane


Combustor development• Limit temperatures much lower with catalyst

200

400

600

800

1000

1200

10 100 1000

Inconel (no cat)Inconel (cat)Ceramic (no cat)Ceramic (cat)

Reynolds number

Propane


Combustor development

• Temperature measurements confirm that catalyst can inhibit gas-phase reaction

0

200

400

600

800

1000

0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7

Center (cat)

1 turn out (cat)

2 turns out (cat)

Center (no cat)

1 turn out (no cat)

2 turns out (no cat)

Equivalence ratio

Centered reactionNo effect of catalyst

C3H

8-air

Re = 45

Catalyst inhibits reaction in center

Centered reactionCatalyst retards

reaction

No stable non-catalytic reactionStable non-catalytic reaction


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?

0

200

400

600

800

1000

2 3 4 5 6 7 8 9

T (Re = 500)T (Re = 458)T (Re = 328)T (Re = 229)T (Re = 199)T (Re = 159)T (Re = 113)

Mole percent propane in air


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


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 Heat release rate


Combustor development

• Model predicts intermediates H2 and CO used in electrochemical cell

H2 CO


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 2

3 4


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

Reactants

T = Ti

(0)

Products

T = Te

(0)

Adiabatic

end walls

Well-stirred

reactor

T = Te

(1)

Area = AR

x = 0 x = 1

Wall temperature = Tw

(x) = ( Tw,e

(x) + Tw

(x))/2

Surface temperature = Tw,e

(x)

Surface temperature = Tw,i

(x)

Heat transfer coefficient to wall = h1

Gas temperature = Te

(x)

Gas temperature = Te

(x)

Heat transfer coefficient to wall = h1

Heat loss coefficient to ambient = h2

Heat loss coefficient to ambient = h2

Wall thickness τ

Channel height d

Channel height d


Heat exchanger / combustor modeling• Results show low-velocity limit requires heat loss (H > 0)

and wall heat conduction (B < ∞)• Very different from burners without heat recirculation!

H = dimensionless heat loss

B-1 = dimensionless wall conduction effect

Da = dimensionless reaction rate

3

4

5

6

7

0.01 0.1 1M (mass flux)

Da = ∞H = 0B = ∞

H = 0.05B = 1000

H = 0B = ∞

H = 0.05B = 100

H = 0.05B = ∞

Da = 107



• 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

1

1.1

1.2

1.3

1.4

1.5

1.6

0.001 0.01 0.1

Temperature rise (

Δ

T

)

M (mass flux)

B = 100

B = 1000

B = 10000

B = ∞



• Much worse performance found with conductive-tube burner

1.8

2

2.2

2.4

2.6

2.8

3

0.001 0.01 0.1 1

Temperature rise (

Δ

T

)

M (mass flux)

B = 0

B = 10

B = 100

B = ∞

B = 1000


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

• Effects studied– Heat loss coefficient (H)– Flow velocity or Reynolds number (2.4 - 60)– Fuel/air AND fuel/O2 ratio

Wall boundary condition H = 0, 5 or10 W/m2˚C

1 mmnon-catalytic wall

9 mmcatalytic wall

Fuel/airinlet

1 mmdiameter



• “Dual-limit” behavior similar to experiments observed when heat loss is present



• Surface temperature profiles show effects of heat loss at low flow velocities



• 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 Surface coverage



• 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)



• Behavior due to transition from O(s) coverage for lean mixtures (excess O2) to CO(s) coverage for rich mixtures (excess fuel)

Lean

Rich



• Predictions consistent with experiments (C3H8-O2-N2) in 2D Swiss roll at similar Re

• Opposite (conventional) fuel:O2 ratio effect seen in gas-phase combustion

400

500

600

700

800

900

1

1.5

2

2.5

3

0.4 0.6 0.8 1 1.2 1.4

Equivalence ratio

Tmax

(non-cat)

Tmax

(cat)

Fuel % (cat)Fuel % (non-cat)


600

620

640

660

680

700

1

1.5

2

2.5

3

0.4 0.6 0.8 1 1.2 1.4

Equivalence ratio

Tmax

(cat)

Fuel % (cat)

Re = 35


• Similar behavior at other Re


750

800

850

900

950

1000

1050

5

5.5

6

6.5

7

0.4 0.6 0.8 1 1.2

Equivalence ratio

Tmax

(cat)

Fuel % (cat)

Re = 35CH

4-O

2-N

2 mixtures


• Also seen with methane - surprisingly low T


Micro-aspirator

• 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

Fuel from supply tank

P = Psat

, V ≈ 0

Air

P = ambient, V ≈ 0 Exit to burner

Fuel/air mixture

P > ambient

V > 0

Propane mass fraction fields for varying inner nozzle diameters (outside dia. 2 mm)


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 Combustion, heat transfer, fluid flow Lead: Paul Ronney Postdoc:...

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