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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

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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 generator1
Integrated Micropower Generator


  • 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

Combustor development
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

Combustor development1
Combustor development

  • Limit temperatures much lower with catalyst

Combustor development2
Combustor development

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

Mesoscale experiments
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?

Mesoscale experiments1
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 development3
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)


Heat release rate

Combustor development4
Combustor development

  • Model predicts intermediates H2 and CO used in electrochemical cell



Combustor development5
Combustor development

  • Individual reactions occur at different locations within Swiss roll - possibility for in-situ reforming of C3H8 and O2 to CO and H2without a catalyst





Heat exchanger combustor modeling
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 exchanger combustor modeling1
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

Heat exchanger combustor modeling2
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

Heat exchanger combustor modeling3
Heat exchanger / combustor modeling

  • Much worse performance found with conductive-tube burner

Catalytic combustion modeling
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

Catalytic combustion modeling1
Catalytic combustion modeling

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

Catalytic combustion modeling2
Catalytic combustion modeling

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

Catalytic combustion modeling3
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



Heat release rates and gas-phase CH4 mole fraction

Surface coverage

Catalytic combustion modeling4
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)

Catalytic combustion modeling5
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)



Catalytic combustion modeling6
Catalytic combustion modeling

  • 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

Catalytic combustion modeling7
Catalytic combustion modeling

  • Similar behavior at other Re

Catalytic combustion modeling8
Catalytic combustion modeling

  • 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

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

Future plans
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