1. 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)

2. 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

3. 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

4. Combustor development
Limit temperatures much lower with catalyst

5. Combustor development
Temperature measurements confirm that catalyst can inhibit gas-phase reaction

6. 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?

7. 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

8. 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

9. Combustor development
Model predicts intermediates H2 and CO used in electrochemical cell H2 CO

10. 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

11. 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

12. 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

13. 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

14. Heat exchanger / combustor modeling
Much worse performance found with conductive-tube burner

15. 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 ( )
Fuel/air AND fuel/O2 ratio

16. Catalytic combustion modeling
“Dual-limit” behavior similar to experiments observed when heat loss is present

17. Catalytic combustion modeling
Surface temperature profiles show effects of heat loss at low flow velocities

18. 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
Surface coverage

19. 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)

20. 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

21. 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

22. Catalytic combustion modeling
Similar behavior at other Re

23. Catalytic combustion modeling
Also seen with methane - surprisingly low T

24. 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)

25. 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