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 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 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 development Limit temperatures much lower with catalyst

Combustor development Temperature measurements confirm that catalyst can inhibit gas-phase 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?

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

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 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 modeling Much worse performance found with conductive-tube burner

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

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

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

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 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

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 modeling Similar behavior at other Re

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 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