Integrated Micropower Generator Combustion, heat transfer, fluid flow Lead: Paul Ronney Postdoc: Craig Eastwood Graduate student: Jeongmin Ahn (experiments) Graduate student: James Kuo (modeling) Collaborator: Kaoru Maruta (Tohoku Univ.) (Catalytic combustion modeling) University of Southern California

Integrated MicroPower GeneratorReview, 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

Integrated MicroPower GeneratorReview, Oct. 18, 2002 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

Integrated MicroPower GeneratorReview, Oct. 18, D 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

Integrated MicroPower GeneratorReview, Oct. 18, 2002 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?)

Integrated MicroPower GeneratorReview, Oct. 18, 2002 Quenching limits in Swiss roll Lower Re - flame always centered - heat recirculation needed to obtain sufficiently high temperature to sustain reaction Maximum temperatures near stoichiometric

Integrated MicroPower GeneratorReview, Oct. 18, 2002 Quenching limits in Swiss roll 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

Integrated MicroPower GeneratorReview, Oct. 18, 2002 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)

Integrated MicroPower GeneratorReview, Oct. 18, 2002 Quenching limits - continued Temperatures dramatically lower with Pt catalyst - < 500 K possible even at Re < 4

Integrated MicroPower GeneratorReview, Oct. 18, 2002 Thermal behavior of Swiss roll Peak temperatures correlate well with heat recirculation parameter =  {Abs(T i -T i-1 )/(T ad -T ∞ )}

Integrated MicroPower GeneratorReview, Oct. 18, 2002 Thermal performance Titanium burner - lower wall conductivity, same wall thickness & number of turns - higher peak temperatures Also lower coefficient of thermal expansion

Integrated MicroPower GeneratorReview, Oct. 18, 2002inletoutlet 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

Integrated MicroPower GeneratorReview, Oct. 18, 2002 Model results - radiation & heat loss Reaction near center (centered for weaker mixtures near extinction limit) Peak T near peak reaction rate

Integrated MicroPower GeneratorReview, Oct. 18, 2002 Model results - radiation, no heat loss Minor effect of heat loss for high Re (=700) case shown, much greater effect for lower Re

Integrated MicroPower GeneratorReview, Oct. 18, 2002 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

Integrated MicroPower GeneratorReview, Oct. 18, 2002 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 T 3 d/

Integrated MicroPower GeneratorReview, Oct. 18, 2002 Numerical modeling - 3D 3D modeling initiated with 4-step chemical model (Hauptmann et al.) –(1) C 3 H 8 (3/2)C 2 H 4 + H 2 –(2) C 2 H 4 + O 2  2CO + 2H 2 –(3) CO + (1/2)O 2  CO 2 –(4) H 2 + (1/2)O 2  H 2 O 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

Integrated MicroPower GeneratorReview, Oct. 18, 2002 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

Integrated MicroPower GeneratorReview, Oct. 18, 2002 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 H 2, CO, H 2 /CO mixtures Hydrocarbons

Integrated MicroPower GeneratorReview, Oct. 18, 2002 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

Integrated MicroPower GeneratorReview, Oct. 18, 2002 Mesoscale burners Wire-EDM fabrication Tungsten carbide, 10% Co ( ≈ 20 W/mK)

Integrated MicroPower GeneratorReview, Oct. 18, 2002 Mesoscale experiments 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?

Integrated MicroPower GeneratorReview, Oct. 18, 2002 Needs from the group Catalyst formulations - we’ve only tested bare metal Pt, Pd, Ni Testable SCFCs with interconnects - macro- and meso/micro-scale Wide range of SCFC –Temperatures –Compositions –Residence times possible by tailoring –Fuel –Flow rate –Catalyst type –Swiss roll construction (# of turns, wall thickness & material) but tell me what you need