2. Microscale combustion & power generation Jeongmin Ahn, James Kuo, Francisco Ochoa, Lars Sitzki, Craig Eastwood, Paul Ronney Dept. of Aerospace & Mechanical Engineering Univ. of Southern California, Los Angeles, CA http://ronney.usc.edu/Research/MicroFIRE Supported by DARPA Microsystems Technology Office and NASA-Glenn

3. Energy storage density of hydrocarbon fuels (e.g. propane, 46.4 MJ/kg) >> batteries (≈ 0.5 MJ/kg for Li-ion)Energy storage density of hydrocarbon fuels (e.g. propane, 46.4 MJ/kg) >> batteries (≈ 0.5 MJ/kg for Li-ion) Methanol 2.3x lower, formic acid 8.9x lowerMethanol 2.3x lower, formic acid 8.9x lower Mesoscale or microscale fuel  electrical power conversion device would provide much higher energy/weight than batteries for low-power applications, even with very low efficiencyMesoscale or microscale fuel  electrical power conversion device would provide much higher energy/weight than batteries for low-power applications, even with very low efficiency Problems at micro-scalesProblems at micro-scales Heat losses to walls - flame quenching, efficiency lossHeat losses to walls - flame quenching, efficiency loss Friction losses in devices with moving partsFriction losses in devices with moving parts Precision manufacturing and assembly difficultPrecision manufacturing and assembly difficult Micro-scale combustion & power generation Swiss roll

4. What is microcombustion? PDR’s definition: microcombustion occurs in small- scale flames whose physics is qualitatively different from conventional flames used in macroscopic power generation devices, specificallyPDR’s definition: microcombustion occurs in small- scale flames whose physics is qualitatively different from conventional flames used in macroscopic power generation devices, specifically The Reynolds numbers is too small for the flow to be turbulent and thus allow the flame reap the benefits of flame acceleration by turbulence ANDThe Reynolds numbers is too small for the flow to be turbulent and thus allow the flame reap the benefits of flame acceleration by turbulence AND The flame dimension is too small (i.e. smaller than the quenching distance, Pe < 40), thus some additional measure (heat recirculation, catalytic combustion, reactant preheating, etc.) is needed to sustain combustionThe flame dimension is too small (i.e. smaller than the quenching distance, Pe < 40), thus some additional measure (heat recirculation, catalytic combustion, reactant preheating, etc.) is needed to sustain combustion

5. Cox Tee Dee.010Cox Tee Dee.010 Application: model airplanes Weight: 0.49 oz. Bore: 0.237” = 6.02 mm Stroke: 0.226” = 5.74 mm Displacement: 0.00997 cu in (0.163 cm3) RPM: 30,000 Power:5 watts Poor performancePoor performance Low efficiency (4-5%)Low efficiency (4-5%) Emissions & noise unacceptable for indoor applicationsEmissions & noise unacceptable for indoor applications Not “microscale”Not “microscale” Re = Ud/ ≈ (2 x 0.6cm x (30000/60s)) (0.6cm) / (0.15 cm 2 /s) = 2400 - high enough for turbulence (barely)Re = Ud/ ≈ (2 x 0.6cm x (30000/60s)) (0.6cm) / (0.15 cm 2 /s) = 2400 - high enough for turbulence (barely) Size > quenching distance even at 1 atm, nowhere near q.d. at post-compression conditionSize > quenching distance even at 1 atm, nowhere near q.d. at post-compression condition Smallest existing combustion engine

6. Some power MEMS concepts Wankel rotary engine (Berkeley) Free-pistonengines (U. Minn, Georgia Tech)

7. Some power MEMS concepts - gas turbine (MIT) Friction & heat losses heat transfer along casing & rotor, from turbine to compressor Made from silicon - very high thermal conductivity - heat transfer along casing & rotor, from turbine to compressor Very high rotational speed (≈ 2 million RPM) needed for compression (speed of sound doesn’t scale!) Manufacturing tolerances Not microscale according to PDR’s definition: Re ≈ 1000, combustor scale > quenching distance Not microscale according to PDR’s definition: Re ≈ 1000, combustor scale > quenching distance Mixing time vs. chemical time - mixing time scales with combustor size but reaction time does not - need larger relative chamber size as scale decreases … Mixing time vs. chemical time - mixing time scales with combustor size but reaction time does not - need larger relative chamber size as scale decreases …

8. H 2 PEM fuel cells - CWRU - Savinell et al. Up to 5 mW/cm 2 demonstratedUp to 5 mW/cm 2 demonstrated Can use borohydride solutions for H 2 storage: ≈ 7 mass % H 2Can use borohydride solutions for H 2 storage: ≈ 7 mass % H 2 NaBH 4 + 2 H 2 O  NaBO 2 + 3 H 2NaBH 4 + 2 H 2 O  NaBO 2 + 3 H 2

9. Direct methanol fuel cell Methanol is easily stored compared to H 2, but has ≈ 6x lower energy/mass and requires a lot more equipment! (CMU concept shown)

10. Scaling of micro power generation Heat losses vs. heat generationHeat losses vs. heat generation Heat loss / heat generation ≈ 1/  at limit (  = non- dimensional activation energy)Heat loss / heat generation ≈ 1/  at limit (  = non- dimensional activation energy) Premixed flames in tubes or channels: Pe  S L d/  ≈ 40 - as d , need S L  (stronger mixture) to avoid quenchingPremixed flames in tubes or channels: Pe  S L d/  ≈ 40 - as d , need S L  (stronger mixture) to avoid quenching S L = 40 cm/s,  = 0.2 cm 2 /s  quenching distance ≈ 2 mm for stoichiometric HC-airS L = 40 cm/s,  = 0.2 cm 2 /s  quenching distance ≈ 2 mm for stoichiometric HC-air Note  ~ P -1, but roughly S L ~ P -0, thus can use weaker mixture at higher PNote  ~ P -1, but roughly S L ~ P -0, thus can use weaker mixture at higher P Also: Pe = 40 assumes cold walls - less quenching problem with higher wall temperature (obviously )Also: Pe = 40 assumes cold walls - less quenching problem with higher wall temperature (obviously )

11. Scaling of micro power generation Gas-phase (volumetric) vs. catalytic (surface) heat release rate H (in Watts)Gas-phase (volumetric) vs. catalytic (surface) heat release rate H (in Watts) Gas-phase: H = Q R *  *(rate/volume)*volume; rate/volume ~ A gas exp(–E gas /RT), volume ~ d 3  H~  Q R A gas exp(–E gas /RT)d 3Gas-phase: H = Q R *  *(rate/volume)*volume; rate/volume ~ A gas exp(–E gas /RT), volume ~ d 3  H~  Q R A gas exp(–E gas /RT)d 3 Catalytic: H =  Q R *(rate/area)*area, area ~ d 2 ; rate/area can be transport limited or kinetically limitedCatalytic: H =  Q R *(rate/area)*area, area ~ d 2 ; rate/area can be transport limited or kinetically limited »Transport limited (large scales, low flow rates) Rate/area ~ diffusivity*gradient ~  D(1/d)Rate/area ~ diffusivity*gradient ~  D(1/d)  Rate/area ~  D/d  H ~  Q R Dd »Kinetically limited (small scales, high flow rates, near extinction) Rate/area ~ A surf exp(–E surf /RT)Rate/area ~ A surf exp(–E surf /RT)  H ~  Q R d 2 A surf exp(–E surf /RT) Ratio gas/surface reactionRatio gas/surface reaction Transport limited: H gas /H surf = A gas exp(–E gas /RT)d 2 /D ~ d 2Transport limited: H gas /H surf = A gas exp(–E gas /RT)d 2 /D ~ d 2 Kinetically limited: H gas /H surf = A gas exp(–E gas /RT)d/(A surf exp(– E surf /RT)) ~ dKinetically limited: H gas /H surf = A gas exp(–E gas /RT)d/(A surf exp(– E surf /RT)) ~ d  Catalytic combustion more may be faster than gas-phase combustion at sufficiently small scales

12. Scaling of micro power generation Flame quenching revisitedFlame quenching revisited Heat loss (by conduction) ~ g (Area)  T/d ~ g d 2  T/d ~ dHeat loss (by conduction) ~ g (Area)  T/d ~ g d 2  T/d ~ d Heat loss / heat generationHeat loss / heat generation ~ d -2 (gas-phase combustion) ~ d -1 (surface, transport limited) ~ d 0 (surface, kinetically limited, relevant to microcombustion)  Catalytic combustion may be necessary at small scales to avoid quenching by heat losses!

13. Scaling of micro power generation Turbulence (example: IC engine, bore = stroke = d)Turbulence (example: IC engine, bore = stroke = d) Re = U p d/ ≈ (2dN)d/ = 2d 2 N/Re = U p d/ ≈ (2dN)d/ = 2d 2 N/ U p = piston speed; N = engine rotational speed (rev/min) Minimum Re ≈ several 1000 for turbulent flowMinimum Re ≈ several 1000 for turbulent flow Need N ~ 1/d 2 or U p ~ 1/d to maintain turbulence (!)Need N ~ 1/d 2 or U p ~ 1/d to maintain turbulence (!) Typical auto engine at idle: Re ≈ (2 x (10 cm) 2 x (600/60s)) / (0.15 cm 2 /s) = 13000 - high enough for turbulenceTypical auto engine at idle: Re ≈ (2 x (10 cm) 2 x (600/60s)) / (0.15 cm 2 /s) = 13000 - high enough for turbulence Cox Tee Dee: Re ≈ (2 x (0.6 cm) 2 x (30000/60s)) / (0.15 cm 2 /s) = 2400 - high enough for turbulence (barely) (maybe)Cox Tee Dee: Re ≈ (2 x (0.6 cm) 2 x (30000/60s)) / (0.15 cm 2 /s) = 2400 - high enough for turbulence (barely) (maybe) Why need turbulence? Increase burning rate - but how much?Why need turbulence? Increase burning rate - but how much? »Turbulent burning velocity (S T ) ≈ turbulence intensity (u’) »u’ ≈ 0.5 U piston (Heywood, 1988) ≈ dN »≈ 67 cm/s > S L (auto engine at idle, much more at higher N) »≈ 300 cm/s >> S L (Cox Tee Dee)

14. Scaling of micro power generation Fricton due to fluid flow in piston/cylinder gapFricton due to fluid flow in piston/cylinder gap Shear stress (  ) = µ oil (du/dy) = µ oil U p /hShear stress (  ) = µ oil (du/dy) = µ oil U p /h Friction power =  x area x velocity = 4µ oil U p L 2 /h = 4µ oil Re 2 2 /hFriction power =  x area x velocity = 4µ oil U p L 2 /h = 4µ oil Re 2 2 /h Thermal power = mass flux x C p x  T combustion =  S T d 2 C p  T =  (U p /2)d 2 C p  T =  Re)dC p  T/2Thermal power = mass flux x C p x  T combustion =  S T d 2 C p  T =  (U p /2)d 2 C p  T =  Re)dC p  T/2 Friction power / thermal power = [8µ oil (Re) ]/[  C p  Thd)] ≈ 0.002 for macroscale engineFriction power / thermal power = [8µ oil (Re) ]/[  C p  Thd)] ≈ 0.002 for macroscale engine ImplicationsImplications »Need Re ≥ Re min to have turbulence »Material properties µ oil,,  C p,  T essentially fixed  For geometrically similar engines (h ~ d), importance of friction losses ~ 1/d 2 ! What is allowable h? Need to have sufficiently small leakageWhat is allowable h? Need to have sufficiently small leakage »Simple fluid mechanics: volumetric leak rate = (  P)h 3 /3µ »Rate of volume sweeping = Ud 2 - must be >> leak rate »Need h << (3   dRe min /  P) 1/3 »Don’t need geometrically similar engine, but still need h ~ d 1/3, thus importance of friction loss ~ 1/d 4/3 !

15. Three projects at USCThree projects at USC Thermoelectric power generation (DARPA)Thermoelectric power generation (DARPA) Single-chamber solid-oxide fuel cell (DARPA)Single-chamber solid-oxide fuel cell (DARPA) Microscale jet or rocket engine (NASA)Microscale jet or rocket engine (NASA) Common themesCommon themes »No moving parts »Use common fuels in air »Spiral counterflow heat exchanger / combustor (“Swiss roll”) for thermal management »Catalytic combustion »Thermal transpiration for pumping of fuel and air »Non-traditional fabrication and materials Micro-scale combustion & power generation

16. “Swiss roll” heat recirculating burner -“Swiss roll” heat recirculating burner - minimizes heat losses Toroidal 3D geometry: further reduces losses - minimizes external temperature on all surfaces Thermal management 1D counterflow heat exchanger and combustor 2D “Swiss roll” combustor (Weinberg, 1970’s)

17. Approach for thermal-fluid design Test macroscale versions of mesoscale combustorTest macroscale versions of mesoscale combustor Use experiments to calibrate/verify CFD simulations at various Reynolds number (Re)Use experiments to calibrate/verify CFD simulations at various Reynolds number (Re) Re  Uw/ ; U = inlet velocity, w = channel width, = viscosity DemonstrateDemonstrate Scale down process (macro  meso or micro)Scale down process (macro  meso or micro) Ability to model (macro, meso) over a range of ReAbility to model (macro, meso) over a range of Re Use CFD models to optimize meso- and micro-scale devices (difficult to use diagnostics at small scales)Use CFD models to optimize meso- and micro-scale devices (difficult to use diagnostics at small scales) Key issuesKey issues Extinction limits, especially at low ReExtinction limits, especially at low Re Catalytic vs. gas-phase combustionCatalytic vs. gas-phase combustion Control of temperature, mixture & residence time for thermoelectric or solid oxide fuel cell generatorControl of temperature, mixture & residence time for thermoelectric or solid oxide fuel cell generator

18. Swiss roll experiments Initial 2-D inconel designs - high thermal conductivity (poor low-Re performance) & thermal expansion coefficient (warpage)Initial 2-D inconel designs - high thermal conductivity (poor low-Re performance) & thermal expansion coefficient (warpage) Titanium - 2x lower conductivity & expansion than inconel, improved performanceTitanium - 2x lower conductivity & expansion than inconel, improved performance Bare metal Pt catalyst in center of burnerBare metal Pt catalyst in center of burner Implementation of experimentsImplementation of experiments 3.5 turn 2-D rectangular Swiss rolls3.5 turn 2-D rectangular Swiss rolls PC control and data acquisition using LabViewPC control and data acquisition using LabView Mass flow controllers for fuel (propane) & airMass flow controllers for fuel (propane) & air Thermocouples - 1 in each inlet & outlet turn (7 total)Thermocouples - 1 in each inlet & outlet turn (7 total)

19. Macroscale experiments Mass Flow Controllers Air PC with LabView FuelO 2 or N 2 Flashback arrestor NI-DAQ board Gas Chromatograph PC with PeakSimple Thermocouples Outgoing products Incoming reactants

20. Macroscale experiments 3.5 mm channel width, 0.5 mm wall thickness3.5 mm channel width, 0.5 mm wall thickness Top & bottom sealed with ceramic blanket insulationTop & bottom sealed with ceramic blanket insulation

21. Quenching limits 0.1 1 10 1 1001000 Equivalence Ratio Reynolds Number Out-of-center reaction zone (cat. & gas-phase) Catalytic combustion only No combustion No combustion NH 3 conditioned catalytic combustion only Catalytic or gas-phase combustion 3.5 turn macroscale inconel burner Propane-air mixtures Pt catalyst where noted

22. Quenching limits Gas-phase extinction limitsGas-phase extinction limits ≈ symmetrical about  = 1≈ symmetrical about  = 1 Minimum Re ≈ 40Minimum Re ≈ 40 CatalyticCatalytic Low ReLow Re »Very low Re (≈ 1) possible »Lean limit rich of stoichiometric (!), limits very asymmetrical about  = 1 - due to need for excess fuel to scrub O 2 from catalyst surface (consistent with computations - later…) »Conditioning Pt catalyst by burning NH 3 very beneficial, »Rearranging catalyst or 4x increase in area: practically no effect! - not transport limited Intermediate Re: only slight improvement with catalystIntermediate Re: only slight improvement with catalyst Still higher Re: no effect of catalystStill higher Re: no effect of catalyst Near stoichiometric, higher Re: strong combustion, heat recirculation not needed, reaction zone not centered, not stable (same result with or without catalyst)Near stoichiometric, higher Re: strong combustion, heat recirculation not needed, reaction zone not centered, not stable (same result with or without catalyst)

23. “Flameless combustion” Combustion usually occurs in “flameless” mode - no visible flame even in darkened room, even without catalystCombustion usually occurs in “flameless” mode - no visible flame even in darkened room, even without catalyst Also seen in highly preheated air combustion (Wünning and Wünning (1997), Katsuki & Hasegawa (1998), Maruta et al (2000))Also seen in highly preheated air combustion (Wünning and Wünning (1997), Katsuki & Hasegawa (1998), Maruta et al (2000)) Residence times longer than in conventional flameResidence times longer than in conventional flame Reaction zone broader than conventional flameReaction zone broader than conventional flame More like plug-flow reactor (consistent with measured temperatures)More like plug-flow reactor (consistent with measured temperatures)

24. Out-of-center combustion regime Near-stoich. mixtures, high Re: heat recirculation not neededNear-stoich. mixtures, high Re: heat recirculation not needed Blue reaction zone propagates upstream (“flashback”), becomes conventional flame, unstableBlue reaction zone propagates upstream (“flashback”), becomes conventional flame, unstable Non-catalytic: flame extinguishes, cannot be re-establishedNon-catalytic: flame extinguishes, cannot be re-established With catalystWith catalyst Reaction can be re-established - catalyst helps flame stabilization and recovery from off-nominal operationReaction can be re-established - catalyst helps flame stabilization and recovery from off-nominal operation Pulsating mode under moderately rich conditions: gas-phase flame flashes upstream to inlet, leaves behind ultra-rich mixture that only burns catalytically, when fresh incoming mixture reaches catalyst, gas-phase flame flashes upstream again…Pulsating mode under moderately rich conditions: gas-phase flame flashes upstream to inlet, leaves behind ultra-rich mixture that only burns catalytically, when fresh incoming mixture reaches catalyst, gas-phase flame flashes upstream again…

25. 1 3 2 4 5 6 7 Thermocouple placements Out-of-center regime Lean or richLean or rich Maximum possible heat recirculation needed to obtain high enough T for reactionMaximum possible heat recirculation needed to obtain high enough T for reaction Flame centeredFlame centered Near-stoichiometricNear-stoichiometric Heat recirculation not needed - flame self-sustainingHeat recirculation not needed - flame self-sustaining Reaction zone moves toward inletReaction zone moves toward inlet Center cool due to heat lossesCenter cool due to heat losses

26. Thermal characteristics - limit temperatures

27. Much lower limit T with catalyst but only slightly leaner mixturesMuch lower limit T with catalyst but only slightly leaner mixtures For a given mixture and Re supporting gas-phase combustion, catalyst actually hurts slightly - only helps when gas-phase failsFor a given mixture and Re supporting gas-phase combustion, catalyst actually hurts slightly - only helps when gas-phase fails Limit temperatures ≈ same lean & richLimit temperatures ≈ same lean & rich Limit temperatures down to 650˚C (non-cat), 125˚C (cat), 75˚C (!) (cat, with NH 3 treatment)Limit temperatures down to 650˚C (non-cat), 125˚C (cat), 75˚C (!) (cat, with NH 3 treatment) Limit temperatures follow Arrhenius lawLimit temperatures follow Arrhenius law Ln(Re limit ) ~ -Ln(residence time) ~ 1/TLn(Re limit ) ~ -Ln(residence time) ~ 1/T Activation energies ≈ 19 kcal/mole (gas-phase), 6.4 kcal/mole (catalytic)Activation energies ≈ 19 kcal/mole (gas-phase), 6.4 kcal/mole (catalytic) Proposed mechanismProposed mechanism At limit, heat loss ~ heat generationAt limit, heat loss ~ heat generation Heat loss ~ T max -T ∞Heat loss ~ T max -T ∞ Heat generation ~ exp(-E/RT max ) ~  ∞ U ∞ AY f Q RHeat generation ~ exp(-E/RT max ) ~  ∞ U ∞ AY f Q R Limit temperatures approx. ~ ln(U ∞ ) ~ ln(Re)Limit temperatures approx. ~ ln(U ∞ ) ~ ln(Re)

28. Thermal characteristics - limit temperatures Temperatures across central region of combustor very uniform - measured maximum T is indicative of true maximumTemperatures across central region of combustor very uniform - measured maximum T is indicative of true maximum

29. Thermal characteristics - continued Heat-recirculating burners transfer heat from products to reactants, increasing reactant total enthalpyHeat-recirculating burners transfer heat from products to reactants, increasing reactant total enthalpy Dimensionless total heat recirculation (Q) ~  (T o -T i )/T ∞Dimensionless total heat recirculation (Q) ~  (T o -T i )/T ∞ T o = temperature of outlet turnT o = temperature of outlet turn T i = temperature of inlet turn adjacent to outlet turnT i = temperature of inlet turn adjacent to outlet turn T ∞ = ambient temperatureT ∞ = ambient temperature Dimensionless excess enthalpy (H) = (T max -T ad )/(T ad -T ∞ )Dimensionless excess enthalpy (H) = (T max -T ad )/(T ad -T ∞ ) T ad = adiabatic flame temperatureT ad = adiabatic flame temperature H & Q well correlated for both catalytic & gas-phase combustionH & Q well correlated for both catalytic & gas-phase combustion Low Re (low Q)Low Re (low Q) H < 0 due to heat losses & wall heat conductionH < 0 due to heat losses & wall heat conduction »Heat generation ~ fuel mass flux ~ Re »Heat loss  constant »(Heat loss) / (Heat generation)  as Re  Need Q > 1 for excess enthalpy combustionNeed Q > 1 for excess enthalpy combustion

30. Thermal characteristics - limit temperatures

31. Exhaust gas composition

32. All cases: > 80% conversion of scarce reactantAll cases: > 80% conversion of scarce reactant Low ReLow Re No CO or non-propane hydrocarbons found, even for ultra- rich mixtures!No CO or non-propane hydrocarbons found, even for ultra- rich mixtures! Only combustion products are CO 2 and (probably) H 2 OOnly combustion products are CO 2 and (probably) H 2 O Additional catalyst has almost no effectAdditional catalyst has almost no effect NH 3 catalyst treatment increases fuel conversion substantially for very low Re casesNH 3 catalyst treatment increases fuel conversion substantially for very low Re cases Moderate ReModerate Re Some CO formed in rich mixtures, less with catalystSome CO formed in rich mixtures, less with catalyst High ReHigh Re Catalyst ineffective, products same with or without catalystCatalyst ineffective, products same with or without catalyst

33. Mesoscale experiments Wire-EDM fabricationWire-EDM fabrication Pt igniter wire / catalystPt igniter wire / catalyst

34. Mesoscale experiments Can’t reach as low Re as macroscale burner!Can’t reach as low Re as macroscale burner! Wall thick and has high thermal conductivity - loss mechanism?Wall thick and has high thermal conductivity - loss mechanism?

35. Effect of wall thermal conduction Simple quasi-1D analytical model of counterflow heat-recirculating burners developed includingSimple quasi-1D analytical model of counterflow heat-recirculating burners developed including (1) heat transfer(1) heat transfer (2) chemical reaction in well-stirred reactor(2) chemical reaction in well-stirred reactor (3) heat loss to ambient(3) heat loss to ambient (4) streamwise thermal conduction along wall(4) streamwise thermal conduction along wall

36. Effect of wall thermal conduction Results show low-velocity limit requires heat loss (H > 0) and wall heat conduction (B 0) and wall heat conduction (B < ∞) Very different from burners without heat recirculation!Very different from burners without heat recirculation! H = dimensionless heat loss B -1 = dimensionless wall conduction effect Da = dimensionless reaction rate

37. Effect of wall thermal conduction High-velocity limit almost unaffected by wall conduction, but low-velocity limit dominated by wall conductionHigh-velocity limit almost unaffected by wall conduction, but low-velocity limit dominated by wall conduction Thin wall, low thermal conductivity material (ceramic vs. steel) will maximize performanceThin wall, low thermal conductivity material (ceramic vs. steel) will maximize performance

38. Effect of wall thermal conduction Predictions consistent with experiments in 2D Swiss roll combustors made of inconel (k = 11 W/mK) vs. titanium (k = 7 W/mK) - higher T, wider extinction limits with lower kPredictions consistent with experiments in 2D Swiss roll combustors made of inconel (k = 11 W/mK) vs. titanium (k = 7 W/mK) - higher T, wider extinction limits with lower k

39. Polymer combustors Experimental and theoretical studies show importance of wall thermal conductivity on combustor performance - counterintuitive: lower is better - heat transfer across thin wall is easy, but need to minimize streamwise conductionExperimental and theoretical studies show importance of wall thermal conductivity on combustor performance - counterintuitive: lower is better - heat transfer across thin wall is easy, but need to minimize streamwise conduction Low T max demonstrated in metal burners with catalytic combustion - no need for high-temperature metals (high k) or ceramics (k = 1 - 2 W/m˚C but fragile, hard to fabricate)Low T max demonstrated in metal burners with catalytic combustion - no need for high-temperature metals (high k) or ceramics (k = 1 - 2 W/m˚C but fragile, hard to fabricate) Use polymers???Use polymers??? Low k (0.3 - 0.4 W/m˚C)Low k (0.3 - 0.4 W/m˚C) Polyimides, polyetheretherketones, etc., rated to T ≈ 400˚C, even in oxidizing atmospherePolyimides, polyetheretherketones, etc., rated to T ≈ 400˚C, even in oxidizing atmosphere Easy to fabricate, not brittleEasy to fabricate, not brittle Key issuesKey issues SurvivabilitySurvivability Extinction limits - how lean or rich can we burn?Extinction limits - how lean or rich can we burn? Control of temperature, mixture & residence time for thermoelectric or solid oxide fuel cell generatorControl of temperature, mixture & residence time for thermoelectric or solid oxide fuel cell generator

40. Plastic combustor - implementation World’s first all polymer combustor?World’s first all polymer combustor? DuPont Vespel SP-1 polyimide (k = 0.29 W/m˚C)DuPont Vespel SP-1 polyimide (k = 0.29 W/m˚C) CNC milling: 3.5 turn Swiss roll, 3 mm channel width, 0.5 mm wall thickness, 2.5 cm tallCNC milling: 3.5 turn Swiss roll, 3 mm channel width, 0.5 mm wall thickness, 2.5 cm tall NH 3 -treated bare metal Pt catalyst in central regionNH 3 -treated bare metal Pt catalyst in central region General performanceGeneral performance Prolonged exposure at > 400˚C (high enough for single chamber SOFCs) with no apparent damageProlonged exposure at > 400˚C (high enough for single chamber SOFCs) with no apparent damage Thermal expansion coefficient of Vespel ≈ 4x higher than inconel, but no apparent warpingThermal expansion coefficient of Vespel ≈ 4x higher than inconel, but no apparent warping Sustained combustion at 2.9 W thermal (birthday candle ≈ 50 W)Sustained combustion at 2.9 W thermal (birthday candle ≈ 50 W) Catalystregion 5.5 cm

41. Results - polymer burner - extinction limits Extinction limit behavior similar to macroscale at Re > 20Extinction limit behavior similar to macroscale at Re > 20 Improved “lean” and “rich” limit performance compared to macroscale burner at 2.5 < Re < 20Improved “lean” and “rich” limit performance compared to macroscale burner at 2.5 < Re < 20 Sudden, as yet unexplained cutoff at Re ≈ 2.5 in polymer burnerSudden, as yet unexplained cutoff at Re ≈ 2.5 in polymer burner

42. Results - polymer burner - temperatures Sustained combustion at T max = 72˚C (lowest T ever self- sustaining hydrocarbon combustion?)Sustained combustion at T max = 72˚C (lowest T ever self- sustaining hydrocarbon combustion?) If combustion can be sustained at 72˚C, with improved thermal management, could room temperature ignition be possible?If combustion can be sustained at 72˚C, with improved thermal management, could room temperature ignition be possible? Peak T at equivalence ratio ≈ 1.5 for all RePeak T at equivalence ratio ≈ 1.5 for all Re Minimum T required to sustain combustion exceeds material limit at Re > 20, even with catalyst - polymers not suitable for high ReMinimum T required to sustain combustion exceeds material limit at Re > 20, even with catalyst - polymers not suitable for high Re Low outer wall temperature (≈ 50˚C) even with 400˚C internal TLow outer wall temperature (≈ 50˚C) even with 400˚C internal T

43. Maximum temperatures - plastic combustor

44. Temperature vs. mixture - plastic combustor

45. Numerical model FLUENT, 2D, 2nd order upwind (3D in work)FLUENT, 2D, 2nd order upwind (3D in work) 32,000 cells, grid independence verified32,000 cells, grid independence verified Conduction (solid & gas), convection (gas), radiation (solid-solid only, DO method,  = 0.35)Conduction (solid & gas), convection (gas), radiation (solid-solid only, DO method,  = 0.35) Steady, no turbulence model - valid only at low ReSteady, no turbulence model - valid only at low Re 1-step chemistry (Westbrook & Dryer), pre- exponential adjusted for agreement between model & expt. at Re = 5001-step chemistry (Westbrook & Dryer), pre- exponential adjusted for agreement between model & expt. at Re = 500 All gas & solid properties chosen to simulate inconel burner experimentsAll gas & solid properties chosen to simulate inconel burner experiments Boundary conditions:Boundary conditions: Inlet: 300K, plug flowInlet: 300K, plug flow Outlet: pressure outletOutlet: pressure outlet Heat loss at boundaries + volumetric term to simulate heat loss in 3rd dimensionHeat loss at boundaries + volumetric term to simulate heat loss in 3rd dimension

46. inletoutlet Numerical model 1 2 3 4 5 6 7 d Thermocouple locations

47. User-Defined Function to simulate heat loss in 3rd dimension (includes radiation to ambient)User-Defined Function to simulate heat loss in 3rd dimension (includes radiation to ambient) T1 temperature of burner T2 temperature of top plate T3 temperature of ambient inletoutlet Heat loss in 3 rd dimension h ambient =10 W/m 2 K T3 T2 T1 H Numerical model Numerical model

48. Results - full model

49. Results - full model - extinction limits Reasonable agreement between model and experiment for all ReReasonable agreement between model and experiment for all Re High velocity “blow-off” limit-insufficient residence time compared to chemical time scaleHigh velocity “blow-off” limit-insufficient residence time compared to chemical time scale Low velocity heat loss induced limitLow velocity heat loss induced limit Model & experiment show low-velocity limit at Re ≈ 40, even for stoichiometric mixturesModel & experiment show low-velocity limit at Re ≈ 40, even for stoichiometric mixtures Up-swing in predicted limit composition at high Re not seen in experimentUp-swing in predicted limit composition at high Re not seen in experiment Probably due to transition to turbulence (not modeled)Probably due to transition to turbulence (not modeled) Higher effective conductivity (k t ~ u’ ~ U ~ Re) for turbulent flow vs. k = constant for laminarHigher effective conductivity (k t ~ u’ ~ U ~ Re) for turbulent flow vs. k = constant for laminar Leads to greater heat recirculation for turbulent flows, higher temperatures, leaner extinction limitsLeads to greater heat recirculation for turbulent flows, higher temperatures, leaner extinction limits

50. Results - full model

51. Results - full model - temperatures “Virtual thermocouples” - 1 mm x 1 mm region at same locations at thermocouples in experiments“Virtual thermocouples” - 1 mm x 1 mm region at same locations at thermocouples in experiments Maximum temperatures at limit higher for 1-step model than experiments - typical result for 1-step model without chain branching stepsMaximum temperatures at limit higher for 1-step model than experiments - typical result for 1-step model without chain branching steps Higher T over-emphasizes radiative effects (≈ 5x at low Re)Higher T over-emphasizes radiative effects (≈ 5x at low Re)

52. Results - lower wall thermal conductivity Lower wall conductivity drastically widens limits, but optimal conductivity is lower than air!Lower wall conductivity drastically widens limits, but optimal conductivity is lower than air!

53. Modeling - effect of heat loss & radiation

54. Effect of heat loss & radiation Radiation: effect similar to heat lossRadiation: effect similar to heat loss Causes heat to be conducted along the walls and subsequently lost to ambientCauses heat to be conducted along the walls and subsequently lost to ambient Less important at smaller scalesLess important at smaller scales »Conduction ~ k(  T/  x) »Radiation ~  (T 4 -T  4 ) »Radiation/Conduction ~  x … but unless you include radiation, you get the wrong answer when you calibrate a macroscale model then apply it to microscales! … but unless you include radiation, you get the wrong answer when you calibrate a macroscale model then apply it to microscales! High Re: convection dominates heat transfer, finite residence time dominates extinction, all models yield almost same predictionsHigh Re: convection dominates heat transfer, finite residence time dominates extinction, all models yield almost same predictions

55. Reaction zone structure Broad, centered reaction zone at low % fuel - maximum heat recirculation needed for high enough T for flame survivalBroad, centered reaction zone at low % fuel - maximum heat recirculation needed for high enough T for flame survival Higher % fuel, less recirculation needed - thin, flame-like reaction zone flame moves away from centerHigher % fuel, less recirculation needed - thin, flame-like reaction zone flame moves away from center High % fuel Low % fuel Low % fuel Reaction rates Temperatures

56. Reaction rate Temperature Temperature profiles Full model, Re = 40Full model, Re = 40 Fuel concentration = 4.0% (near limit)Fuel concentration = 4.0% (near limit) Peak temperature = 1556 KPeak temperature = 1556 K Reaction located at the center of burnerReaction located at the center of burner Peak temperature near peak reactionPeak temperature near peak reaction Monotonic decrease in T moving outward from center - minimal heat recirculationMonotonic decrease in T moving outward from center - minimal heat recirculation

57. Reaction rate Temperature Full model, higher Re Full model, Re = 100Full model, Re = 100 Fuel concentration = 1.1% (near limit)Fuel concentration = 1.1% (near limit) Peak temperature = 1418 KPeak temperature = 1418 K Reaction zone more spread out - higher u, shorter residence timeReaction zone more spread out - higher u, shorter residence time More T contrast between inlet & outlet turns - less effect of wall heat conduction, but still nearly monotonic decrease in T moving from center outwardMore T contrast between inlet & outlet turns - less effect of wall heat conduction, but still nearly monotonic decrease in T moving from center outward

58. Reaction rate Temperature Full model, “very” high Re Full model, Re = 1000Full model, Re = 1000 Fuel concentration = 1.3% (near limit)Fuel concentration = 1.3% (near limit) Peak temperature = 1862 KPeak temperature = 1862 K Reaction zone still more spread outReaction zone still more spread out Still more T contrast between inlet & outlet turns - finally getting alternating low T / high T in inlet / outlet turnsStill more T contrast between inlet & outlet turns - finally getting alternating low T / high T in inlet / outlet turns

59. Reaction rate Temperature Comparison of different models at fixed Re Full model, Re = 50Full model, Re = 50 Fuel concentration = 2.0% (near limit)Fuel concentration = 2.0% (near limit) Peak temperature = 1541 KPeak temperature = 1541 K

60. Reaction rate Temperature Full model without radiation Full model without radiation, Re = 50Full model without radiation, Re = 50 Fuel concentration = 0.8% (near limit)Fuel concentration = 0.8% (near limit) Peak temperature = 1388 KPeak temperature = 1388 K Not significantly different structure, but central core is less isothermal due to absence of radiationNot significantly different structure, but central core is less isothermal due to absence of radiation Limit fuel concentration is WAY different!Limit fuel concentration is WAY different!

61. Full model without heat loss Full model without heat loss, Re = 50Full model without heat loss, Re = 50 Fuel concentration = 0.7% (near limit)Fuel concentration = 0.7% (near limit) Peak temperature = 1104 KPeak temperature = 1104 K Central region nearly isothermal (no 3D loss)Central region nearly isothermal (no 3D loss) As expected, low limit fuel concentrationAs expected, low limit fuel concentration Reaction rate Temperature

62. Catalytic combustion modeling Collaborator: Kaoru Maruta (Tohoku Univ., Sendai, Japan)Collaborator: Kaoru Maruta (Tohoku Univ., Sendai, Japan) Detailed catalytic combustion model (Deutschmann et al.) integrated into FLUENTDetailed catalytic combustion model (Deutschmann et al.) integrated into FLUENT ModelModel Cylindrical tube reactor, 1 mm dia. x 10 mm length, no wall thermal conductionCylindrical tube reactor, 1 mm dia. x 10 mm length, no wall thermal conduction Platinum catalyst, CH 4 -air and CH 4 -O 2 -N 2 mixturesPlatinum catalyst, CH 4 -air and CH 4 -O 2 -N 2 mixtures Heat loss to ambientHeat loss to ambient

63. Catalytic combustion modeling “Dual-limit” behavior similar to experiments observed when heat loss is present“Dual-limit” behavior similar to experiments observed when heat loss is present Heat release inhibited by high O(s) coverage (slow O(s) desorption) at low T - need Pt(s) sites for fuel adsorption / oxidationHeat release inhibited by high O(s) coverage (slow O(s) desorption) at low T - need Pt(s) sites for fuel adsorption / oxidation

64. Catalytic combustion modeling Computations with fuel:O 2 fixed, N 2 (not air) dilutionComputations with fuel:O 2 fixed, N 2 (not air) dilution Minimum fuel concentration & T needed to sustain combustion much lower for even slightly rich mixtures!Minimum fuel concentration & T needed to sustain combustion much lower for even slightly rich mixtures! Behavior due to transition from O(s) coverage for lean mixtures (excess O 2 ) to CO(s) coverage for rich mixtures (excess fuel)Behavior due to transition from O(s) coverage for lean mixtures (excess O 2 ) to CO(s) coverage for rich mixtures (excess fuel)

65. Catalytic combustion modeling Behavior due to transition from O(s) coverage for lean mixtures (excess O 2 ) to CO(s) coverage for rich mixtures (excess fuel)Behavior due to transition from O(s) coverage for lean mixtures (excess O 2 ) to CO(s) coverage for rich mixtures (excess fuel) Lean Rich

66. Modeling - comparison with experiments Predictions qualitatively consistent with experiments (propane- O 2 -N 2 ) in Swiss roll (not straight tube) at low RePredictions qualitatively consistent with experiments (propane- O 2 -N 2 ) in Swiss roll (not straight tube) at low Re No analogous behavior without catalystNo analogous behavior without catalyst Comment: typical strategy to reduce flame temperature: dilute with excess air, but with catalytic combustion: better strategy is to dilute with material containing no O 2, i.e. slightly rich mixtures with exhaust gas dilutionComment: typical strategy to reduce flame temperature: dilute with excess air, but with catalytic combustion: better strategy is to dilute with material containing no O 2, i.e. slightly rich mixtures with exhaust gas dilution

67. Same principal as thermocouple, material optimized for power generationSame principal as thermocouple, material optimized for power generation Imbed in wall between hot (outgoing product) and cold (incoming reactant) streamsImbed in wall between hot (outgoing product) and cold (incoming reactant) streams US Patent No.US Patent No. 6,613,972 (9/2/2003) Power generation - thermoelectrics Overall configuration - Wall itself is electrical conductor Typical thermoelectric configuration - alternating n- and p-type elements Products Reactants Combustion volume 16001200400300K500 14006005007001600 Thermoelectric elements

68. Widely used in deep space missions, some commercial applications (mostly used in reverse for cooling, not power generation)Widely used in deep space missions, some commercial applications (mostly used in reverse for cooling, not power generation) TE efficiency typically 15% of Carnot with same  TTE efficiency typically 15% of Carnot with same  T Recent development: “quantum well” thermoelectrics, many bold claimsRecent development: “quantum well” thermoelectrics, many bold claims Thermoelectrics

69. Thermoelectric microgenerator problem TE wall material: thermal conductivity k ≈ 1 W/m˚CTE wall material: thermal conductivity k ≈ 1 W/m˚C Gas: k ≈ 0.025 - 0.1 W/m˚CGas: k ≈ 0.025 - 0.1 W/m˚C  Thermal resistance between gas & TE wall >> resistance across TE  Most  T between gas & TE wall, not across TE  No power generation! Need “dirty tricks” for microscale devices!Need “dirty tricks” for microscale devices! Macroscale devices - strong turbulence, convective heat transfer, low thermal resistance, but microscale Reynolds # too low!Macroscale devices - strong turbulence, convective heat transfer, low thermal resistance, but microscale Reynolds # too low!

70. “Dirty tricks…” Integrated TE wall & T-fin design greatly reduces R gs /R TE - without massive pressure drops due to aggressive fins in flow channelIntegrated TE wall & T-fin design greatly reduces R gs /R TE - without massive pressure drops due to aggressive fins in flow channel Metal fins (blue) have high thermal conductivity - act as thermal short-circuitMetal fins (blue) have high thermal conductivity - act as thermal short-circuit Air acts as thermal open-circuitAir acts as thermal open-circuit Elongating base of T-fin and TE walls reduces R gs /R TEElongating base of T-fin and TE walls reduces R gs /R TE US Patent No.US Patent No. 6,613,972 (9/2/2003)

71. Power generation - SOFC in a Swiss roll PI: Sossina Haile, CalTechPI: Sossina Haile, CalTech Solid Oxide Fuel Cells (SOFCs) use hydrocarbon fuels directly, but need high TSolid Oxide Fuel Cells (SOFCs) use hydrocarbon fuels directly, but need high T Swiss roll for thermal managementSwiss roll for thermal management No need for T gradientNo need for T gradient Need rich mixtures for in situ reforming to CO & H 2Need rich mixtures for in situ reforming to CO & H 2 Catalytic after-burner with secondary air to oxidize rich productsCatalytic after-burner with secondary air to oxidize rich products Single Chamber Fuel Cell (SCFC) design to minimize cracking/sealing problemsSingle Chamber Fuel Cell (SCFC) design to minimize cracking/sealing problems No reformer of any kind - direct utilization of hydrocarbon!No reformer of any kind - direct utilization of hydrocarbon! Patent pending (filed 6/23/04)Patent pending (filed 6/23/04)

72. Single Chamber Solid Oxide Fuel Cells CH 4 + 1/2 O 2  CO + 2H 2 H 2 + O =  H 2 O + 2e - CO + O =  CO 2 + 2e - 1/2 O 2 + 2e -  O = fuel + oxidantby-products Introduced by Hibino et al. Science (2000)Introduced by Hibino et al. Science (2000) Fuel & oxidant mixed - no sealing issues, no coking problemsFuel & oxidant mixed - no sealing issues, no coking problems Highly selective anode & cathode catalysts essential since fuel & oxidant exposed to both anode & cathodeHighly selective anode & cathode catalysts essential since fuel & oxidant exposed to both anode & cathode conventional SOFC fueloxidant CH 4 + 4O =  CO 2 + 2H 2 O +8e - 1/2 O 2 + 2e -  O = seals

73. World’s smallest self-sustaining SOFC (?) 7 cm 1.3 cm 0.71 cm 2

74. Single Chamber Fuel Cell in Swiss roll Maximum power density ≈ 375 mW/cm 2 at T ≈ 540˚C demonstrated with direct utilization of hydrocarbon fuelMaximum power density ≈ 375 mW/cm 2 at T ≈ 540˚C demonstrated with direct utilization of hydrocarbon fuel

75. Effect of cell temperature and O 2 :fuel ratio Much lower T than conventional SOFC; significant power production even at 400˚CMuch lower T than conventional SOFC; significant power production even at 400˚C Performance not to sensitive to temperature - range of T within 20% of max. power ≈ ±50˚CPerformance not to sensitive to temperature - range of T within 20% of max. power ≈ ±50˚C Performance sensitive to O 2 :fuel ratio - best results at lower O 2 :fuel ratio (closer to stoichiometric but still fuel-rich)Performance sensitive to O 2 :fuel ratio - best results at lower O 2 :fuel ratio (closer to stoichiometric but still fuel-rich)

76. Summary Microscale power and propulsion devices may requireMicroscale power and propulsion devices may require Heat recirculation (e.g. Swiss roll)Heat recirculation (e.g. Swiss roll) Catalytic combustion, slightly rich mixturesCatalytic combustion, slightly rich mixtures Thin walls made of low conductivity materialsThin walls made of low conductivity materials Non-conventional power generation & pumping concepts - no moving partsNon-conventional power generation & pumping concepts - no moving parts Combustion behavior different from “conventional” macroscale systemsCombustion behavior different from “conventional” macroscale systems Air (or aerogels) are good insulators - ceramics are not, even plastics are marginalAir (or aerogels) are good insulators - ceramics are not, even plastics are marginal Polymer meso- or micro-combustors appear feasible and may be advantageousPolymer meso- or micro-combustors appear feasible and may be advantageous 72˚C - 400˚C sustained operation demonstrated72˚C - 400˚C sustained operation demonstrated Good low-Re performance (low k)Good low-Re performance (low k) Inexpensive, durable, many fabrication optionsInexpensive, durable, many fabrication options

77. Summary - general Study and applications of microscale thermal/chemical systems is a promising new technology in its infancyStudy and applications of microscale thermal/chemical systems is a promising new technology in its infancy Many potential applicationsMany potential applications Power generationPower generation PropulsionPropulsion Chemical or biological sensorsChemical or biological sensors … Not as crowded as “traditional” MEMSNot as crowded as “traditional” MEMS

78. Blue-sky pipe dream complete system Polymer 3D Swiss rollPolymer 3D Swiss roll Hydrocarbon fuel, self-starting at room temperatureHydrocarbon fuel, self-starting at room temperature Single-chamber solid oxide fuel cell or quantum well thermoelectrics for power generation - direct utilization of hydrocarbonsSingle-chamber solid oxide fuel cell or quantum well thermoelectrics for power generation - direct utilization of hydrocarbons Thermal transpiration pumping of fuel/air mixture - no moving parts, uses thermal energy, not electrical energyThermal transpiration pumping of fuel/air mixture - no moving parts, uses thermal energy, not electrical energy

79. USC contributions to microthermochemical systems DesignsDesigns Thermoelectric Swiss roll generatorThermoelectric Swiss roll generator Fin design for thermoelectric power generationFin design for thermoelectric power generation Use of SOFC in a Swiss rollUse of SOFC in a Swiss roll Catalytic combustion based thermal transpiration propulsionCatalytic combustion based thermal transpiration propulsion Multi-stage thermal transpiration pumping using Swiss rollMulti-stage thermal transpiration pumping using Swiss roll FundamentalsFundamentals Identified flameless combustion in broad reaction zones in heat-recirculating burnersIdentified flameless combustion in broad reaction zones in heat-recirculating burners Stability of gas-phase & catalytic modesStability of gas-phase & catalytic modes Tradeoffs between gas-phase & catalytic combustionTradeoffs between gas-phase & catalytic combustion Effect of equivalence ratio (independent of flame temperature) on catalytic combustionEffect of equivalence ratio (independent of flame temperature) on catalytic combustion Effect of wall thermal conductivityEffect of wall thermal conductivity Effect of heat losses in 3rd dimensionEffect of heat losses in 3rd dimension Importance of radiation in scale-downImportance of radiation in scale-down