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AME 514 Applications of Combustion

AME 514 Applications of Combustion. Lecture 5: Microcombustion science II. Microscale reacting flows and power generation. Micropower generation: what and why (Lecture 4) “ Microcombustion science ” (Lectures 4 - 5) Scaling considerations - flame quenching, friction, speed of sound, …

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AME 514 Applications of Combustion

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  1. AME 514Applications of Combustion Lecture 5: Microcombustionscience II

  2. Microscale reacting flows and power generation • Micropower generation: what and why (Lecture 4) • “Microcombustion science” (Lectures 4 - 5) • Scaling considerations - flame quenching, friction, speed of sound, … • Flameless & catalytic combustion • Effects of heat recirculation • Devices (Lecture 6) • Thermoelectrics • Fuel cells • Microscale internal combustion engines • Microscale propulsion • Gas turbine • Thermal transpiration AME 514 - Spring 2013 - Lecture 6

  3. Heat recirculating burners “Swiss roll” heat recirculating burner - minimizes heat losses - can be used as heat source for thermoelectric or other power generator Toroidal3D geometry: further reduces losses - minimizes external T on all surfaces 2D “Swiss roll” combustor (Lloyd & Weinberg, 1974, 1975) 1D counterflow heat exchanger and combustor AME 514 - Spring 2013 - Lecture 5

  4. Heat recirculating “Swiss roll” reactors • Use experiments to calibrate/verify CFD simulations at various Reynolds number (Re) Re  Uw/; U = inlet velocity, w = channel width,  = viscosity • Key issues • Extinction limits, especially at low Re • Catalytic vs. gas-phase combustion • Control of temperature, mixture & residence time for thermoelectric or solid oxide fuel cell generator (Lecture 6) • Implementation of experiments • 3.5 turn 2-D rectangular Swiss rolls • PC control and data acquisition using LabView • Mass flow controllers for fuel (propane) & air • Thermocouples - 1 in each inlet & outlet turn (7 total) • Bare metal Pt catalyst in center of burner AME 514 - Spring 2013 - Lecture 5

  5. NI-DAQ board PC with LabView Thermocouples Flashback arrestor Outgoing products Incoming reactants Mass Flow Controllers Air Fuel O2 or N2 PC with PeakSimple Gas Chromatograph Swiss roll experiments AME 514 - Spring 2013 - Lecture 5

  6. Swiss roll experiments 3.5 mm channel width, 0.5 mm wall thickness Top & bottom sealed with ceramic blanket insulation AME 514 - Spring 2013 - Lecture 5

  7. Swiss roll experiments (Ahn et al., 2005) AME 514 - Spring 2013 - Lecture 5

  8. Quenching limits • Gas-phase extinction limits • ≈ symmetrical about  = 1 • Minimum Re ≈ 40 • Catalytic • Low Re • Very low Re (≈ 1) possible • Lean limit rich of stoichiometric (!), limits very asymmetrical about  = 1 - due to need for excess fuel to scrub O2 from catalyst surface (consistent with computations - Lecture 4) • Conditioning Pt catalyst by burning NH3 very beneficial, • Rearranging catalyst or 4x increase in area: practically no effect! - not transport limited • Intermediate Re: only slight improvement with catalyst • Still 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) AME 514 - Spring 2013 - Lecture 5

  9. “Flameless combustion” • Combustion 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), Cavaliere and Joannon, 2004) • Reaction zone much more distributed than conventional combustion (residence time at high temp. ≈ 50 ms vs. 0.1 ms for conventional flame) • Chemical mechanism different - less CH & C2 formation • More like plug-flow reactor (consistent with measured temperatures) AME 514 - Spring 2013 - Lecture 5

  10. “Flameless combustion” • When combustion process relies on transverse heat transfer from channel walls for its existence, no means to sustain steep temperature gradient in streamwise direction • No convection-diffusion zone, more like “convection-reaction zone” • Maruta et al. (2000): nonpremixed counterflow flame with highly preheated air (1500K) vs. ambient (300K) CH4-N2 • High fuel concentration: thin reaction zone • Low fuel concentration: broad reaction zone, CH4 & O2 exist together over a spatially distributed region • No similar behavior with ambient air temperature (of course); only thin reaction zones Maruta et al. (2000) AME 514 - Spring 2013 - Lecture 5

  11. Thermal characteristics - limit temps. AME 514 - Spring 2013 - Lecture 5

  12. Thermal characteristics - limit temps. • Much 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 fails • Limit temperatures ≈ same lean & rich • Limit temperatures down to 650˚C (non-cat), 125˚C (cat), 75˚C (!) (cat, with NH3 treatment) • Limit temperatures follow Arrhenius law • Ln(Relimit) ~ -Ln(residence time) ~ 1/T • Activation energies ≈ 19 kcal/mole (gas-phase), 6.4 kcal/mole (catalytic) • Mechanism • At limit, heat loss ~ heat generation • Heat loss ~ Tmax-T∞ • Heat generation ~ exp(-E/RTmax) ~ ∞U∞AYfQR • Limit temperatures approx. ~ ln(U∞) ~ ln(Re) AME 514 - Spring 2013 - Lecture 5

  13. Thermal characteristics - limit temps. • Temperatures across central region of combustor very uniform - measured maximum T is indicative of true maximum AME 514 - Spring 2013 - Lecture 5

  14. Out-of-center regime • Lean or rich • Maximum possible heat recirculation needed to obtain high enough T for reaction • Flame centered • Near-stoichiometric • Heat recirculation not needed - flame self-sustaining • Reaction zone moves toward inlet • Center cool due to heat losses 1 2 3 4 5 6 7 Thermocouple placements AME 514 - Spring 2013 - Lecture 5

  15. Exhaust gas composition • All cases: > 80% conversion of scarce reactant • Low Re • No CO or non-propane hydrocarbons found, even for ultra-rich mixtures! • Only combustion products are CO2 and (probably) H2O • Additional catalyst has almost no effect • NH3 catalyst treatment increases fuel conversion substantially for very low Re cases • Moderate Re • Some CO formed in rich mixtures, less with catalyst • High Re • Catalyst ineffective, products same with or without catalyst AME 514 - Spring 2013 - Lecture 5

  16. Exhaust gas composition AME 514 - Spring 2013 - Lecture 5

  17. Mesoscale experiments • Wire-EDM fabrication • Pt igniter wire / catalyst AME 514 - Spring 2013 - Lecture 5

  18. 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 Re - 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! AME 514 - Spring 2013 - Lecture 5

  19. 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 conduction • Low Tmax 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??? • Low k (0.3 - 0.4 W/m˚C) • Polyimides, polyetheretherketones, etc., rated to T ≈ 400˚C, even in oxidizing atmosphere • Easy to fabricate, not brittle • Key issues • Survivability • Extinction limits - how lean or rich can we burn? • Control of temperature, mixture & residence time for thermoelectric or solid oxide fuel cell generator AME 514 - Spring 2013 - Lecture 5

  20. Plastic combustor - implementation • World’s first all polymer combustor? • 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 tall • NH3-treated bare metal Pt catalyst in central region • General performance • Prolonged 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 warping • Sustained combustion at 2.9 W thermal (birthday candle ≈ 50 W) 5.5 cm Catalyst region AME 514 - Spring 2013 - Lecture 5

  21. Results - polymer burner - extinction limits • Extinction limit behavior similar to macroscale at Re > 20 • Improved “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 burner AME 514 - Spring 2013 - Lecture 5

  22. Maximum temperatures - plastic combustor AME 514 - Spring 2013 - Lecture 5

  23. Temperature vs. mixture - plastic combustor AME 514 - Spring 2013 - Lecture 5

  24. Numerical model • Kuo and Ronney, 2007 • FLUENT, 2D, 2nd order upwind • 32,000 cells, grid independence verified • Conduction (solid & gas), convection (gas), radiation (solid-solid only, DO method,  = 0.35) • k- turbulence model - useful for qualitative evaluations but not quantitatively accurate for low Re • 1-step chemistry, pre-exponential adjusted for agreement between model & expt. at Re = 1000 • All gas & solid properties chosen to simulate inconel burner experiments • Boundary conditions: • Inlet: 300K, plug flow • Outlet: pressure outlet • Heat loss at boundaries + volumetric term to simulate heat loss in 3rd dimension AME 514 - Spring 2013 - Lecture 5

  25. Numerical model Thermocouple locations inlet outlet 7 6 5 4 3 2 d 1 AME 514 - Spring 2013 - Lecture 5

  26. T_ambient T_outside T_plate T_blanket T_gas Numerical model • User-Defined Function to simulate heat loss in 3rd dimension (includes radiation to ambient) Intake h = 10 W/m2K  = 0.35 Exhaust • T_ambient • T_wall • T_plate • T_blanket T1 • T_gas Heat loss in 3rd dimension blanket AME 514 - Spring 2013 - Lecture 5

  27. Results - full model - extinction limits Temperatures too high to conduct experiments above this Re! AME 514 - Spring 2013 - Lecture 5

  28. Comparison of model & experiment • Reasonable agreement between model & experiment for all Re when turbulence included • High-Re “blow-off” limit - insufficient residence time compared to chemical time scale • At high Re, wider limits with turbulence - increases heat transfer (gas  wall), thus heat recirculation • At low Re, limits same with or without turbulence (reality check) • Low-Re limit due to heat loss • Heat generation ~ mass flow ~ U ~ Re • Heat loss ~ (Tmax - Tambient) ≈ const •  Heat loss / heat generation  at low Re - need more fuel to avoid extinction • Model & experiment show low-U limit at Re ≈ 40, even for stoichiometric mixture (nothing adjusted to get this agreement at low Re!) AME 514 - Spring 2013 - Lecture 5

  29. Turbulence effects • Extinction limit with laminar flow deviates from turbulent flow at higher Re • Higher heat transfer coefficient (h ~ u’ ~ u) for turbulent flow vs. h = constant for laminar flow • Adiabatic reactor temperature (homework…): • If h ~ u ~ , Treactor (thus limit Yfuel) ≈ independent of u (thus independent of Re) • Vital to include turbulence effects in macroscale model to obtain correct pre-exponential factor AME 514 - Spring 2013 - Lecture 5

  30. Results - temperatures Tmax Tad AME 514 - Spring 2013 - Lecture 5

  31. Results - full model - temperatures • “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 steps • Low Re: Tmax < Tad due to heat loss - even with heat recirculation • Higher Re: heat loss less important, Tmax > Tad due to heat recirculation • Tmax at extinction nearly same with or without turbulence even though limit mixtures (thus Tad) are different • At high Re, extinction is caused by insufficient residence time compared to reaction time - determined by flow velocity (Re) • Reaction time far more sensitive to temperature than mixture • Re determines T required to avoid extinction, regardless of transport environment required to obtain this temperature AME 514 - Spring 2013 - Lecture 5

  32. Modeling - effect of heat loss & radiation AME 514 - Spring 2013 - Lecture 5

  33. Effect of heat loss & radiation • Radiation: effect similar to heat loss • Causes heat to be conducted along the walls and subsequently lost to ambient • Less important at smaller scales • Conduction ~ k(T/x) • Radiation ~ (T4-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! • High Re: convection dominates heat transfer, finite residence time dominates extinction, all models yield almost same predictions AME 514 - Spring 2013 - Lecture 5

  34. Reaction zone structure • Broad, 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 center High % fuel Low % fuel Reaction rates Temperatures AME 514 - Spring 2013 - Lecture 5

  35. Results - out of center modeling • Model shows that when fuel mole % increases, reaction zone moves out of center - consistent with experiments • Semi-quantitative agreement between simulations & experiments - NO ADJUSTABLE PARAMETERS • Again need to include turbulence at high Re AME 514 - Spring 2013 - Lecture 5

  36. Results - effect of wall conductivity • Heat recirculation requires spanwise conduction across wall from products to reactants • … but conduction to wall also causes streamwise heat conduction - removes thermal energy from reaction zone which can be lost to ambient, narrows extinction limits (Ronney, 2003; Chen & Buckmaster, 2004) • BUT if wall k = 0, no heat recirculation •  THERE MUST BE AN OPTIMUM WALL THERMAL CONDUCTIVTY • Computational predictions • High Re: convection >> conduction, wall k doesn’t matter unless it’s too small • Lower Re: convection ≈ conduction, heat loss dominant; optimal k exists, but is less than air! • Optimal k roughly where thermal resistance across wall ≈ thermal resistance air  wall AME 514 - Spring 2013 - Lecture 5

  37. Results - lower wall thermal conductivity AME 514 - Spring 2013 - Lecture 5

  38. 3D effects • Q: Does 2D model properly account for heat loss in 3rd dimension? • A: (Chen & Ronney, 2011) Generally yes, but new effects arise - Dean vortices in flow in curved channels - additional heat transport - heat recirculation (thus extinction limits) similar with or without turbulence (RSM = Reynolds Stress model) included, whereas 2D model (no Dean vortices possible) shows very different results! Equivalence ratio at ext. limit Equivalence ratio at ext. limit Upper: no turbulence Lower: with turbulence AME 514 - Spring 2013 - Lecture 5

  39. Chemistry effects • Q: One-step model: pre-exponential term (Z) adjusted to match experiments – can Swiss-roll combustors be modeled without adjustable parameters and/or complex chemistry? • A: Yes – 4-step model (Hautmann et al., 1981) designed to model flow reactor experiments (not flames) works well with no adjustable parameters 4-step 1-step Reaction rate map: Re = 55 4-step 1-step Equivalence ratio at ext. limit Reaction rate map: Re = 1760 AME 514 - Spring 2013 - Lecture 5

  40. Scale effects in heat-recirculating combustors • Simplified analysis (Chen and Ronney, 2013) • Adiabatic energy balance across heat exchanger: equating heat transfer QT to enthalpy increase of reactants due to QTyields excess enthalpy (E) UT = overall heat transfer coefficient, AT = exchanger area N = number of transfer units from heat exchanger literature • Non-adiabatic analysis using “mixing cup” (average) temperatures AME 514 - Spring 2013 - Lecture 5

  41. Scaling of heat-recirculating combustors • Heat transfer • Laminar flow: UT~ h ~(k/d)Nu ~ (k/d)Re0 h = heat transfer coefficient, Nu = Nusselt number N ~ UTAT/mdotCP~ (k/d)d2/(rUd2)CP ~ Re-1 ~ 1/d • Turbulent flow: UT~(k/d)Nu ~ (k/d)Re0.8, N ~ Re-0.2 • Either way, Re (which is known a priori) is uniquely related to N, so can use Re as a scaling parameter instead place of N (which depends on h and isn’t known a priori) • Heat loss • UL generally independent of scale (for buoyant convection or radiation), AL ~ AT, thus for laminar flow with UT ~ 1/d, a ~ 1/d • Thus, at low Re, for the same Re performance is poorer forlarge scale combustors AME 514 - Spring 2013 - Lecture 4

  42. Scaling (continued) • Chemical reaction • Reaction_rate/volume ~ Yf,∞Zgasexp(–Egas/RT) ~ 1/(Reaction time) • Residence time ~ V/(mdot/) ~ V/((uA)/) ~ (V/A)/u (V = volume, u = velocity) • V/A ~ d3/d2 = d1 Residence time ~ d/u • Residence time / reaction time ~ Yf,∞Zgasd/uexp(–Egas/RT)] ~ Da/(exp(–Egas/RT)])Red-1; Da = Yf,∞Zgasd2/n • Blowoff at high u occurs more readily for small d (small residence time / chemical time); at same Red, need Z ~ 1/d2 to maintain same extinction limit • Radiation • Convective transfer per unit area between walls iand j ~ UT(Ti – Tj) • Radiative heat transfer ~ [e/(2-e)]s(Ti4– Tj4) • Radiation / convection • Surface radiation effects more important at larger scale; as previously discussed, hurts performance in a manner similar to streamwise wall heat conduction AME 514 - Spring 2013 - Lecture 4

  43. Scale effects • Simulations in 3D, 3.5 turn Swiss roll, without and with property values adjusted to obtain constant a, Da and R • Without adjustments, at small Re heat loss effects result in worse performance for large combustor whereas at large Re, residence time (Da effects) results in worse performance for small combustor; with adjustments, all scales similar With property adjustment Without property adjustment AME 514 - Spring 2013 - Lecture 5

  44. Linear vs. spiral (Swiss roll) • Create pseudo-3-turn spiral exchanger from linear exchanger cut into 3 pieces, again use mixing-cup temperatures AME 514 - Spring 2013 - Lecture 5

  45. Linear vs. spiral (Swiss roll) • Adiabatic linear exchanger performance much better than spiral exchanger at large N (low Re) • With increasing heat loss (a), linear exchanger performance deteriorates substantially compared to spiral exchanger (homework problem!) • … but this is all just heat transfer, what about with chemical reaction? Simulated spiral Linear AME 514 - Spring 2013 - Lecture 5

  46. Scale effects • Consistent with detailed calculations including chemical reaction (Chen & Ronney, 2013) • Adiabatic • Linear better at low Re (large N) • Same performance at high Re (small N) (Swiss roll has 2x larger AT than linear device, so 2x lower equivalence ratio at limit) • Non-adiabatic • Swiss roll MUCH better at low Re (need to reduce for linear device heat loss coefficients by 4x just to get plots on the same scale!) AME 514 - Spring 2013 - Lecture 5

  47. References Ahn, J., Eastwood, C., Sitzki, L., Ronney, P. D. (2005). “Gas-phase and catalytic combustion in heat-recirculating burners,”Proceedings of the Combustion Institute, Vol. 30, pp. 2463-2472. Cavaliere, A., de Joannon, M., Prog. Energy Combust. Sci. 30:329-366 (2004). Chen, C.-H., Ronney, P. D. (2013), “Scale and geometry effects on heat-recirculating combustors,” to appear in Combustion Theory and Modelling Chen, C.-H., Ronney, P. D. (2011) “Three-dimensional Effects in Counterflow Heat-Recirculating Combustors,”Proceedings of the Combustion Institute, Vol. 33, pp. 3285-3291. Hautman, D. J., Dryer, F. L., Schug, K. P.,Glassman, I. (1981). “A Multiple-stepOverall Kinetic Mechanism for the Oxidation of Hydrocarbons,” Combustion Science and TechnologyVol. 25, pp. 219-235. Katsui, M., Hasegawa, T., Proc. Combust. Inst. 27:3135-3146 (1998). Kuo, C.-H., Ronney, P. D. (2007). Numerical Modeling of Heat Recirculating Combustors, Proceedings of the Combustion Institute, Vol. 31, pp. 3277 - 3284. Lloyd, S.A., Weinberg, F.J., Nature 251:47-49 (1974). Lloyd, S.A., Weinberg, F.J., Nature 257:367-370 (1975). Maruta, K., Muso, K., Takeda, K., Niioka, T., Proc. Combust. Inst. 28:2117-2123 (2000). Targett, M., Retallick, W., Churchill, S. (1992). “Solutions in closed form for a double-spiral heat exchanger,” Industrial and Engineering Chemical Research 31, 658-669. Wünning, J.A., Wünning, J.G., Prog. Energy Combust. Sci. 23:81-94 (1997). AME 514 - Spring 2013 - Lecture 5

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