ame 514 applications of combustion lecture 5: microcombustion science ii

AME 514

Applications of Combustion

Lecture 5: Microcombustion science II

2AME 514 - Spring 2015 - Lecture 5

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 combustionEffects of heat recirculation

Devices (Lecture 6)ThermoelectricsFuel cellsMicroscale internal combustion enginesMicroscale propulsion

» Gas turbine» Thermal transpiration


Heat recirculating combustor - minimizes heat losses - can be used as heat source for thermoelectric orother power generator

Toroidal 3D geometry:further reduces losses - minimizes external T on all surfaces

Heat recirculating combustors

1D counterflow heat exchanger and combustor

2D “Swiss roll” combustor(Lloyd & Weinberg, 1974, 1975)

Cold reactantsHot Products

Combustion zone

Heat exchange


“Swiss roll” combustors - methods Use experiments to calibrate/verify CFD simulations at various

Reynolds number (Re)Re Ud/; U = inlet velocity, d = 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


Mass Flow

Controllers

Air

PC with LabView

Fuel O2 or N2

Flashback

arrestor

NI-DAQ board

Gas ChromatographPC with PeakSimple

Thermocouples

Outgoing

productsIncoming reactants

Swiss roll experiments


Swiss roll experiments

• 3.5 mm channel width, 0.5 mm wall thickness

• Top & bottom sealed with ceramic blanket insulation


Swiss roll experiments (Ahn et al., 2005)


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)


Thermal characteristics - limit temps.


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)


Thermal characteristics - limit temps. Temperatures across central region of combustor very uniform -

measured maximum T is indicative of true maximum


1

32

45

67

Thermocouple placements

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


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


Exhaust gas composition


Scale-down experiments Wire-EDM fabrication, Pt igniter wire / catalyst Can’t reach as low Re as macroscale burner! Wall thick and has high thermal conductivity - loss mechanism!

2D mini Swiss Roll


Polymer combustors

Theoretical study showed 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 (DuPont Vespel SP-1 polyimide, k = 0.29 W/m˚C), rated to

T > 400˚C, even in oxidizing atmosphere Easy to fabricate, not brittle


Plastic combustor - implementation

World’s first all polymer combustors? (Sanford et al., 2008) 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

No damage even at T > 400˚C (high enough for SOFCs) Thermal expansion coefficient of Vespel ≈ 4x inconel, but no warping Sustained combustion at 2.9 W thermal (birthday candle ≈ 50 W)

Catalystregion

5.5 cm


Results - polymer burner - extinction limits Extinction limit behavior similar to metal burner at larger Re 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

Sanford et al., 2008


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


inlet outlet

Numerical model

12

34

56

7

d

Thermocouple locations


User-Defined Function to simulate heat loss in 3rd dimension (includes radiation to ambient)

Numerical model

Intake Exhaust h = 10 W/m2K = 0.35

T1

Heat loss in3rd dimension

blanket

T_gas

T_blanket

T_plate

T_wall

T_ambient

T_gas

T_blanket

T_plate

T_outside

T_ambient


Model results - comparison to experiment

Temperatures too high to conduct experiments above this Re!


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!)

Model results - comparison to experiment


Model results - 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


Model results – temperatures at extinction

Tmax

Tad


Model results – temperatures at extinction “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


Temperatures too high to conduct experiments above this Re!

Model results - extinction limits


Model results - 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


Model results - out of center limit 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


Model results - 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


Model results – wall conductivity


Model results - 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!

Equ

ival

ence

rat

io a

t ex

t. li

mit

Equ

ival

ence

rat

io a

t ex

t. li

mit


Model results - 3D effects

No turbulence

With turbulence


Model results - 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

Equ

ival

ence

rat

io a

t ex

t. li

mit Reaction rate map: Re = 55

Reaction rate map: Re = 1760

4-step

4-step

1-step

1-step


Scale effects - revisited Simplified analysis (Chen and Ronney, 2013)

Adiabatic energy balance across heat exchanger: equate heat transfer QT to enthalpy increase of reactants due to QT yields 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


Scale effects - revisited

Heat transfer Laminar flow: UT ~ h ~ (k/d)Nu ~ (k/d)Re0

h = heat transfer coefficient, Nu = Nusselt number N ~ UTAT/ CP ~ (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 ~ d Thus, at low Re, for the same Re performance is poorer for large scale

combustors


Scale effects - revisited 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/U exp(–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 i and j ~ UT(Ti – Tj) Radiative heat transfer ~ [e/(2-e)]s(Ti

4 – 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


Scale effects - revisited 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

Property Half Full Double

hL (W/m2K) 10 5 2.5

εL (external wall) 0.8 0.4 0.2

εL (insulation) 1 0.5 0.25

Z (m-sec-kmole units) 1.44 x 1011 3.6 x 1010 9.0 x 109

εi (internal wall) 0.8 0.5 0.2857

Without property adjustment With property adjustment


Linear exchanger vs. spiral Swiss roll Create pseudo-3-turn spiral exchanger from linear exchanger cut into 3

pieces, again use mixing-cup temperatures


Linear exchanger 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?

Linear Simulated spiral


Linear exchanger vs. spiral Swiss roll Consistent with detailed calculations (Chen & Ronney, 2013)

Adiabatic»Linear better (leaner extinction limit) 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!)


Model results - number of turns Fair comparison – same overall dimension and wall thickness

(fabrication limitation) Ronney, 2015: More turns means larger N but more material, thus

more thermal conduction (and heat loss) in 3rd dimension – optimum exists, but relatively flat; optimal n larger at higher Re (lower N, more “starved” for additional heat recirculation)


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.

Chen, C.-H., Ronney, P. D. (2013), “Scale and geometry effects on heat-recirculating combustors,” Combustion Theory and Modelling, Vol. 17, pp. 888-905 (2013)

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-step Overall Kinetic Mechanism for the Oxidation of Hydrocarbons,” Combustion Science and Technology Vol. 25, pp. 219-235.

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).Ronney, P. D. (2015). “Heat-Recirculating Combustors,” Chapter 8 in Microscale Combustion and

Power Generation (Y. Ju, C. Cadou and K. Maruta, Eds.), Momentum Press LLC, New York. Sanford, L. L., Huang, S. Y. J., Lin, C. S., Lee, J. M., Ahn, J. M., Ronney, P. D. (2008). “Plastic

mesoscale combustors/heat exchangers,” Proceedings of the ASME International Mechanical Engineering Congress and Exposition, Nov. 11 – 15, 2007, Seattle, WA, pp. 141 – 145.

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.

ame 514 applications of combustion lecture 5: microcombustion science ii

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