Microscale reacting flows and power generation

1
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,
• Catalytic combustion
• Effects of heat recirculation
• Devices (Lecture 6)
• Thermoelectrics
• Fuel cells
• Microscale internal combustion engines
• Microscale propulsion
• Gas turbine
• Thermal transpiration
• Rocket
• Reference Fernandez-Pello, A. C., Micropower
  Generation Using Combustion Issues and
  Approaches, Proc. Combust. Inst., Vol. 29, pp.
  883 - 899 (2002)

2
Heat recirculating burners

• Swiss roll heat recirculating burner -
• minimizes heat losses - can be used
• as heat source for thermoelectric or
• other power generator
• Toroidal 3D geometry
• further reduces losses -

2D Swiss roll combustor (Lloyd Weinberg,
1974, 1975)
1D counterflow heat exchanger and combustor

3
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

4
Swiss roll experiments
5
Swiss roll experiments

• 3.5 mm channel width, 0.5 mm wall thickness
• Top bottom sealed with ceramic blanket
  insulation

6
Swiss roll experiments (Ahn et al., 2005)
7
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)

8
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)

9
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)
10
Out-of-center combustion regime

• Near-stoich. mixtures, high Re heat
  recirculation not needed
• Blue reaction zone propagates upstream
  (flashback), becomes conventional flame,
  unstable
• Non-catalytic flame extinguishes, cannot be
  re-established
• With catalyst
• Reaction 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

Non-catalytic Catalytic
11
Thermal characteristics - limit temps.
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 650C (non-cat), 125C
  (cat), 75C (!) (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-T8
• Heat generation exp(-E/RTmax) ?8U8AYfQR
• Limit temperatures approx. ln(U8) ln(Re)

13
Thermal characteristics - limit temps.

• Temperatures across central region of combustor
  very uniform - measured maximum T is indicative
  of true maximum

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
15
Exhaust gas composition

• All cases gt 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

16
Exhaust gas composition
17
Mesoscale experiments

• Wire-EDM fabrication
• Pt igniter wire / catalyst

18
Mesoscale experiments

• Steady combustion obtained even at lt 100C 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?
• Cant reach as low Re as macroscale burner!
• Wall thick and has high thermal conductivity -
  loss mechanism!

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/mC but fragile, hard to fabricate)
• Use polymers???
• Low k (0.3 - 0.4 W/mC)
• Polyimides, polyetheretherketones, etc., rated to
  T 400C, 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

20
Plastic combustor - implementation

• Worlds first all polymer combustor?
• DuPont Vespel SP-1 polyimide (k 0.29 W/mC)
• 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 gt 400C (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
21
Results - polymer burner - extinction limits

• Extinction limit behavior similar to macroscale
  at Re gt 20
• Improved lean and rich limit performance
  compared to macroscale burner at 2.5 lt Re lt 20
• Sudden, as yet unexplained cutoff at Re 2.5 in
  polymer burner

22
Results - polymer burner - temperatures

• Sustained combustion at Tmax 72C (lowest T
  ever self-sustaining hydrocarbon combustion?)
• If combustion can be sustained at 72C, with
  improved thermal management, could room
  temperature ignition be possible?
• Peak T at equivalence ratio 1.5 for all Re
• Minimum T required to sustain combustion exceeds
  material limit at Re gt 20, even with catalyst -
  polymers not suitable for high Re
• Low outer wall temperature ( 50C) even with
  400C internal T

23
Maximum temperatures - plastic combustor
24
Temperature vs. mixture - plastic combustor
25
Smaller plastic combustors

• Smaller combustors difficult to CNC mill - need
  very thin walls for geometric similarity
• Use folded Kapton (another type of polyimide)
  plastic sheet (k 0.12 W/mC !!!) (0.25 mm
  thick sheets tested to date, thinner in progress)
• Same NH3-treated Pt catalyst

26
Smaller plastic combustors - performance

• Similar performance to large plastic burners
• Sustained temperatures gt 400C
• Slightly higher T for same Re
• but minimum temperatures much higher ( 320C)
  - need thinner walls, less streamwise wall
  conduction

27
Numerical model

• Kuo and Ronney, 2007
• FLUENT, 2D, 2nd order upwind (3D in work)
• 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

28
Numerical model
Thermocouple locations
inlet
outlet
7
6
5
4
3
2
d
1
29
Numerical model

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

Intake
Exhaust
h 10 W/m2K ? 0.35

• T_ambient

• T_wall

• T_plate

• T_blanket

T1

• T_gas

Heat loss in 3rd dimension
blanket
30
Results - full model - extinction limits
Temperatures too high to conduct experiments
above this Re!
31
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!)

32
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

33
Results - temperatures
Tmax
Tad
34
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 lt Tad due to heat loss - even with
  heat recirculation
• Higher Re heat loss less important, Tmax gt 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

35
Modeling - effect of heat loss radiation
36
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

37
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

38
Out of center limit

• For higher Re near-stoichiometric mixtures,
  heat loss is not dominant thus the flame is
  robust enough to exist without heat recirculation
• The reaction zone may propagate out of the center
  of Swiss roll, an out of center or flashback
  limit
• Definition out of center when last inlet or
  first outlet turn T higher than center T, i.e
  Max(TC1, TC2) lt Max(TC3TC7)

39
Out of center limit
40
Out of center limit
Out of center limit
41
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

42
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

43
Results - lower wall thermal conductivity
44
Results - lower wall thermal conductivity

• Optimal conductivity is lower than air!
• High Re convection gtgt conduction, wall k doesnt
  matter unless its 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
• As Re decreases, optimal k decreases and limit is
  more sensitive to k

45
Size effects

• Titanium (Ti) Swiss roll is employed to study
  size effects - everything 2x or 4x smaller
• Model for full-size Ti Swiss roll predicts the
  low-velocity limit at Re 36 - consistent with
  experiment
• Low-Re extinction limit for stoichiometric
  mixtures is 46 for half-size burner 58 for
  quarter-size vs. 36 for full-size - smaller
  burner actually needs larger Re!
• Heat loss is more significant for smaller burner
• At high Re, the effects of heat loss decrease,
  extinction limits independent of size

46
Size effects
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. 30329-366 (2004).
• Katsui, M., Hasegawa, T., Proc. Combust. Inst.
  273135-3146 (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 25147-49
  (1974).
• Lloyd, S.A., Weinberg, F.J., Nature 257367-370
  (1975).
• Maruta, K., Muso, K., Takeda, K., Niioka, T.,
  Proc. Combust. Inst. 282117-2123 (2000).
• Wünning, J.A., Wünning, J.G., Prog. Energy
  Combust. Sci. 2381-94 (1997).