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, …
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 2015 - Lecture 5
2
Heat recirculating combustors
Combustion zone
Heat recirculating combustor minimizes heat losses - can be used
as heat source for thermoelectric or
other power generator
Hot Products
Cold reactants
1D counterflow heat
exchanger and combustor
Heat exchange
2D “Swiss roll” combustor
(Lloyd & Weinberg, 1974, 1975)
Toroidal 3D geometry:
further reduces losses minimizes external T
on all surfaces
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“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
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Swiss roll experiments
NI-DAQ board
PC with LabView
Thermocouples
Flashback
arrestor
Incoming reactants
Outgoing
products
Mass Flow
Controllers
Air
Fuel
O2 or N2
PC with PeakSimple
AME 514 - Spring 2015 - Lecture 5
Gas Chromatograph
5
Swiss roll experiments
• 3.5 mm channel width, 0.5 mm wall
thickness
• Top & bottom sealed with ceramic blanket
insulation
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Swiss roll experiments (Ahn et al., 2005)
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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)
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Thermal characteristics - limit temps.
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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)
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Thermal characteristics - limit temps.
 Temperatures across central region of combustor very uniform measured maximum T is indicative of true maximum
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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
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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
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Exhaust gas composition
Re
Percent (Molar Basis)
Combustion Catalyst
2
mode
area (cm ) [C3H8]inlet [C3H8] [CO2] [CO] [HC]*
Catalytic
10
Catalytic
Catalytic
Catalytic
Catalytic
16
Catalytic w/
NH3 treatment
Catalytic
100
1000
Gas-phase
Catalytic
Gas-phase
%
conv.
30
6.26
2.69 10.7
X
X
94.4
120
6.17
2.40 11.3
X
X
97.4
30
35.0
33.3 5.21
X
X
78.2
120
35.3
33.6 6.27
X
X
86.2
30
4.03
0.822 9.63
X
X
87.9
30
4.08
0.108 11.9
X
X
30
30
n/a
n/a
1.81
7.57
1.83
7.84
0.332
3.66
X
3.60
4.44 X
X
8.29 2.54 0.520
5.50 X
X
4.02 7.79 0.704
99.2
81.7
89.3
100.0
81.0
30
n/a
10.8
10.8
6.64 5.71 3.33 1.75
6.47 5.92 3.36 2.08
79.4
80.6
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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
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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
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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)
5.5 cm
Catalyst
region
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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
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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
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Numerical model
Thermocouple locations
inlet
outlet
7
6
5
4
3
d
2
1
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Numerical model
 User-Defined Function to simulate heat loss in 3rd
dimension (includes radiation to ambient)
SRth,solid =
t
H
1
+ plate +
2ksolid k plate hambient
Þ q¢¢¢ = -
2 T1 - T3
×
H SRth
Intake Exhaust
SRth,gas =
t
1
1
+ plate +
hgas k plate hambient
T_ambient
h = 10 W/m2K  T_ambient
 = 0.35
 T_wall
 T_plate
 T_blanket
T1
T_outside
T_plate
 T_gas
Heat loss in
3rd dimension
T_blanket
T_gas
blanket
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Model results - comparison to experiment
Temperatures too high to
conduct experiments above
this Re!
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Model results - comparison to 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!)
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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
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Model results – temperatures at extinction
2500
T
(model)
T
(model, no turbulence)
T
(experiment)
max
Temperature (K)
max
2000
max
Tmax
T (model)
ad
T (model, no turbulence)
ad
T (experiment)
ad
1500
1000
Tad
500
100
1000
Reynolds number
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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
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Mole % fuel at extinction limit
Model results - extinction limits
With radiation & heat loss
Without radiation
Without heat loss
4.0
3.0
2.0
Temperatures too high to
conduct experiments above
this Re!
1.0
0.0
1
10
100
1000
Reynolds Number
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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
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Model results - out of center limit
Mole % fuel at 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
Full model
Model with turbulence suppressed
Experiment
4.0
3.0
2.0
1.0
0.0
100
Reynolds number
AME 514 - Spring 2015 - Lecture 5
1000
29
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
AME 514 - Spring 2015 - Lecture 5
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Model results – wall conductivity
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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 o
o
- additional heat transport iDean vortices in flow in curved channels
i
theat recirculation (thus extinction limits)
similar with or without
t
a
turbulence (RSM = Reynolds Stressamodel) included, whereas 2D
R
R
model (no Dean vortices possible) shows very different results!
e
c 0.45
n
e 0.4
l
a 0.35
v
0.3
i
u
q 0.25
E
3D with RSM
2D with RSM
2D without RSM
0.2
0.15
100
1000
Re
Equivalence ratio at ext. limit
Equivalence ratio at ext. limit
e
c 0.45
n
e 0.4
l
a 0.35
v
i 0.3
u
q 0.25
E
Experiment
3D with RSM
3D without RSM
0.2
0.15
100
AME 514 - Spring 2015 - Lecture 5
1000
Re
32
Model results - 3D effects
No turbulence
With turbulence
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Model results - chemistry effects
 Q: One-step model: pre-exponential term (Z)
adjusted to match experiments – can Swissroll combustors be modeled without adjustable
parameters
and/or complex chemistry?
o
 A: iYes – 4-step model (Hautmann et al.,
t
1981)
a designed to model flow reactor
experiments
(not flames) works well with no
R
adjustable parameters
Equivalence ratio at ext. limit
e
c 0.45
n
e 0.4
l
a 0.35
v
i 0.3
u
q 0.25
E
4-step
1-step
Reaction rate map: Re = 55
2D 1-step model
2D 4-step model
4-step
1-step
0.2
0.15
100
1000
Re
Reaction rate map: Re = 1760
AME 514 - Spring 2015 - Lecture 5
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Scale effects - revisited
 Simplified analysis (Chen and Ronney, 2013)
T3 = T1 + (T2 -T1 ) + (T3 -T2 ) = T1 + (T2 -T1 ) +Yf ,¥QR / CP
 Adiabatic energy balance across heat exchanger: equate heat transfer
QT to enthalpy increase of reactants due to QT yields excess enthalpy (E)
QT = UT AT (T3 - T2 ) = mCP (T2 - T1 ) Þ E º
T2 - T1 UT AT
=
=N
T3 - T2 mCP
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
æT +T T +T ö
æT +T
ö
QT - QL,inlet _ side = UT AT ç 3 4 - 2 1 ÷ -U L AL ç 2 1 - T1 ÷ = mCP (T2 - T1 )
è 2
è 2
ø
2 ø
æT +T T +T ö
æT +T
ö
QT - QL,outlet _ side = -UT AT ç 3 4 - 2 1 ÷ -U L AL ç 3 4 - T1 ÷ = mCP (T4 - T3 )
è 2
è 2
ø
2 ø
T -T
4N
ÞEº 2 1 =
T3 - T2 4 + a N éë4 + N ( 2 + a )ùû
where a º
U L AL
UT AT
AME 514 - Spring 2015 - Lecture 5
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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/(Ud2)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
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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/
 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 ~ [/(2-)](Ti4 – Tj4)
s ei
1
2
2
R
=
T
+
T
T
+
T
~
~d
 Radiation / convection
i
j
i
j
UT 2 - ei
k/d
 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 2015 - Lecture 5
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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
0.3
0.35
Equivalence Ratio
Equivalence Ratio
0.4
Half
Full
Double
0.3
0.25
0.2
Without property adjustment
0.15
100
Re
1000
Half
Full
Double
0.25
0.2
0.15
0.1
100
With property adjustment
Re
AME 514 - Spring 2015 - Lecture 5
1000
38
Linear exchanger vs. spiral Swiss roll
 Create pseudo-3-turn spiral exchanger from linear exchanger cut into 3
pieces, again use mixing-cup temperatures
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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
AME 514 - Spring 2015 - Lecture 5
Simulated spiral
40
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!)
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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)
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References
Ahn, J., Eastwood, C., Sitzki, L., Ronney, P. D. (2005). “Gas-phase and catalytic combustion in heatrecirculating 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 - Spring 2015 - Lecture 5
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