AME 514 Applications of Combustion Lecture 4: Microcombustion
science I
Slide 2
2 AME 514 - Spring 2015 - Lecture 4 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
Slide 3
3 AME 514 - Spring 2015 - Lecture 4 Paper review format Prepare
a critical review of the article, not to exceed 2 pages, structured
as follows: 1.Motivation: Why the author(s) conducted the work
2.Summary of the methods and results 3.Summary of the conclusions
4.Merits: Your opinion of the merits of the work 5.Weaknesses: Your
opinion of the shortcomings of the work Suggestions: Don't repeat
text that is in the paper. Summarize in your own words it shows me
that you really do understand the paper. Don't use buzz words from
the paper without defining them. If you don't understand them and
don't feel inclined to learn what they are (which is ok, I don't
expect you to understand every detail of the paper) then leave the
buzz words out! In other words: everything you say can and will be
used against you (Sounds harsh, but that's the way real science is
anything you write in a paper is subject to evaluation and
criticism). Points 1 and 5 are the most important. Say more than 1
line about item 5, in particular. This really shows what you
learned from the paper. It also helps you to generate your own
ideas for research.
Slide 4
4 AME 514 - Spring 2015 - Lecture 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, 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 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 combustion
Slide 5
5 AME 514 - Spring 2015 - Lecture 4 The seductive lure of
chemical fuels
Slide 6
6 AME 514 - Spring 2015 - Lecture 4 The challenge of
microcombustion Hydrocarbon fuels have numerous advantages over
batteries 100 X higher energy density Much higher power / weight
& power / volume of engine Inexpensive Nearly infinite shelf
life More constant voltage, no memory effect, instant recharge
Environmentally superior to disposable batteries > $40
billion/yr of disposable batteries ends up in landfills > $6
billion/yr market for rechargables (increasing rapidly due to
Electric vehicles)
Slide 7
7 AME 514 - Spring 2015 - Lecture 4 The challenge of
microcombustion but converting fuel energy to electricity with a
small device has not yet proved practical despite numerous
applications Foot soldiers (past DARPA funding: > 25 projects,
> $50M) Portable electronics - laptop computers, cell phones,
Micro air and space vehicles (enabling technology) Most approaches
use scaled-down macroscopic combustion engines, but may have
problems with Heat losses - flame quenching, unburned fuel & CO
emissions Heat gains before/during compression Limited fuel choices
for premixed-charge engines need knock-resistant fuels, etc.
Friction losses Sealing, tolerances, manufacturing, assembly
Slide 8
8 AME 514 - Spring 2015 - Lecture 4 The challenge of
microcombustion Other issues Modeling - gas-phase & surface
chemistry submodels Characterization of catalyst degradation &
restoration Heat rejection - 10% efficiency means 10x more heat
rejection than battery, 5% = 20x, etc. Auxiliary components -
valves, pumps, fuel tanksvalves Packaging
Slide 9
9 AME 514 - Spring 2015 - Lecture 4 Application: model
airplanes Weight: 0.49 oz. Bore: 0.237 = 6.02 mm Stroke: 0.226 =
5.74 mm Displacement: 0.00997 in 3 (0.163 cm 3 ) RPM: 30,000 Power:
5 watts Ignition:Glow plug Typical fuel: castor oil (20%),
nitromethane (10%), balance methanol (much lower heating value than
pure hydrocarbons, 22 MJ/kg vs. 45 MJ/kg) Poor performance Low
efficiency ( 5%) Emissions & noise unacceptable for indoor
applications Not microscale 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 at 1 atm, nowhere near
quenching post-compression Test data (for 2.45 cm 3 2-stroke
engine) (Menon et al., 2007): max. efficiency 8%, max. power 140
Watts at 10,000 RPM (Brake Mean Effective Pressure = 3.38 atm, vs.
typically 8 - 10 atm for automotive engines) Smallest existing
combustion engine Cox Tee Dee.010
Slide 10
10 AME 514 - Spring 2015 - Lecture 4 Wankel rotary engine
(Berkeley) Free-piston engines (U. Minn, Georgia Tech) Some power
MEMS concepts
Slide 11
11 AME 514 - Spring 2015 - Lecture 4 Pulsed combustion driven
turbine (UCLA) Some power MEMS concepts Liquid piston
magnetohydrodynamic (MHD) engine (Honeywell / U. Minn)
Slide 12
12 AME 514 - Spring 2015 - Lecture 4 Some power MEMS concepts -
gas turbine (MIT) Friction & heat losses Manufacturing
tolerances Very high rotational speed ( 2 million RPM) needed for
compression (speed of sound doesn't scale!)
Slide 13
13 AME 514 - Spring 2015 - Lecture 4 Some power MEMS concepts -
P 3 - Wash. St. Univ. P 3 engine (Whalen et al., 2003) -
heating/cooling of vapor bubble Flexing but no sliding or rotating
parts - more amenable to microscales - less friction losses Layered
design more amenable to MEMS fabrication Stacks - heat out of
higher-T engine = heat in to next lower-T engine Efficiency?
Thermal switch? Self-resonating? To date: 0.8 W power out for 1.45
W thermal power input
Slide 14
14 AME 514 - Spring 2015 - Lecture 4 PEM fuel cell Solid Oxide
Fuel Cell Fuel cells Basically a battery with a continuous feed of
reactants to electrodes Basic parts Cathode: O 2 decomposed,
electrons consumed, Anode: fuel decomposed, electrons generated
Membrane: allows H + or O = to pass, but not electrons Fuel cells
not limited by 2nd Law efficiencies - not a heat engine Several
flavors including Hydrogen - air: simple to make using Proton
Exchange Membrane (PEM) polymers (e.g. DuPont Nafion, but how to
store H 2 ?) Methanol - easy to store, but need to reform to make H
2 or find holy grail membrane for direct conversion (Nafion:
crossover of methanol to air side) Solid oxide - direct conversion
of hydrocarbons, but need high temperatures (500 - 1000C) Formic
acid (O=CH-OH) - low energy density but good electrochemistry
Slide 15
15 AME 514 - Spring 2015 - Lecture 4 Hydrogen storage Hydrogen
is a great fuel High energy density (1.2 x 10 8 J/kg, 3x
hydrocarbons) Much higher than hydrocarbons ( 10 - 100x at same T)
Excellent electrochemical properties in fuel cells Ignites near
room temperature on Pt catalyst But how to store it??? Cryogenic
liquid - 20K, = 0.070 g/cm 3 (by volume, gasoline has 64% more H
than LH 2 ); also, how to insulate for long-duration storage?
Compressed gas, 200 atm: = 0.018 g/cm 3 ; weight of tank >>
weight of fuel; spherical tank, high-strength aluminum (50,000 psi
working stress), (mass tank)/(mass fuel) 15 (note CH 4 has 2x more
H for same volume & pressure) Borohydride solution or powder +
H 2 O NaBH 4 + 2H 2 O NaBO 2 (Borax) + 3H 2 (mass solution)/(mass
fuel) 9.25 4.05 x 10 6 J/kg bonus heat release Safe, no high
pressure or dangerous products, but solution has limited lifetime
Palladium - absorbs 900x its own volume in H 2 (
www.psc.edu/science/Wolf/Wolf.html ) - but Pd/H = 164 (mass basis)
www.psc.edu/science/Wolf/Wolf.html Carbon nanotubes - many claims,
currently < 1% plausible (Benard et al., 2007) Long-chain
hydrocarbon (CH 2 ) x : (Mass C)/(mass H) = 6, plus C atoms add
94.1 kcal of energy release to 57.8 for H 2 !
Slide 16
16 AME 514 - Spring 2015 - Lecture 4 Methanol is much more
easily stored than H 2, but has 6x lower energy/mass and requires a
lot more equipment! (CMU concept shown) Direct methanol fuel
cell
Slide 17
17 AME 514 - Spring 2015 - Lecture 4 Formic acid fuel cell Zhu
et al. (2004); Ha et al. (2004) HCOOH H 2 + CO 2 - good hydrogen
storage, chemistry amenable to fuel cells, low crossover compared
to methanol, but low energy density (5.53 x 10 6 J/kg, 8.4x lower
than hydrocarbons) but it works!
Slide 18
18 AME 514 - Spring 2015 - Lecture 4 Scaling of micro power
generation - quenching Heat losses vs. heat generation Heat loss /
heat generation 1/ at limit Premixed flames in tubes: 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-air Note ~ P -1, but roughly S L ~ P -0.1, thus can use weaker
mixture (lower S L ) at higher P Also: Pe = 40 assumes cold walls -
less heat loss, thus quenching problem with higher wall temperature
(obviously)
Slide 19
19 AME 514 - Spring 2015 - Lecture 4 Scaling - gas-phase vs.
catalytic reaction Heat release rate H (in Watts) Gas-phase: H = Q
R * *(reaction_rate/volume)*volume Reaction_rate/volume ~ Y f, Z
gas exp(E gas /RT), volume ~ d 3 H ~ Y f, Q R Z gas exp(E gas /RT)d
3 d = channel width or some other characteristic dimension
Catalytic: H = Y f, 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 ~
DY f, (1/d) H ~ ( D/d)*d 2 *Q R H ~ Y f, Q R Dd Kinetically limited
(small scales, high flow rates, near extinction) Rate/area ~ Z surf
exp(E surf /RT) H ~ Y f, Q R d 2 Z surf exp(E surf /RT) Ratio
gas/surface reaction Transport limited: H gas /H surf = Z gas exp(E
gas /RT)d 2 /D ~ d 2 Kinetically limited: H gas /H surf = Z gas
exp(E gas /RT)d/(Z surf exp(E surf /RT)) ~ d Catalytic combustion
will be faster than gas-phase combustion at sufficiently small
scales
Slide 20
20 AME 514 - Spring 2015 - Lecture 4 Scaling - flame quenching
revisited Heat loss (by conduction) ~ k g (Area) T/d ~ k g d 2 T/d
~ k g d T Define = Heat loss / heat generation (H) Gas-phase
combustion ~ (k g d T)/( Q R Z gas exp(E gas /RT)d 3 ) fQ R ~ C P
T; S L ~ ( g ) 1/2 ~ ( g Z gas exp(E gas /RT)) 1/2 ~ ( g /S L d) 2
~ (1/Pe) 2 (i.e. quenching criterion is a constant Pe as already
discussed) Surface combustion, transport limited ~ (k g d T)/( Q R
Dd) ~ (C P T/Q R )(k g / C P )/D ~ 1 (i.e. no effect of scale or
transport properties, not really a limit criterion) Surface
combustion, kinetically limited, relevant to microcombustion ~ (k g
d T)/ Q R d 2 Z surf exp(E surf /RT) ~ (k g / C P )(C P T/Q R )(1/Z
surf d) ~ g /Z surf d ~ 1/d Catalytic combustion: decreases more
slowly with decreasing d (~ 1/d) than in gas combustion (~1/d 2 ),
may be necessary at small scales to avoid quenching by heat
losses!
Slide 21
21 AME 514 - Spring 2015 - Lecture 4 Scaling blow-off limit at
high U Reaction_rate/volume ~ Y f, Z gas exp(E gas /RT) ~
1/(Reaction time) Residence time ~ V/(mdot/ ) ~ V/(( UA)/ ) ~
(V/A)/U (V = volume) V/A ~ d 3 /d 2 = d 1 Residence time ~ d/U
Residence time / reaction time ~ Y f, Z gas d/U exp(E gas /RT)] ~
(Y f, Z gas d 2 / )Re d -1 Blowoff occurs more readily for small d
(small residence time / chemical time)
Slide 22
22 AME 514 - Spring 2015 - Lecture 4 Scaling - turbulence
Example: IC engine, bore = stroke = d 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 flow 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
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) Why
need turbulence? Increase burning rate - but how much? Turbulent
burning velocity (S T ) turbulence intensity (u') u' 0.5 U p
(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)
Slide 23
23 AME 514 - Spring 2015 - Lecture 4 Scaling - friction
Friction due to fluid flow in piston/cylinder gap Shear stress ( )
= oil (du/dy) = oil U p /h Friction power = x area x velocity = 4
oil U p L 2 /h = 4 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/2
Friction power / thermal power = [8 oil (Re) ]/[ C p Thd)] 0.002
for macroscale engine Implications Need Re Re min to have
turbulence Material properties 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 leakage Simple fluid mechanics: volumetric leak rate = ( P)h
3 /3 Rate of volume sweeping = Ud 2 - must be >> leak rate
Need h