AME 514 Applications of Combustion Lecture 4: Microcombustion science I

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

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.

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

5 AME 514 - Spring 2015 - Lecture 4 The seductive lure of chemical fuels

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)

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

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

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

10 AME 514 - Spring 2015 - Lecture 4 Wankel rotary engine (Berkeley) Free-piston engines (U. Minn, Georgia Tech) Some power MEMS concepts

11 AME 514 - Spring 2015 - Lecture 4 Pulsed combustion driven turbine (UCLA) Some power MEMS concepts Liquid piston magnetohydrodynamic (MHD) engine (Honeywell / U. Minn)

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!)

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

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

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 !

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

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!

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)

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

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!

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)

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)

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

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AME 514 Applications of Combustion Lecture 4: Microcombustion science I