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1 AME 514 - Spring 2013 - Lecture 15 Emerging Technologies in Reacting Flows (Lecture 3)   Applications of combustion (aka “chemically reacting flow”)

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Presentation on theme: "1 AME 514 - Spring 2013 - Lecture 15 Emerging Technologies in Reacting Flows (Lecture 3)   Applications of combustion (aka “chemically reacting flow”)"— Presentation transcript:

1 1 AME 514 - Spring 2013 - Lecture 15 Emerging Technologies in Reacting Flows (Lecture 3)   Applications of combustion (aka “chemically reacting flow”) knowledge to other fields (Lecture 1)   Frontal polymerization   Bacteria growth   Inertial confinement fusion   Astrophysical combustion   New technologies (Lecture 2)   Transient plasma ignition   HCCI engines   Microbial fuel cells   Future needs in combustion research (Lecture 3)

2 2 AME 514 - Spring 2013 - Lecture 15 Combustion synthesis of thin-film photovoltaic cells  Courtesy of Prof. Hai Wang  Current photovoltaic (solar) cells are reasonably efficient but very expensive to produce (≈ $10/watt vs. $1/watt for conventional electric power); net cost of solar ≈ 5x conventional power  Dye-sensitized solar cells not as efficient but cheap to manufacture  First proposed by O’Regan and Grätzel (Nature 343, 737-740, 1991)  Somewhat like fuel cell  Anode: transparent, conductive glass, coated with TiO 2 nanoparticles in turn coated with fluorescent dye to absorb incoming photons  Electrolyte: I - / I 3 - oxidation/reduction reaction – basically a diode so current can only flow one direction  Cathode: Pt-coated transparent, conductive glass

3 3 Dye-Sensitized Solar Cell S electrolyte Transparentconductingglass dye TiO 2 S* hhhh ox (I 3 - ) red (I - ) Redox mediator e-e-e-e- e-e-e-e- e-e-e-e- -0.5 0.0 0.5 1.0 E (V) maximumVoltage ~0.75 V Transparentconductingglass AME 514 - Spring 2013 - Lecture 15

4 4 TiO 2 particle considerations  TiO 2 has advantages over silicon - TiO 2 “work function” such that once an electron jumps to conduction band it stays; cannot fall back down to valence band (if particle truly crystaline)  Ideal particle size < 10 nm  Too large: low surface/volume ratio, don’t get good electron collection  Too small: too many contacts between particles, causes high resistance to electron flow  Current technique for anode fabrication  Commercial TiO 2 powder (> 20 nm)  Making a paste/paint & screen printing  Sinter at 450 ◦ C (glass substrate only)  Stain with dye  USC method  Particle synthesis and film deposition in one step  No need to sinter AME 514 - Spring 2013 - Lecture 15

5 5 Tubular burner Shielding Ar C 2 H 4 /O 2 /Ar Synthesis method – stagnation flame Flame Stabilizer TTIP Carrier gas Ar TTIP/Ar Electric mantle Particle properties controlled by Flame temperature (argon dilution) Flame temperature (argon dilution) Reaction time (flow rate) Reaction time (flow rate) Ti precursor concentration Ti precursor concentration vOvOvOvO vOvOvOvO T max burner-stabilizedflame Stagnation flame AME 514 - Spring 2013 - Lecture 15

6 6 Flame Structure (C 2 H 4 -O 2 -Ar,  = 0.4) Computations used the Sandia counterflow flame code and USC Mech II 10-4 10-3 10-2 10 100 2.72.82.93.03.13.23.3 Mole Fraction O 2 C 2 H 4 H H 2 CO H 2 O CO 2 Distance from nozzle, x (cm) AME 514 - Spring 2013 - Lecture 15

7 7 Aspects of particle growth  Growth time limited to 2 ms because of thermophoresis (TP) – moves particles to from high T to low T in gas  On increasing T side of flame, convection is rapid and TP can’t hold particles in place against convection  As particle approaches stagnation plane, U decreases and TP force pushes particle along faster (about 1 m/s), limiting growth time and thus particle size  Very uniform residence time for on-axis and off-axis particles – characteristic of stagnation flow AME 514 - Spring 2013 - Lecture 15

8 8 Increase Ti Precursor Concentration Particle size distributions Particle size can be well controlled Size distribution widens as median size increases but the size variation still small compared to other methods AME 514 - Spring 2013 - Lecture 15

9 9 Flame Stabilized on Rotating Surface Want boundary layer thickness  ~ (  /  rad ) 1/2 < distance from flame to stagnation surface, so rotation doesn’t affect particle formation & growth ~0.3 cm AME 514 - Spring 2013 - Lecture 15

10 10 Stationary vs. Rotating Stagnation Plate Rotating the stagnation surface results in smaller particles and narrower distributions AME 514 - Spring 2013 - Lecture 15

11 11 Comparison with commercial TiO 2  Tested under the standard AM1.5 solar light  Use Solaronix purple dye, comparisons made under comparable conditions  FSTS films (largely unoptimized) outperform Degussa powder with screening printing  Method allows continuous, reel-to-reel fabrication of DSSC photoanodes in one step AME 514 - Spring 2013 - Lecture 15

12 12 Future needs…  Inertial confinement fusion - What is optimal size of fusion shell to avoid instabilities yet allow ignition?  Hypersonic propulsion  Inlet designs (steady or PDEs) - minimize stagnation pressure losses, stress concentrations on airframe due to shocks  Mixers (steady) - how to get fuel and air mixed without massive stagnation pressure losses?  Detonation initiation schemes - transient plasma ignition or ???  HCCI engines: identify “radical buffer” (analogous to pH buffer) to limit rate of heat release, allow slower combustion once reaction starts  Probably must not generate any solid particles  Probably must contain C, H, O, N atoms only

13 13 AME 514 - Spring 2013 - Lecture 15 Future needs…  Transient plasma discharges  Possible way of exploiting faster burn in IC engines - reverse engineer engine for lower turbulence (lowers both burning rate and heat losses), restore high burning rate using transient plasma discharges  Need detailed chemical models like “GRI Mech” for ionized species  Microbial fuel cells - kinetic and transport laws for bacterial metabolism and electron production - what are the equivalents of  Navier-Stokes equations  Fick’s law of diffusion  Arrhenius law of reaction rates

14 14 Microbial fuel cell modeling - objective  Improve the mechanical, electrical, hydraulic, and diffusive aspects of the fuel cell until the bacterial activity is the only rate-limiting step  Computational, with experimental calibration & verification (both)  Toward this goal, study effects of  Anode and cathode geometry - shape, thickness, porosity, electrical contacts  Biofilm community »Species & strain »Growth method - anaerobic, aerobic or with a cell voltage bias  Planktonic bacteria  Mixing rates in anode and cathode chambers  Anolyte - nutrient type and concentration, pH  O 2 crossover - membrane thickness, N 2 purging AME 514 - Spring 2013 - Lecture 15

15 15 Computational model  FLUENT computational fluid dynamics software package - flow, convective & diffusive transport, chemical reaction  One-dimensional, steady state (easily extended to 2D or 3D, transient)  3-step chemical model  (Slow) oxidation reaction occur only at the anode Nutrient (R1) + bacteria  Product (P1) + 2H + (I) + 2e - + bacteria  (Faster) reduction reaction occur only at the cathode Oxygen (R2) + 4H + (I) + 4e - + Pt  2H 2 O (P2) + Pt  O 2 crossover - competition with anode - no power production 2 Nutrient (R1) + Oxygen (R2) + bacteria  2 H 2 O (P2) + bacteria  Membrane approximated as permeable only to selected species (intermediate (I) = H +, Reactant 2 (R2) = O 2 )  Anode and cathode reaction rate constants adjusted to get agreement between model and experiment at ONE condition, same rate constants applied for varying parameters AME 514 - Spring 2013 - Lecture 15

16 16 Model parameters  Dimensions  Anode and cathode chamber sizes  Anode and cathode thickness and surface area per unit volume  Membrane thickness  Concentrations  Nutrient, dissolved O 2  Reaction rate constants (connection with “microscale” modeling effort, A. Lüttge)  (1): per unit molarity per unit surface area  (2): per unit (molarity) 2 per unit surface area  (3): per unit (molarity) 2  Diffusivities of all species  Boundary conditions (next slide) AME 514 - Spring 2013 - Lecture 15

17 17 5mm(20cells) 100μm (10 cells) 5mm(20cells)5mm(20cells)5mm(20cells) 12345 Anode Wall R1 = 8.6x10 -6 M/m 3 I = No Flux Others = 0 Cathode Wall R2 = 8.3x10 -6 M/m 3 I = No Flux Others = 0 Anode Chamber Turbulent diffusion 1 Cathode Chamber Turbulent diffusion 5 Anode electrode with bacteria Reaction: R1  2 I + P2 Nutrient H+ Waste Nutrient H+ Waste Molecular diffusion 2 Cathode electrode Reaction: R2 + 4 I  2 P2 Oxygen H+ H2O Oxygen H+ H2O Molecular diffusion 4 Membrane Reduced diffusion 3 1D FLUENT computational model AME 514 - Spring 2013 - Lecture 15

18 18 Nutrient Boundary conditions: 8.6e-3 at the Anode wall 0 in the Cathode wall Consumption No diffusion through the membrane Nutrient concentration profiles Concentration (M)Concentration (M) AME 514 - Spring 2013 - Lecture 15

19 19 Anode reactionAnode reaction R1  P1 + 2 IR1  P1 + 2 I Nutrient Part. Nutrient H +Nutrient Part. Nutrient H + Product & intermediate concentration profiles Concentration of P1 (M)Concentration of P1 (M) Concentration of I (M/m 3 )Concentration of I (M/m 3 ) AME 514 - Spring 2013 - Lecture 15

20 20 Computed effect of nutrient diffusivity  N 2 flow alone in anode chamber alone does not provide reaction-limited power, but magnetic stirring does (consistent with experiments) AME 514 - Spring 2013 - Lecture 15

21 21 Experiments - apparatus  10 complete cells & data acquisition  Glass anode & cathode chambers  Carbon felt electrodes; Pt coating on cathode  Ports for N 2 (anode) & air (cathode)  LabView data acquisition - automatic generation of polarization curves AME 514 - Spring 2013 - Lecture 15

22 22 Results - transport effects  N 2 flow noticeably increases power due to stirring, but magnetic stirring much more effective AME 514 - Spring 2013 - Lecture 15

23 23 Scaling – “small,” “average,” “large” MFCs  “Large” scales  Turbulent flow - more rapid mass transfer  Buoyancy effects - more rapid mass transfer  “Average” scales  Laminar flow - slower mass transfer  “Small” scales  Diffusion rate high (~ D/d 2 )  Characteristic reaction rates independent of scale  Multiple transitions between mixing-limited and reaction rate- limited operation as scale changes AME 514 - Spring 2013 - Lecture 15

24 24 Future needs…  “In situ” enhanced oil recovery via subsurface combustion  See e.g. Mahinpey, N., Ambalae, A., Asghari,K., “In situ combustion in enhanced oil recovery (EOR): A review” Chem. Eng. Commun. Vol. 194 pp. 995-1021 (2007)  Heavy-oil reservoirs containing high-viscosity oil are impossible to produce via conventional pumping  Viscosity decrease via steam injection expensive & of limited effectiveness  Can inject air and combust a portion of oil  Has seen limited field use but can be effective  Limited laboratory experiments, in small diameter tubes  Real situation: large cross-section – instabilities  Similar to “filtration combustion” of porous solid  Very similar to flames in Hele-Shaw cell (see lecture 8) »Flow described by Darcy’s Law »Buoyancy (RT), thermal expansion (DL), viscosity change (ST) instabilities »…. But a non-premixed, 3-phase (air, oil, inert porous solid) system

25 25 AME 514 - Spring 2013 - Lecture 15 Future needs…  “In situ” enhanced oil recovery via subsurface combustion http://www.netl.doe.gov/technologies/oil-gas/publications/eordrawings/BW/bwinsitu_comb.PDF

26 26 AME 514 - Spring 2013 - Lecture 15 Future needs… M. V. Kök, G. Guner, S. Bagci, Oil Shale, Vol. 25, No. 2, pp. 217–225 (2008)


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