Emerging technologies in reacting flows lecture 3
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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)

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Emerging Technologies in Reacting Flows (Lecture 3 )

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Emerging technologies in reacting flows lecture 3

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)

AME 514 - Spring 2013 - Lecture 15


Combustion synthesis of thin film photovoltaic cells

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 TiO2 nanoparticles in turn coated with fluorescent dye to absorb incoming photons

    • Electrolyte: I- / I3- oxidation/reduction reaction – basically a diode so current can only flow one direction

    • Cathode: Pt-coated transparent, conductive glass

AME 514 - Spring 2013 - Lecture 15


Dye sensitized solar cell

E (V)

S*

-0.5

maximum

Voltage

~0.75 V

0.0

hu

red (I-)

ox (I3-)

0.5

Redox mediator

1.0

e-

e-

Dye-Sensitized Solar Cell

Transparent

conducting

glass

Transparent

conducting

glass

TiO2

dye

electrolyte

e-

S

AME 514 - Spring 2013 - Lecture 15


Tio 2 particle considerations

TiO2 particle considerations

  • TiO2 has advantages over silicon - TiO2 “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 TiO2 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


Emerging technologies in reacting flows lecture 3

Synthesis method – stagnation flame

Tmax

Flame Stabilizer

Stagnation flame

vO

burner-stabilized

flame

Tubularburner

Carrier gas Ar

vO

C2H4/O2/Ar

Shielding Ar

TTIP/Ar

Particle properties controlled by

Flame temperature (argon dilution)

Reaction time (flow rate)

Ti precursor concentration

TTIP

Electric mantle

AME 514 - Spring 2013 - Lecture 15


Emerging technologies in reacting flows lecture 3

Flame Structure (C2H4-O2-Ar, f = 0.4)

Particle nucleation/

2500

Stagnation surface (x = 3.4 cm)

growth region

2000

(K)

1500

T

1000

500

500

Particle nucleation/

growth region

400

300

(cm/s)

Axial Velocity

200

v

100

Laminar flame speed

0

10

0

CO

H

O

2

2

O

2

10

-1

C

H

2

4

CO

Mole Fraction

10

-2

10

-3

H

2

H

Computations used the Sandia counterflow flame code and USC Mech II

10

-4

2.7

2.8

2.9

3.0

3.1

3.2

3.3

Distance from nozzle, x (cm)

AME 514 - Spring 2013 - Lecture 15


Aspects of particle growth

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


Emerging technologies in reacting flows lecture 3

Particle size distributions

Increase Ti Precursor Concentration

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


Emerging technologies in reacting flows lecture 3

Flame Stabilized on Rotating Surface

Want boundary layer thickness d~ (u/wrad)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


Emerging technologies in reacting flows lecture 3

Stationary vs. Rotating Stagnation Plate

Rotating the stagnation surface results in smaller particles and narrower distributions

AME 514 - Spring 2013 - Lecture 15


Comparison with commercial tio 2

Comparison with commercial TiO2

  • 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


Future needs

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

AME 514 - Spring 2013 - Lecture 15


Future needs1

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

AME 514 - Spring 2013 - Lecture 15


Microbial fuel cell modeling objective

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

    • O2 crossover - membrane thickness, N2 purging

AME 514 - Spring 2013 - Lecture 15


Computational model

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 2H2O (P2) + Pt

    • O2 crossover - competition with anode - no power production

      2 Nutrient (R1) + Oxygen (R2) + bacteria 2 H2O (P2) + bacteria

  • Membrane approximated as permeable only to selected species (intermediate (I) = H+, Reactant 2 (R2) = O2)

  • 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


Model parameters

Model parameters

  • Dimensions

    • Anode and cathode chamber sizes

    • Anode and cathode thickness and surface area per unit volume

    • Membrane thickness

  • Concentrations

    • Nutrient, dissolved O2

  • 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


1d fluent computational model

2

4

Cathode electrode

Reaction: R2 + 4 I  2 P2

Oxygen H+ H2O

Molecular diffusion

Anode electrode with bacteria

Reaction: R1  2 I + P2

Nutrient H+ Waste

Molecular diffusion

1

3

5

Anode Chamber

Turbulent diffusion

Membrane

Reduced diffusion

Cathode Chamber

Turbulent diffusion

Cathode Wall

R2 = 8.3x10-6 M/m3

I = No Flux

Others = 0

Anode Wall

R1 = 8.6x10-6 M/m3

I = No Flux

Others = 0

5mm

(20cells)

5mm

(20cells)

5mm

(20cells)

5mm

(20cells)

100μm

(10 cells)

1D FLUENT computational model

1

2

3

4

5

AME 514 - Spring 2013 - Lecture 15


Emerging technologies in reacting flows lecture 3

Nutrient concentration profiles

Nutrient Boundary conditions:

8.6e-3 at the Anode wall

0 in the Cathode wall

Consumption

No diffusion through the membrane

Concentration (M)

AME 514 - Spring 2013 - Lecture 15


Product intermediate concentration profiles

Product &intermediate concentration profiles

Anode reaction

R1  P1 + 2 I

Nutrient Part. Nutrient H+

Concentration of P1 (M)

Concentration of I (M/m3)

AME 514 - Spring 2013 - Lecture 15


Computed effect of nutrient diffusivity

Computed effect of nutrient diffusivity

  • N2 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


Experiments apparatus

Experiments - apparatus

  • 10 complete cells & data acquisition

  • Glass anode & cathode chambers

  • Carbon felt electrodes; Pt coating on cathode

  • Ports for N2 (anode) & air (cathode)

  • LabView data acquisition - automatic generation of polarization curves

AME 514 - Spring 2013 - Lecture 15


Results transport effects

Results - transport effects

  • N2 flow noticeably increases power due to stirring, but magnetic stirring much more effective

AME 514 - Spring 2013 - Lecture 15


Scaling small average large mfcs

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/d2)

    • 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


Future needs2

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

AME 514 - Spring 2013 - Lecture 15


Future needs3

Future needs…

  • “In situ” enhanced oil recovery via subsurface combustion

http://www.netl.doe.gov/technologies/oil-gas/publications/eordrawings/BW/bwinsitu_comb.PDF

AME 514 - Spring 2013 - Lecture 15


Future needs4

Future needs…

M. V. Kök, G. Guner, S. Bagci, Oil Shale, Vol. 25, No. 2, pp. 217–225 (2008)

AME 514 - Spring 2013 - Lecture 15


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