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Combustion of Energetic gMaterials
Richard A. YetterPenn State Universityy
Princeton‐Combustion Institute 2018 Summer School on Combustion
June 25‐29, 2018Princeton NJPrinceton, NJ
1
M l C b iMetal Combustion
2
Heats of Oxidation of Several Materials
120
140Gravimetric (kJ/gm
fuel)
80
100
120 fuel
Volumetric (kJ/cm3fuel
)
idat
ion
40
60
Hea
t of O
x
0
20
Al)
B) e) C) e) Li) g) S
i) Ti)
W)
Zr)
lum
inum
(A
Bor
on (B
Ber
yliu
m (B
e
Car
bon
(C
Iron
(F
Lith
ium
( L
gnes
ium
(Mg
Sili
con
(S
Tita
nium
(T
Tung
sten
(W
ircon
ium
(Z
A B
Mag T Zi
3
Thermodynamic Considerations• The determination of which metals will burn in the vapor phase or heterogeneously can be made from knowledge of the thermodynamic and physical properties of the metal andthe thermodynamic and physical properties of the metal and its oxide product.
• For oxygen containing environments, early studies ( )(Glassman, Von Grosse and Conway) recognized– The importance of the volatility of the metal vs the metal oxide– The relationship between the energy required to gasify the metal– The relationship between the energy required to gasify the metal or metal oxide and the overall energy available from the oxidation reaction – a limiting flame temperature
Δ Q ( ) ΔΔHvap‐dissoc > QR – (HoT,vol – Ho
298) = ΔHavail
QR = heat of reaction(Ho Ho ) th l i d t i th d t id t it(Ho
T,vol – Ho298) = enthalpy required to raise the product oxide to its
volatilization temperatureΔHvap‐dissoc = heat of vaporization‐dissociation 4
Thermodynamic Considerations –OxidesOxides
Metal Tbp(K)
Oxide Tvol(K)
ΔHf,298(kJ/mol)
ΔHvol(kJ/mol)
HTvol-H298 + ΔHvol
(kJ/ l)(kJ/mol)
Al 2791 Al2O3 4000 -1676 1860 2550B 4139 B2O3 2130 -1272 360 6402 3
Be 2741 BeO 4200 -608 740 1060Cr 2952 Cr2O3 3280 -1135 1160 1700Fe 3133 FeO 3400 -272 610 830Hf 4876 HfO2 5050 -1088 1014 1420Li 1620 Li2O 2710 -599 400 680
Mg 1366 MgO 3430 -601 670 920Mg 1366 MgO 3430 601 670 920Si 3173 SiO2 2860 -904 606 838Ti 3631 Ti3O5 4000 -2459 1890 2970Zr 4703 ZrO2 4280 -1097 920 1320Zr 4703 ZrO2 4280 1097 920 1320
Glassman, Combustion, Academic Press, 3rd Edition, 1996 5
Classification of Metal Combustion
I Volatile Product Nonvolatile ProductII V l til t l N l til t l V l til M t l N l til M t lII Volatile metal
Gas‐phase combustion
Nonvolatile metalSurface combustion
Volatile MetalGas‐phase combustion
Nonvolatile MetalSurface or condensed phase combustioncombustion
III Soluble Nonsoluble Soluble Nonsoluble Soluble Nonsoluble Soluble Nonsoluble
Product No flux of Product may No product If product Disruption Metal may ProductProduct may dilute metal during burning d
No flux of product to metal
Product may build up in metal during burning
No product penetration into metal
If product returns to metal it may dilute it and
Disruption strongly favored if product returns to
l
Metal may diffuse through growing product l l
Product coating makes ignition difficult
and cause disruption if its BP exceeds that of
cause disruption
metal layer, purely condensed phase combustion possible
metalp
F.A. Williams, "Some Aspects of Metal Particle Combustion," Physical and Chemical Aspects of Combustion: A Tribute to Irv Glassman, F.L. Dryer and R.F. Sawyer, eds., Gordon and Breach, The Netherlands, 1997, pp. 267‐289. 6
Thermodynamic Considerations – Effect of Pressure and OxidizerPressure and Oxidizer
6000
2Al+1.5O2
(K) 5000
2Al 1.5O2
2Al+1.5(O2+3.76N2)
pera
ture
4000
2Al+3CO2,Reactants at 298K
Tem
p
30002Al+3H2O(g),
0 1 1 10 1002000
Al vaporization2Al+3H2O(l),Reactants at 298K
2 (g)Reactants at 398K
Pressure (atm)0.1 1 10 100
Glassman and Yetter, Combustion, Academic Press, 4th Edition, 2008 7
Micron Aluminum Particle CombustionOxidizer Effects Pressure Effects
Air
60 t2 t 60 atm2 atm1 atm
R.A. Yetter and F.L. Dryer, in Fire in Free Fall: Microgravity Combustion, H. Ross, Ed., Academic Press, Chapter 6, 2001. 8
Equilibrium Product Composition and Temperature vs Input EnthalpyTemperature vs. Input Enthalpy
1 4500
Stoichiometric Mixture of Al and O2 at 298 K and 1 atm
0.8
1
4000
4500
T
TAl2O
3(l)
0.63500
4000
Frac
tion
Tempera
O
0 2
0.4
3000
Mol
e F ture (K)
Al
AlO
O
0
0.2
2500-15 -10 -5 0 5 10 15
O2
Al2O
-15 -10 -5 0 5 10 15
Assigned Enthalpy (kJ/g)Glassman, Combustion, Academic Press, 3rd Edition, 1996 9
Flame Structure of Micron Aluminum Particle Burning in AirParticle Burning in Air
ctio
ns
1 0 ]Natural Luminosity
Mol
e Fr
ac
0.60.81.0
erat
ure
[K]
3500
4000
T AlO
Simulated
AlO PLIF
Al2O3 EPMA
rmal
ized
M
0 00.20.4
Tem
pe
2000
2500
3000
Al2O3
T PLIF
AlO PLIF
r / rp
1 2 3 4 5 6 7 8 9 10Nor 0.0 2000AlO PLIF
Burn interrupted particleon silicon waferr / rp on silicon wafer
Bucher, P., et al. Proc. Combust. Inst., 27, 2421, 1998.
EPMA
10
Predicted burning rates as a function of pressure for aluminum combustion in 21% O2 and 79% Ar2
2 20d d tβ= −
4000
5000
)
1.2T Flame (Limit)
T Al B il
T (K
)
2000
3000
mm
2 /s)
0.8T Al Boil
T Particle Surface
T
1000
2000
β (
0.4
symbols - exp.lines - modelβ
P (atm)1 10 100
0 0.0symbols exp.
P (atm)Also shown are the particle surface and maximum flame temperatures and the vaporization/decomposition temperatures of Al and Al2O3 11
Modes of Particle Combustion
11
Diffusion‐Controlled with Envelope Flame
Diffusion‐Controlled without Envelope Flame
Kinetically‐Controlled
T/T’
T/T’1
T/T’fuel
products
1 T/T1
oxidizerproducts
∞s0
oxidizer
products
∞s0
oxidizerfuel
products
∞s0ss
12
Diffusion vs Kinetic Control
dρ 20 dρ 2
Diffusion controlled combustion with vaporization or surface reaction
B)1(ρD8dρ
t 0pb,diff +
=ln )iY1(ρD8
dρt
O,
0pb,diff
∞+=
ln
Kinetic controlled surface reaction
= 0pb kin kPXMW2
dρt
Kinetic controlled surface reaction
∞O,pb,kin kPXMW2
Transition may be defined through a Damköhler number
)iY1(ρD4XkPdMW
tt
Da O,0pb,diff ∞
+==
l )iY1(ρD4t O,b,kin ∞+ln
13
Burning Times of Boron Particlese
and
X O2
(ms)
1000
10000ur
ning
Tim
xyge
n, t b
X
10
100
d2 - law
ct o
f the
Bu
ctio
n of
Ox
0.1
1 P = 1 atmd - law
Pro
duc
Mol
e Fr
ac
0.001
0.01 P = 35 atmd1 - law
Particle Diameter, d (μm)
0.1 1 10 100 1000
Li, S.C. and Williams, F.A. Proc. Combust. Inst. 23, 1147‐1154, 1991.14
Effect of Particle Diameter on Al Burning TimeBurning Time
100000Wilson and Williams [7]Wong and Turns [2]Prentice [4]
Davis [8]Parr et al. [9] (T=1500 K)Parr et al. [9] (T=2000 K)
1000
10000Prentice [4]Olsen and Beckstead [1]Hartman [3]Friedman and Macek [5,6]
Parr et al. [9] (T 2000 K)Park et al. [10] (T=1173 K)Eisenreich et al. [11] (T=900 K)
me
[ms]
100
urni
ng T
im
1
10Bu
d1.8T = 1500K
0.10.01 0.1 1 10 100 1000
Diameter [μm]
T = 2000K
T. Bazyn, H. Krier, and N. Glumac, Proc Combust. Inst. 31 (2007) 2021 & Combust. Flame 145 (2006) 703.Y.L. Shoshin and E.L. Dreizin, Combust. Flame 145 (2006) 714. 15
Effect of Particle Diameter on Al Ignition TemperatureTemperature
3500Parr et al. [20]
3500Derevyaga et al. [34]
e,K
3000
[ ]Bulian et al. [38]Assovskiy et al. [40]Yusasa e tal. [37,39]Brossard et al. [36]Ermakove tal. [35]
e,K
3000
y g [ ]Merzhanov et al. [33]Friedman et al. [31,32]Trunov et al. [26]CurveFit
Tem
pera
ture
2000
2500
Tem
pera
ture
2000
2500
Igni
tion
T
1000
1500
Igni
tion
T
1000
1500
i l i10-2 10-1 100 101 102 103 104500
1000
i l i10-2 10-1 100 101 102 103 104500
1000
Particle Diameter, μmParticle Diameter, μm
16
N i l C b iNanoparticle Combustion
17
Surface to Bulk Atom Ratio for Spherical Iron CrystalsCrystals
100
60
80 bulk atoms
40
60
0
20 surface atoms
00 5 10 15 20 25 30
particle size (nm)
K.J. Klabunde, J. Stark, O. Koper, C. Mohs, D.G. Park, S. Decker, Y. Jiang, I. Lagadic and D.J. Zhang, J. Phys. Chem. 100, (1996) 12142‐12153.
18
Melting Point and Normalized Heat of Fusion for Tin NanoparticlesFusion for Tin Nanoparticles
480
500
4050
60
440
460
2030
40
4200 20 40 60 80 100
particle diameter (nm)
100 20 40 60 80 100
particle diameter (nm)
S.L. Lai, J.Y. Guo, V. Petrova, G. Ramanath, and L.H. Allen, Phys. Rev. Lett. 77 (1996) 99‐102.
For aluminum particles:Al i S d Th D L JPC A 110 1518 2006Alavi, S. and Thompson, D.L., JPC A 110, 1518, 2006.Puri, P. and Yang, V., JPC C 110, 11776, 2007. Also AIAA 2008‐938 and AIAA 2007‐1429.
19
Other Features of Nanoparticles
• Increased specific surface area• Increased specific surface area• Increased reactivity• Increased catalytic activity• Increased catalytic activity• Lower melting temperatures• Lower sintering temperatures• Lower sintering temperatures• Superparamagnetic behavior• Superplasticity• Superplasticity
20
Energy Content as a Function of Particle DiameterDiameter
1
0.8
0.6 2 nm
3
0 2
0.4 3 nm
0
0.2
9 8 7 610-9 10-8 10-7 10-6
particle diameter (m)21
A Brief History of Nanoenergetic MaterialsnAl RESS nRDX
• Synthesis of Energetic Nanoparticles– Nanoscale particles readily available– Simple substitution for micron sized
100nm40 nmSimple substitution for micron sized materials
• Assembly of Energetic MaterialsV i bl d dditi
Fe0‐AOTL‐ALEX
1
A. Cortopassi, T. Wawiernia, J. T. Essel, P. Ferrara, K. K. Kuo, and R. M. Doherty, 8‐ISICP, 2009
Nanotechnology
20 nm
– Various assembly and additive manufacturing processes: self assembly, sol‐gel chemistry, ink‐jetting, vapor deposition processes electro and cold C.E. Bunker and J.J. Karnes,
1 μm
10 nm
T. Sippel and S.F. deposition processes, electro and cold spray, arrested reactive milling, supercritical processing
– Multilayer foils, core shell structures,Al‐C13F27COOH CL‐20/NC Cryogel
,JACS 126, 10852, 2004Son, TEM of palmitic
acid coated ALEX
Multilayer foils, core shell structures, reactive nanowires, directly assembled particles, dense nanocomposite powders produced by arrested reactive milling,
200nmT. B. Brill, B. C. Tappan and J. Li (2003). MRS Proceedings,800,
100nm
R.J. Jouet, A.D. Warren, D.M. Rosenberg, V.J.
nanoporous particles and substrates, clusters
( ) g , ,AA2.1doi:10.1557/PROC800AA2.1
g,Bellitto, K. Park, and M.R. Zachariah, Chem. Mater. 17 (2005) 2987‐2996.
22
Nanometer ParticlesAnother length scale of importance to the combustion of small particles is the mean g p pfree path in the surrounding gas‐phase. A comparison of this length scale to the particle diameter defines whether continuum conditions exist (i.e., the particle may be distinguished separately from the gas molecules). The key dimensionless group that defines the nature of the surrounding gas to the particle is the Knudsen numberdefines the nature of the surrounding gas to the particle is the Knudsen number,
pdλ2Kn =
where λ is the mean free path of the gas phase
Nπσ21λ 2=
where λ is the mean free path of the gas‐phase
21MW8μ2λ /⎞⎛
=
where σ is the molecular diameter of the molecule and N is the gas concentration (number density)
RTMW8P
/
⎟⎠⎞
⎜⎝⎛π
where μ is the gas viscosity and MW is the molecular weight of the gas. At atmospheric pressure particles of 100 nm and smaller are characterized by Kn numbers greater thanpressure, particles of 100 nm and smaller are characterized by Kn numbers greater than unity for the entire temperature range from room to combustion flame temperatures, indicating that they can no longer be considered as macroscopic particles in a continuum gas.
23
Nanometer ParticlesFor Kn numbers of the order of 1, expressions used to model heat and mass transfer in the continuum limit are no longer applicable.• appearance of a discontinuity in temperature, velocity, and concentration profiles at
the gas‐surface interface often referred to as slip or jumpthe gas surface interface, often referred to as slip or jump. • magnitude of slip depends on the efficiency of exchange of energy and momentum
between the gas and surface. • efficiency is quantified in accommodation coefficients α of a gas‐surface interface. For
example, the thermal accommodation coefficient αT is defined as:
f gs g
s g
( , )T
T TT T
T T−
=−
α
where Ts is the surface temperature, Tg is the temperature of the gas incident on the surface, and Tf is the temperature of the gas after scattering from the surface. The value of αT depends on the gas and surface temperatures in addition to the nature of the gas d f Th ti h t t f ffi i t h th b l t d t i thand surface. The convective heat transfer coefficient h can then be related to αT in the
transition regime by1
8 1T
g
PchT
α γγ+
=−
where c is the mean speed of the gas.
g γ
24
Nanometer ParticlesNanometer Particles• Experimental measurements of thermal accommodation coefficients
suggest that α values at elevated temperatures for some systems are ofsuggest that αT values at elevated temperatures for some systems are of the order of 0.1, and some may be much lower.
• As the Knudsen number increases, heat and mass transfer are reduced significantly by non‐continuum effects. g y y
• In the kinetic limit, transport of mass (and heat for Le ~ 1) is rapid, and concentration and temperature profiles are relatively flat.
• However, for isolated burning of nanoparticles, particle temperatures can rise hundreds of degrees above ambient during combustion at elevated pressures, while for lower pressures the temperature overshoot (as well as the combustion rate) is reduced. Th b i h l h h K 1 f i l• These observations suggest that although Kn > 1 for some nanoparticle systems, the combustion has not reached the kinetic limit due to inhibited transport in the non‐continuum regime. Such particles can be considered to be burning in a “transition” regime between the diffusion and kineticto be burning in a transition regime between the diffusion and kinetic limit, in which neither the d2 nor d burning rules apply.
25
Shrinking Core Model for Spherical Particles of Unchanging Size
Product Unreactedcore
Low HighUnreacted
core
Unchanging SizeDiffusion through Product Layer Controls
tion of
reactant Typical position
in diffusion region
Lowconversion conversion
Time Time
Product
Reactionzone
R Rrc rc0
Concen
trat
gas‐ph
ase r
CAg
CA
0 r
region
ntratio
n of
reactant
Product Unreactedcore
Radial position
R R R R R R00 0
Concen
solid
Chemical Reaction Controls
entration of
ase reactant
CAg
Radial position
R R R R R R00 0
Radial positionR Rrc rc0
Conce
gas‐ph
a
See for example, Levenspiel, O., Chemical ReactionEngineering, Third Edition, J. Wiley and Sons, 1999
26
Shrinking Core Model for Spherical Particles of Unchanging Size
2pRρ
τ =pRρ
τ =
Diffusion through Product Layer Controls Chemical Reaction Controls
Unchanging Size
Ag2 3
2 / 3
6bDCt r r1 3 2
R Rt
τ
⎛ ⎞ ⎛ ⎞= − +⎜ ⎟ ⎜ ⎟τ ⎝ ⎠ ⎝ ⎠
Ag
1/ 3
bkCt r1
Rt 1 (1 X)
= −τ=
1 1 0
2 / 3t 1 3(1 X) 2(1 X)= − − + −τ
1 (1 X)= − −τ
Radius vs. Time Conversion vs. TimeRate of Change of
Conversion vs. Conversion
0.6
0.8
R
0.6
0.8
X
-0.4
-0.2
/d(t/τ)
Diff i
Diffusion‐controlled
Kinetic‐controlled
Kinetic‐controlled
0.2
0.4
r/R
0.2
0.4
1-X
-0.8
-0.6d(1-
X)/Diffusion‐
controlled
Diffusion‐controlled
Kinetic‐controlled
00 0.2 0.4 0.6 0.8 1
t/τ
00 0.2 0.4 0.6 0.8 1
t/τ
-10 0.2 0.4 0.6 0.8 1
X
See for example, Levenspiel, O.,Chemical Reaction Engineering, Third Edition, J. Wiley and Sons, 1999 27
Schematic of Melt‐Dispersion Mechanism of Combustion of Nano Aluminum Particles at HighCombustion of Nano Aluminum Particles at High
Heating Rate (>106 K/s)
Al i dAluminum core covered by initial alumina shell
Fast melting of Al leads to Unloading wave propagatesFast melting of Al leads to spallation of the alumina shell
Unloading wave propagates to the center of Al molten core and generates tensile pressure which disperses
Levitas, Combust. Flame, 156, 2, 543, 2009
p psmall Al custers
28
Molecular dynamic simulation snapshots of the central slice of a 26 nm Al particle covered with an oxide shell 3 nm thick
•Oxidation starts after core & shell heated to 1100 K
•As oxidation begins, due to expansion of molten core, the size grows
•Oxidation dynamics starts with the Al-NP at the corewith the Al NP at the core shell interface when molten Al reacts with alumina shell taking Ο to f Al h Al O lform Al rich AlxOy clusters, which generates heat
• Shell does not break or shatter, only deforms, y
•When T ~ 2900 K, the alumina shell melts and Al ejections occur from Al-NPi t i t
Blue = envirnomental oxygenRed = core Al atomsBl k h ll Al tinto oxygen environment
producing oxygen rich AlxOy clusters
Black = shell Al atomsGreen = shell O atoms
Li et al., J. Applied Phys. 114, 13, 134312, 2013 29
Thermal Gravimetric Analysis of Nanoaluminum with CONanoaluminum with CO2
1 6
1.8 7
38nm
1.2
1.4
1.6
5
6
Mass (TG)
10μm
3μm
108nm
45nm (2.5nm)45nm (3.6nm)
1.0
3
4
45nm (2.5nm oxide)38nm
10μm
1
2
nergy (DSC)
3μm108nm
45nm (3.6nm oxide)
( )
250 450 650 850 1050 12500
1
En10μm
Temperature (oC)
K. Brandstadt, D. L. Frost, and J. A. Kozinski, in Advancements in Energetic Materials and Chemical Propulsion, ed. K.K. Kuo and J. de Dios Rivera, Begall House, 2007.Il’in AP, Gromov AA, and Yablunovskii GV (2001) Combustion, explosion, and shock waves, 37 n.4: 418‐422. 30
Polymorphic Phase Transformations in Al2O3 LayerAl2O3 Layer
M. A. Trunov, M. Schoenitzand E. L. Dreizin, Combustion Theory and Modelling, 10, 2006, 603–623
31
Aluminum Nanoparticle Oxidation with Single Particle Mass Spectrometry
2
Particle Mass SpectrometryIncreased rate constant with
decreasing particle size Shrinking core model
0.5
0.6
0.7
rsio
n, s
T = 700oC0
2
sec)
< 50 nmE ~ 32kJ/ l
0 2
0.3
0.4
for h
alf c
onve
r
t ~ r1.58
-4
-2
(Kco
nv),
Ln(1
/s
50 - 100 nm
Ea ~ 32kJ/mol
0
0.1
0.2
0 5 10 15 20 25 30 35
Tim
e f
time(s) = 0.0025r(nm)1.582
-8
-6
Ln(
Ln(Kconv
) = 17.8 - 21000(1/T)
Ln(Kconv
) = 5.2 - 6847(1/T)
Ln(Kconv
) = 3.2 - 3824(1/T)100 -150 nm
Ea ~ 95kJ/mol
radius, nm-80.0008 0.0009 0.001 0.0011 0.0012
1/T (1/K)
Effective diffusion coefficient (De) of oxygen in ash layer = 10‐9 ~ 10‐8 cm2/s for < 50 nm particlesAt temperatures above ~ 1000oC hollow particles formedAt temperatures above ~ 1000oC, hollow particles formed.
K. Park, D. Lee,† A. Rai, D. Mukherjee, and M. R. Zachariah,J. Phys. Chem. B 2005, 109, 7290‐7299A. RAI, K. PARK, L. ZHOU and M. R. ZACHARIAH, Combustion Theory and Modelling, Vol. 10, No. 5, October 2006, 843–859 32
Combustion of Aluminum Particles inCombustion of Aluminum Particles in Energetic Materials
33
Combustion of Aluminum Particles in Energetic MaterialsEnergetic Materials
• Aluminum is used as an ingredient in solid propellants because of its high energy density, high heat release during oxidation, low cost, and relative safety
• Because most unaluminized propellants are already fuel rich, it might seem counterproductive to add more fuel (Al)
• However oxidation reduces the oxidizer vapors (H2O and CO2) to H2 and CO, better propulsive fluids
• This in combination with a large increase in temperature,This in combination with a large increase in temperature, results in an increase in Isp of around 10% and a net gain in propellant mass fraction in the motor of typically 15%
• Aluminum in even the modest amounts suppressesAluminum in even the modest amounts suppresses combustion instability problems common in motor development programs
• There are also problems with use of aluminum such asThere are also problems with use of aluminum, such as smoky, luminous exhaust trails, two‐phase flow, and slag accumulation
34
Combustion of Aluminized Solid Propellants• Aluminum does not turn into vapor at the propellant
surface like other primary ingredients (BPAl = 2467oC, usually propellant Ts ~ 600oC, which is ~ 60 deg below the y p p s , g660oC MP of Al; ρAl = 2.7 g/cm3 which is ~ 1.38 times that of AP and 3 times that of HC binders)
• Its reaction or ignition on the surface is inhibited by theIts reaction or ignition on the surface is inhibited by the refractory oxide coatings on the particle surfaces (MPAl2O3=2072oC; Al2O3 constitutes about 0.5% of the particle mass for 25 μm particles)μ
• The particles tend to adhere to the surface in binder melts, leading to locally higher concentrations
• At temperatures between the Al and Al2O3 melting points,At temperatures between the Al and Al2O3 melting points, leaking of Al from the oxide coating can lead to sintering in assemblages of particles into sintered accumulates. The leakage can be so great that the Al engulfs the entire g g gassemblage to form spherical agglomerates.
35
Combustion of Aluminized Solid Propellants• When aluminum particles are heated the oxide structure
changes, and the aluminum expands. The coefficient of expansion of the oxide skin is less so that it must be stressed. At the melting point of Al, it expands in volume about 6%, thereby stressing the oxide skin further and potentially causing cracks.
• The concentrations of typically 20 μm particles can lead to coalescence into large droplets (e.g., typically 200 μm containing 103 original particles).
• Large droplets leave the surface and burn in the gas flow field (typically with H2O and CO2), whereas any original particles that leave the surface individually burn up within 1 t 2 f th f ( ith di t1 to 2 mm of the surface (with a corresponding greater effect on the propellant burning)
• In small motors, large droplets may not burn up completely, d i Ireducing Isp
36
Aluminum Particle Combustion
Image from a small sample of the space shuttle propellant
Magnified video image from a small sample of a nitraminepropellant
oxide cap
streamer
Al agglomerate
oxide cap
Al2O3
approximate
flow direction
200μm1 mmapproximate
surface location
37
Accumulation and AgglomerationOxide skins on particles dropped on 1400oC
Sintered accumulate, quenched in transition to an agglomerate. Note spherical structures where
p ppplate showing crack pattern and bridging between particles.
spherical structures where coalescence of liquid Al has started.
Agglomerate quenched before complete transition to a molten droplet. Al surface t i h d d th l t i ttension had drawn the accumulate into a sphere, but not yet closed the surface. Un‐melted oxide shells still visible in openings.
38
Particle Packing• The stoichiometry of an AP/binder system dictates that a maximum achievable content of AP be used with required propellant processing and mechanicalrequired propellant processing and mechanical properties
• Typically, a coarse (e.g. 200μm) AP portion is combined with finer particles to fill the spaces left in the coarsewith finer particles to fill the spaces left in the coarse particle array
• By careful blending of sizes (e.g., using trimodalparticle size), formulations are processed with 86‐88% AP by weight (still fuel rich), yielding cured propellants with acceptable mechanical properties
• When Al is added (typically 10‐30 μm), a corresponding volume of fine AP is removed
• Thus the Al particles are preconcentrated in pockets of• Thus the Al particles are preconcentrated in pockets of coarse AP packing pattern
39
Progression of burning front into an aluminized propellantaluminized propellant
The size distribution of agglomerates on the surface depends on• The structural features of the Al particle packing• The surface retention characteristics of the binder (e.g., particle adhesion in a binder melt layer, retention in a carbonaceous surface structure, direct adhesion to underlying Al particles, centrifugal force in spinning motors)
• The susceptibility of individual particles to ignition• Possible connective processes between concentrating particles that lead to groupPossible connective processes between concentrating particles that lead to group behavior
• Evolution of the underlying structure and hot overlying gas flamelets that lead to detachment‐ignition events 40
Effect of pressure on mass‐average agglomerate size for bimodal propellantagglomerate size for bimodal propellant
Coarse/fine AP of /ratio 8/2
Coarse size 390 μmCoarse size 390 μm
Fine size as indicated
41
Typical burn‐out oxide distribution
• Generally, the distribution below 2 μm results from the Al2O3 formed in the cloud around the burning particles (smoke), whereas the larger sizes result from Al2O3 accumulation on the original Al particles
• From a practical viewpoint, a mean size distribution of 10‐25μm is ideal for stabilizing combustor instabilities in the 200‐400 Hz range 42
Factors influencing agglomerates• Large agglomerates result in large residual oxide droplets• High aluminum content in the propellant leads to large
agglomeratesagglomerates• Factors that increase burning rate reduce the amount and
size of agglomerates• Ingredient particle size distributions that yield large
concentrations of aluminum in the propellant particle packing pattern give large agglomerates
• Use of burning rate suppressants and binders that produce increased melts and carbonaceous residue on the burning surface usually lead to increased agglomeration and
l t iagglomerate size• Modification of the aluminum particles in a way to
minimize inter‐particle sintering reduces agglomeration ( i f idi i i i id i )(e.g. coatings for aiding ignition, pre‐oxidation)
• Cross‐flow environments tend to decrease agglomeration43
Example of burning propellants with nanometer and micron sized Al particlesnanometer and micron sized Al particles
2~1.8 ×’s increase in rb
1.5 rb = 2.538P0.618
9% nAl (ALEX) / 9% μAl
0.5
1
43 μm Al 80 nm Al
5 mms
s
00 0.5 1 1.5 2
rb = 2.092P0.390
18% μAlCombustion of μm Al far from surface
Combustion of nm Al close to surface
log (P) [MPa]T. R. Sippel, S. F. Son, L. J. Groven, Aluminum agglomeration reduction in a composite propellant using tailored Al/PTFE particles, Combustion and Flame 161 (2014) 311–321.Mench, M.M., Yeh, C.L., and Kuo, K.K., “Propellant Burning Rate Enhancement and Thermal Behavior of Ultra‐fine Aluminum Powders (ALEX),” in P f th 29th A l C f f ICT 1998 30 1 30 15Proc. of the 29th Annual Conference of ICT, 1998, pp. 30‐1 – 30‐15.G. V. Ivanov, and F. Tepper, Challenges in Propellants and Combustion 100 Years after Nobel, pp.636‐645, (Ed. K. K. Kuo et al., Begell House, 1997).
44
Al Burning Times
10000
Experiment
100
1000m
s]1173 K
1
10
00
ng T
ime
[m
d21500 K
0 01
0.1
1
Burn
i
d1
2000 K
Anticipated
0.001
0.01
0.01 0.1 1 10 100 1000
d AnticipatedTimes
Diameter [μm]
45
Example of Burning Propellants: the SurfaceSurface
aggregate of nmAlμmAl agglomerate
surface
50 μm500 μm
50‐250 nmAl emerging from surface as aggregates (pre‐
50 μmAl spherical agglomerateformed by inflammation of an
agglomerate)aluminized aggregate
L. T. De Luca, L. Galfetti, F. Severini, L. Meda, G. Marra, A. B. Vorozhtsov, V. S. Sedoi, and V. A. Babuk, Burning of Nano‐Aluminized Composite Rocket Propellants, Combustion, Explosion, and Shock Waves, Vol. 41, No. 6, pp. 680–692, 2005
46
Why n ~0.3Why n 0.3
• Surface roughness and presence of cracks inSurface roughness and presence of cracks in oxide layer may alter relationship between dpand A for chemical reactions (Buckmaster andand As for chemical reactions (Buckmaster and Jackson, Comb Theory Model 17, 335, 2013)
fracDd⎛ ⎞ frac
pref
ref
dA A
D⎛ ⎞
= ⎜ ⎟⎜ ⎟⎝ ⎠
• n varies from 1 to 0 as Dfracchanges from 2 to 3
3
2 3f
frac
D
D⎝ ⎠
< <g
• For non‐fractal surfaces, tb ~ d
3 fracDb pt ~ d −
47
Sintering and reaction times are close that nanostructure is lost before it can be utilized
Sintering of NanoAluminum
0.68( 1)Dfpd
CoalescenceTime N= −ησ
t t d d t i it
• Sintering time consists of two times: time to heat the particles and the fusion time
η – temperature dependent viscositydp – particle diameterσ – surface tensionDf – aggregate fractal dimensionN ‐ number of primaries
P. Chakraborty and M. Zachariah, Combustion and Flame 161 (2014) 1408.
• Sintering time consists of two times: time to heat the particles and the fusion time • Loss of nanostructure (surface area) in < 20 μs• Experimental burning times 50‐500 μs
Wh i d dWhat is needed:• Approaches that enable smaller length scales, disable sintering, and enable distribution 48
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